Fuel Economy and MPG

Can I tune my engine for better gas mileage?  Yes, you can.  And you can do it without affecting performance.  If you don’t have the discipline to change the way you drive, however, don’t bother.  You don’t have to give up the performance and you can still put your foot down once in a while but if your two speeds are go and stop then you aren’t capable of doing this.

Disclaimer: What I am about to describe requires making changes to your engine management system.  By following these directions you may completely blow up your engine, damage your car, wreck, start an apocalypse, or begin the Rapture.

The Concept

The idea is to change the fuel consumption of your car while cruising and leave the performance when you hit the gas.  This is something that I read about other people doing and wanted to see if I could do it on my own car.  I have a B18 LS with stock compression and non-VTEC Crower 403 (stage 2) cams.  My rings are already smoked and I need a reason to put a B20 in the car so I really didn’t care that much about blowing it up.  I made only mild changes in a relatively small part of the fuel/timing map and netted a 10% increase in fuel economy.  I can now achieve 30+ MPG city/hwy combination driving.  That’s decent for an LS with aftermarket cams and a semi-heavy foot.  More effort can probably gain a lot more than this but I’m satisfied at this point with 300 miles per tank.  No pun intended but, uh, your mileage may vary.

EMS: Crome, Hondata, AEM, NepTune

This writeup is about making changes to your engine management system to increase MPG without affecting performance.  If you are looking for car mods other than fuel management, try a website like http://ecomodder.com/.

You must have an engine management system that can be tuned.  This can be a chipped ECU or one of the aftermarket EMS systems, a wideband O2, and a datalogging system.

If you don’t already know how to use your EMS or if you don’t know anything about working on your engine you should not be doing this.  The very fact that you are here to learn how to do something because you don’t already know makes you unqualified to do this.  So stop reading, give up on tuning for better MPG, and just put a 2×4 under your accelerator to keep you from pressing it too hard.

What not to do

Do not modify the map of an un-tuned car.  Your car must already be properly tuned to run safely and definitely before trying to modify it for fuel economy.  Do NOT modify any boost columns.  You really should not be modifying any columns over about 750 mb (millibars).

You should also avoid modifying your idle.  Changing this can affect start-up  and warm-up on extreme weather days and could make it difficult for the car to run.  If you end up modifying the car’s idle then you’ll inevitably have to play with the ECT compensation maps but you’ll have to wait for a very cold day to get this right.  If you modify your idle then you’ll have a lot more tinkering to do at some point instead of just changing it once and leaving it.

Be Careful

The big danger when making adjustments to your EMS is detonation.  Higher compression engines are more prone to detonation.  Leaning out the AFR makes an engine more prone to detonation.  Increasing ignition timing makes an engine more prone to detonation.  The steps described here require leaning out the mixture in certain areas and increasing timing so detonation is a real concern.  This is why you should not modify columns 7-10 because higher loads (higher load = more air) promote detonation.  But there is still is risk of detonation at lower loads.  I don’t know at what point you’ll reach the limit and start detonating and nobody will really be able to tell you because the modifications you will be making are not recommended.  Nobody makes any extensive tests of where detonation will occur in the cruising loads which makes this an “at your own risk” activity.

Set Up a Baseline

Tuning an engine is about measuring, making changes, then measuring again.  You need to set a baseline so you can measure to find out if your fuel economy is getting better – or worse.  Fill up your tank and do a normal tank’s worth of driving city/highway or whatever you normally do.  During this test keep your acceleration limited to columns 7 and 8 (800-900 mb) and don’t go above column 6 under cruising conditions.  Datalog your driving to find your average AFR in each cell for columns 3-6.

You’ll quickly get a feel for how much you should be pressing the gas pedal.  You will learn patience as you watch some cars blow by you after the light turns green but you probably will still be accelerating at an average pace.  You will learn patience.  If you can’t make it through this step without hammering the gas all the time then you’re done.  You don’t have what it takes to tune your engine for better gas mileage because you’ll always be in columns 9 and 10 and if you lean out those columns your engine will go boom.

Fill up your tank after you’ve emptied it and calculate your gas mileage for that tank.  Make sure that your AFR in each cell in columns 3-6 is as close to stoich ( 14.7:1 ) as you can get it.  If your AFRs are not consistent then you can’t reliably adjust them.

Carefully Modify your Fuel Map

Fill up your gas tank so you can measure your results on a complete tank of gas. Adjust the fuel in the cells you have logged in columns 3-6.  Do not modify any cells which have not been datalogged.  Leave the idle alone for now (under 1500 RPMs).  You may want to modify the idle later but do one thing at a time so you don’t screw it up.  I started by taking away around 10% of the fuel.

Now drive the car and log your new AFR.  I ended up with about 15.5:1 AFR but some people have gone as lean as 16.0:1.

Timing – Don’t Blow your Motor

A leaner AFR takes longer to burn so your old timing values for the modified cells are now wrong.  Getting the correct ignition timing is the most important part of tuning your car and making power.  The fuel air mixture burns over a period of time and generates a prolonged pressure wave pushing down on your piston.  It does not explode, it burns.  If the pressure wave builds too early then it is fighting the piston instead of pushing it, or worse, detonating.  If the pressure wave builds too late then it is chasing the piston instead of pushing it.  The correct ignition timing is the most important part of engine efficiency, which is fuel economy in this case.  Getting this timing wrong can cause detonation and blow up your motor which is more likely over columns 7 and up.

Adjust the timing of the modified cells while driving the car at a fixed RPM.  The safest thing to do is have someone help you with this.  Drive within the cell range and increase the timing by 2 degrees.  Did you have to slightly let off the gas to maintain the same speed?  Use your MAP readings to confirm that you are using less load on the engine.  If you lost speed then try going the other direction.  If there was no change then you should probably go back to the original values.  Sometimes I confirm no difference by going +/- 4 degrees which should definitely show something.  Too much timing increases the chance of detonation so be careful.  If the 2 degrees was an improvement, try another 2 degrees.  Repeat this process until you have no more improvements.

Compare to the Original Baseline

Drive the car for the full tank of gas in the same way as before.  Compare your fuel economy to the original baseline.  Check regularly for detonation to be absolutely safe.  You can do this by reading your spark plugs.  If your fuel economy did not change then go back to the original map.  If there’s no benefit then your engine should be run as normally as possible.


You may end up with a scenario where very light pressure on the gas pedal causes the car to accelerate and backing off the gas causes deceleration.  This makes it very difficult to maintain a constant speed. This is a good sign because it means that your engine is more than powerful enough to do what you want.  But it’s very hard to drive while always either increasing or decreasing speed.

If this happens then you need to modify columns 3 and 4.  Back the timing off on these columns until the car is capable of maintaining a constant speed.  Columns 1 and 2 are only for decelerating so don’t touch them.  Adjust your columns 3 and 4 until you can smoothly move from accelerating in columns 5 and 6 to constant speed in columns 3 and 4 and then to very slight deceleration in columns 3 and 4.  Once you hit columns 1 and 2 you’ll be decelerating pretty hard.

Better Fuel Economy, Same Performance

If done properly this should net some increase in fuel economy.  Since you didn’t modify columns 7 and up you still have the full acceleration characteristics of the engine.  While cruising you should have the benefits of increase fuel economy and when you put the pedal to the metal you should still have all the performance you want.  Always beware detonation when making any changes to AFR and timing.  Now go enjoy your leaner, greener, cheaper, yet still fast ride.

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