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Discussion Starter · #1 ·
So about a month ago I had my car tuned for an A/F ratio of 11.8 at WOT @ 13 psi- this was using 91 octane from ESSO. Well I decided to try some 94 octane from the Mowhawk here- they basically take 91 octane and add 10% ethanol blend to make 94 octane. Knwoing the ethanol blend would make me run leaner I added some fuel accross the board on my tune and brought up the boost to about 15 psi. At this point I mt A/F ratio was about 12.2 which made me a little uncomfortable... I'm not a big fan of how the ethanol blend made me run leaner and in the next monrth or so i plan on adding methanol injection so I decided to steer away from from the ethanol blend and fill up with Shell V-power 91 octane. Surprisingly, I found myself running JUST as lean as I was with the 94 (ethanol blend) if not even leaner! Concerned about how high the a/f ratios are i brought boost down to 10 psi and i'm still running 12.0-12.1 a/f....

Obviously some tuning is going to be required to clean things up here but I just cant believe the differences various types of fuel are making! The only other thing im going to double check is that i dont have a loss of fuel pressure in the system which could expain why its running leaner..
 

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what tuner are you using?
mine did the same..
the dyno tuned my car super rich and i lost my s/c whine..
the other day i accidentally put 87 octane in the tank and drove it about 15miles and the car began to pull better and whine!
i don't have a wideband yet so i couldn't tell the a/f...
but when i did have a wideband, i tried tuning the car enough to know what sound it makes, runs,etc whether i am running rich or lean.
obviously, i try not to push it without a wideband, but i know the closer i get to 12.1 the louder my supercharger is.
with that said, i am interested in finding this out as well../
 

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Discussion Starter · #3 ·
One other possibility is that one of my primary injectors (190cc) isn't firing... (I run 290's in my secondary rail setup)
 

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A 10% ethanol blend would not do much to change the AFR. It's only when you get between 30-50% Ethanol do things really get whacky. Its all about energy content of the fuel as to how much it takes to maintain a lambda of 1 or stoich ratio of 14.7:1. The OE ECU does not read AFR it reads lambda values using a 0-1volt spread. The higer the octane level you got the lower the energy content of the fuel. The higher octane levels in gasoline are all about detonation resistance and NOT power production. Additional power on high octane fuels comes from being able to advance timing and run a leaner mixture due to the higher detonation threshold.
Chris, the difference between and AFR or 11.8 and 12.2 is VERY small and was likely caused by differences in ambient temps than because of the 94 octane fuel. The ECU will add fuel under cooler temps and pull fuel out under hotter temps. Try it out. Do a run early in the morning and then midday when its hot outside. Your AFRs will vary.
Mark, you have many issues, first and foremost not having a wideband yet. ;) The increase in power felt is due to 87 octane having a higher energy content. The dangerous part is that it has a lower detonation threshold which could mean kaboom. With that said Hyundai's timing is VERY conservative so you may be able to get away just fine with 87 octane and 5-6psi of boost pressure.
You'll see more and more auto makers certifying their cars on both 87 and 91-93 octane. You'll find that power production doesn't fall as much as you would expect. Look at the new genesis. The power figures they post were done on 87 octane. Switching to 91 might give you 15 wheel horsepower. For some people that's not enough to justify the price difference between 87 to 91 fuel.
The best thing to do if you can is switch to E85 (Ethanol) which is about 15% Gasoline and 85% Ethanol. It has 106 Octane and costs less than 87 octane in most parts of the US. It has a much lower energy content than gasoline so you need about 30-35% more fuel to maintain stoich AFR but the benefits of having pretty much zero chance of detonation pay off big time!!
 

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Discussion Starter · #5 · (Edited)
thanks, good info dave...however, as im running 91 octange again i reverted back to my original tune from the dyno and even brought boost down to 11 psi. It is alarming how off the tune is now... between 3500-5000 rpm a/f spikes as high as 13.0 before finally making its way back down to high 11's. What could explain this? When I left the dyno just over the month ago the tune was very solid and held right around 11.8 throughout the power band- now its just plain ugly between 3500-5000. The air might be slightly more dense these days but I find it hard to believe it could throw off the tune that much?? Is gasoline brand simply the cause of this? or could there be an issue with my fuel system (partially clogged primary injector, etc.)
 

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Have you reset your ecu at any point between now and the dyno? did you reset it before you dyno'd?
Perhaps your fuel trim's are to blame.
 

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Discussion Starter · #7 · (Edited)
Have you reset your ecu at any point between now and the dyno? did you reset it before you dyno'd?
Perhaps your fuel trim's are to blame.
havent reset the ecu...haven't changed anything other than boost levels...I can try reseting the ecu tonite to clear the stored fuel trims.
 

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If you had injector problems it would cause all sorts of issues. Do you have a CEL? Does it go to 13 AFR at initial throttle tip in? Does your fuel pressure drop right when it goes lean?
 

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Discussion Starter · #9 ·
If you had injector problems it would cause all sorts of issues. Do you have a CEL? Does it go to 13 AFR at initial throttle tip in? Does your fuel pressure drop right when it goes lean?
NO CEL

AT WOT the a/f basically starts out around 12.3 @ 3500, peaks around 12.9-13.0 @4500, then drops back to 12.2 @5500 and in the 11's by 5750+

I currently do not have instrumentation to measure fuel pressure.
 

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NO CEL

AT WOT the a/f basically starts out around 12.3 @ 3500, peaks around 12.9-13.0 @4500, then drops back to 12.2 @5500 and in the 11's by 5750+

I currently do not have instrumentation to measure fuel pressure.
Hammer has a OBD-II scantool... you may want to talk to him about using it to check your fuel trims.
could just be a sticky injector or something too... mite wanna run er down to 1/8 tank add some injector cleaner and another 1/8 tank of fuel and run er down bone dry before filling up... that should clean just about anything out.... if not shiftless and I have a system to clean injectors with carb cleaner... but it's a royal pain in the ***... and messy.... but it works.
 

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well i figure i should put this out b/c i didnt see it anywhere for guys running larger boost numbers i know a lot of guys running e85 i know this sound weird at first but there is some logic to it. with it the larger amount of ethanol allows them to run more timing and higher boost levels with no knock, draw back is 30% more fuel is needed.
so try advancing your timing if you are running more ethanol fuels
 

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Discussion Starter · #13 ·
Hammer has a OBD-II scantool... you may want to talk to him about using it to check your fuel trims.
could just be a sticky injector or something too... mite wanna run er down to 1/8 tank add some injector cleaner and another 1/8 tank of fuel and run er down bone dry before filling up... that should clean just about anything out.... if not shiftless and I have a system to clean injectors with carb cleaner... but it's a royal pain in the ***... and messy.... but it works.
ya i'll check them with my OBD-II tonite and reset the ecu-and the injector cleaner sounds like a good idea too thanks.
 

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ya i'll check them with my OBD-II tonite and reset the ecu-and the injector cleaner sounds like a good idea too thanks.
Just watch your A/F when you reset the ECU... it could throw things way outta whack.
 

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An oil leak would not affect AFR. Period. Now a crack in the intake manifold would be a different story. I bet all that happened was that your fuel trims got whacked and now you need to either reset the ECU or add more fuel where it gets a little lean.
 

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Discussion Starter · #17 ·
ya but couldn't a vacuum leak be the cause of the problem- its awfully suspicious that I started experiencing these a/f issues right when my repair job on the valve cover failed.

BTW the hairline crack is my own fault- last year I was careless when taking off the valve cover and forgot a bolt and cracked it when trying to pull it off. I had a difficult time locating a substitute so I reinforced the hell out of it and gave it a shot- and obviously it finally failed.
 

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... you had a good run on that repair so you cant be totally angry about it.
 

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Discussion Starter · #19 ·
ya not too shaby hey... i found a replacement cover off an elantra last year for $5 at a pick-ur-part.. but when i examing it more closely i noticed a small chunk (about 1 cm) around the INNER edge which is BEFORE the gasket and outer edge. Ideally id like to find one in perfect shape and not have to worry about it but i'm strongly considering giving this one a shot.
 

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A vacuum leak can only happen pre intake valves like between the throttle body and head. Anywhere between there can have a leak. The valve cover is not part of the intake system so it cannot cause a vac leak.
 
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