L78 & Today's Pump Gas?

[Bill Pritchard]

Will a stock 69 L78 with 11:1 compression run without problems on today's premium pump gas?

[mockingbird812]

Hey Bill. Sorry I missed yr email on this subject. You probably know this, but the bottom line is you can run whatever octane that allows you to avoid detonation. Here are some things to consider:
What is the true comp ratio for the engine. An L78 is supposed to be 11:1, but manufacturing standards indicate in reality that may not be true (less). Also, if the engine has been worked on, the published factory 11:1 may have changed (different pistons, heads shaved, etc. In practice, I cut my 93 octane (available here in OH) with a quarter tank of 110 VP racing fuel (you are lucky to have access to racing fuel on the reservations out in Phoenix area - it ain’t cheap though). In a pinch, I have gotten away with just 93 octane, but I’ve listened carefully for detonation.

Tom (Wilma’s Boy) has has/had a few.....

[WILMASBOYL78]

Trying to cut corners on fuel quality for a vintage L78 engine is a risky adventure. I understand some folks may not have access to race fuel, but finding it is worth the effort. These motors crave octane and run best with an aggressive timing curve...no place for low octane fuel. We have 112-115 Sunoco five minutes away...it ain't cheap, but it smells great, burns cool and make the L78 come alive. In a pinch I use 100 octane av gas...

Bottom line...find the best fuel...it's worth the time and money :3gears:

wilma

[1971ls6]

Find a small airport and buy the 100 for $4.00 a gallon

[Xplantdad]

Bill...maybe try calling the RideNow Powersports location on the 202 and see if they still sell Sunoco racing fuel there? I know they used to sell it...as I would give it away as a prize when I put on car shows for Ridenow when I worked there :beers:



RideNow Powersports Chandler/Euro

2677 E Willis Road, Chandler, AZ 85286
Phone: 833-515-0876 (tel:8335150876)

[Lynn]

I am fortunate enough to have access to 100LL at a local municipal airport.

About 4 bucks a gallon. I don't even need to run it at 100%, but sometimes do.

About half and half with 92 works just fine. I don't have access to 93 at the pump around here.

IIRC I calculated the actual CR on my LT-1 spec engine at 11.25, and I simply could not run with aggressive timing on straight 92. Runs like a scalded dog on the AV gas and the timing bumped a bit. Aggressive curve, all in at 3k rpm, total mechanical advance of 38.
It was still pinging slightly even with timing back at 32 to 34. Tried Torco octane booster. I cured the pinging, but fouled plugs something awful. Still have about 3 gallons I would give away if someone wanted it.

[CanCOPO]

I also run 100 Low Lead (actually more lead than leaded gas back in the day) from my local Executive Airport. It's a decided difference in my 69Z 302 which has issues with 93 pump gas. I also have great luck adding tetraethyl lead.

[CanCOPO]

Available from Wild Bill's Corvette. Don't get it on your hands. I find it works as good as the avgas and much more convenient.

[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.

[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.

[WILMASBOYL78]

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 would say those open chamber aluminum heads will definitely make a difference. Your actual compression is probably closer to 9:1 and aluminum allows for better cooling, so detonation isn't as much of an issue.

[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.

[1971ls6]

With the alluminum heads it should run on 93

[WILMASBOYL78]

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 :frown:

[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

[big gear head]

You could just put thicker head gaskets on it. Cometic makes many different thickness gaskets. That may make enough difference.

[olredalert]

----Build the engine you really want and find some place that sells 91 octane non ethanol. Then spend the bucks to get 110 in five gallon cans and add 5 gallons to the 91. You will be happy with the outcome.....Bill S

[WILMASBOYL78]

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

----------------------------------------------------------------------------------

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. :3gears:

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 :cool2:

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]

----------------------------------------------------------------------------------

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. :3gears:

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 :cool2:

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]

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.