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.

 

Many modern head designs want less total timing than the old heads. AFRs and Vortecs run only about 24 degrees total at max power because flame propagation is faster in the newer combustion chambers. Don't know if the LT1 heads have the better chambers.

 

I hadn't heard of that before, but I'll check into that.

 

Hmmm...on AFRs website their dyno test with a 355 Chevy running AFR 195 heads they used 34 degrees total advance. I am running the AFR 195s and started at 34 degrees total. Picked up 2 mph in the 1/8 mile y advancing to 36 total. I haven't done any other tuning, but I doubt 24 total would be any where near max power.
Jeff

 

I asked around in C4 tech about timing w/ LT1s and the only reply so far says he runs 40* at WOT!?

 

Guys, I've very interested in this conversation, but I'll admit I lack the technical sophistication you have, and I don't plan to race my car. I have a '69 with an L36 engine. I've only had this car since last November and I've been a little frustrated trying to get it to run smoothly and perform. I've heard lots of suggestions, through this forum and just talking to guys around my area, regarding timing adjustments and fuel blends. Currently, I'm having a nearby Corvette shop replace the carb. They told me that the one currently installed on my car is not correct for the '69 model. I didn't realize that getting this car to run properly was going to require so much voodoo. Any suggestions to remove the curse?

 

Welcome to the automotive hobby. :seeya

Street or strip and without regard to the particular vehicle's manufacturer we are all in search of the "Daemon Tweak": The magic combo; The Holy Grail; The Lost Chord.

It'll drive you nuts. It'll tear the skin off your knuckles. It'll ruin your clothing. It'll stain up the driveway and the garage. It'll create funny smells that'll get into the house sometimes. There will be smoke of many colors. There will be spilled fluids and ruined dishtowels, t-shirts and shop rags. It'll frighten little children and it'll anger soccer moms. It'll make your neighbors who aren't involved in the automotive hobby either think you're out of your mind or resent you. It'll lighten your wallet and checkbook. It'll fatten up both your credit card balance and the assortment of stuff on your shelves that didn't work. It'll make you buy books and magazines. It'll make you consider advice from people you've never seen face-to-face. It'll keep you up until 3 a.m. on Friday night so you can have it running at the track the next morning. It'll send you to work on Monday morning with faint traces of grime under your fingernails. It'll take time away from your yard work. It'll make you think that owning industrial quality shop equipment is a good thing.

But it's fun. Enjoy.

 

I have seen on a dyno with flat top piston 12:1 engines of different varieties that with octain and timing you get botom end power but the top end (above peak torque) you lose horse power from the extra timing. (too much head psi.) ?Just a thought on why maybe it feels slower or is slower. I have also seen 11-12:1 engines that don't seem to care much what fuel/timing combo they have (probably because of other design issues!)
Overlay dyno runs with 1-2 deg. timing changes and see where it wants it and doesn't. MSD along with others, has upper rpm timing controles to let you drop timing a chosen amount above a determined rpm.

 

Amen, brother. I spent months trying to make my L36 happy, tweaking everything from the distributor springs to the carb jetting to the thermostat rating trying to get it to stop knocking without losing a buttload of power to severely retarded timing. As the first post suggested, the only thing that makes the monster happy is octane. I too, am running blue low-lead avgas in a 50/50 mix, and boy she runs beautiful. The other changes helped, too, but the octane was the only sure cure.

No offense, but a '69 big block might not be the best choice for someone who needs a mechanic to replace a carb for him. I sure don't mean that as a put down or anything--it's just that they are kind of tempermental beasts and do need lots of tweaking, and even with a "correct" carb for a '69, you may need different jetting to get it to run really well (mine had the stock jetting, but runs better with a different combo).

JB


[Modified by JB, 11:59 PM 4/18/2003]

 

Langadorf,

I am actually able to run 91 octane with my 12:1 427 on the street. I use 118 octane race gas at $4 per gallon (before Gulf War II) for dyno runs and extra hard play. Adding 3-5 gallons per tank will work for anything but extended high output driving. A quart of Real Lead 130 works very well with each tank too adding about 4 full numbers to the octane level.

The dynamic pressure in the cylinder is more than just the compression ratio. My L88 cam has so much overlap the dynamic pressure is actually a bit less than a 11:1 motor with the 435 horse cam, at least up to about 5,000-6,000 rpm. My new shorter duration solid roller cam will make much more dynamic pressure if I ever get around to installing it. I will really need the good stuff with this combination or the pistons will be history.

Chuck

 

Here's what I did. I limited the total advance to 28 deg. with a distributor spring kit bushing. I used the spring that allowed the advance to be all in by 3000 rpm. I do not use the vacuum advance. Also fuel is very important.. Amoco 93 is the only fuel that works without pinging in warm weather. Also ... get Lars cab tuning paper from http://www.corvettefaq.com and set your carb up per the correct specs for carb 7029215 or 7029201. No black magic here... Just a case of completely different fuel available today. :cheers:


[Modified by silvervetteman, 6:20 AM 4/19/2003]

 

JB,

"I too, am running blue low-lead avgas in a 50/50 mix, and boy she runs beautiful."

Where do you get this fuel? There is small airport in my city. Is it simply a matter of going to the airport and driving my car up to a gas pump, or should I show up with a 5-gal gas container? What's the protocol? And how much does this fuel cost per gallon - is it really practical to run this fuel in my car on a regular basis?

 

OK, I guess AFRs don't use the reduced timing. Vortecs and GM Fast Burns do. For any engine the best thing is to set the timing on the dyno where the engine likes it best, with the gas you are going to run.

 

Well, you probably don't want to drive the car up to the pump. A lot of small airports don't even have "pumps"--they've got gas trucks that pull up to the planes and fuel them. Don't tell them you want it for a car, because they can't legally sell it to you for automotive use since no road tax is paid on avgas. Best bet is to show up with cans and tell them you want some "low lead". If they ask you what you're using it in, tell them an experimental home-built aircraft. If you're lucky, they'll have self-service pumps, but you still don't want to pull the car up to them. I'd call the airport and ask.

Best bet, though, is to have a buddy who works at the airport like I do. :D

Whether you ought to run it regularly in your car depends on who you talk to. I've discussed it with a lot of motorheads and aviation guys, and the concensus is, it's fine to run it, but not at 100% because it doesn't contain all the same additives that automotive gas does. It's also supposed to be a little hard on the plastic parts in your carb. I've never had a problem. I also mix in a half-bottle of Marvel Mystery oil with each tank at the suggestion of several folks I've talked to about this.

JB


[Modified by JB, 10:47 AM 4/19/2003]

 

I have been hanging out at drag stips for 35 years and anytime fuel comes up you get a number of opinions. My personal experience is to bite the bullet and buy good fuel. I run VP and have found it to be very consistent. They have a good website that gives specific gravities, caloric values, etc. http://www.vpracingfuels.com They sell the stuff all over the world so it should be available in your area. Its not cheap but then neither are our engines, use the good stuff.

 

I'd probably use race gas rather than the avgas, but there's just no place within a reasonable driving distance of where I live that I get the stuff.

JB

 

The jury's still out on my combo.Thaught I had nailed the distributer recurve perfectly till the warmer weather hit and I detected slight ping on a few hills after running hard on the highway.I refuse to run additives,so timing is what will have to do the trick. For what it's worth I'm running 35 total now,with 10:1 flats.The Griffin radiater works like a heat magnet and I'm sure that helps with the ping thing too.