60 270hp 283 compression ratio

[Ted Stock]

What was the compression ratio for the 2x4 270hp 283? I get different answers when I look at a Motors manual, Colvin's book, etc. For example in the Motors manual I have, it even shows two different compression ratios for various years of the 283 270hp engine and that doesn't seem correct. The Motors manual I have is from '62 and it states '57 to '59 had 10.5:1 and '60 and '61 had 9.5:1. I would have thought the pistons may have been different due to chamber size but the compression ratio would have been static. For the 230, 245, and 250hp it's shows 9.5 for all years. It shows the FI 290hp as 10.5:1 and the '61 275hp and 315hp as 11:1. So I'm confused at this point. It would seem that GM would have made two basic versions of the engine (i.e. hyd cam w/low compression vs solid cam w/high compression) and from there changed induction systems. Specifically I'm interested in a '60 270hp.

Thanks,

Ted
NCRS #30057

[Dennis A.]

Re: 60 270hp 283 compression ratio

Ted ...

All Standard engines had 9.5:1, the high-lift cam was 10.5:1. The 61' Fuel-Injected may be 11.0:1

[Loren L.]

The Engine sheet in the Chevrolet

Engineering specifications sheet, as revised in June 1960, in the blank for compression ratio states "9.5:1*" and the * footnote reads "11.0:1 with all Fuel Injection engines".

[Joe L.]

Re: 60 270hp 283 compression ratio

Ted-----

Based upon some further reserach that I did on this subject, I, too, feel very confident that the compression ratio for the 270 hp engine in 1960 was 9.5:1.

I agree that there does seem to be conflicting information, but I'm confident in the 9.5:1 rating.

[Tom B.]

Re: 60 270hp 283 compression ratio

Ted,

Many of the aftermarket manuals like Motors listed incorrectly that the hyd. cam 283s were 9.5 cr and the solid lifter cammed engines were 10.5 cr. This is wrong although it is easy to believe that the 270 was just a fuelie with carbs. Unfortunately that is not the case.

All 1957 to 1960 Corvette 283s used piston original GM part number 3739886 (std.) for both left and right banks. It was a flat top with 4 valve reliefs and produced a 9.5 advertised compression ratio. I say advertises as that is the listing in the technical data. True actual compression ratio is a product of combustion chamber volumn, comressed head gasket volumn and piston installed deck height. Most of these engines were somewhat less than advertised.

1961 carbed engines, including the 270 were the same also.

The only engine with 10.5 CR (1957-1960) was the Fuel injection with the high lift cam. That would be the 283 horse 1957 and the 290 horse 1958-1960 engine. They used piston original GM part numbers 3746289 (LH) and 3746290 (RH). The reason for the left and right hand pistons was due to the pop up dome on the piston head that prevented them from being interchangable left to right. They would fit but pistons are marked with a notch to designate front. The dome raised the CR to 10.5 with the same volumn head.

Tom

[Tom B.]

Re: 60 270hp 283 compression ratio

For a little further explaination 1957 used a seperate casting number for the Fuelie head even though the chamber volumn was the same. Although there were 4 different casting numbers of heads used from 1958 up through all of the 1960 engines and the 1961 carbed engines they were used universally throughout the horsepower ratings. In other words, for example, all 1960 engines used 3774692 irreguardless if it was a 230 base engine or a 290 Fuelie. The pistons made the difference on the high lift cam fuelie only.

The only other head was the 3782461 casting that was introduced on the 1961 fuelies, both 275 and 315 horse, and both with the fuelie pop up pistons this year only on 283s. Of course thew 461 casting was continued into 327 usage in a couple different configurations.

There is a lot of wrong info found in both GM literature and elsewhere about the 1960 315 horse Fuelie with 11:1 compression option. This was intended to happen and many GM lists (such as charts in GM parts books) show the figures but it didn't happen. All 1959 engines were carried over to 1960 production. If you recall the details of the aluminum head fiasco you will understand what happened. I believe that the 461 casting introduced in 1961 on the Fuelies was basically the 1960 aluminum head cast in iron with some small differences.

This all might be way more than you wanted to know so, bottom line, your 1960 270 has flat top pistons and 9.5:1 CR.

Tom

[Mike McKown]

Re: 60 270hp 283 compression ratio

Tom:

"Atta Boy"! You're right on the money!

[Tom B.]

Re: 60 270hp 283 compression ratio

My wife says I get long winded sometimes. Wonder where she gets that?

[Roy Braatz]

Re: 60 270hp 283 compression ratio

I don't know what your checking but if you have low compression you first.
1.Check all 8 to get a reading on each.
2.add engine oil in each plug hole and recheck compression on all 8.
3.if compression is the same (low) on any one cylinder your valves are burned or not seating.
4.if one cylinder compression increases the rings are bad.
Also if any one cylinder read higher then the book calls for you have carbon build up.