Too much octane cause a misfire?

B

BlownZ

Moderator
Joined
Aug 8, 2003
I thought I had cured my misfire....tooling around tonight with the wife a little bit to show off my skillz fixing the car...:D A couple of full throttle blasts and no problem...no misfire...17lbs of boost. I stopped at a little station that had VP 108 out of a pump. I put 6 gallons of gas in ...10 minutes later I cracked on it and psi hit 14 and it started flattening out and misfiring just like before.

I figure too much octane maybe so I add 6 gallons of 93 from another pump....should give a mix of abot 100 or so I assume. It is still misfiring. What gives guys....bad gas? Too much octane?
 
I am no expert but I would think ignition problem? How are your wires? Cracked or old. Maybe shorting out?

Plugs may need a tighter gap with higher octane??
 
I have seen a street chip cause that problem with too much octane. The problem was insufficent timing in the chip for the octane. (Too little timing). We put the race chip in and all was well.

Dont know if its your problem, but it could be.

Bob
 
Bob...that is what I suspect. It is a Jay Carter 93 chip from what I am told in the car. It was running mint until I put the 6 gallons of 108 in the car...then it started misfiring at 15 lbs or so. It almost feels like it hits a rev limiter at 15lbs...it just lays down and pops a little until you let off of it. At part throttle it runs great...at WOT throttle forget it. The plugs and wires are new and the grounds have been redone...they are tight and clean. The only ignition part that I have not replaced is the coil pack and module but it just seems like WAY too much of a coincidence that the car was running great...put 108 in and it starts running like crap. I have to say it is the gas.
 
Don't be too sure about the new gas being the culprit. You may have driven around long enough to reveal the heat sensitivity of a failing module/coil pack. My car is going thru a similar problem even though I am just running street gas. All I can do is try replacing the module and if that doesn't do it, then try replacing the coil pack. Maybe someone has other ideas? Good luck with correcting your problem.
 
You are right. At that point we had driven approximately 20-25 minutes or so...enough time to get up to full underhood temperature. It's raining this morning but I think I am going to try closing the gap on my plugs first and then replace the module and coil pack. If nothing else I will have a spare module/coil which from what I understand is not a bad thing with a turbo buick. :(
 
THis happen to a buddy's 87GN going down the track. We dialed the car in on 104 unleaded and the car did not miss a beat. However spark knock was occurring in 3rd gear (5deg) so he decided to put 118 in. The car backfired and severely and would not run good the rest of the night. Got back home purged the 118 out of the tank, and put in 104. Car run flawlessly. It was explained to me that 118 race gas burns slower than 104 unleaded which may be the reason it ran so shi**y. He was running a 100 octane chip. Just my experience.. Oh yea our plug gap is .029"-.030" cause we are running alot of boost.

Mike Banas
87GN
 
???

Sounds bogus to me. Higher octane gas is more resistant to pre-ignition, and to dieseling, but it doesn't burn slower. If the chip is not set up for the high octane, you are wasting money on expensive gas you don't need, but it should NOT affect the way the engine runs. You advance the timing with high octane because high octane fuel ALLOWS the timing without knock, not because it requires the extra timing to burn. If you got an ignition problem, which just happened to show up at the same time you put in the high octane gas, well, the gas was just an innocent bystander. If you maybe got a tank of "bad" gas, maybe with water in it, that didn't have anything to do with octane, it was just a coincidence.
 
I think Ormand is right. The only reason higher octane fuel would burn slower (and the amount would be insignificant) is if you weren't reaching a high enough cylinder pressure before you lit it off. In other words, the closer you get it to spontanious combustion before sparking it, the faster it burns. There is an optimum octane rating for any particular combo, and to much octane can slow you down. However, if your problem was that the gas had too high of an octane rating and that's why your car wasn't running right, it would start to run BETTER as the boost increased. Sounds like either bad gas, or to little fuel to me.
 
If I were getting too little fuel I would have been getting too little fuel before adding the race gas too.

I agree that too much octane doesn't seem feasible to create a misfire condition...I could see it maybe making the car lay down a little or lose some top end mph or something like that but not a distinct misfire. I think Ihave found my problem though, I think Airbuick hit it on the head....at the time of adding the race gas we had been driving 25 minutes or so...the underhood temps would be getting up there and might have been heat soaking the coil/module.

I just went over the connections and found one plug wire that pulled off the coil too easily, it did not feel like it was getting a good solid connection and the ring inside looked a little gunky. I cleaned and tightened it and then took it for a ride and voila...no misfire. I only drove it about 5 minutes though as the roads are wet....I am taking my life in my hands goofing around on wet roads at 17lbs of boost. It did not misfire on either of two 17lb runs. I am going to wait until it dries out and then drive the car for a half an hour or so and see if it is heat sink on the module/coil or if this funky wire was the culprit. Time will tell.

I can say one thing I will definitely be doing the happy dance when and if my $%^ scanmaster ever shows up. I hate being blind trying to diagnose a driveability issue. The only scantool I have now is OBD11.

Thanks guys...if this cured it I will let you know...thanks for the advice.
 
fuel

The higer octane fuels need more timing because the fuel is harder to ignite,if it takes more time to ignite the mixture at before TDC you must light the mixture earlier. so higher octane does take longer to ignite but burns quicker than 93, also this very fast burn also creates excess carbon. The top fuel cars run nitromethane and will not ignite to a flame without some compression.:D
 
Higher octane fuels do NOT NEED MORE timing. Read my post above. Higher octane fuels can be used with more timing, but it is because they have higher resistance to knock/pre-ignition, NOT because they are harder to ignite, or slower to burn. As timing is increased, or compression, the higher combustion temperatures/pressures make knock or detonation more likely, and this can be offset by the use of higher octane. Top fuel cars run nitromethane because they can get a lot more power than with gasoline, and they run a tremendously rich mixture, as is required by the chemical make-up of nitromethane. They also run lots of boost, and the rich mixture and high boost require things like dual magnetos to get good ignition. But that has no bearing on the question of missing vs octane. Suggest that anyone who is still confused do a google on "octane", and find out what's really involved.
 
fuel

what your saying is corrcet the reasoning is wrong ask yourself why is higher octane less likely to preignite/detonate? Higher octane fuels wont detonate beacause they are harder to ignite or start the combustion process. 93 detonates because its very easy to ignite,pressure,hot spots,advanced timing,thin air all cause the 93 to ignite without the spark from the ignition. the cure is to put in a fuel which will start to burn to the above. So the fuel is harder to ignite. also to make all the power the fuel has to offer the timming will always need to be advanced Vs 93. This is beacause the the spark timing needs to be earlier to start the combustion process so the fuel lights a TDC. why do you think most people upgrade coils when running race fuels, more spark is need to ignite the race fuel
 
Look It Up!!!

Chevy too, you are sadly misinformed, or maybe you've read the information, just didn't understand. I'm a graduate engineer, with an MS, and I've been doing this for a long, long time. As I said, if you don't understand, and clearly you don't, use Google, and do a search. Higher octane is a measure of the resistance to "spontaneous" ignition, in which the fuel ignites from heat/pressure in the combustion chamber. It does NOT mean that the fuel is more difficult to ignite by spark. People who know what they're doing upgrade the ignition system when they increase compression, or the operating rpm range, NOT because the higher octane fuel requires it. The ignition needs are more critical because of the higher combustion chamber pressure, and the same engine parameters which bring on the need for higher quality ignition also bring on the need for higher octane fuel. If, for some reason, you choose to put higher octane fuel in an engine than it needs, based on compression, combustion chamber design, etc, that does NOT mean that the ignition needs to be upgraded, or that the engine won't run as well. It just means that you spent more on fuel than you needed to.
 
fuel

you just proved my point:If octane is a measure of resistence to spontaneous combustion meaning its less likely to ignite without spark. do you think the minimum amount of spark energy to ignite the different fuels will be the same? its not
 
Grape Ape??

Try this one, it offers a statement that contradicts the one linked above. That doesn't mean that either is absolutely correct, however.Octane
Here's another:
.Octane, again

There isn't much difference in resistance to ignition from a spark, or from a hot-spot in the combustion chamber, from high octane to low octane. The primary difference is the resistance to AUTO ignition, from pressure/temperature alone, without a spark or a hot spot to trigger the combustion process. That does not correlate to an increased resistance to spark ignition, since the two processes depend on different chemical phenomena to get them going. The temp/pressure required for auto-ignition just don't correlate directly to "the minimum amount of spark energy to ignite the different fuels". If you want to learn more, continue to look, but be careful of web sites not connected to a University, a government agency, or some other source of "peer reviewed" information, such as the Society of Automotive Engineers, or DIN, etc. I suspect, however, that some are not interested in learing anything, only in reinforcing their misinderstanding.
 
fuel

i just read the 76 page you posted. It also says that the burn will be faster but says nothing about what it takes to light the mixture they are totaly different processes. i think were talking apples and oranges. I stated its harder to ignite but burns faster once lite. neither one of these pages says anyhtingh about lighting the fuel only about burn rate
 
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