'64 L-76 piston question

[Laurence E.]

Rebuilding the engine in my "64. Original L-76. Bored the block +.020

My dilemma is should I use the original style domed pistons or the flat top version. I know dome are correct but am concerned about detonation with todays gas.

Plan on using Speed-Pro products and with the 461 heads at about 64 CC's, -.025" piston deck height and a .032" gasket have calculated the following

Dome piston would give me an effective CR of 10.56:1
Flat top piston - effective CR of 9.34:1

I am aware no one will see the difference, but I hate to change from original. I am looking for a dependable engine, as I am older than the car and not rebuildable.

Also using the stock 30-30 cam. Car has a 4:11 rear

Any experiences, thoughts, insight appreciated

[Duke W.]

Re: '64 L-76 piston question

I don't think the Federal Mogul OE replacement forged, 5.3 cc dome pistons are available in .020" oversize. The first oversize is .030".

You should consider the Keith Black KB 157, 0.5 cc dome hypereutectic piston. If you have 93 PON fuel limit true, measured CR to no more than 10.5, and reduce 0.1 for every octane point below 93. Head gasket thickness choice is the way to achieve the target CR.

I highly recommend you install a LT-1 cam in lieu of the 30-30. It will make more low end torque with about the same top end power.

In addition the early 327 rods are weak and subject to fatigue failure, especially on high revving mechanical lifter engines. A new set of Eagle SIR 5700 rods is about $250 and is the cheapest insurance policy you will ever buy.

The machine shop should not do the final honing until they have the replacement pistons in hand, so they can hone each cylinder for a specific piston to achieve the proper clearance range. Also, KB pistons have a high mounted top ring that requires greater than OE clearance to avoid ring butting, which will break the top ring land. Be sure your machine shop understands this. The instructions on the United Machining Web site are very clear.

Duke

[Laurence E.]

Re: '64 L-76 piston question

Duke, thanks for taking the time to respond. I appreciate the in-depth technical answer. Besides your answer, when i look at GM crate motors none are over 9.6 and many have aluminum heads. So flat top it is.

Thanks for the idea about the cam. 3972178 unit, yes?

Great idea about the rods. Was planning on reconditining and new bolts. At $242 from Sumit why bother.

[Jack H.]

Re: '64 L-76 piston question

FederalMogul/SpeedPro L2166NF are available in Std, .020, .030, .040, and .060. Last I bought them, they were about $300/set and I happened to get the .020's.

[Richard G.]

Re: '64 L-76 piston question

We have learned a lot since 1963.
We know, for the most situations, piston domes do not promote flame propagation and flame propagation makes horsepower.
I believe it is the major reason that modern high compressions engines have small combustion chambers and no piston domes when compared to the 60's counterparts with the same compression ratios. Use flat top pistons with the thinnest gasket available and you may not loose as much power as one might think because of better flame propagation. The overbore will also slightly increase compression. Might even be an argument for going .030 over instead of .020, if not purchasing domed pistons. (if the piston compression height doesn't change) See below;
It would make an interesting dyno study.

Read the fine print on the replacement pistons literature. Often the compression height is lower than stock to account for blocks that have been decked. Of course customers would blame the piston manufacture if the pistons hit the heads. GM didn't have to account for this.
I would avoid the lower deck height pistons at all costs. A lot of variables. Do the measurements and the math.
Best of luck on your project.
Rick

[Duke W.]

Re: '64 L-76 piston question

Duke, thanks for taking the time to respond. I appreciate the in-depth technical answer. Besides your answer, when i look at GM crate motors none are over 9.6 and many have aluminum heads. So flat top it is.

Thanks for the idea about the cam. 3972178 unit, yes?

Great idea about the rods. Was planning on reconditining and new bolts. At $242 from Sumit why bother.

If memory serves, the Speed Pro (Federal Mogul) CS-1145R is the number for the LT-1 cam. Verify by inputing the '70 Corvette 350/370 engine into the napaonline.com Web site, and you can buy it from any outlet that sells Federal Mogul parts. It may also be available in a "kit" that includes both the cam and piddle valve lifters. Also buy a set of F-M VS677 valve springs.

Go over to the Corvette Forum and search for threads stated by me, "SWCDuke". You will find several that are useful including an article on how to manage the compression ratio to achieve your target. GM crate engine CRs are on the conservative side and some may even operate on regular octane fuel (87 PON). Flattop pistons with valve notches will leave torque/power on the table. Compare the Keith Black KB 156, KB157 and F-M L2166 with various gaskets and don't go below 10:1, but up to 10.5 is okay with the LT-1 cam 93 PON fuel.

Also search for threads started by Dave McDufford and ghostrider 20 to learn about the "327 LT-1" configuration. If you massage the heads you'll have close to 300 SAE corrected RWHP and 7200 useable revs. It you don't massage the heads figure 10 percent less top end power (but similar low end and peak torque) and revs, but the Eagle rods will yield in a much more durable engine.

The performance and quality of the final product will be proportional to your planning effort. You should have a parts list that contains the brand/part number for every part that might need to be replaced and manage the machine shop to only do machining operations that are absolutely necessary, and for most rebuilds that is only boring/honing of the cylinders for the new pistons.

Piston choice is yours, but depending on your blocks deck height it will probably be easier to achieve the target CR with a thin head gasket and the Keith Black piston KB 157 piston, and they are about half the price of the L-2166 OE replacement type. Flattop pistons will valve notches that yield a net "dish" will be leaving torque/power on the table.

Duke