Brumbach

What is the lowest CR that the L79 will perform acceptably?

Powershift

My opinion is that you need an honest 10 to 1 compression ratio to make things work correctly with the L-79 cam.

Let's see if others agree.

I believe 9 to 1 won't make you happy.

Larry

MikeM

Dunno what the lowest is but mine runs well on 89 octane with the factory domed pistons advertised at 11.25-1. I'm sure that's not actual ratio but whatever it is, it works. I wouldn't worry about the lowest compression, I'd build it back to stock spec but that wasn't your question.

When you take compression away from these performance camshafts, you'll wind up with a soggy feeling engine.

I have no idea how low "acceptable" is but it wouldn't be any less than a stock spec build for me.

Brumbach

I agree with you but unfortunately, sometime ago I was convinced to go with flat top pistons on my build. Since then, I've had the pistons mounted on the rods and the rotating assembly balanced. Soon I'll know my achievable CR when I determine piston to deck clearance. If I can't achieve an "acceptable" CR that is compatible with the L79 cam, seems that I have two choices...1) change the cam or 2) purchase dome pistons and have the rotating assembly rebalanced. Am I right?

midyearvette

lower than stock compression with that cam will kill your lower end performance and also much of your mid range performance....you will be able to wind her really tight but it will take forever to get there....i know there are those who do this on these cams and swear by it, but i can't see how that claim can be made....now if you had more displacement, that could be a different story...jmo..good luck....

mikem350

You could find a chart and using the cc's of your heads and flat tops get a ballpark number...or ask the engine builder

MikeM

Like I said, I can't comment on what is acceptable. I can comment on some actual experience for a comparison.

A long time ago, I put an L-79 cam in a 300 hp '65 Chevy II. The car ran noticeably better. I then put a Rochester FI on the engine. It ran better yet.

It would not keep up with my car, which was a virtual twin except I had the factory domed pistons and slightly larger valves, but it wasn't far behind. I believe that would make the engine in that 300 similar to yours except for valve size.

Even if you are using the stock 62-64 cc heads, a block deck that hasn't been cut, factory flattop pistons with the stock compression height and typical thick, blue, Felpro head gaskets, I think you'll be happy. You just won't have all you could have. If you've decked the block, so much the better.

Too late to worry about it now unless you like spending a bunch of money for little gain.

Scott Marzahl

With flat tops you are about 9:1 and as said you'll have a dog with that cam. You would be better off running the 300hp cam or swap out for the correct pistons. I run a true 10.25:1 in one of my cars with the L79 cam and it runs awesome on premium gas.

novadude

Something like a 214 to 220 adv duration cam on a 110 LSA will work very well in a flat-top piston 327 w/ ~9.5:1 CR (where they typically end up with flat-tops and OEM advertised 64cc heads). A cam like this will give you a slight bit of a lumpy idle due to the 110 LSA, and will sound very similar to an L79 cam. The earlier intake valve closing allows the cam to perform well with the low CR.

I have an actual 9.3:1 327 in my Chevy II w/ a Reed Cams 264/272, 214/218, .450/.460, 110 LSA cam. If you search You Tube, there is a 'stock 65 Chevelle L79 idle video on there. The sound of my car is not too far from the way this Chevelle sounds. Idle is basically smooth with just a hint of lope - enough to let you know it has a cam, but not enough to shake the car.

Scott Marzahl

If you are running stock exhaust manifolds do not use a 110 LCA hot rod cam, they are fine for headers but not stock manifolds.

MikeM

Don't know what measured compression we had but your comment is contrary to my experience I posted above. And that's okay!

A 300 hp cam is not a performance cam by any stretch.

Brumbach

If I could achieve 10:1 with the FTs, would the performance with the L79 cam be respectable? Heads and block have been decked to clean up the surfaces. Don't know if they were decked before me.

MikeM

With cleaned up deck, cleaned up heads, stock compression height on flat top pistons, you'll be okay.

Are you looking for more opinions or what?

I remember when you asked about the choice of pistons for this engine and a number of people told you to use the domed replacement but you chose the flattops. Deal with it, you'll be okay but would have been better off with the domed pistons.

MiguelsC2

72 LT-1s were 9.5 to 1 compression and ran the 70 1/2 Mech. cam. They ran exceptionally well. Was the L-79 cam that much different?

Brumbach

Actually, I think the earlier inquiry you refer to was an attempt to validate my purchase of the flattops. (I was influenced to buy the FTs by a quench article I read plus a few other things.) I've been at this build for nearly two years-- sort've paying as I go. I'm now assembling the engine. I'll not undo the work on the rotating assembly, including the flattops. My review of numerous comments on the forum regarding FTs is that they'll fall short of the 11:1 CR and will perform badly with the L79 cam. (I understand the true CR is actually 10.5:1 according to another posting). GM L79 cam gives a CR range of 9.5:1 to 11.5:1. And finally, using the CR calculator, I should be able to achieve 10+:1 CR with the flattops that I've purchased. So it would seem to a novice like me, that my flattops ought to work fine. You seem to agree. I'm asking just in case I'm missing or misunderstood something. And if I am, changing the cam won't cost that much and it's a perfect time to do it now instead of later. Sorry for rambling. Hard day at the office, I guess...TGIF.. Thanks,

MikeM

There is no line drawn in the sand at which point compression ratio becomes unacceptable/acceptable.

I think you will be fine unless you want to spend a lot of money to fix something that isn't really broken.

Brumbach

I put a lot of stock in your opinion. Thanks

aaronz28

The L79 is a smaller hydraulic cam, however
25 more cubes for that 350 LT1 makes a world of differnce though.

Scott Marzahl

If you are at 10:1 you will be great, factory cars were closer to 10.25-10.5:1 than their advertised ratio anyway. If you need a thinner head gasket you can use the hypalon coated fel pro 1094 at .015" or anywhere in between the standard thickness fel pro's.

One thing to pay attention to since the block has been decked and maybe the heads milled is your intake to head seal since the mating geometry has been altered. Not knowing how much material has been removed, you may need to have your intake milled to match.

MiguelsC2

Prior to the LT-1 and L-88, I get the Ls confused sometimes.I thought they were refering to the mech 365HP cam.
The L-79 is the 350hp cam. Which was also running in the L-82s until 79. If my memory serves me correctly. Which it sometimes doesn't.