Thread: Tuning for Gas Mileage

Let me be the first one to start a gas mileage tuning thread.

In this thread I plan on listing techniques and see what seems to be working best. Please note this will be mainly ECU tuning.

Prerequisites
Before you start tuning, you need to make sure your car is in good condition. Having broken/old parts means the engine not running efficiently. Inefficient engine means bad power and gas mileage.

Maintenance Parts

• Spark Plugs
  - Indexed
  - Side Gaped
• Ignition System Parts
  - Wires
  - Rotor
  - Cap
• Fuel Filter
• MAF in a good Voltage Range
• Alignment (Mine was off by by 1", caused some nasty tire wear and bad gas mileage)
  

Performance Parts that Help with Gas Mileage

• Intake - CAI / WAI (Flow stack is a big plus)
  - CAI / WAI
  - Flow Stack
• Good Header (Free Flowing, while still good at producing Vacuum to help preserve the scavenging effect)
• Free Flowing Exhaust (Keep withing proportion of your platform (DE, VE, DET, VET) - Too big of an exhaust for a DE motor would loose velocity, thus not help much. 2.25-2.5 for a DE should be a good size
• Reduce Rotation Mass
  - Lightened Flywheel
  - Lightweight Pulleys
  - Lightweight rims/tires. Keep it at 15" rims, anything bigger then that hurts performance and gas mileage.
  

ECU Tuning
I will be looking at two ways of ECU Tuning; A/F and timing.

Timing

A/F
For the safety of the engine a turbo engine under boost should be around 12.0:1. DE is around 13 (stub). This is cooling the engine by fuel. For cruising you can use another cooling method, air cooling. By leaning out the mixture to be above 14.7 A/F you can achieve air cooling.

I must say all of this is not safe for the CAT. The fuel burns the best at 14.7 in the cat, richer is better, but too rich will clog it up. My buddy's MAF went out, thus his car was too rich. After a two hour drive, his headers were glowing red, reason for the trouble is the CAT got clogged, he is lucky the fuel didn't explode inside the CAT.

Cold Throttle Enrichment
When you first cold start your engine, it will enrich the mixture, and the IACV will do it's work to keep the RPM's up until the motor warms up. From what I understand so far, this is done to warm up your engine quicker, and to warm up the CAT.

Well if you don't have a CAT, then this is not a requirement. Unless the motor really like to running rich until it warms up. Still researching this topic.


To be continued

what kind of mpgs you getting now that ur boosted?

timing is goingto be tricky unless oyu have a knock alert/knock sensor or something that will give you the ability to monitor your iginiton timing and knock to make sure you are in the good, while iginiton timing can benifit you, there comes a point where you may gain no more power be increasing your timing in certain areas.

if your timing isnt overly aggressive but not weak, so basicly averagei dont see a need for really tweaking it, unless the timing gets adjusted on the fly on a dyno.

as far as the fuel, the obviuous is going to be idle, i would lean out the idle, lean out the cruise, those 2 places are where you are going to see the most gain

boost is going to e tricky to tune becuase depending on the boost psi you are running that will determine what AF will be good at WOT, IMO 12:5:1 is overly lean for WOT, where as most of the tuners i have talked with like to go for a well rounded 11:8:1 its not overly rich but more overly lean either, i will say tho that tuning for boost is hard beucase without being able to do a full run and datalog the run then monitor the air fuels and make map changes to it, its really going to be hard to do

Boosted and 10lb flywheel is giving me 15MPG city with a lead foot, 19 Mixed with city lead foot, 27 Highway with a semi led foot.

Since I've switched to blow through, my A/F have been a bit off. Like 10.5 A/F at WOT. I'm going to up it up to 12:1 for WOT, and over 15 for cruise if it handles it .



Timing is indeed a tricky subject. But I personally like timing more then A/F's. I can get to to a very close spot on the map with timing. Using DataScan logs for RPM and Timing, I am able to pin point the cells that I touched the most while cruising and while at WOT.

Now since I do have a DET motor, 8.5 CR, that means I have a bigger pillow for both timing and leaning out. As I mentioned above I'm going to attempt to stay close to 12:1 at WOT. I'll do as much as I can via wideband, but I will save the fine tuning for the dyno

Thanks to BenFenner I have a pretty good idea on when to look for error with timing, while on the street I wont do anything too aggressive.

I will try my fuel map tomorrow and see how the car runs, I will post pictures of what I have if it still runs

You can actually probably go way leaner then that on cruise.
Basically, lean out as much as you can get away with and not get a stumble.

I was almost to 17:1 AFR cruise on my 280z, and got 32mpg freeway out of the 2.8l turbo (7.5:1 CR with almost 40deg of timing at cruise).

It's also important to note that you will also need to learn to "drive" for MPG. If you lean out the cruise this much, you must make sure that you are in the proper gear at all times. If not, you can cause knock by forcing the motor if in too high of a gear, or end up not even being in that part of the map if you're in too low of a gear.

Also, accel and decel timers on both OEM and aftermarket ECUs can have a big impact on MPG. Stay out of the accel enrichments by using smooth throttle transitions. And on the other end, try to stay in decel zones, especially when braking. I think the stock ECU starts injecting fuel again at around 1.5k RPM, so keep it above that for fuel shutoff.

Why does timing make such a big difference in fuel mileage? I'll tell you.

Timing is about making power. The more power you make while cruising per combustion event, the less fuel you will use. Here's how that works.

While cruising you're basically interested in maintaining your speed. This means continually overcoming air and rolling resistance. This takes a certain amount of power. Say at 45 MPH it takes 50 WHP to accomplish this. If tuned poorly you might make 50 WHP at 60% throttle position. There is a load on the engine associated with this steady throttle position, but I'm going to keep it simple for now and ignore that. Basically you're drawing in a certain amount of air over time with this 60% throttle angle and that amount of air is always going to be mixed with a certain amount of fuel.
Now, if we tune the timing and start to make more power at lower throttle angles you could conceivably make the 50 WHP you need to overcome the resistances at 50% throttle angle. You're drawing in less air so you're using less fuel, but you're still making the power you need to maintain speed.

How much less fuel are you using?

Well, just keeping things simple you're drawing in 17% less air, so you should be injecting 17% less fuel, and you should see a gain in fuel mileage of 17%. That's quite a bit of savings. That would be like going from getting 30 MPG to 35 MPG. I don't think that's unrealistic either.

I agree with you on the power deal. This is why I am so concerned with low end torque. The more torque it has, the less time I will spend in higher RPM's trying to get up to the same speed.

A while back when I had my 1.6L 200sx. My dad would say that having a small engine would only mean that it has to work twice as hard to go up a hill then an engine that had bigger displacement/more power. I would disagree with him in the case of my 200sx, because while it was small displacement and I beat on it it newer went below 30mpg city and 39mpg highway.

But as I was watching Top Gear and comparing other cars, I also noticed that once you go below a specific displacement you loose too much power and don't get all that great of a gas mileage. Like my friend's Ford Festiva has a 1.3L Automatic and it gets worse gas mileage then my 200sx did, while their Manual one did get about the same as my 200sx did.

Anyway back on track, my ScanGauge II gives me a real time BHP output, granted it is estimated, but it's a good start. I can start comparing how much power I'm making at highway cruising speeds.

Oh the idea of this thread is to have some information available to other members, and work more of a How To on increasing gas mileage, since there are not many articles about this specific way of tuning.

On another note, I set my idle to be at 17 A/F, TunerPro corrected that to be 16.95, then cruising to be 15.68 all from 14.7. For WOT I set it to be .50 A/F bigger then regular, so for the most part it went from 10.xx to 11.xx.

First impressions after cold starting the car, it wasn't vibrating as much. After it fully warmed up, it would still be idling at 900rpm, but very smoothly compared to before. I think the A/F at idle on the Wideband were just around 15 A/F, I will check in detail today.

Other then that the car run just fine to work, at WOT the A/F's were just under 12's, so might be perfectly fine to leave them there now.

Now when my air gets blown off, I noticed my A/F drop to around 10. This was the case before, is this pretty normal, or does that mean my BOV is too close to the MAF?

The rich condition immediately after throttle close is normal and intended. It's a combination of deceleration enrichment from the ECU and natural xTau fuel evaporation. It is intended as a buffer to lessen the jerky motion experienced when closing the throttle and completely removing fuel.

Remember, I was talking about power or torque output at less than WOT, and with no acceleration. Just steady state cruising, not getting up to speed or using high throttle angles.