Special High Performance and FI compression ratios

[Duke W.]

The '62 and '63 Special High Performance and Fuel Injection engines have an advertised compression ratio of 11.25:1, but we know that at some point production engines were a bit lower (about half a point) because Flint began to build them with two head gaskets per side to reduce detonation complaints, and a 1962 service bulletin called for field installation of two gaskets per side to ameliorate customer detonation complaints.

For 1964 both engines were upgraded with hotter cams and larger valves with an attendent increase in rated power, and a new Special High Performance with Hydraulic Lifters engine (L-79) was added to the lineup in '65.

All these engines have the same head casting and pistons, so the question is: Why was the compression ratio for these engines reduced to 11:1 beginning in 1964. Did it have something to do with the extra head gasket?

A member's recent inspection of a '66 L-79, which is believed to be an original untouched Flint built engine showed only one head gasket per side, but the '66 head and piston carried over and the CR was still advertised at 11:1.

This whole CR/head gasket deal in the sixties has me both confused and amused. Does anyone have any explanation/speculation/insight into this issue.

Duke

[dale pearman]

Re: Special High Performance and FI compression ra

Duke the trap here is the difference between static and dynamic compression ratios. The 30-30 cam, as well as the 151 cam close the intake valve much later in the cycle thereby moving the peak volumetric efficiency higher in the power band. At lower rpm cylinder filling is relatively poor and pressures lower resulting in no "knocking complaints".

Dealing with static compression ratios can be a serious derailment in the understanding of engines. By definition the cylinder is at 100% volumetric efficiency, which is impossible to achieve with a running, naturally aspirated engine.

Varooom!

[Jeff]

Re: Special High Performance and FI compression ra

I assume Duke is refering to the computation of static compression and not a direct measurement of effective cylinder pressure but anyway, FWIW, in 76 I had 64 and 65 375s and a 65 365 with 232,000 miles. I can't recall the VIN but I remember it was late because I was going to swap the body onto the 65 FI which was very low miles but had a horribly custom body so I had done some comparisons of the cars and bodies.

At 232K, I figured it was time to hit 'refresh' and when I took it apart it only had one original GM gasket. Based on how long I had had it and the way it came apart I am pretty sure it had never been apart until then but can't say absolutely. Never took the other 2 apart.

JP

[Bill Clupper]

Re: Special High Performance and FI compression ra

I can vouch for '65 Corvette VIN 12088, 365 HP 1 head gasket per side, (thin) head pulled at 60K miles

[Gene M.]

Re: GM's optimistic ratios

GM's figures on compression are based on perfect cylinder volumes. If you actually fill the head cc's and the gasket thickness and piston in the bore dome size actuals the compression is closer to 10.5 at best. A "blue printed" motor could achieve 11 to 1 static compression.

[dale pearman]

Re: Special High Performance and FI compression ra

Bill I can guarantee that if you did nothing else to your engine except change the mechanical phasing of the cam (advance it 4 or 6 degrees) you would be knocking, rapping and tapping like Edgar Allen Poe never dreamed! Static compression ratio has a lot to do with detonation but the cam is the controling factor. The sooner you close the intake valve, the greater the cylinder pressure, and with today's crummy gas, KA-BOOM! The 097 cam closes a lot sooner than the 30-30 and as a result detonation is much more likely. That's why the 1962 327 needed two gaskets to cut the DYNAMIC compression ratio down. Dynamic is related to static of course through piston dome shape and cylinder head volume but it's the cam that controls things.

Varooom.

[Joe L.]

Re: Special High Performance and FI compression ra

Duke------

The reason for the differences in compression ratios between 62-63 special high performance (SHP) engines and later versions is the pistons. Although the pistons APPEAR the same, they ARE NOT the same.

The 1962-63 SHP PRODUCTION piston was GM #3799501. This is the 11.25:1 piston. It was also available in SERVICE until late 1964.

The 1964-65 SHP PRODUCTION piston was GM #3850137. This is the 11.0:1 piston. In late 1964, this piston replaced the 3799501 piston for SERVICE of all 1962-1965 SHP Corvette applications. So, if the 3850137 piston was installed in a 62-63 engine, the compression ratio was automatically, and usually unknowingly, reduced.

In 1966 the SHP PRODUCTION piston was changed again. It remained an 11.0:1 piston but it now it became GM #3871208. In late 1966 this piston replaced the GM #3850137 piston for SERVICE of all 1962-1966 Corvette SHP applications.

The GM #3871208 piston remained the PRODUCTION piston for Corvette L-79 applications during 1967 and 1968 model years. It also remained the SERVICE piston for all 1962-68 SHP applications until it was discontinued in 1987.

So, now you know the rest of the story.

[Duke W.]

Re: GM's optimistic ratios

It looks like GM eliminated the double gasketing in '64. This makes sense because the 30-30 cam was more aggressive than the Duntov and killed some low rev compression pressure, which may have eliminated the prior detonation problem. The L-79 cam was probably a bit less aggressive than the Duntov, but fuel quality had improved since the early sixties, and super premiums were more common.

My 1995 Federal Mogul Performance Parts catalog (They now own TRW Automotive) lists calculated CRs using a .038" thick compressed gasket, which is about equal to the thickness of the two steel shim gaskets. For the .125" dome pistons (OEM replacement) they list a compression height of 1.675" and this would yield a crown to deck clearance of .025" with a nominal 9.025" crankshaft C/L to deck dimension. Their calculated CR is only 10.23 with a nominal bore 327. If a single gasket was used and the deck cut down to achieve the minimum recommended quench zone height of .035" the total chamber volume would be reduced about 6 ccs which would bump the CR to just about an even 11:1.

Yeah, it looks like the advertised static CRs were about as realistic as the advertised SAE gross horsepower.

Thanks for the input, guys, but I've got one other question. I've seen the early 1.94/1.50 valve 461 heads quoted at 61 ccs for the head chamber volume, but 64 ccs for the later 2.02/1.60 valve versions. Anyone have any insight on this specification change?

Duke

[Bill Clupper]

Re: Special High Performance and FI compression ra

With respect to the cyl head cc question, the difference is due to the factory maching some metal off the inside of the chamber to clear the 2.02 intake valve

[Duke W.]

Thanks, guys!

This long running mystery is now solved. I replaced my original pistons with GM service replacements purchased in the late seventies, so I must have the late L-79 pistons. I also beveled the chamber on the exhaust side to get rid of the overhang, so with a .030" overbore, the late pistons, a bit more chamber volume and double gaskets I must not be any higher than 10.25, and with the LT-1 cam I may even be able to run the beast on unleaded pump premium. I'm not holding my breath on that one, but maybe all I'll have to do is slow down the ignition advance from the late '64-'65 L-76/L-84 curve.

I've got one more question, Joe. Do you know what the actual change in the piston configuration is? Comparing them they looked the same, but I was not looking for differences. I figure they must have reduced the dome height slightly or reduced the dome plan view area. Did the later pistons come from the same forging with just machining changes, or was it a different forging?

Duke

Duke

[Joe L.]

Re: Thanks, guys!

Duke-----

I do not know if the pistons were all produced from the same forging. However, I expect that the difference was due to machining. Whatever the difference is, it is very slight and subtle. But, to shave only 1/4 point off the compression ratio would not require much of a change.