Piston question

[Mike R.]

I disassembled my engine today and found Keith Black pistons with a .100 dome. They look virtually new with no skirt wear and the ring grooves are perfectly clean. Unfortunately, they will only give 10.4 compression with the thinnest head gasket. Should I keep these or is it worth another 400 to get another 0.6 of compression?

[William C.]

Re: Piston question

Use them and be happy

[Joe L.]

Re: Piston question

I disassembled my engine today and found Keith Black pistons with a .100 dome. They look virtually new with no skirt wear and the ring grooves are perfectly clean. Unfortunately, they will only give 10.4 compression with the thinnest head gasket. Should I keep these or is it worth another 400 to get another 0.6 of compression?

Mike-----


Quite frankly, the only reason I would be reluctant to use these pistons is because they produce TOO HIGH of a compression ratio. Personally, I would NEVER, EVER build a big block engine today with a compression ratio of 11:1. I think that 10.4:1 is even too high. Big blocks are very susceptible to detonation and pre-ignition and it's a MISERABLE thing to have to live with. Once you build the engine with a certain compression ratio, if you can't tune the engine to reduce or eliminate the detonation or pre-ignition problem, you're stuck with it. Too much of a risk for me.

Here's a "clue" that I've mentioned before: GM does not today build a crate big block engine with cast iron heads that has a compression ratio exceeding 8.75:1 and even those engines require 91 octane fuel. I feel very confident that if GM engineers thought they could reliably get away with more compression without causing unhappy customers, they would. After all, building more compression into the engine basically costs nothing.

Also, quite a few years ago I had a discussion with John Erb, chief engineer for Silvolite/Keith Black. John is VERY knowledgeable about engines, especially performance engines and owned several classic "muscle cars" himself. John told me that he would never build or recommend building a street big block Chevrolet engine with a compression ratio exceeding 9:1.

[Mike R.]

Re: Piston question

Its a small block

Mike-----


Quite frankly, the only reason I would be reluctant to use these pistons is because they produce TOO HIGH of a compression ratio. Personally, I would NEVER, EVER build a big block engine today with a compression ratio of 11:1. I think that 10.4:1 is even too high. Big blocks are very susceptible to detonation and pre-ignition and it's a MISERABLE thing to have to live with. Once you build the engine with a certain compression ratio, if you can't tune the engine to reduce or eliminate the detonation or pre-ignition problem, you're stuck with it. Too much of a risk for me.

Here's a "clue" that I've mentioned before: GM does not today build a crate big block engine with cast iron heads that has a compression ratio exceeding 8.75:1 and even those engines require 91 octane fuel. I feel very confident that if GM engineers thought they could reliably get away with more compression without causing unhappy customers, they would. After all, building more compression into the engine basically costs nothing.

Also, quite a few years ago I had a discussion with John Erb, chief engineer for Silvolite/Keith Black. John is VERY knowledgeable about engines, especially performance engines and owned several classic "muscle cars" himself. John told me that he would never build or recommend building a street big block Chevrolet engine with a compression ratio exceeding 9:1.

[Joe L.]

Re: Piston question

Its a small block

Mike------


OK, I forgot that. Anyway, my advice does not change too much. I would not build a small block with an 11:1 compression ratio, either. As a matter of fact, most original small blocks with an ADVERTISED 11:1 compression ratio did not have a REAL 11:1 compression ratio, either.

For a street small block with cast iron heads I'd be reluctant to go as high as 10.4:1. I MIGHT chance that, though. Actually, what I would do if I were you would be to use the pistons you have in conjunction with a thick head gasket, like a Fel-Pro Permatorque with 0.039" compressed thickness or an even thicker Victor-Reinz VictorCore gasket. That should get you down to 10:1 or, hopefully, even a little less. Like I say, you absolutely don't want to live with pre-ignition.

Some folks say that an engine has to have high compression to sound right. In my opinion, one of the best sounding engines i ever heard was a 1971 Corvette LS-6 (with 9.0:1 compression ratio).

[Clem Z.]

Re: Piston question

Its a small block

use this website and see how much you have to cut the heads to get the desired CR. http://cochise.uia.net/pkelley2/crc.htm

[Mike R.]

Re: Piston question

Joe, thanks for your thoughts. The static compression ratio is only one factor that determines the dynamic compression ratio. Altitude is also a big factor. The lowest altitude around here is 2100' so the pressure here is equal to about 3" of vacuum at sea level. The cam is improtant too. Obviously you can't run 11:1 with the 250hp cam and pump gas. It is a different story with long duration camshafts like the 30-30 and LT-1. A lot of people are stock L84 cars on pump gas without problem. I have been driving around all summer with about 10:1 (the existing KB pistons with a .039 gasket) and have never heard it ping even flooring the throttle at low rpm in fourth gear.

BTW reducing compression by using thicker gaskets is a compromise. There is an optimium piston to head clearance which is around 040" which is the nominal deck height .025 plus a .015 gasket. As you increase this dimension, you reduce turbulence in the combustion chamber. The right way to do it is to change pistons.

Mike------


OK, I forgot that. Anyway, my advice does not change too much. I would not build a small block with an 11:1 compression ratio, either. As a matter of fact, most original small blocks with an ADVERTISED 11:1 compression ratio did not have a REAL 11:1 compression ratio, either.

For a street small block with cast iron heads I'd be reluctant to go as high as 10.4:1. I MIGHT chance that, though. Actually, what I would do if I were you would be to use the pistons you have in conjunction with a thick head gasket, like a Fel-Pro Permatorque with 0.039" compressed thickness or an even thicker Victor-Reinz VictorCore gasket. That should get you down to 10:1 or, hopefully, even a little less. Like I say, you absolutely don't want to live with pre-ignition.

Some folks say that an engine has to have high compression to sound right. In my opinion, one of the best sounding engines i ever heard was a 1971 Corvette LS-6 (with 9.0:1 compression ratio).

[Joe L.]

Re: Piston question

I have been driving around all summer with about 10:1 (the existing KB pistons with a .039 gasket) and have never heard it ping even flooring the throttle at low rpm in fourth gear.

BTW reducing compression by using thicker gaskets is a compromise. There is an optimium piston to head clearance which is around 040" which is the nominal deck height .025 plus a .015 gasket. As you increase this dimension, you reduce turbulence in the combustion chamber. The right way to do it is to change pistons.

Mike------


Then I would thank my lucky stars for what you've got and keep everything exactly where it is. Why risk getting into detonation just to gain 0.6 CR increase? Sure, it might not happen, but what if it does? Just because a particular engine runs ok at 11:1 does not mean that another will. There are other variables and, even, "intangibles".

To alter CR a piston change is definitely the right way to do it. However, you don't find pistons available for every possible incremental CR ratio. This is especially true for pistons for older applications and 327 pistons definitely fall into that category.

[Timothy B.]

Re: Piston question

Mike,

What camshaft are you going to use in the engine? If I understand correctly you want to reduce quench by using shim gasket and that will increase compression from where it's been running.

I don't know what the cost would be but can the pistons be turned to reduce the dome so the piston volumn is the same and placing the compression where you want. May be cheeper to buy new pistons in this case..

I thought the stock hp pistons had a .125 dome so you may be where you need to be, the GM shim gasket I have is .022 thick so think about that..

Do you know for sure the compression height of the Keith Black pistons??

[Joe C.]

Re: Piston question

I am not an advocate of replacing a 30-30 cam with an LT1 cam, but in this case, I'll make an exception!

I realize that in a prior post, you made no mention of replacing your 30-30 cam with something different, however, I can attest to the fact that a properly built engine equipped with a 30-30 cam will run perfectly on 93 PON gas with an aggressive advance curve and no detonation and an ACTUAL, verified and calculated SCR of 11.0:1. Since you will be shy of this ideal number by more than 1/2 point, and if you wish to keep a stock GM cam, then replacing the 30-30 with the LT1 is certainly much cheaper and easier than losing those nice KB pistons. The LT1, because of its earlier closing intake valve, requires slightly lower SCR than the 30-30 in order to produce comparable DCR.

Of course, going with an aggressive aftermarket cam can boost both torque and extended range power output (later and less rapid torque drop-off past the torque peak). You will sacrifice a small amount of durability as a result............

Optimal quench can safely be as low as .035" with decent rods and .025" with racing rods and cast pistons (low skirt clearance). If you use the Eagle rods, then I recommend .035" quench. If you mill the heads, as Clem suggests, then I'll guess that with small domed pistons such as yours (.1cc vs 5.3cc stock pistons), you'll be looking for chamber volumes in the vicinity of 58cc to get you back to 11:1 SCR with .035" quench. I don't think that it is possible to get your chambers down to 58cc unless you have them angle milled. You'll then have to use a .038" compressed thickness MLS gasket with deck height of 9.022" to do this. If you don't surface your decks, then the thinnest gasket you can use, a shim steel, will give you quench somewhere in the vicinity of .046" +/- .015".

[Joe C.]

Re: Piston question

Mike,

What camshaft are you going to use in the engine? If I understand correctly you want to reduce quench by using shim gasket and that will increase compression from where it's been running.

I don't know what the cost would be but can the pistons be turned to reduce the dome so the piston volumn is the same and placing the compression where you want. May be cheeper to buy new pistons in this case..

I thought the stock hp pistons had a .125 dome so you may be where you need to be, the GM shim gasket I have is .022 thick so think about that..

Do you know for sure the compression height of the Keith Black pistons??

Timothy,

He needs more SCR, not less.

Original pistons had 5.3cc domes net (less reliefs), which are duplicated by FM Speed Pro L2166NF.

The compression height had better be the same for any SBC piston used with a 3.25" stroke and 5.7" rod: 1.675" - 1.678".