The pumps for 87 were full the other day and I was in a hurry so I just pumped the car with $20 of 92 octane.

For some reason the car almost immediately started trying to stall from a stop and was a lil more stubborn about firing right off.

Refilled it with 87 from the same place yesterday and its back to running perfectly.

I'm pretty sure that it was bad fuel or anything, what else could cause the car to run worse on better fuel? Last summer I ran 92 all the time because it helped my MPG a bit.

Should I check my timing? I was thinking it might be off just enough that I don't notice it with the 87 or something?

Carb might need a bit of adjustment?

This may be a really stupid question...

Oh, I wanted to add that I usually run 87 with a touch of lead substitute every 2nd or 3rd fillup

Bad stale fuel. It's getting to be a problem finding fresh high octane. With the high price of gas, everyone uses the cheapest fuel, so much so, that some of the local stations drop their prices on high test to equal regular in order to get rid of it.

I just didn't know if it could be some kinda telltale of bad tuning as well.

Same thing happened when I filled up at a Shell right before going to the strip in the fall. Car would stumble and try to stall off the line unless I eased into it up to about 15-20 MPH then stomped it

Led to dissapointing times and looking like a jakass in front of a lot of people.

Originally posted by TJC:
Bad stale fuel. It's getting to be a problem finding fresh high octane. With the high price of gas, everyone uses the cheapest fuel, so much so, that some of the local stations drop their prices on high test to equal regular in order to get rid of it. Could be. Friend of mine that does marine work told me not to add stabil to the tanks for winter storage anymore. Instead try to run them dry, gas formulations have changed and it won't last as long, even with Stabil added.

Just because the octane is higher doesn't mean your engine will run better. Low octane fuel burns more rapidly than high octane fuel, producing higher cylinder pressures earlier in the cycle. If your engine is optimized for low octane gas and you swap to high octane gas, your peak cylinder pressure will be way past the optimum point, resulting in lost power.
We had a car on the dyno that went from 470 rwhp to 430 rwhp with the simple addition of 5 gallons of 100 octane to the existing 10 gallons of 91 octane.

Look, no offense but I find it hard to believe that you lost 50-60 flywheel HP just because of a fuel change.

I've been involved in several hundred engine dyno pulls and have NEVER seen an engine lose more that a couple of # because of the fuel.

A stocker motor we did made the same HP with 91 pump gas as it did with C-12. The only reason the C-12 was used is the consistency of it passing tech.

Did you change the timing?
Was the Specific Gravity close to 1.00 or something?
Was it AVGAS, that stuff is CRAP.
It doesn't add up...

And actually, higher octane fuels burn at the same or slightly quicker rate than lower ones. The higher octane fuels have higher Flash Point so they don't burn as EARLY. Hence the higher temps in a high compression motor.

You are right about octanes and power though. Even the guys from VP say to run as low an octane as possible(motor octane that is)in your performance car. The only reason to run a higher octane is to take advantage of high compression and more timing(where some engines make the most power) and to avoid detonation is those particular cases.

Rob

I'd be skeptical about a 40 rwhp drop also. Don't forget that if the H to C ratio is different between the fuels and you don't change the figures in your analyzer, you might be inadvertantly running at a different A/F ratio without knowing it. Could account for a substantial change.

Mike

When I change from C16 to 93 octane it is always necessary for me to fatten up the idle a LOT for the 93 fuel for some reason. If I just swap fuel it will idle like crap.

I also run the same timing N/A when I drop the octane and it seems to tolerate it - plugs look ok.

I only change the jets 1 or 2 sizes but it will run almost the same WOT with both fuels so I doubt the hp loss also.

Originally posted by mfsr:
Look, no offense but I find it hard to believe that you lost 50-60 flywheel HP just because of a fuel change.

I've been involved in several hundred engine dyno pulls and have NEVER seen an engine lose more that a couple of # because of the fuel.

A stocker motor we did made the same HP with 91 pump gas as it did with C-12. The only reason the C-12 was used is the consistency of it passing tech.

Did you change the timing?
Was the Specific Gravity close to 1.00 or something?
Was it AVGAS, that stuff is CRAP.
It doesn't add up...

And actually, higher octane fuels burn at the same or slightly quicker rate than lower ones. The higher octane fuels have higher Flash Point so they don't burn as EARLY. Hence the higher temps in a high compression motor.

You are right about octanes and power though. Even the guys from VP say to run as low an octane as possible(motor octane that is)in your performance car. The only reason to run a higher octane is to take advantage of high compression and more timing(where some engines make the most power) and to avoid detonation is those particular cases.

Rob No offense taken. The car was a Z06 Corvette owned by Tim Kerrigan of Red Line Oil. We did before and after tests for the installation of a belt-driven supercharger, installed by Wayne Torkelson. We did no tuning, no timing, just strapped it to the dyno and ran it up. The 'before' power was about 340 rwhp and after was about 470 and climbing when we shut it off. Tim had 5 gallons of race gas in a can that he added to the approx 10 gallons already in the tank and we ran the fuel pump for a few minutes to mix the gas. The next pull, about 15 minutes later, resulted in a loss of about 40 rwhp. It was an across the board loss of torque all the way up the scale. We weren't in a position to drain all the gas and re-do the 91 octane test, so we called it a day.

I would then have to say the computer didn't like that change but it could be re-mapped to accept that fuel, although un-necessary.

I would be curious as to the avg. HP loss.

66,

I'm not 100% sure but the lighter fuel in the idle feed slots could have something to do with that. The heavier 93 may move slower in the passages? Good to research........

Rob

Thunderstruck, Maybe the high octane has gotten water in the storage tank. Sometimes that happens when it sits too long, if your local station doesn't sell enough to keep it fresh it might be going bad before it can be sold.

What Harold said for sure. I had the same issue last year on occasion. Chekced plugs, timing lash etc. Just stale gas here and there. in a high compression engine it'll fire fine, but it hesitated, pinged, and just behaved poorly lacking its typical snot.

Keep in mind that onovakind said, best power is made with the minimum octane required.

Thanks for the replies

i have been racing at the track for about 13 years and myself i have found thru trying higher octane then needed fuel would slow my cars down anywhere from 1-3 mph trap speeds. so were talking about 10 to 30 hp. i run the lowest octane fuel possable with out hurting my engines and one of the engines i had was a 11.7 to 1 bbc 396 and it like the 92 octane fuel at the time when 92 was at the pumps.....trying 114 octane slowed it down 3 mph. of course that was 5 gallons mixed in with about 5 or so gallons in the tank of 92 pump gas.....as soon as i got that gas mix ran out and put in my 92 my car started to pick up the mph it lost. didnt do that again on that engine....other mild engines i lost less mph thru the trap........doing trial and errors for finding more hp. i just concluded that if i had ping problems or bad plug reading from preignition then i would just add alittle at a time til i find the right mix.

i'm no fuel expert....just letting some know what i have found thru my own trials and errors.....

Could be the 92 octane had a higher ethanol content and leaning the AF/R.

Sounds to me like pump fuel and race fuel don't mix. A good question for the race fuel manufacturers.
Also, depending on where you are, isn't the fuel winterized? Maybe the 92 mix is different than the 87 mix?
I think you got bad fuel. It happens.
I always try to buy from high volume service stations. They probably have the tanks filled way more often.
Ron