L78 & Today's Pump Gas?

[Pro Stock John]

Originally Posted by big gear head

My 427 is a true 11:1 engine and I'm running 91 alcohol-free fuel in it with the timing turned down to 35 degrees. When I go to the track I use 110 with the timing at 39 degrees. No problems so far. I have open chamber aluminum heads which may make a difference.

I had been wondering how much timing y'all are running.

[Chuck_Burg]

California 91 does NOT work in our L78! It is a 67 with closed chamber heads and NOS .030" over TRW pistons. 5 gallons of 110 leaded in each full tank make it run beautifully, 36 degrees total timing.

[big gear head]

I'm running open chamber pistons. I measured the combustion chamber volume, head gasket volume and piston dome with a piston deck height of about -.005 and did the math. It came out to 10.99:1, which is pretty much 11:1. The open chamber shape and aluminum material may help. I also probably have more valve overlap with my cam than what a stock L78 cam has, which would lower the cranking compression.

[WILMASBOYL78]

Originally Posted by big gear head

I'm running open chamber pistons. I measured the combustion chamber volume, head gasket volume and piston dome with a piston deck height of about -.005 and did the math. It came out to 10.99:1, which is pretty much 11:1. The open chamber shape and aluminum material may help. I also probably have more valve overlap with my cam than what a stock L78 cam has, which would lower the cranking compression.

Sounds like you have a nice setup on that motor...the cam duration will 'bleed off' some cylinder pressure and help the cause. What kind of HP does it produce??? Enjoy all this stuff now...with the way things are going, our hotrods will only exist in museums or magazines

[1971ls6]

With the alluminum heads it should run on 93

[big gear head]

Yea, limited time to enjoy them. It made 563 HP on the pump gas, but I didn't get any dyno time with race gas. 3600 pound car ran 10.85 at 125 with race gas, 11.17 at 123 on pump gas.

[65blackcoupe]

I run half 110 racing fuel and half ethanol free gas. I never use regular ol pump gas of any grade. Ethanol free can be found at co ops, etc.

[JRSully]

I use AV gas 100LL as well. I have a true 11:1 and the timing "all in" at 3k (timing is a big diff maker). I did put some 93 in it a few years ago, no good. Another advantage to AV100LL is it lasts a very long time without breaking down (ie: ethanol laced fuels), should you not use your car much. I get all I want at a local airport for $5 gallon, load up the jugs and leave them in the garage.

[Chevelle SS 396 L78]

I am in the process of organizing parts for my 1967 EG Block (L78) build for a Chevelle SS. I am on the fence as to whether proceed full steam ahead with a genuine parts correct (as close as possible) L78 build, or keep it as stock as possible only "lower" the design compression so it will easily run on pump gas (93 Octane). I've talked with owners who swear their L78's run fine on 93 without detonation, and others say "don't do it!" So, I'm conflicted as to what to do. I just did a compression calculation with my assumed parts and gasket/deck thicknesses and calculated 10.8:1 SCR.

I want to build a correct cone (starting with a true 138 car Chevelle), but paint it and trim it out to match my original 1967 Chevelle SS 396 L78 car (very rare and long gone!) I've been looking for it for more years than I can remember! So, the L78 in '67 was the same engine in both the Camaro and the Chevelle.

Has anyone been successful building their engine with lower compression (say 9.0:1 and how did it run? I want to "re-live" my youth in a clone of the car I "still remember" how powerful and fast it was back in the day! If it;s going to run like a "dog" instead of a "scalded dog" due to low compression, I don't want to throw money at this project at this point in my life. But, if it will run the same or very close?...I'm all in. 4.10 Posi, Muncie M20 4-speed. I was also thinking of shooting for maybe 10.2 to 10.5 compression.

Any advice would be very much appreciated! Thanks! John

[WILMASBOYL78]

Originally Posted by Chevelle SS 396 L78

I am in the process of organizing parts for my 1967 EG Block (L78) build for a Chevelle SS. I am on the fence as to whether proceed full steam ahead with a genuine parts correct (as close as possible) L78 build, or keep it as stock as possible only "lower" the design compression so it will easily run on pump gas (93 Octane). I've talked with owners who swear their L78's run fine on 93 without detonation, and others say "don't do it!" So, I'm conflicted as to what to do. I just did a compression calculation with my assumed parts and gasket/deck thicknesses and calculated 10.8:1 SCR.

I want to build a correct cone (starting with a true 138 car Chevelle), but paint it and trim it out to match my original 1967 Chevelle SS 396 L78 car (very rare and long gone!) I've been looking for it for more years than I can remember! So, the L78 in '67 was the same engine in both the Camaro and the Chevelle.

Has anyone been successful building their engine with lower compression (say 9.0:1 and how did it run? I want to "re-live" my youth in a clone of the car I "still remember" how powerful and fast it was back in the day! If it;s going to run like a "dog" instead of a "scalded dog" due to low compression, I don't want to throw money at this project at this point in my life. But, if it will run the same or very close?...I'm all in. 4.10 Posi, Muncie M20 4-speed. I was also thinking of shooting for maybe 10.2 to 10.5 compression.

Any advice would be very much appreciated! Thanks! John

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Sounds like a great project...one that deserves all the right parts and outcome. First, the street mechanical solid cam [143] was meant to run with high compression, good fuel and plenty of gear in the rear.

To be true to your vision of recreating the old days, the motor should be done right...that means the correct solid lifter cam, true 11:1 compression, big port rectangular heads, 163 hi-rise, Holley, etc...

It was fairly common for the L78 to be under the 11:1 from the factory...usually because of chamber volume in the heads. If you really want to do the motor justice, get a copy of Bill Thomas's book on the 396 and build it to blueprinted specs. Make sure you have a true 11:1 motor that has been built the right way. That motor will make 425hp if it is built right...maybe more.

You mentioned using a M-20 with the 4.10 rear...depending on your final tire size, that will be a hell of a first gear launch...you might want to consider an M-21, or at least a taller rear tire.

Hopefully we can follow along with your build...all of us enjoy a good build thread. Build it big and buy the good gas

Good luck,

-wilma