L78 & Today's Pump Gas?

[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

[1971ls6]

I ran an M20 with a 4.30 gear in my vette, I broke it at the track and put in an m21 and lost 2/10ths and 2mph

I had the same m20 with a 4.10 in the Camaro before going to an automatic

In the vette, I was in 3rd gear by the end of a left hand turn from a light

[Chevelle SS 396 L78]

Originally Posted by WILMASBOYL78

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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

Thanks for the info. I actually have the Bill Thomas book...it's my original copy that is 51 or so years old and covered with my greasy fingerprints! LOL! I will utilize it during the engine build! I met with my engine builder today and he knows his stuff....he said to go stock 11:1 compression and build a full stock L78 engine...he's done several with success. He has racing gas at his shop so no worries about gas for the dyno. I can mix once I have it in the car! An L78 with low compression just won't do! I bought the M20 trans when I ordered my car in 1967, not knowledgeable enough to know about the M21 at the time. But with the 4.10 rear end it was super quick out of the hole! I likely would prefer the M21 more, but that's not what I had back then. My build will have headers right from the start, as I had the M/T Super Scavenger headers on my engine...nothing like that sound. I'll likely go with Hooker Competition Headers, since it's impossible to find M/T Headers anymore. I'm anxious to see what it dynos...likely at least 450hp or more.

[big gear head]

The Mickey Thompson headers are being reproduced.

[firstgenaddict]

How much are you actually going to drive it?
2000 miles a year - running straight 110 @$10/gal getting about 8-10 miles per gallon would run approx 2000-2200 annual fuel costs. or approx $1/mile.

Drums of 100LL last I checked (late 2020) were $170-$180 - same fuel consumption would run 30 cents per mile.
The way I figure even on the high side - $1/mile -is worth the fun factor of running the best advance curve and not having to worry about pinging nor run on etc etc.

[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.

[big gear head]

Open chamber and closed chamber heads require different amounts of timing advance. Also piston dome size and shape will have an affect on it. What works on mine might not work on yours. 35 degrees might be a good place to start. Be sure to check your timing marks with a piston stop to be sure that they are accurate.

[Pro Stock John]

Cam will make a difference too.