I'm running a carb'ed 383 LT1 (modern) engine w/ ~11.7:1 compression. On street gas, I can get this setup to run great but have to retard timing about 8* (27-28* total) to avoid pinging. It really runs like a bat outta hell like this.

However, I know that those 8* of retard are probably worth 40+ HP and I'd like to be able to get those back when I go to the track. In order to do this, I have to run higher octane fuels. So far I've tried several things.

First, I tried Xylene. I would mix in about 5 gallons of Xylene with 15 of 93 octane which would give me around 100 octane total. With this setup, I was able to run full timing, but felt no seat-of-the-pants difference and actually was a little slower at the track.

Currently, I'm running 100LL avgas (no, I don't have cats). Again, I'm able to run full timing, but it actually feels slower than running pump gas.

In both of these cases, I've adjusted jetting as needed to get good plug color.

What gives? I know higher octane fuels produce less power (burn slower) than pump gas, but I thought the power gained from running full timing would more than compensate for this. My only other choice now is to either try toluene (which is very similar to xylene chemically), or go to the local circle track and buy some $5+ race gas.

I was always under the impression that race gas was simply a blend of normal pump gas with extra toluenes, xylenes, and/or lead to reach the desired octane equivalent. Is there something else "special" that they put in race gas to make it different?

I know avgas has extra stabilizers in it for cold/high altitude/etc. However, everyone I've ever talked to that runs it, it never seems to be a problem and these guys run low 11s/10s.

With xylene, I've read/heard different things. Most things I've heard about it are good. Only once did I read that the Xylene you buy in paint stores wasn't the same as "chemical grade" Xylene. Apparently this was supposed to make some difference and if true, could be the reason why it didn't help me any.

Should I try blending toluene next or would my results probably be similar to my experience with Xylene?

If race gas is the answer, what octane do you think (I was thinking 100) I should run and leaded/unleaded?

Or, is it possible that my setup just likes running on pump gas w/ 8* of timing retard for some weird reason? (unlikely)

Thanks for any input.

Jason

 

I think you might be laboring under a few erroneous assumptions....

It might help if you had specific numbers from the track (which you indicate you have) rather than "seat-of-the-pants" and "feels slower".

I think you are also assuming that your engine will make more power with more timing, but that isn't necessarily the case.

You can NOT correlate fuel burn rate or energy content to octane. They are not absolutes. High octane fuel does not necessarily burn any slower than street pump gas.

Blending high octane fuels is a lot more complex than adding toluene, xylene or lead to "normal pump gas". If "normal pump gas" includes compounds that break down under heat and pressure to form autoignitable "end gases", adding xylene of toluene will accomplish little. Often higher octane fuels are blended using ligher cut aromatics, which alter the density of the fuel, and alter the BTU/# energy content. So you need to look at specific energy (BTU/unit volume) to understand the impact on a volume-based delivery system like a carb or an injector. You need a fuel that is blended by fuel "engineers" for a specific purpose.

You seem pre-disposed to "home brew" and mis-applied fuels (avgas). Why not just try a very high quality unleaded race gas... VP fuels has 102.5, Sunoco has something slightly higher (104?). When looking at fuel octane, focus on "motor" octane, not research octane or (R+M)/2. Using high octane fuel blended by "professionals" may cost more, but it would provide a baseline which would allow you to test your theory about more timing and establish a valid baseline.

I realize there are no "answers" in here, just a few random thoughts.

 

I know numbers from the track are the best way to test modifications. However, I don't frequently have the chance to run out and test every modification I make. While not very precise, I think the butt-o-meter can still tell if something "feels much slower".

From the literature I've read, I've been lead to believe that most SBC motors liked around 34-36* total timing. Is this possibly not correct for some reason in my application? If this is indeed so, what unusual factors about my engine might contribute to this? Higher compression and/or displacement?

Well, you might say I'm pre-disposed to saving some money, if possible. I have realized that race gas might be my final solution, but I wanted to try cheaper options first.

After reading this, I did some research and it seems I was indeed mistaken. I believe I was thinking that most "octane boosters" use chemicals that are high in octane but lower in energy content.

Aren't aromatics benzene, toluene, and xylene? Or is a "lighter cut aromatic" something different?

Again, thanks for the input.

 

I'm not sure if you realize this but when you "retard" timing, you will gain more rpms up high; "advance" the timing and you gain down low but less up high. Are you running a distributor? If so you might want to check your springs in the cap and you may need 1 stiff and one medium. 34-36 degrees is just a base point for sbc's. My 76 I ran once with 46 degrees total advance and I had no detonation; but it was progressive.