C2 SHP compression ratio, 63 v 64

[Alan Drake]

For a 64 FI what it the correct compression ratio?

The 63 manual shows 11.25 for the 360hp FI, while the 64 manual shows 11.00 for the 375hp FI. Always thought the ratio was 11.50 and that's where the extra 15 hp came from!

Thanks
Alan

[Duke W.]

Re: C2 SHP compression ratio, 63 v 64

Due to the chamber relief machined in the 461 heads to unshroud the 2.02" valve, the chamber size increased from about 61.5 to 63.4 cc on '64 SHP/FI, which dropped the nominal CR about a quarter point.

Also, Chevrolet began double gasketing SHP/FI engines in mid-'62 to address customer detonation complaints, so the actual as built CR was typically less than 10.75 in '63 and 10.5 on later SHP/FI engines.

Another cc got added with the 462 heads due to elimination of the small quench zone on the spark plug side of the head, and about that time the OE shim head gasket thickness was increased, so after '65, SHP, which was then only L-79 received single head gaskets, and also about that time the advertised ratio for 300 HP engines was reduced from 10.5 to 10.25:1, which reflected the larger chamber and slightly thicker gasket.

But keep in mind that the advertised ratio is ONLY achieved if the block deck height is at the nominal 9.025" dimension for .025" deck clearance and ONE .018" steel shim gasket, and most decks are high by a few thou, and it's not uncommon to see decks .010" high or more.

So the extra advertised 15 HP (about 4 percent) on FI came from the combination of the larger valves and 30-30 cam, but it took a big hit in low end torque.

Today's premium unleaded gasolines will handle a TRUE CR of about 10.25 with the Duntov or L-79 cam and 10.5 with the 30-30 or LT-1 cam and 9.75 with the base cam. This is achievable on most OE machined blocks with the OE pistons and double shim gaskets or one composition gasket. Chamber size increase from bevel grinding the overhang on the chamber sides also helps keep CR inbounds as this increases chamber size up to one cc.

Installing "low compression pistons" on SHP/FI engines is a big mistake unless you want to run regular unleaded fuel. The combination of high overlap cams and low compression is a recipe for poor torque bandwidth and mediocre top end power.

But since the above specified CRs are pretty much the limit for current unleaded premium, you have to pay very careful attention to achieving the target CR, which means having accurate values of deck clearance and head chamber volume, then selecting a suitable head gasket to achieve close to the target without exceeding it.

Most guys don't understand the sensitivity of actual CR to a small change of chamber volume or gasket thickness. If you want to investigate this, go to:

http://www.csgnetwork.com/compcalc.html

and run some numbers. Use 4.1" for gasket diameter and 5.3cc for SHP piston dome volume.

Duke

[William C.]

Re: C2 SHP compression ratio, 63 v 64

The larger valves in the 375 hp engine require the edge of the combustion chamber to be machined in the intake valve area to unshroud the 2.02 inch intake. That costs a little compression, but the HP gain is made up by the 30-30 cam used in the 375 hp engines, more lift and more duration.

[Alan Drake]

Great info as always, thanks! *NM*