Jetting and Octane Rating


I understand octane rating in relation to compression ratio and ignition timing.
Speed of flame-front under different pressures and so on.
I've determined "and please correct me if I'm wrong" 87 Ethanol free pump fuel to be the ideal choice for a stock Pred 212 with stock flywheel max timing 16 degrees advanced, 10.8's on a stock cam, air filter adapter, any header, and max jet size .040 on a stock carb with stock E-tube.
I personally run a smaller jet than .040 just to be clear.

That being said....I witnessed an experienced racer using VP 94 bottled fuel. I didn't hesitate to question Him.
The response I got was "We're still running the stock Jet"
The karts in question ran very well though I felt like I ran very well compared to them also.
It didn't sit well with me and I'd like to learn if I'm mistakenly suspicious or do I have some homework to do?
This was Not at any of my local tracks and is completely unrelated to any of my usual competitors.
This was at an event that was far from where we typically race.

I've never been one to point the finger and the word cheater is a very strong word in my opinion.
I've been called a cheater and I take it as a major compliment.
I like to sleep at night win or lose.

Just to get it out there; my suspicion would be smaller combustion chamber be it milled/welded or thinner head gasket, offset flywheel key beyond 16 degrees, or stroker crank among other possibilities.

My other suspicion is that I have something to learn.

I'm asking this to learn. It'd be great if one of you more experienced racers could help me understand.

Thank you!!


Dawg 89
Interesting Question .
To surmise , you would like too know if the 94 octane fuel could produce more hp in a stock engine . Even possibly altered fuel ?
Any funny yellow/green flame come out of the pipe ?
Altered fuel , possibly if tuned correctly .
Higher octane , I do not believe is a benifit .


I didn't notice any flames, it was during the day.
My knowledge suggests that a higher octane if anything would be a hindrance to HP in this application.
I also watched him crack the seal on the bottle. It's about a 2qt shelf bought VP product 94 Octane.
Who knows how long it sat on the shelf which I assume would affect the true octane rating.
The response he gave me about the "stock jet" made me want to research the concept.
I'm not an engine builder, but I believe I've got a good understanding about how Octane rating effects engines......or rather what octane rating an engine would demand....regardless of jetting.


Thank you.
Very informative article.
My knowledge falls very much in line with the information in the article, but I'm always eager to take in information.
As far as the VP brand....I must be mistaken. The 94 Octane rating was very clearly written on the can and I am 99% sure of it in that aspect.
In fact I said to him "94?...What are you running that for?" He's a super nice guy and a good driver. I believe I'll see him again in the near future and I even invited him to come race at our track at some upcoming dates.
It's just the "stock size jet" response didn't add up for me.


I recently was forced to change my allowance on my digitron 64 tester from an allowable 25 points to only 10 points. I found out many guys were showing up with a mixture that was reading 17 to 19 points when others in the same evening were showing up in the area of 0 to 4/5.
I discovered what it was and did some testing on the dyno with different levels of the mixture. When I tested the 17 to 19 mixture that was showing on my gauge, I was surprised to see as much as 1/4 horse gain in unrestricted clone and animal ( 206 ) without any changes to jet or timing. When I dropped the mixture to 10/11, the gain was no more. Thus the reason I went to only allowing 10 points on my gauge.

alvin l nunley

Site Supporter
Tell me how closely you monitor the air density. I believe that's really important. Tell me how closely you monitor your exhaust gas temperature (EGT). Tell me how thorough the track you were running, where you met this guy, conducts post race tech. This competitor we're talking about, how does his .040" main jet compare to yours? There is a difference in area of 5.19% between a .040" and a .039" jet! I have my suspicions that two "stock" engines, running in the same air, could be even close to competitive with that big a disparity between setups. Let's assume, for this conversation, that your setup was right and you're both equally skilled at driving and set up. That means; he was running too much octane, and rich. For a stock engine that is. Or maybe, his jet selection was correct?