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.

 

What is your quench/squish?

 

Stock deck height plus .039 gasket, so about .060.

 

Have you considered using a thinner head gasket to get to the ideal .035.045 quench?

 

I had considered it. If I were to use the thin Impalla LT1 gasket (.029"), I would have a .049 which would have been better as far as quench, but would have put me at around 12:1 compression. I thought it'd be better to keep my compression lower. Was this wrong?


[Modified by Langadorf, 3:21 PM 4/18/2003]

 

I don't know where you got the 40 hp number, but it's not that much. Assuming your current setup makes best power with 36 degrees of timing, then 28 degrees is only going to loose about 3 percent, 5 percent at most. This is based on a rule of thumb that varying timing about 2 to three degrees from optimum losses about one percent, and the the percent loss of power increases with increasing deviation from optimum.

The SOTP can often be misleading. I think your best bet would be to dyno the car with 28 degrees and pump gas, then redyno with racing gas an 36 degrees to see what the difference it. I would guess in the range or 10 to 15 RWHP at the top end and less as you go down the rev scale.

Duke

 

I agree with the lower compression, I am in that same boat right now, My deck is .021 and my stock head gasket is .039 so I am going to a .021" in order to get a 10.5:1.

I just sent my 64 CC heads back because I would have had @ 12.1:1 with a thinner head gasket the same as you.

I think thinner quench could help but you have a compression issue.

 

I can't help you blend a mixture, and I am not familiar with the various combinations you have tried. As a test, I would try a load of the highest octane race gas, readily availible, and time it for the highest 1/4 mph or dyno HP.

The idea that it runs best with less timing is a testament to a good burn and combustion efficiency. I'm a little surprised if this is the case; however, given your large quench distance. Often, knock can be decreased by installing a thinner head gasket. The benefit of a tight quench can overcome the minor incease in compression ratio. The less advance required for max power, the better. Rejoice! Good luck, and...

RACE ON!!!

 

I can attest to that. Years ago I had 396 Chevelle with some mods that was running 13.1's with 38º total. I bumped it to 42º and could feel a very noticeable increase in SOTP feel at low-mid rpms. At the track though, my times slowed to 13.3's despite "feeling" more powerfull. Put the timing back to 38º and my et's went back to 13.1's. That's when I discovered the value of recurving a distributor so that I had the benefits of more timing earlier without giving up any power at the top end.

 

Interesting example. By increasing the total timing you were improving the low end and mid range before the centrifugal was totally in, but hurting the top end because there was too much total advance once the centrifugal was all in. Being as how the low end/midrange is only an issue in first gear, you were actually making less average horsepower during the run.

The best way to arrive at the optimum WOT ignition map is to determine the total timing for peak power and adjust the initial so the initial plus full centrifugal gives you this amount of timing.

Then install lighter springs to get the centrifugal in as quicky as possible without signficant low end detonation. Usually the OEM centrifugals are fairly slow to ward off detonation, but they can usually be brought in quicker and this will yield more SOTP low end and mid range as long as the centrifugal is not so aggressive that you get detonation.

Duke

 

What you say is true, Vetterodder. The correct total, does not mean the curve is correct. That is why I said to tune to the MPH for total advance. Quarter mile MPH is a pretty good indicator of horsepower. The origional question was one of total advance and the resultant pinging. Good luck, and...

RACE ON!!!

 

Good discussion! Okay, I can help illustrate what's happening. Picture the torque curve. Horsepower follows torque, but it's torque that you feel first.
When you retard the timing, you are pushing that same curve slightly higher in the rpm band. When that happens, your feel-of-the-pants launch will feel weaker because at a given rpm, there is less torque. BUT that torque band has been shifted more upstairs, resulting in higher peak horsepower numbers. That would explain why retarding the timing improves your quarter mile et by 2/10's of a second, yet it feels weaker.

When you advance the timing, you're bringing your torque curve down the rpm band, so your off-idle torque IS stronger. But horsepower being a factor of torque and rpm, your peak horsepower is reduced, resulting in the 2/10's slower et.

I think this is more a result of your timing experiments and less of the type of high octane fuel you use. Although there are differences between these different types of fuel. Av-gas, if I remember, has a different evaporation coefficient than race gas. In other words, if your car requires race gas, it'll run better on race gas than it would on Av-gas. I wish I remembered better.

Anyway, if you're getting better times with less timing, then I suggest that your LT1 is better optimized with less timing, NOT more. If that is the case, then you want your total advance earlier, not later, and therefore lighter springs. But be conservative. Maybe make a run between changes and record the results. Do what works.

 

For every every speed at WOT there is a timing value that will provide peak power, and it doesn't vary that much from low to high revs. Engines that are designed to operate at constant speed or a narrow speed band sometimes have fixed timing.

The WOT timing curve on an automotive engine is not fixed for a couple of reasons. First, the engine would be tough to start and might even try to run backwards. Second, detonation limits the amount of spark advance the engine will accept at low revs. Detonation is more of an issue at low revs than high revs, so OEMs install a centrifugal curve although modern engines often operate right at the ragged edge of detonation under all conditions because this will yield maximum efficiency.

On older cars the centrifugal curve is usually lazy to ward off detonation, so if you increase total timing there will be more at 2500 which improves torque/power at this speed, but if it's more than the amount required to achieve peak power in the upper rev scale the engind can actually loose top end power.

It's not about shifting the torque curve. It's about raising the torque/power level at each point in the rev scale, and once you know the total timing value to make peak power at high revs, getting the total advance in as quickly as possible will yield the greatest torque/power at each engine speed, which yields the greatest torque bandwidth, and that's the ultimate objective on a steet engine.

Duke

 

Bearcat,

Most of what you are saying, here, about shifting the torque curve by advancing and retarding, applies to CAM timing, rather than ignition timing. Incorrect ignition timing can only reduce the power (or torque) potential built into the engine. Because the ignition timing IS externally variable and adjustable, compromises are not necessary. Cam timing (on our engines) is a set it and forget it proposition. Of couse cam timing CAN be altered to best suit the application, it isn't conviently "adjustable". Good luck, and...

RACE ON!!!

 

I run 100+ avgas with my 12.5 engines and normal lead of 36-38 degrees. L88`s, LS7, an a Race Hemi. Some critics will say you need fancy valve seats, :bs I dont agree it`s necessary. :thumbs:

 

Your 1st paragraph sums it up.
Tune it for 28 on pump gas and you are done.
You are not likely to find more power with race gas. Or the tiny amount you find will be a waste of time IMO.