BB rebuild - too much compression?

[Joe R.]

I am in the process of rebuilding my L-72 (427 / 425) engine. While rebuilding the short block (it's now complete), my machinist suggested dropping the compression. I said let's keep it at 11 to 1 because I wanted maximum power. I was planning to run aviation gas due to its higher octane and better stability. So far I have never taken the car out of the area. But I am starting to doubt the wisdom of my decision. If I wanted to go on a road trip, av gas might be a real hassle. Is there any octane booster that works well? The block is on the stand currently.

Joe

[Donald T.]

Re: BB rebuild - too much compression?

I am in the process of rebuilding my L-72 (427 / 425) engine. While rebuilding the short block (it's now complete), my machinist suggested dropping the compression. I said let's keep it at 11 to 1 because I wanted maximum power. I was planning to run aviation gas due to its higher octane and better stability. So far I have never taken the car out of the area. But I am starting to doubt the wisdom of my decision. If I wanted to go on a road trip, av gas might be a real hassle. Is there any octane booster that works well? The block is on the stand currently.

Joe

Joe,

You have options besides AV gas. First, try running on just premium pump gas. If you don't experience knocking, then there is no need for higher octane. There is a very good chance that it will run fine, especially as long as you keep the RPMs up and don't lug the engine. Octane is just an anti-knock rating - nothing more. If there's no knock then there's no need for more anti-knock additives. Second, if knock is an issue then you can just retard the ignition timing to compensate. Third, if you are convinced that you really need higher octane then you can use a product like lead supreme 130 which contains real TEL.

http://www.kemcooil.com/product_info.php?pId=130

[Joel F.]

Re: BB rebuild - too much compression?

Joe,

I'm with Don completely. Do you know for sure what your compression ratio is, or are you going by the piston spec plus what your builder is saying? The reason I ask is that there are a lot of little things that will affect compression besides the piston. Things like chamber volumes, which can easily get bigger if the valves were sunk at all, a thicker head gasket (factory for a big block was only .028 if I recall), even the gasket bore size and how far the first ring land is will affect compression to a small extent. There are many websites that will allow you to put in all your specs and come up with a truly accurate compression instead of a swag.

Joel

[Michael H.]

Re: BB rebuild - too much compression?

I am in the process of rebuilding my L-72 (427 / 425) engine. While rebuilding the short block (it's now complete), my machinist suggested dropping the compression. I said let's keep it at 11 to 1 because I wanted maximum power. I was planning to run aviation gas due to its higher octane and better stability. So far I have never taken the car out of the area. But I am starting to doubt the wisdom of my decision. If I wanted to go on a road trip, av gas might be a real hassle. Is there any octane booster that works well? The block is on the stand currently.

Joe

Joe,

I agree with Joel and Don. I have a 425 HP 66 that I drive often and have been doing so for the last 25 years. I completely rebuilt the engine when I bought the car and went right back to the original 11.0-1 pistons. Using pump gas, I never have an issue with pinging as long as I don't get on it. If I use AV gas, I can drive it just like it was 1966 again. No pinging, at all.
I don't have a functioning vacuum advance unit though. (plugged)

[Clem Z.]

Re: BB rebuild - too much compression?

if the block was not decked and the heads cut to the CC specs you do not have a ture 11:1 CR

[Joe R.]

Re: BB rebuild - too much compression?

Guys,

Thanks for the valuable input.

Joel,

I don't really know what the compression ratio is. What is a good web site that calculates it?

Michael,

How does the plugged vacuum advance unit come into play here?

Clem,

The block was not decked. I'm not sure what you mean by heads cut to CC specs.

Joe

[Joel F.]

Re: BB rebuild - too much compression?

Joe,

The following site can help you calculate your compression ratio. This is one but there are many others out there that require more or less info.

http://www.crt-performance.com/compression.htm

I think your heads would have had a combustion chamber volume of 106.9 cc's stock. If they have ever been milled that chamber volume will decrease. If the valves seats have been cut the valve will be somewhat sunken and increase the volume. Clem is talking about using a burrette and doing a precision measurement of each chamber on both heads, then equallizing them to within maybe a half a cc. They do not need to be the 106.9 but knowing what the volume is will help ensure you end up with the desired compression ratio, as the chamber volume is an input to the compression calculation (play around with it on the calculator and you'll see what I mean).

Joel

[Duke W.]

Re: BB rebuild - too much compression?

You have to make some measurements and compute the actual CR. My recommendation for L-72 is no more than 10.25:1 true, which is doable with the OE pistons and proper head gasket selection and maybe a little chamber grinding.

This subject has been discussed extensively including an easy to use calculator that you input your exact measurements into to detemine the true CR and the head gasket thickness you need to achieve the target range. Do some archive searching.

Unfortunately most "engine builders" just throw the engine together without any measurements and you have no idea what you end up with.

Duke

[Clem Z.]

Re: BB rebuild - too much compression?

every "new" cylinder head chamber i ever CCed had a volume larger that the one stated on the AMA specs.

[Duke W.]

Re: BB rebuild - too much compression?

In most of the AMA specs I've seen they list total combustion chamber volume, not the head chamber.

The NHRA minimums are a lot less than a typical production head, and I have no idea how NHRA comes up with the minimums unless they assume a certain amount of head surface "cleanup".

Duke

[Clem Z.]

Re: BB rebuild - too much compression?

In most of the AMA specs I've seen they list total combustion chamber volume, not the head chamber.

The NHRA minimums are a lot less than a typical production head, and I have no idea how NHRA comes up with the minimums unless they assume a certain amount of head surface "cleanup".

Duke

when i was a drag race tech inspector we got the specs from the manufactures. used to go every winter to detroit and talk to the guys in charge of drag racing at the manufacturers and get the spec books. some of the AMA specs i have list the chamber volume and the volume of the head gasket. some just list the chamber volume and these are the same volume as the NHRA specs on the few i have compared.

[Ted S.]

Re: BB rebuild - too much compression?

Are you planning on running the factory grind on the cam? If so you may not need AV gas. The factory HP grinds have such a late intake valve closing that they bled off alot of compression. Many times the valve was almost half way up the cylinder before the intake valve seated. You need to check the dynamic compression ratio which takes into account the valve timing verses the 11:1 static compression ratio. I haven't done the calcs on the 427 425hp but the L78 396 375hp engines will run on premium unleaded with a factory cam.

[Joe R.]

Re: BB rebuild - too much compression?

Duke,

I suspect this may be hard to answer, but all else equal, roughly how much horsepower and torque is lost by backing the compression ratio down from 11:1 to 10.25:1?

Ted,

I am planning to run a cam from Comp Cams. It has the same specs as the factory cam. I can dig up the part number if that would help.

Joe

[Clem Z.]

Re: BB rebuild - too much compression?

Duke,

I suspect this may be hard to answer, but all else equal, roughly how much horsepower and torque is lost by backing the compression ratio down from 11:1 to 10.25:1?

Ted,

I am planning to run a cam from Comp Cams. It has the same specs as the factory cam. I can dig up the part number if that would help.

Joe

try this. http://www.bgsoflex.com/auto.html scan down to CR influence

[Duke W.]

Re: BB rebuild - too much compression?

Duke,

I suspect this may be hard to answer, but all else equal, roughly how much horsepower and torque is lost by backing the compression ratio down from 11:1 to 10.25:1?
Joe

Maybe two percent, at most...

Keep in mind that as built by Tonawanda the true CR was very likely less than 11:1 - as low as 10.5 depending on how high the decks are relative to the nominal dimension, which I think is 9.800". SB nominal deck height is 9.025", and it's not unusual to see them .005-.015" high, which is up to half a point.

You need to spend some time with the CR calculator I've listed in the archives to understand reality and how sensitive true CR is to small dimension changes - like deck clearance and gasket thickness. Advertised compression ratios from that era are about as realistic as advertised gross horsepower. It was all a big numbers game.

Duke