MPGs run, Aspire SE

[mattdickmeyer]

The cam that I have have developmed for Tominmo is specifically designed to create a highly detination tolerant combustion enviroment. This will allow a much leaner idle & cruise circiut with more ignition advance for more complete combustion which would otherwise ping in a normal application. Reducing the FP is the only way to control fuel adjustment on this application. The stock ECU is a simple EFI stradagy system that only knows to target a narrow AFR range & has no ability to compinsate for changes. The stadagy is programmed to follow a preprogrammed load table using the VAF to assume load. Using this info, the ECU knows with a given injector size (for the B3) of 10.4 PPH how to calculate injector on time (pulse width)to maintain a desired AFR. If a larger injector were installed, the ECU has no way of knowing & will calculate the same injector on time but flowing thru a larger injector will create an extreme rich condition. As far as fuel pressure is concerned, it is commonly thought that the injector sprays a nice fuel mist. Most injectors spray a single stream which is rapidly broke up in turbulant intake flow. The primary reason for elivated FP is in boosted applications when delta pressure has to be taken into consideration. What this is is if you have 40 PSI of rail pressure & 10 PSI of boost your effective delta pressure is only 30 PSI. Which if not calculated will cause a devistating lean condition. As far as Festiva's & Aspire's are concerned, their EFI system works much like an on/off switch at WOT the ECU targets approximately 12.2-1 AFR. Since the injector on time cannot be reduced, the next option is to reduce FP. I have found 12.8-13.0 to have the best results. But this is all relative to the mods done to the engine. If engine performance is desired, air flow has to be increased as well as fuel volume. In this instance, bigger cams & ported heads with stock FP will create a lean condition, especially when throttle is rapidly opened as the EFI system has no "accelorator pump funtion" to compinsate enrichment when airflow is instantly increased, except the spike in FP when vacuum drops at WOT. To address this, FP has to be increased for best results. But there is a place where too much FP can cause excessive fuel heat retention & reduce the overall volume of fuel that flows thru the injector. Correct tuning for the application will vary but is paramount to achive best results.

[getnpsi]

Using a static fuel pressure at the same settings and your mileage goes up may mean the stock FPR sucks and isn't holding steady, the new FPR is getting a better solid vaccum signal if you tee'd from another part of your plumbing. The more a computer has to hunt and play with its curves to hit its targets the more it adjusts fuel and timing, even though it has the best of intentions for emissions. This is also a dinosaur computer older than 50% of our members. Burning 14.:1 or stabbing at stoich is not the best for mpg, it's just the best for a cat converter. This is why some things are not for sale on california highway use vehicles, even tho it increases power and mileage...If you sit at a traffic light and press the brake all the way to the floor your vacuum changes because unmetered air is doing havoc. Some cars may even stumble. The computer has to play with its idle and timing to get it back. That itself is an emission spike. Air getting in the computer doesn't know about is a lean condition and if the o2 is working properly it will try its hardest to adjust what it can to come back. With a speed density and map sensor you can easily fool the amount of air measured. the VAF CAN be tuned as well, there are threads on it, mostly the turbo guys mess with it because they use a larger injector from another engine, hit their boost target on the fat side, and tweak VAF down to nail a fuel ratio.

I need to go to home depot and get an air dam myself, I think ill get great gains cleaning up my airflow. My aspire sure got quieter when i had the back end buttoned up.

[FestYboy]

i wonder then why there is a variable signal from the VAF sent to the ECU, and why the car runs like poop when there is a hole in the intake tube after the VAF (mind you, the VAF door still stays open enough to trigger the fuel pump)?

[TominMO]

Festiva (Skippy, the one with the B3 head work).

Yeah, IDK if it makes a difference if you have a VAF. I would bet not tho, since these are very simple EFI systems; just a step up from SPFI.

[FestYboy]

Festiva (Skippy, the one with the B3 head work).

[TominMO]

hmmm, FSM suggests otherwise in the adjustment arena. i understood that that's the reason for the VAF in the loop, otherwise i would have expected to see a speed density system (MAP only).

so after reading the last few posts, is Matt hinting that i should lower the rail pressure back to or below stock to gain MORE mpg? as stated before, i've seen a steady 2mpg GAIN since adding the FPR at 42psi. if lowering the pressure will gain me more without loosing power, i'm all about it.

I'd say try it. Are you talking Festiva or Aspire for the MPG increase? Since Aspires have MAFs not VAFs.

BTW, since adding my front air dam, my average MPG for a tank of gas, while driving a combination of city/hwy, has risen from the 35-36 range to 39-40.

[FestYboy]

hmmm, FSM suggests otherwise in the adjustment arena. i understood that that's the reason for the VAF in the loop, otherwise i would have expected to see a speed density system (MAP only).

so after reading the last few posts, is Matt hinting that i should lower the rail pressure back to or below stock to gain MORE mpg? as stated before, i've seen a steady 2mpg GAIN since adding the FPR at 42psi. if lowering the pressure will gain me more without loosing power, i'm all about it.

[TominMO]

I had a good talk with Matt today, and based on that, I will do the following:
1. Install the FPR, and run it at stock pressure. (No need for another test here.)
2. Install the cam, and test it at stock fuel pressure. Advance ignition timing a bit, maybe 3 degrees.*
3. Do a test at reduced fuel pressure (~27 at idle, ~35 at WOT). Matt told me this is the way to go. Atomization is not really an issue at this range of pressure; it still just sorta squirts out, and gets atomized mostly by the airflow. Greater pressure for a stock, NA application is basically just wasted gas (for burning purposes), tho it does help cool the combustion chamber if not ignited. The cat takes care of the unburned gas, tho excessive gas will clog the cat up.
4. Finally, install the header and run another test.

* Matt says I can safely advance the ignition timing a bit, so I will do that. (Stock is 10 BTDC for EFI Festys and Aspires). I think I have it at 10 now.

EDIT: Forgot to add that Matt said that our simple little gen1 economy-car EFI computers are not able to compensate for injector flow, by opening the injector longer or shorter. So for example a B6 injector would always run too rich. You would need an aftermarket system like Megasquirt to play with innjector parameters.

[zoom zoom]

running the FPR is going to smooth things out and should NOT be installed last (last should be the header). i would START with the FPR, that way it's there to support the cam and header as they are installed.

I would do it last, its the only way to test individual parts, the fpr adds a pretty big variable that will throw off the numbers. Keep everything else the same except the individual pieces being tested, then when those two are tested toss the fpr on and get your max potential for all the parts combined. Sure, probably won't happen like that, but it should imo. These mpg numbers are already somewhat doctored because the trips are too short, should be averages of 3-4 tanks of gas to get parts broken in and see how things hold up over time, you can't tell me those spark plugs are still putting out the same mpg numbers after the first couple thousand miles, I've done my own testing to know they get good numbers for a couple months then start to drop off.

[mattdickmeyer]

After reading the last few pages it's evident there is much confusion in EFI terminology and operation.I'm a tech adviser for FAST and see this everyday.

[Zanzer]

Ahhhhhhhhhhh. Hehehehe, OK I also wondered if that may have been what you were alluding to as well. Sorry but my ESP license doesn't kick in until next week LOL :mrgreen:


You needs to PM a brotha!

[FestYboy]

If you were referring to the other post then we were only looking for pulses to verify the ECU and spout signal was not dead and my method worked just fine, don't knock it till ya try it :p

i was trying to hint at the way we may be able to work arround the 6200 rpm fuel cut....

[getnpsi]

In open loop cold start you hve the pressure lower to burn a little bit less your overall tanks would be better. If you use 5-10 percent less fuel at WOT thats some more savings albeit flooring it is bad for mileage unless you couple it with some nicely timed glides, which does work well. Another possibility is a voltage offset to the ecu provided by a lm1 or similar wideband with that feature. The fpr anf a wideband and a mpg cam...500 bones, a lot of cost but rewards would come over time.

[rmoltis]

The stock pressure on my aspire is 30-38psi in the factory service manual.
30 at idle and (high vacuum)
&
38 at wot (low vacuum)
Assuming this is for your aspire.

My other question is.
Since the ecu manages the injector pulse time and whatnot.
What happens at WOT when the o2 sensor goes into open loop?

Could this make things run overly rich if turned up too high?
Since it has no feedback on air/fuel values?

[Zanzer]

after (pay attention Zanzer!!!) 5000 rpm, the injectors fire ONCE per combustion event per cyl, because otherwise the injectors would start to go 100% and that makes for poor power.

If you were referring to the other post then we were only looking for pulses to verify the ECU and spout signal was not dead and my method worked just fine, don't knock it till ya try it :p

100% duty cycle also makes for a short injector life.


Tom, you typically want to keep the duty cycle at 80% or below. Running a higher pressure will help you on 2 fronts because the duty cycle will be shorter and the spray more atomized. The ECU will take feedback from the O2 and adjust as needed for mixture. As long as it has good inputs an ECU can compensate for a good bit more than people generally give them credit for....well unless it's a speed density system and you install a huge cam