How are engine displacement and power/torque related?

I got involved in a discussion elsewhere on this topic, and wanted to share my response here as well.  This is meant to be a solid explanation in layman’s terms, for those who don’t want to dive down a big physics and thermodynamics rabbit hole!

While I’m an automotive engineer I’m ashamed to say that I still don’t really understand the relationship between displacement and power/torque produced. While I assume that the difference between the 1000+hp – 8l engine in the Veyron and the 645hp – 8.4l engine in the Viper is mostly determined by turbos I would prefer a more detailed explanation.

Leaving out for a moment questions of efficiency, turbocharging, and a lot of other smaller factors:

  • Torque is most proportional to displacement. This is mostly a matter of how much fuel you can burn per cycle of the engine. Torque is a force, and applies to questions like, “how heavy a car can I push up this slope?”
  • Horsepower is proportional to the product of torque and engine rpm. There’s a constant in the equation, but otherwise it’s a direct relationship. Power applies to the question, “How fast can I push this 4000lb car up this slope?”

Everything else is just a factor that modifies those two variables. Let’s take the steady-state example of a truck climbing a steady grade at a steady speed – it’s actually simpler to understand than everyone’s favorite “drag race” example. Want to increase the amount of load you can carry up the hill at a given speed (increase the power)? Here are the ways you can do it:

  • Make the engine bigger. If everything is proportional so that your efficiency is the same, your torque will go up proportionally as well, because you’re ingesting more oxygen and burning more fuel. This means your power will also increase proportionally. More torque at the same speed (more power) means you can pull a heavier load up the hill.
  • Spin the engine faster for the same road speed (RPM). You’re still making roughly the same torque at the engine, but to maintain the same road speed, you will have had to change the axle/transmission gearing. This gives your same engine torque more “leverage” on the road. This example both shows the difference between torque and power, and shows you why it’s power that matters for climbing hills. Looking directly at the power really tells you what your engine can do at a given road speed once you’ve factored in all the gearing – it simplifies everything (better tool for analyzing that type of job).
  • “Fake” making the engine bigger. You can do this with turbocharging, supercharging, nitrous oxide … your choice. Either way, you’re using an external component to force additional oxygen and fuel into your engine, faking the behavior of larger displacement. The result is more power. This solution will almost always be more efficient for some operating conditions and less efficient for others, so you get to pick where you gain and lose economy, too. You have to do more work “stuffing” in the extra air, which reduces efficiency, but it can let you tune for better efficiency when you don’t need full power. Ford Ecoboost is a good example of this idea.
  • Improve overall efficiency. You can do this by increasing compression, tweaking your spark timing, mechanical/frictional tweaks, anything that gets more of the energy from your fuel to your tires instead of going out the tailpipe and radiator. You tend to be pretty limited by your fuel quality here compared to the first three options.
  • Improve efficiency at the engine speed you’re operating. Change your valve timing. Here, you’ll trade better efficiency at the RPM you care about for worse efficiency elsewhere. Your limit here is that you still have a “peak” torque value proportional to displacement, which you can move around with valve timing but not really increase. Assuming you don’t change your gearing (RPM) at the same time, once you get to the point where your peak torque is at the RPM you’re climbing the hill, you’ve gained all you can with this option.

In short, power is everything. Torque only really matters in that you’d like most of it to be “well distributed” across engine RPM, instead of very concentrated in a narrow band – this just makes your engine more versatile and nicer to drive. However, for pulling a hill, etc, the question of “not enough torque” is always solved by “more gear”, because the power is the same either way; that power is really just a matter of how much oxygen you can stuff in, and how much heat you lose from there to the tires.

For a good comparative example, consider the difference between the 110ci engine in a Miata and the 300ci engine in a mid-90’s Ford. I have both. Both make roughly the same HP, plus or minus a few – around 140.

The Miata has high compression, good mechanical efficiency, and all of its variables (valve timing, etc) are tuned to maximize the available torque and power from 5,000 to 7,000 RPM. It’s torque curve is very peaky, maxes out at about 115 lb-ft, and below 3,000 it’s essentially worthless. This is okay for acceleration, because everything is lightweight, and the car has very steep axle gearing (4.56:1) to try to keep it where it makes some power. However, you’d never want to tow anything with this engine, because the high RPM and compression really limit reliability if you needed to make the full 140 horsepower long enough to, say, climb a 10 mile hill, something you’d never need in a 2500lb car even at full speed. You need five (efficient, manual) gears at a minimum to keep this little engine where it will get out of its own way, and you’re shifting constantly in hilly terrain and traffic.

In contrast, the 300 is in a 90’s van with a three speed automatic, probably the most reliable but inefficient transmission Ford ever produced. Because of the massive energy-suck of the transmission, considerably less of this engine’s power gets to the road than the Miata’s. It’s in a vehicle that weighs double what the Miata does, and which will happily tow its own weight – so this engine is happy moving four times the load of the Miata. Why? Rather than focusing on a narrow “happy” spot, the design focused on distributing it’s torque out well. It doesn’t have overwhelming “go” anywhere, with only 260 ft-lb of peak torque limited largely by very low compression compared to the Miata; at the same time, what “go” it has is available everywhere (over 200 for almost the entire operating range). It makes its maximum power at only 3500 RPM, which it will happily do all day long, on crappy fuel, in lousy, hot, humid weather. Because the torque curve is so flat, you almost never find yourself shifting for any hill but the most extreme. It’ll never get anywhere as fast as the Miata, but it will go everywhere with extreme reliability doing four times the actual work, strolling along like a big, dopey draft horse.

You can dive down the rabbit hole all day with the hundreds of smaller variables that affect torque and power, but sometimes the basics are better summed up with no math and a little example or two. If nothing else, hopefully this version was entertaining.

Second failed distributor

So far the parts stores are 0 for 2 on good distributors.

Failed distributor drive gear
Another failed distributor! This one a reman.

That’s what’s left of the bronze distributor gear on the NAPA Echlin reman I installed in the last post.  This gear survived around 1,000 miles at most.

On the other hand, a postmortem on the pair of dead distributors, and a review of the Ford documentation on distributor drive gears, has shown me a likely common cause for both failures.

Ford indicates in the linked document that “very little or no shaft endplay… has been found with new and remanufactured distributors. Improper endplay may force the gear against the support in the block or hold it up off the support, causing damage.”

Before I began the repair, I checked the distributor shaft end play on both the NAPA with the failed gear, and the Rich Porter with the failed rotor plate (after pressing it back on).  Both were in the neighborhood of 0.010″ to 0.012″, which is substantially less than the 0.024″ to 0.035″ called for by Ford.

The distributor is a steel shaft in an aluminum housing.  As the assembly heats up, the aluminum grows more than the steel shaft, and the end play measurement decreases.  If you have too little end play, the end result can be that your clearance goes to zero, trapping the housing between the rotor plate and the drive gear.  This could easily either press the rotor plate off its splines, or in the case of the NAPA unit, put so much load on the softer drive gear that it wore out almost immediately.

I needed a rapid fix, and swapped the perfectly good steel gear from the failed Rich Porter onto the NAPA distributor.  Since I had to re-drill the roll pin hole in the process anyway, it let me set my own clearance, and set it properly.  I set the clearance to 0.032″, and so far I’ve had zero issues since the repair (approximately 1000 miles).

As the NAPA gear wore, it manifested in progressive loss of base timing as the teeth wore away.  When I sorted out the cause of the problem I was having (misfire, loss of power and fuel economy), I measured a loss of 6º of base timing on the #1 cylinder.  However, #1 was one of the least worn teeth, visible at the bottom in the photo.  Based on the wear in the other teeth and the difference in rotational play with the distributor still installed, I was losing at least 10º on the #6 cylinder, where I was seeing the most misfires.

Currently, after a timing re-check yesterday, I’ve lost less than 1º of timing since I set it after breaking the gear back in.  Actually, I’d say zero, but my timing light just isn’t that accurate.

As a note on the Ford 300 inline six, there’s very little drawback to setting your distributor shaft end play high.  Unlike the V8 engines, the distributor rotates clockwise from the top, and as you can see from the wear on the gear, that means the gear rides up on the plain bearing surface at the bottom of the distributor housing, not on the gear support block inside the engine block.  Because of the load of the oil pump, the gear will stay up against the housing steady as the engine is running, so you won’t have a timing variation.  A bit more end play just puts your rotor a tiny bit higher in the cap – nowhere near enough to cause an interference.

Failed TFI aftermarket distributor

I experienced a new-to-me form of failure a week or so before Christmas, and thought I’d share the details, since even a pretty detailed Google hunt failed to turn up any other account of this problem.

The vehicle is a 1996 Ford Econoline with a 300 I-6. After driving around perfectly for an hour, it suddenly lost most of its power mid-drive, running smoothly but unable to exceed about 10mph. Manual shifting of the C6 proved we still had both first and second, and it still started acceptably (if weak), with no sign of engine shakes or cylinder misfires. A quick roadside diagnosis showed no new codes, nothing out of the ordinary in the OBD II data stream, and a look at the distributor indicated it was still tight and hadn’t shifted from the previous owner’s paint mark (which was correct, I’d checked timing in November after purchasing it).

After a tow home, I began diagnosis. After eliminating some of the other basics, I got the timing light out, and found it was running with a base timing of about 20* ATDC. Loosening the distributor hold down and twisting in about 30* more timing immediately removed the symptoms. Then, it was time to find the cause.

With a loose hold down bolt ruled out, the usual suspects would be the timing set, the distributor gear at the cam, or the shear pin that holds the gear to the distributor shaft. A dead timing set or stripped distributor gear usually mean no start, not timing slipped. I suspected the shear pin might have went, with the gear just tight enough on the shaft to have “stuck” after losing some timing.

I pulled the cap and rotor, and everything looked normal at first glance. Here’s a shot after having pulled the distributor.

Richporter TFI distributor
This is the failed distributor, which looks innocuous with the shutter wheel still on the shaft.

However, once I grasped the shutter wheel and gave it a bit of light torque, I immediately felt a “notchy” click, and was able to rotate it.  The possibility of a magic “half-stripped” distributor gear went briefly through my head, but it didn’t take long to realize the distributor shaft wasn’t turning at all.  In fact, the shutter wheel popped right off in my hand.

Failed distributor
This shot shows the failed component. Notice the stripped splines where the TFI shutter wheel should press onto the shaft.

At that point, it was obvious what had happened, though I still can’t point to why.

I pulled the distributor, verified the gear and shear pin were in fact fine, and popped in a NAPA reman, which was the only thing I could get locally that day.  The failed unit was a Richporter Technologies, and the NAPA is a reman Motorcraft.

I still have no clue why the original distributor was replaced by the previous owner – I’ve never had an original actually fail, and this engine has pretty low mileage for a 300.  I’m guessing his mechanic swapped it in when they were trying to hunt down a SPOUT circuit error, which I suspect is part of why I got this van so cheap.  That was something simple I fixed five minutes after we bought it – a slightly loose terminal at the back of the SPOUT connector.  Haven’t had a single real issue with it other than the SPOUT issue and the newly failed aftermarket distributor.

Identify your axle gear ratio

Have an axle with an unknown ratio that you’d like to identify? Want accurate results? Here’s a simple, dead accurate method that will give you results on any rear wheel drive axle (or any front axle for a four wheel drive), as long as the axle in question is driven by a driveshaft. Sorry, front wheel drive folks, you’re generally out of luck on this one. This is a single-person technique. Helpers are not required, though one does make the counting a bit quicker.

Tools required:
Chalk, crayon, or paint pen
Jack stands

I make the general assumption that you already know to follow all the necessary safety techniques. If you don’t already know how to do anything required here safely, find a friend to learn from, or another source of knowledge. General work safety practices for cars abound on the internet. Hence, work at your own risk.


1. Determine if you have an open differential, or a traction aid (limited slip, locker, etc.). If you don’t already know, here’s how. Jack up the axle and put both sides on stands, so both wheels are off the ground. Leave the transmission in park or in gear (manual). Rotate one wheel by hand, while watching the opposite wheel. If the other wheel rotates easily in the opposite direction, you have an open differential. If there is resistance to rotation or no rotation, you have a LSD or locker. To verify a traction aid, place the transmission in neutral, and both wheels should rotate together when you turn one by hand. I’ll note differences in later steps between open and traction aid techniques.

2. LSD/Locker: leave the car on the stands. Open diff: lower one tire to the ground, leave the other on a stand.

3. Mark the tire and driveshaft. Put one mark on the tire sidewall where you can easily see it, and another on the driveshaft near the axle. You’ll need to be able to see both from where you’re working unless you have a helper, so they should both start out facing you. If you have a lot of tire clearance to your fenders, it’s often easiest to start with the marked spot toward the ground, and a block of wood or rock to use as a reference point. You want to be able to count tire turns to within a few inches of your starting point; more accurate than that won’t really be necessary.

4. Now, you’ll need to rotate the tire while counting turns of the driveshaft using your mark. Rotating forward or backward doesn’t matter. Turn the tire 10 turns for an LSD/Locker, 20 turns for an open differential. When finishing, try to count your last driveshaft turn to the nearest 1/4 revolution.

5. Divide the number of driveshaft turns you counted by 10 (same for both LSD/Locker and open). That’s your gear ratio.


  • You determine your pickup has an open diff, so lower one tire to the ground. You turn the tire 20 turns, and count 35-1/2 turns of the driveshaft. 35.5 / 10 is 3.55 – a common stock gear ratio for Ford pickups.
  • Your car has a limited slip, so you leave both tires up. You turn the tires 10 times and count about 27-1/4 to 27-1/2 turns. This will be either 2.73 or 2.75 depending on your axle make and what ratios are available.

The last number is always a bit of a fudge with this method, but always close enough, as you’ll basically never encounter a single axle make that has two different ratios available that are so close. You can always count on the first two numbers, such as 2.7x and 3.5x, being dead accurate, and that’s always close enough to identify the exact ratio once you know what axle family you have. For instance, with a Ford, the 2.7x example is going to be 2.73 if you have an 8.8″ or 7.5″ axle, or 2.75 if you have an 8″ or 9″ axle.

I’ve used this method many times over the years, since it’s dead on and works great whether the vehicle still has its stock gears or not; axle tags are only useful if no other previous owner decided to regear. I also take a crayon with me any time I’m headed to a junkyard or to purchase an axle, for the same reason.

Happy counting!

Exporting from Revit 2014 BIM to ArcGIS

And now, for something completely different.

This is an issue I ran into at work over the past week. We needed to get topography from a project I was working in BIM into ArcGIS for our GIS modeling folks. Days worth of googling revealed absolutely nothing useful – it was evident that someone had kludged together a few successful techniques back in 2009 and 2010, but there was absolutely nothing recent. Either BIM-based engineers no longer needed to get their info out to big-world GIS folks (unlikely), or no one who’s solved this problem since 2010 has taken the time to document it where the rest of the world can find it. It turns out the entire process takes about 15 minutes, but sorting out the process can take a week of frustration based on the information that’s readily available.

The problem: A Revit 2014 model, consisting mostly of site topography with a few “building”-like features. The model is built on a 2D survey with a vertical datum of NOAA NOS MLLW, and a horizontal datum of Virginia State Grid (South Zone) NAD 83. None of that really means anything electronically to anyone who isn’t trying to get a conversation going between BIM and GIS. The relevant part of the model in Revit looks like this, in 2D and 3D:


Revit-ArcGIS Example - Raw REVIT 2D
Example File – Revit 2D


Revit-ArcGIS Example - Raw REVIT 3D
Example File – Revit 3D

It’s important to note before we begin that you may need to export in both 2D and 3D to get your GIS coworkers everything they need.  The 3D export will carry your model elements over, but they will often need contour lines for your topography, and contour lines are a 2D only element.  The only way we’ve found to get both your elements and contours into GIS is by exporting a 2D contour set to go with the 3D model.

Revit has a lot of user confusion due to multiple coordinate systems. There’s an “internal” system that the user never sees, a “Project Coordinate” system controlled by the Project Base Point, and a “Shared Coordinate” system controlled by a Survey Point. Revit has limitations on how far parts of the model can lie from the internal origin, so it’s best to keep your Project Base Point somewhere near most of your actual model. In our case, the Project Base Point (and internal origin point) is a circular symbol to the left of the 2D, near the corner of a large flat spot that’s actually a helipad. We can’t move the Project Base point to our survey grid origin, since that’s 12 million feet to the west and far out of Revit’s comfort zone.

What we can do is put the Survey Point out there, thus setting the Shared Coordinates to match NAD83. This seems to work just fine. My file has a 1000′ grid of coordinates which are just out of view in the 2D above. To set the survey point, find the nearest convenient known point, and use the following tool: Manage tab > Project Location > Coordinates > Specify Coordinates At Point. You’ll need to use this tool once each for Northing and Easting.

What if you’re already using Shared Coordinates to coordinate between several discipline files? In that case, create an alternate Location. Go to Manage tab > Project Location > Location > Site:

Revit Site Dialog
Revit Site Dialog

Click Duplicate… to make a new site, give it the name of your datum, and make it current.  Now, you can go back to moving the survey point, and you’ll only affect the survey point for this saved Site.  Once you’re done exporting, go back to this dialog, make your previous coordinate set current, and you’re back as you were.

Next, let’s export the 2D first.  2D exports are per view, so you’ll need to create a view at 12″:1′ scale containing exactly what your GIS team needs to see.  Use VG to turn off everything extraneous, and set your crop region to exclude the large area of survey you might have in the file outside your project area.

Now, we need to set a few options.  Head to R > Export > Options > Export Setups DWG/DXF (in my case at least – my GIS coworker knew she could work with DWG data to get the results she needed) to set up your 2D export options.

Revit Export DWG Setup Dialog
Revit Export DWG Setup Dialog

You could obviously spend half a day in here tweaking colors, layer names, and lineweights, but your GIS folks can override all that when they import anyway.  The only important things to change are all buried in the depths of this dialog.  First, hit the Units & Coordinates tab:

Revit Export DWG Setup Dialog - Units Tab
Revit Export DWG Setup Dialog – Units Tab

Change the DWG unit to Foot, and the Coordinate system basis to Shared, as shown.  Our first attempts at this had the 2D importing at 1/12 scale, hundreds of miles away – these settings fixed that.  Next, go to the General tab:

Revit Export DWG Setup Dialog - General Tab
Revit Export DWG Setup Dialog – General Tab

The important one here is the file format.  My coworker had the best luck with AutoCAD 2007, based on the version of ArcGIS she’s currently using.

Next step, export your 2D.  R > Export > CAD Formats > DWG gets you here:

Revit DWG Export Dialog
Revit DWG Export Dialog

I didn’t have to make any changes.  Hit Next… to export, select your file location and name.  If all went well, once your GIS friend imports and tweaks, you should be able to get something a bit like this:

Revit-ArcGIS Example - Resulting 2D
Revit-ArcGIS Example – Resulting 2D

Now we can move along to exporting the 3D model.

My GIS coworker had the best luck with an IFC import.  This exports your entire BIM model, so there’s no need to create a special view.  However, you can control what exports in R > Export > Options > IFC Options:

Revit IFC Options Dialog
Revit IFC Options Dialog

You could spend all day in there tweaking, but starting out, the default arrangement worked pretty well for us on this project.  All of the settings we used for the 2D export in terms of units and coordinates seemed to carry over well to the 3D export.

Cross your fingers and head to R > Export > IFC.

With everything already set up, my coworker was able to import the IFC to ArcGIS, and everything still projected correctly.  Here’s the final result, 2D topography contours (dark) and a 3D topography mesh (light) overlaid on a satellite photo.

Revit-ArcGIS Example - Resulting GIS
Revit-ArcGIS Example – Resulting GIS


Machining differential carriers for the Ford IRS

Using a non-IRS limited slip in a 7.5 or 8.8 Ford IRS axle requires the builder to choose between leaving out the axle retainer clips, or machining the differential side gears to accept them.

For those who want to keep their retainer clips, I’ve come by the following diagram for machining the side gears:

Machine work diagram
Machining Differential Side Gears for the Ford 7.5 and 8.8 IRS

I originally found this drawing on TCCOA in a post here, courtesy of 392Bird.  No idea where it originally came from.

The side gear in the drawing is straight cut, unlike the gears used in the Trac-Loc, but that doesn’t really matter.  What’s important here is the bevel machined between the inboard side of the splines and the gear face for the c-clip.  This bevel will allow your IRS retainer clip to compress, so that you can remove the half shaft without having to disassemble the center section to compress the clip.

Measure twice, cut once

This is an example of why you always check your assembly tolerances as you’re putting an engine together.  The image that follows is the cam sprocket from an Edelbrock 7814 set I purchased last week.

Defective cam gear
Defective Edelbrock 7814 cam sprocket

Can you spot what’s wrong already?  If you can’t see it in the view above, click the link to the full resolution of the photo.

Ford uses a simple system to control camshaft end play.  The sprocket face above bolts to the end of the front cam bearing journal, creating a gap between the second step on that sprocket and the journal face.  Sandwiched in that gap is a hardened steel or cast iron camshaft retainer plate.  The difference in thickness between the gap and the plate thickness ends up being the end play.

When putting together an engine, my normal procedure is to install the cam, bolt up the cam sprocket temporarily so that I can check end play while it’s easy to get to, and then remove the cam sprocket and proceed to the crank and the rest of the rotating assembly.

Since I always measure and remove anyway, I didn’t pay too much attention to the new cam sprocket when I was installing it last night, which gave me ZERO end play once installed.  The cam would turn by hand, but I was probably less than a thousandth of an inch of luck from full bind.  Of course, running the engine in this state probably would have lunched the build very quickly, which is why it’s best to measure everything as it goes together.

I was re-using the original cam with a new camshaft retainer plate, so my first thought was a machining error in the Ford plate.  I swapped to the old Ford retainer plate and got the exact same result.

I’d have saved myself about ten minutes of head scratching if I’d looked at the sprocket first.  If you look at the two protruding faces, neither is machined.  They are both as-cast surfaces.  You can see a bit of the hub of the original sprocket in the bottom left of the photo, for a comparison of what that finish should look like.  Or, just take a quick peek below.

Simply put, someone at the factory making this part for Edelbrock just missed a step.  At some point during manufacture of this piece, it should be chucked in a lathe, and those two surfaces turned to meet the end play tolerance.  Someone just missed it.  This surprised me, since it’s the only defective part I’ve ever personally gotten from Edelbrock.

Luckily, a phone call quick trip to Advance the this morning before work got me a part that was completed.  I’d bought their only 7814 in the region, so I paid another $12 and swapped to an Edelbrock 7811, which is interchangeable with the 7814 other than a pair of extra keyways to advance and retard the cam timing.  Problem solved, no hassle, and my one Edelbrock “oops” will go back to them.

Here’s the 7811 with the correct machining.  You can see there are four lathed surfaces here that were missed on the first one.

Edelbrock 7811 sprocket
Edelbrock 7811 cam sprocket

A Note on Measuring Camshaft End Play

I’ve noticed a very common and very critical omission in most of my shop manuals that turns up on forums fairly often.  Off the top of my head, this tidbit is missing in two Ford factory shop manuals I own (1993 Mustang and 1991 Truck), as well as a smattering of Haynes and other manuals I have.

All of these references instruct you to install the cam, install the retainer plate, check the camshaft end play (the tolerance is 0.0055-0.009 for fuel injected 302’s), and then install the timing set.

If you follow these instructions as they appear in every manual out there, you will do a lot of head scratching, because you’ll see anywhere from 1/10 to 1/4 of an inch of camshaft end play.  Don’t worry!

How to measure camshaft end play (the one and only way that actually works):

  • Install camshaft
  • Install retainer plate
  • Install and torque cam sprocket
  • Measure end play
  • Remove sprocket so that you can install remainder of timing set

Without the sprocket installed, the only thing controlling your end play is the plug in the camshaft bore all the way in the back of the block.  The sprocket must be installed in order to measure, but I have yet to find a manual that points that fact out, at least in my collection.

This little anecdote is personal experience, as I was one of the head scratchers the first time I ran into this.

Fuel octane myths

Higher octane fuel will give me more power!

This is the big one.  Unfortunately, higher octane fuels do not generally give you more power.  If only it were so easy.  A higher octane fuel has almost exactly the same energy as a lower octane fuel, and if all goes well, it burns identically.

There are exceptions to this in the automotive world — quite a few modern cars run knock sensors, and with a lower octane fuel, the computer will be forced to retard timing to prevent knock, which will typically cause power to fall off.  HOWEVER, this does not mean that a higher octane fuel will bring more power, only that running the grade of fuel the engine was designed for will provide optimum power on a car equipped with a knock sensor.  Once you reach a point where octane is sufficient such that the computer doesn’t have to pull timing, there are no more gains to be had.

Higher octane fuels burn hotter.

One common myth concerns the ignition/burn temperature of higher octane fuels.  Many people, including quite a few who should know better by now, continue to perpetuate the myth that higher octane fuels require more energy to ignite, or burn at a different temperature or rate (usually “more slowly”).  This is simply not true.  Though burn rate can vary between fuels with all other things being equal, this is not linked to the fuel’s octane rating, and other variables like mixture quality and distribution have a MUCH greater effect on burn rate.
Octane is also not directly a measure of the amount of compression it takes to initiate burn (preignition or detonation).  Although they are often connected, knock is not necessarily directly linked to preignition and detonation.  Knock, specifically, is a violent resonance of the gases in the combustion chamber, causing severe spikes in pressure and temperature, and usually audible from outside the engine.

It is possible for detonation (spontaneous ignition and simulaneous burn of the entire air-fuel mix ahead of the flame front created by the spark plug) or preignition (spontaneous ignition of pockets of flame ahead of the main flame front, caused by heat and pressure, which then burns at a normal rate) to occur without knock, and it is also possible for knock to occur without preignition or detonation.

This phenomenon was studied extensively in the 40’s via high speed camera by NACA (predecessor to NASA) when they were developing many of the more sophisticated late WWII era piston aircraft engines.  However, with the massive shift in research from piston to turbine engines at the end of the war, a lot of the NACA research was filed away and forgotten.  Their information, which is now freely available from NASA over the web, is a treasure trove of information on detonation, supercharging (including turbocharging, which is simply a form of supercharging), fuels, water injection, and even things as odd as using nozzles on jetted exhaust pipes to gain thrust (don’t get too excited, it doesn’t benefit much below about 250mph, and is useful mostly because propellers start to lose efficiency at higher speeds).

For more information, the NACA article covering knock and detonation can be found on the NASA Technical Reports server.  Search for NACA-TR-912 and enjoy!

Simple electric fan control

Want to make an electric fan control harness without having to buy someone’s expensive solid state setup?  See below for three ways you can go about it, depending on how many features you want.

  • All of these setups are based on a Ford Taurus fan, or something with a fairly equivalent amp draw (approx. 38A continuous once it’s running).
  • Relay SS74 marked in the diagrams is also equivalent to a NAPA ST85 or GP Sorensen SF74.  You need a continuous-duty rated relay, with a capacity of at least 60A and referably closer to 80A for reliability.  You also need the relay to have an isolated ground to work with these schematics.  You can do this with a BWD SS664, which is easier to find, but the diagrams would need to be modified to suit the different relay design.
  • FS8 is a temperature switch.  Very important, you need a switch that is normally open, not a temperature sender.  Here is where you’ll need to do some hunting, because you need to find something with the right thread for you to mount somewhere you can use, and with the temperature range you want.  FS8 used to be an easy pick for Fords with an adjustable temperature, but I haven’t been able to find it lately.
  • Other relays shown here can all be a standard Bosch style auto relay.  I’ve been getting mine from All Electronics for years.  No financial interest in them, I just find them to be a good supplier, and they seem to have the best price on these.

Scheme 1

As basic as it gets, here.  This one will run any time the coolant is over the set temperature, including with the car turned off.

Scheme 1
Electric Fan Control – Scheme 1

Scheme 2

This adds an AC Demand input to Scheme 1.  This wire is usually easiest to find at the system pressure switch in your underhood A/C system.  This wire will be powered all the time when your A/C is turned on from inside.  For most cars, there are two wires here – make sure you tap into the wire that heads for the interior, not the one that runs from the pressure switch to the A/C compressor clutch.  The compressor clutch wire will cycle on and off with A/C system pressure, the wire to the interior will not.

Scheme 2
Electric Fan Control – Scheme 2

 Scheme 3

This is the most versatile setup.  Here, we add a switch (run to the interior) which will allow you to turn the fan on manually, off completely, or allow the same automatic operation as the first two schemes.

Scheme 3
Electric Fan Control – Scheme 3

You can easily build off these schemes for more complex setups.  A relay added off the I terminal of the SS74 will provide the ability to turn off the fan automatically when the ignition is off, for instance.  One more relay, another sender, and a second SS74 will use both available speeds of the fan, though I’ve never personally seen much use for the low fan speed.

Please feel free to disseminate, modify, and enjoy!


Chassis Dyno technology

I’ve gotten into a surprising number of discussions over the years about dynamometer technology and sources of error.

Most research facilities and race teams using engine dynamometers have the expensive, high-tech kind.  They are basically an energy absorber, which can dissipate the engine’s energy real-time by turning it into heat, using electricity, hydraulics, or a number of other systems.  You crank up the engine power, set the dynamometer to hold the engine at a fixed engine speed, and measure the engine torque at either the engine mounts or the mounts for the energy absorber.  Math does the rest.  These systems are obviously relatively expensive, but they can measure engine output real-time at a steady state for hours, days, weeks, or sometimes months.

Because these systems are expensive, and obviously require you to have your engine out of the car, most enthusiasts do most of their measurement using chassis dynos.  Many chassis dynos are inertial dynos.  The drum you’re riding on is a tuned mass flywheel.  You measure torque and horsepower by using an accurate speed pickup on the drum to measure instantaneous speed and acceleration.  Rotational intertia of the drum times acceleration gives you torque applied to the drum (force).  Torque applied to the drum times the rotational speed of the drum gives you power.  The natural units for this system to work in are power and accelerative force vs. vehicle speed, because these are the things measured directly by the dyno — you don’t need any additional equipment or knowledge (of gearing, for instance) to figure this out.  This is also why a lot of not-well-set-up dynos like you might find at votech schools, etc. are only set up to give you power vs. road speed.

Now, how do you get this in terms of engine RPM?  You can either measure RPM directly or with an inductive tachometer, or you can calculate your RPM based on gearing and road speed.

This is all necessary to understand why you get different power outputs measured in different gears, which is a frequent source of questions I’ve encountered over the years.  You’re measuring power by measuring the acceleration of a system with a known rotational inertia (the dyno flywheel).  Now, your engine is linked to the drum via the transmission and driveline, so some of the power being produced goes into accelerating the crankshaft, clutch assembly, tranny innards, and wheels.  These automatically get factored out of the equation because you’re using the inertia of the flywheel alone to figure power — that’s why you’re getting wheel horsepower numbers, since it’s power after you subtract what is required to accelerate the driveline and wheels.  Basically, your first major error occurs because inertial dynos can’t measure at a static speed.

In lower gears, your engine/flywheel/input shaft are obviously turning faster in relation to the dyno flywheel.  Hence, you have to accelerate the drivetrain more in order to achieve the same dyno flywheel acceleration.  That takes power.  In lower gears, more power is taken up in accelerating the engine, leaving less to accelerate the car.  That’s why lightweight flywheels make a difference in low-gear acceleration, but practically none in high gear.  This shows up on the inertial chassis dyno as well.

Why might your dyno show less of a power loss in low gears, compared to someone else’s dyno?  Well, it depends on the flywheel mass of the dyno in relation to your power level.  You need a heavier flywheel to measure a more powerful car accurately, because you have more time during the runup to take accurate readings.  If you run on a heavy flywheel, the inertial mass of your driveline is small by comparison.  For the same power output, the entire system will accelerate more slowly, so you lose proportionally less power to accelerating the drivetrain.

This is certainly not the only way to measure power.  It’s cheap and simple, but it’s definitely not ideal, because as I mentioned the mass of the drivetrain/powertrain creates an inaccuracy.  To measure the true wheel horsepower (after friction losses, but before inertial losses), you need a dynamometer with an energy absorber – similar to the engine dynamometer we first discussed, but built into a chassis dyno.  These chassis dynos are based on driving the drum with a fixed, applied resistance from an absorber that can hold the drum speed steady.  These absorber systems can be water brake dynos, electric dynos, or even old school friction brake dynamometers, just like the options for engine dynos.  You measure the force needed to counteract the car’s output and maintain a constant measured speed.  It eliminates inertia from the picture, since the dyno will allow you to maintain a constant speed indefinitely (well, in theory, but the equipment has to dump all that waste energy somewhere ).  These are the dynos you use for serious development like tuning intake and exhaust systems, where you need the engine installed in the car but also need to maintain a given speed while measuring.

On the other hand, inertia dynos have a real advantage in telling you what your actual on-road behavior would be — assuming you have a drum the right inertia for the car.  If you tune it so that the engine on the dyno accelerates in gear at the same speed it would in that gear on the road (restrained by the mass of the car), then you will get a fairly accurate measurement of how much power you’re losing in that gear to rotational inertia.