CR on 1970 454

[Duke W.]

Re: CR on 1970 454

My 1995 F-M Performance Parts catalog (X-3009) lists the L2304F dome height at .100 and the dome volume at 2.4 cc.

It's always been my assumption that the "dome volume" is NET of the dome and valve notches.

These are different than the NHRA data, so the question is: Is there an error, or is there a difference in measurement method?

There must be something to this as I remember that John McRae measured the net dome volume of L-72 pistons and got a few cc less than what the F-M catalog listed, so maybe the F-M catalog does not subtract the effect of valve notches from gross dome volume. It's not at all clear.

Duke

[Patrick B.]

Re: CR on 1970 454

What I believed to be AMA specs came from an old NCCC book of 1977 called "Corvettes Technically Speaking" and they are more internally consistent than those cited by Duke. Perhaps they they were messaged by NHRA because they agree exactly with the specs Clem cites.

Here is what they give for Duke's Example of a 1972 454/270.

Max. CR 9.3:1
Nom. CR 8.5:1
Cyl Head Volume 105 cc (Casting No. 3999241)
Head Gasket .021
Head Gasket Vol. 7.09 cc
Deck Clearance Min. .008 Below Block

Min. Comb. Chamber Vol. 112.1 cc
(Max. CR may not be Exceeded)

The minimum combustion chamber volume includes the head gasket and piston dome (when one is present for other engines in the spec book) and it is the number used to compute the max CR.
In this case: (930cc + 112.1cc)/112.1 cc = 9.3 , the max CR

I believe 105 cc is the minimum volume spec for the 3999241 head casting (Colvin's book cites a nominal 113 cc for this head) and the Min Combustion chamber volume is this number plus the gasket volume for an engine with flat top pistons. The deck clearance and piston notch volumes are somehow neglected. Anyway, I'll bet legal NHRA 454/270 engines are built to the 9.3 CR adhering to a minumum head volume of 105 cc.

Not suprisingly, this source of specs describe the 70 454/390 pistons as having a .094 high dome with 12.1 cc volume in agreement with the NHRA specs cited by Clem. Also, I'll bet it's no coincidence that the KB203 pistons for this engine also have a dome volume of 12 cc, although the dome may be a little higher and the valve notches a little deeper.

[Patrick B.]

Re: CR on 1970 454

My 1995 F-M Performance Parts catalog (X-3009) lists the L2304F dome height at .100 and the dome volume at 2.4 cc.

It's always been my assumption that the "dome volume" is NET of the dome and valve notches.

These are different than the NHRA data, so the question is: Is there an error, or is there a difference in measurement method?

There must be something to this as I remember that John McRae measured the net dome volume of L-72 pistons and got a few cc less than what the F-M catalog listed, so maybe the F-M catalog does not subtract the effect of valve notches from gross dome volume. It's not at all clear.

Duke

I have the 1986 TRW Catalog, also called X 3009, which listed the L2304F LT-1 piston as 2.4 cc "effective dome volume" and the L2417F forged replacement for the 350-300 flat top with notches as -3.4 cc "effective dome volume."
It looks like the 2.4 cc number includes the notches and maybe the NHRA chart has a typo.

[Duke W.]

Re: CR on 1970 454

The specs I quoted are directly from the GM-published AMA specs that you can download from the GM Heritage Web site for any year Corvette.

When you compare AMA to NHRA specs it's clear that NHRA allows decking the block, milling the head, and use of a thinner than OE head gasket.

Conflicting specs that are often unclear as to what is actually being measured is why it's important to make all the requisite measurements on each individual engine beginning with measuring and recording deck clearance prior to block teardown in order to get a handle on the compression ratio management process and end up with a completed engine that is accurately within your target range that you established during the engine restoration planning process.

That last term may be a new idea to some, but it's critical to achieving objectives like maximum broad range performance and fuel economy in your restored engine. At the other extreme is the pull the engine and give it to "bubba the engine builder" crowd. This happens WAAAAAY too often and many get posted on the Web as another "engine builder" horror story.

Duke

[Patrick B.]

Re: CR on 1970 454

The specs I quoted are directly from the GM-published AMA specs that you can download from the GM Heritage Web site for any year Corvette.

When you compare AMA to NHRA specs it's clear that NHRA allows decking the block, milling the head, and use of a thinner than OE head gasket.

Conflicting specs that are often unclear as to what is actually being measured is why it's important to make all the requisite measurements on each individual engine beginning with measuring and recording deck clearance prior to block teardown in order to get a handle on the compression ratio management process and end up with a completed engine that is accurately within your target range that you established during the engine restoration planning process.

That last term may be a new idea to some, but it's critical to achieving objectives like maximum broad range performance and fuel economy in your restored engine. At the other extreme is the pull the engine and give it to "bubba the engine builder" crowd. This happens WAAAAAY too often and many get posted on the Web as another "engine builder" horror story.

Duke

Most of the "Corvettes Technically Speaking" book is a compilation of the AMA specs for 1953 to 1977 Covettes exactly like the GM Heritage site, but there is a section in the rear with a lot more detail about 1962 to 1977 engines. I had always thought this info also had AMA as the source, but I now think the source was NHRA as you suggested. The pages are not identified as NHRA material, but NHRA is listed in the credits of the book.

However, the GM heritage info is sufficient to answer the question under discussion: Were the 1970 LS-5 pistons domed or flat top? Page 4 of the AMA specs describe the LS-5 pistons as "domed head, valve cut out." Page 9 of the Power Train section included in the 1970 GM Heritage info lists all 454 pistons as domed. With the GM info confirming that the pistons were domed, I would not doubt the NHRA info that the dome volume was 12.1 cc.

[Joe L.]

Re: CR on 1970 454

Most of the "Corvettes Technically Speaking" book is a compilation of the AMA specs for 1953 to 1977 Covettes exactly like the GM Heritage site, but there is a section in the rear with a lot more detail about 1962 to 1977 engines. I had always thought this info also had AMA as the source, but I now think the source was NHRA as you suggested. The pages are not identified as NHRA material, but NHRA is listed in the credits of the book.

However, the GM heritage info is sufficient to answer the question under discussion: Were the 1970 LS-5 pistons domed or flat top? Page 4 of the AMA specs describe the LS-5 pistons as "domed head, valve cut out." Page 9 of the Power Train section included in the 1970 GM Heritage info lists all 454 pistons as domed. With the GM info confirming that the pistons were domed, I would not doubt the NHRA info that the dome volume was 12.1 cc.

Patrick-----

I now expect that the 1970 LS-5 pistons were domed (which is actually what I thought for years until I recently found what I believed was credible information from multiple sources to the contrary). However, I'd still like to see one of these pistons. I've tried to locate an NOS example of either bank (these pistons had an offset pin), standard or any oversize, but I've been so far unsuccessful.