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 much 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

 

You also posted this in Advanced Tech, right? Fred posted an excellent answer there.

Rich Krause

 

maybe you connected your msd lines reversed? you want to connect the wires to what gives the most retard then it is correct

 

What are you talking about concerning the MSD?!? It's hooked up correctly.

 

Using a distributor or opti?

 

Distributor.

 

how are you checking full advance? Is it vacuum advance or mechanical? what size springs are on the weights? you want full advance by 2500 rpm's so set the idle screw at 2500 rpms and check the timing. Do you have a dial indicator timing gun? If not invest in one to solve the timing.. try and hook the vacuum port to ported vacuum if its not already. If that fails try full vacuum; If that fails do like I did---I used no vacuum advance at all--I used lighter springs to have my timing all in by 2600rpms and I advanced her 12 degrees with 38 total. Buy the advance kit and do not remove the weights that came with your distributor, just change the springs and look for your combo.. Also what size jets are in the holley? What about power valve? you could be lean and not know and it's screwing up your burn.. What about your squirter size(s)? If it's a double pumper what size lift is on the cam? (I am a Holley guru) I hav e alot of experience with carbed and distributor cars not electronics so thats why I ask all these questions. You should be getting away with 92 octane gas. Running that much compression you should be jetting on the richer side let me know

 

I've installed a timing tab and timing marks and use an adjustable timing gun. To check full advance, I rev the engine up until I see the timing stops advancing. I have vacuum advance disconnected. With my setup, I can't seem to get the full advance in any quicker than 3000 RPM w/out knock.

I'm running a Holley 750 DP w/ 74 jets on the front and 83 jets on the rear and a 6.5" powervalve. I'm probably a little rich if anything. The primary shooter is a 28 w/ the blue cam in the #2 position. The secondary shooter is a 32 w/ the pink cam in the #1 position.

My cam is a Comp Cams cc306 w/ 1.6 RRs. Specs are:
Duration @ .50 230/244
Valve Lift .544/.576
Lobe Seperation 230/244

Thanks.

 

Are you getting knock adjusting the timing? medium springs (blue) and start initial @10 and rev to 2500 see 34; If not adjust by jetting lower or go 2 inches below idle vacuum on the power valve and check the timing again. You have a motor that runs good with semi-lean cylinders (high cr-alum heads-reverse cooling) you will notice the change at WOT. All else fails black springs and no vacuum advance!! You want 36-38 degrees @2500 for streetability and 40 strip.. Just from my experience.. One more thing--your secondaries are way too rich they should only be 4 sizes larger then primaries.

 

The secondaries come 10 jet sizes bigger than the primaries STOCK. I hardly doubt this is too rich.

 

When I bought my 750 doublepumper new it came with 72 primary and 76 secondary and according to every holley guru I know that is how it is not 10 sizes bigger!! Right now I run moroso jets that are 82 primary and 86 secondary with a 3.5" power valve with a primary squirter of 37 with the 50cc pump and the same out back. I converted it to 850 as you see in my sig the pics with an 1050 dominator base. It never ran better. If you get knock retard even with it running that rich then your timing is off quite a bit.

 

Do you hav an actual quantifiable way to gauge the difference? I am not overly technical but after 5 years of racing every week I know that my "butt-o-meter" sin't necessarily accurate to 1/20 of a second.

I also highly recommend race gas that is blended for a specific purpose. I tried AV gas a few times a couple of years ago and what the aviation industry calls "low lead" is just as bad as leaded if you still are relying on 02 sensors for anything. I know you have a carb so I'm not sure thats applicable.

If you had actual results it would be easier to provide accurate advice.