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Optimal A/F Ratio? (quite long)

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andymoraitis

15+ Year Contributor
3,241
153
Jan 25, 2004
Utica, Michigan
I was wondering if some of you more experienced tuners could give me your feedback in an attempt to solve some of the issues I'm having. Here's the short mod list to save you time

Big 16G at 20psi
Stock SMIC
Full 3" exhaust with 3" high flow cat
ACT 2100 w/chromoly flywheel
FIC 550cc injectors
Injen Intake
Buschur Upper I/C pipe
1G Bov
Walbro 190
Keydiver Stage III chip w/EPROM ECU

Now, to the problems. I've tracked this car twice and both times I had poor results in terms of getting any serious top end performance from it. The first time, I ran 14.0 @16psi and trapped 100 MPH. The second time, I tried a better track and ran 13.6 @20 psi trapping the same 100 MPH. While I have launch issues which may be due to an improperly broken in clutch (best 60' is 1.98 and I KNOW how to launch), the slow traps are bothering me. In an effort to check everything, I did a boost leak test and the system held 20psi for 30 seconds easily. Compression across all four is 175-175-180-175.

I consulted with Jeff at DSM Chips and his feeling is the car is running too rich and that we can adjust for it in the chip. FYI, he runs the factory A/F at 9.5:1 with maximum timing of 16 degrees. I recently replaced all plugs with fresh NGK's before running it again and upon removal, all of them had some carbon fouling around the lower threads and around the insulator. The inside of the tailpipe is black and if I rev it to 3K, it will splatter black garbage all over the driveway. In addition, I also get some mild rich backfires when shifting between 3500-4000 RPM in any gear.

Anyway, we're adjusting the chip, and Jeff worked all night and created three new chips for me:

1. 11:1 A/F with max 16 degrees timing
2. 11:1 A/F with max 22-23 degrees timing (for race fuel)
3. 10.5:1 A/F with increased timing (I'm assuming somewhere around 18-19 degrees)

I'll be picking up a logger in the next week so that I can have a better read of what's happening, however I wanted to ask the people who are making strong horsepower on their stock blocks what A/F ratio they tune for. I've corresponded with David Buschur and he thinks 11:1 is still too rich, but then again, I can turn up the boost to lean it out on pump gas (as long as I encounter no knock).

Any feedback on the optimal tuning ratio would be helpful. While I understand that there's no perfect combo, I'd like to feedback from the experienced tuners on what they shoot for so that I have a baseline.

Thanks!

Andy
 
I am at 10.9:1 from 3500 to 5500 then I am little leaner 11.5:1. This is 21 psi on 94 pump and 16-17 degrees of timing. I am trying to get 11.5:1 straight across the board. I see little dips in timing through the midrange but nothing really drastic yet so I will continue to lean it out till I see significant timing being pulled.

Tuning is done with an AFC, wideband (wired to my ECU) and a pocketlogger.
 
Thanks for getting back to me. Based on what I was originally thinking, the 11:1 at 16 degrees will likely be my best bet on pump gas. The fact that you're shooting for 11.5:1 tells me I'm not that far off. Again, I can always up the boost in small increments to lean the car out until I see knock and then back it down to a comfortable level.

Thanks again for your insight,

Andy
 
No problem Andy. I was always told that 11.5 is where to start your tuning and go up from there. I read in this thread here low 11's is way to safe and you are sacrificing too much much power to be that safe.
 
Steve,

I agree based on that thread that 11:1 is probably still a little rich, but I do still have additional options available to me. First, I can add an AFPR and turn down the fuel pressure a little bit, log it and see what it does. Also, I'm currently running 93 pump. 91 octane is available here and I can run that instead to lean out slightly. In addition, I can also add a bit more boost. Jeff and I are of the opinion that my current setup will run 18-19psi without issue at 11:1 and 16 degrees, however if I run 18-19 and it still doesn't trigger any knock, I can bump the boost to 20-21.

More importantly, I need a logger and an AFPR to have some tuning ability and dial the car in correctly. Thanks again very much for your help. I REALLY like your ride by the way!

Cheers,

Andy
 
Andy you will not run leaner with 91 octane. You will knock easier causing you to run RICHER! I run 93 octane and in order not to knock I run an A/F in the mid 10's. i have a wideband, scanmaster(1g's) and tune with a blow thru MAF-T with a keydiver chip stage 3. I put in some 112 octane the other day and bumped my boost to 23 psi and leaned it 5%. No knock, may lean some more. I have had 4 turbo's on my car and all liked to run in the mid 10's for the a/f ratio. My muffler is also sooty. Get the biggest FMIC you can and you will be able to run leaner.
Yours and my charge air are probably too high and only fuel will help prevent knock with that. My bet is the 10.5 chip runs the best. I have a Walbro 255/AFPR also and I run a cold air intake with my K&N sitting where the stock IC once was. Get a BIG FMIC and you will be able to get leaner. Mark
 
Thanks for your insights Mark. I was always under the misconception that if I caused a hotter combustion cycle with lower octane, that it would be a more complete burn. I guess you learn something new every day. Really, what I need to do is have a way to tune the car and this is likely best done through a logger and at least an AFPR. A much larger sidemount or FMIC will soon be on the way.

Thanks for jumping in here!

Andy
 
ecoli said:
I usually got for around 11.2:1 on pump gas on DSMs and around 12:1 on race gas.

I guess it somewhat varies on what works better for some cars and combinations. I'll try both chips and see which one produces better results overall.

Thanks for your feedback,

Andy
 
Wow, I'm just not too bright this evening. Or maybe it's all the time? For some reason, I was always under the mistaken impression that higher octane burned cooler.

Thanks for the correction,

Andy
 
I do believe the higher octane, at least leaded octane burns cooler. That's why Hi-po cars hadto hav different valve seats to run unleaded when the switch was made back in he 70's. I was there. Andy even if 91 burned hotter that would be cause for preignition/knock. Mark
 
I think it boils down to higher octane fuels burn slower and more controlled than lower octane levels. This is why you need to lean out with race gas. You can get a more stable burn out of less gas therefore you can run a leaner mixture. No need to dump a ton fuel at it to keep it from knocking.
 
Also the higher the octane fuel, the more timing you want to run.
 
Staytuned said:
I think it boils down to higher octane fuels burn slower and more controlled than lower octane levels. This is why you need to lean out with race gas. You can get a more stable burn out of less gas therefore you can run a leaner mixture. No need to dump a ton fuel at it to keep it from knocking.

Now THAT makes perfect sense and explains why you'd need to lean out a bit. I always thought that high octane required one to lean out the A/F because it burned cooler. Thanks to everyone for setting me straight. Still feeling kinda dopey though!

Cheers,

Andy
 
trbotaln said:
Also the higher the octane fuel, the more timing you want to run.

I agree, that's why my race gas only chip is calibrated for 11:1 A/F with 23 degrees of timing on 110+ while the pump gas chip uses the same A/F and 16 degrees.

Good point,

Andy
 
The reason why you can run a leaner A/F ratio on high octane gas is because it has better knock resistance and has a slower controlled burn. Often you have to lean out race gas to even get back to your normal A/F though. That is because race gases often have a different specific density than your normal pump gas. A denser fuel will get more actual fuel injected during the fuel injector pulses. To counter act the slower burn of the race gas, you can either advance the timing to start the burn earlier or turn up the boost and make use of the higher knock resistance of the race gas.
 
That's a great explanation and I can't thank all of you for setting me straight and helping me get a better understanding of how tuning really works relative to octane, A/F and timing. My chips should be in tomorrow and I'll see how they do.

Thanks again!

Andy
 
You need a front mount intercooler, nuff said. Thats why with more boost you were running a little faster ET but same MPH.

But on the airfuel thing. Im running about 11.5-11.7:1 on pump gas at 20-21psi. On race gas im at 12.5:1 at 26psi dropping fast.
 
NewB2dsm said:
You need a front mount intercooler, nuff said. Thats why with more boost you were running a little faster ET but same MPH.

But on the airfuel thing. Im running about 11.5-11.7:1 on pump gas at 20-21psi. On race gas im at 12.5:1 at 26psi dropping fast.

From your standpoint, is this an issue of flow with the SMIC or the inability for such a small unit to cool the air charge sufficiently (efficiency loss)?

My original intent with the car was to run in the 12.7 to 12.8 range on the current parts and correct tuning. While I knew that the SMIC would be a restriction, I didn't think it would keep me from running 12.s since my biggest issue had to do with my clutch adjustment (now resolved) and the fact that I was running pig rich on a 9.5:1 A/F. I've also referenced the DejonTool website and verified that they've run 12.2's on their sidemount with a setup similar to mine.

Let me know your thoughts and congrats on running excellent times without a huge list of mods.

Andy
 
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