comp. ratio

[enduro74]

does any one know the stock comp. ratio for a 77 buick 350 vin j engine,and where i can find the specs. thanks Tim

[Turbo Loyd]

[color:"blue"]An excellent place to look for such information is: NHRA Specifications for 1977 Buick

[color:"blue"]Hope this information helps. I always thought the compression ratio in the smogger years was 8.0 to 1

[enduro74]

well if c.r stands for comp.ratio its says 9.06 to 1 up to 10.21 to 1.

thanks

[NTX5467]

The NHRA specs are NOT GM production specs, but what NHRA will accept for a particular engine and nothing more. The NHRA specs do have some basis in the OEM production specs, but they don't have to follow them in all cases (at NHRA's discretion). IHRA and other sanctioning bodies have their own respective rules that might not always coincide with NHRA.

Starting in April, 1971 (or a little earlier, depending on production schedules and car line), all GM engines were lowered to a specified 8.5 to 1 compression ratio and had induction hardened valve seats (except Buick delayed using the induction hardened valve seats until a few years later, as noted in the V-8 Engine seminar at the national meet) to deal with unleaded fuel of 91 Research Octane rating (87 posted "pump octane"). There were a few exceptions to the compression ratio rule back then, notably some Corvette motors that had a stated 9.0 to 1. That was the end of "high compression" as we previously knew it back then.

All of the passenger car V-8s in '77 were still at or below the 8.5 to 1 compression ratio deal. Seems like some Pontiacs were at 8.0 to 1. The "official manufacturer's" numbers will be in the respective vehicle service manuals, if there is any doubt.

In many cases, there was a fudge factor in the compression ratio and how it was made (i.e., cylinder head combustion chamber volume) such that engines usually were a little lower in actual mechanical compression ratio. Many heads usually held a had a few more cc of volume than the minimum spec (which is what's quoted in the manufacturer's specs) and even could have had some variation from cylinder to cylinder. Of course, if the chambers were fully machined in shape instead of being cast to shape, there would be less variation.

If you're going to build a motor, then you can first get the main saddles "line honed" to make sure that area is correct, then get the motor "decked" off of the verified crank centerline. Usually, the higher car line motors were decked at the factory, but this action during rebuilding makes sure that everything is "square" in geometry and architecture. Then, you can measure the deck to see how far it is from the stated manufacturer's spec.

Then, chuck the heads onto a flat lathe and get them surfaced too, just enough to true the surface and make sure the angle between the intake manifold and the head's deck surface is where it needs to be. Then, after you get the valve job done, you can put the valves into the heads and check the chambers for their actual cc volume. Also, with the pistons installed, you can then check for the distance the piston is below (typically) the deck surface at TDC.

Now that you know what the pistons' deck height is, the combustion chamber volume, and the compressed thickness of the head gasket (when installed), you can plug all of these numbers into the Compression Ratio Formula and see where things land. Also, if the piston crown has valve relief cut-outs or sumps, those volumes need to be added into the cylinder volume amount. When done, you'll know what the actual compression ratio is for your particular motor.

Before you finish assembly, you might cut the heads a little more to decrease the chamber volume or possibly find a shim steel head gasket instead of a composition head gasket (steel shim gaskets are usually .020" whereas the composition gaskets are usually in the .040" range). A few little tricks that might help you get toward the middle 9.0 range for a performance street motor to run on available super unleaded fuel.

Just some thoughts . . .

NTX5467

[enduro74]

i was pumping 9.3 to 1 with dish pistons and 60 cc heads.