C1 283 270 hp

[Leon Brown]

Hello all, what is the piston compression ratio for the 270hp 283 used in the C1
thanks. CU engine code. I've been able to find most everything except for this.
Thanks
Leon

[Joe L.]

Re: C1 283 270 hp

Leon------

GM says that it was 9.5:1

[Duke W.]

Re: C1 283 270 hp

The 9.5:1 mentioned by Joe applies to all 283/270 '57 to '61. Since decks were typically machined ten to fifteen thou higher than the 9.025" nominal blueprint dimension, actual CR is probably closer to 9.0:1. With the late closing inlet valve of the Duntov cam, dynamic CR is probably only around 6.7:1 and modern midgrade fuel probably has sufficient octane for detonation free operation, and they may even operate on regular.

If I was restoring a 270 I would do some head work and raise the actual CR to a level not to exceed 10.5:1.

Duke

[Leon Brown]

Re: C1 283 270 hp

Thanks guys for the quick response. Here's a one-off question. Using pop up pistons 11:1 CR with the 692 59cc heads(1.72/1.50 valve)and 270 hp cam (394/400 lift). Would there be enough room in the combustion chamber to prevent the valve from tatoo'ing the piston?
Thanks again
Leon

[Duke W.]

Re: C1 283 270 hp

Most likely, yes, but it depends on piston design including the valve notches.

I would limit CR to 10.5. You do this by determining piston/deck clearance on disassembly, understand compression height and dome volume for both old an new pistons, volume of head chamber, then select a suitable gasket thickness to achieve the target value.

The following is an easy to use online CR calculator.

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

Duke

[Roy B.]

Re: C1 283 270 hp

Originally head gaskets were STEEL

[Duke W.]

Re: C1 283 270 hp

Yes, as was the case with 327s. As a running change in '62 SHP/FI engines were double gasketed to address customer detonation complaints, and I think this practice extended to at least '65.

Typical aftermarket composition gaskets are about .038" compressed thickness compared to about .015" for the production steel shim gaskets.

Duke

[Michael H.]

C/R For 64

Duke,

At SOP of the 1964 model year, the cyl head combustion chamber was changed slightly, adding a relief to the outboard side which, among other things, increased the chamber volume by a few CC's. The increase in size eliminated the need for the second head gasket.

The original owner of my 63 told me that when he brought his car in for some misc warranty repairs, they also installed a second head gasket. He said he never mentioned anything about engine problems and it was only when he looked at the warranty receipt that he discovered the repair. When he questioned the service mgr, he was told that they added the additional gasket to all the mech lifter 63 cars, even if it wasn't a complaint. The original service repair order stayed with the car and shows the parts list and warranty operation numbers etc.

[Duke W.]

Re: C/R For 64

My understanding is that the beveling of the spark plug side quench zone did not occur until the 462 head casting, which went into production in 1966. This increased the nominal chamber volume about 2-3 cc and required a new casting number due to the different chamber geometry.

IIRC '65 original SHP/FI engines (461 heads) have been disassembled and two head gaskets were installed.

My '63 L-76 had two steel shim head gaskets, and all '63 SHP/FI were built with two as this change went into effect in '62. There was a TSB on the subject, so dealers should have installed two head gaskets if they had to remove a head, and they should have installed two on any '62s that did not originally have two.

Duke

[Michael H.]

Re: C/R For 64

I thought it was SOP of 64 but you may be correct. Although, the C/R was reduced from 11.25 to 11 for the new 64 engine. I don't remember a number change for pistons for 64 and the relief would add just enough to produce the 2 CC increase to drop the CR that amount. The 64 head was the beginning of the new 2.02/1.6 valve sizes also.

[Duke W.]

Re: C/R For 64

The 461 chamber radius machining on the inlet side with the larger inlet valve would have added a cc or two, which may account for the quarter point CR reduction in '64. The later closing inlet valve with the 30-30 cam also reduces DCR, but I think they still double gasketed.

A couple of years ago Joe Lucia reviewed the part number changes for 327 SHP/FI pistons through the sixties, but I don't know the differences in design.

This is one of those design/build detail evolution stories that has never been fully sorted out.

If you spend some time playing around with the CR calculator I referenced in a prior post in this thread, you can get a feel for the effect of a few thou change in deck clearance, gasket thickness, or piston dome or head chamber volume has on CR. It's VERY sensitive!

Duke

[Roy B.]

Re: C1 283 270 hp

Ok then this brings up a question, judging is looking at GM logo marking of the intake gasket ,so how many points are taken off for not having steel head gaskets? which I dont think many restored Corvettes have??? and paint over spray is a big deal now.

[Verle R.]

Re: C/R For 64

Duke,

When you refer to "beveling of the spark plug side quench zone" are you referring to the machined side of the combustion chamber next to the 2.02 intake valve?

If so, my late Feb 65 L79 had that machining.

Verle

[Michael H.]

Re: C/R For 64

Duke,

Ok, here's what I've found on pistons. The original 62-63 design, 3799501, was replaced in production wih a 3850137 for the 64 MY. The 137 also became the service replacement for the 501 in Oct 64. Possible then that the combination of a slightly larger combustion chamber and a slightly smaller piston dome for 64 may have resulted in the .25 drop in CR.

The 64 design piston, 3850137, was replaced in service with the 3871208 in July of 66, and probably had something to do with the new 462 cyl head casting that appeared around that same time?

[Duke W.]

Re: C/R For 64

No. If you compare a 461 and 462 head, you will note that the 461 had a small quench zone on the spark plug side. On the 462 this quench zone is beveled to the edge of the cylinder wall. The change to achieve this is in the casting mold tooling.

The "radius cut" is a machining operation to remove material from the side of the chamber to unshrould the 2.02" inlet valve. Both 461 and 461 heads originally equipped with 2.02" inlet valve have this machining, however IIRC the early cutter radius was 2.37" and it was later reduced to 2.34".

Both these changes increase chamber volume, which is why early 461s with 1.94/1.50" valves have about 61cc chambers. If machined for the big valves chamber size increases to about 62 and 462 heads with large valves are about 64 cc.

Duke