Thread: Compression Ratio

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On Pump gas, how high of a compression ratio can i run with

Cast heads?

Aluminum?

Does a slightly higher, or lower CR make a very big change in performance?

I believe that 10:1 and premium gas is about the max with cast heads.

ummm I think you can actualy get away with 10.5:1 compression

With a stock rebuild, 9.0 is the generally accepted limit for iron heads and 10.0 with aluminum heads using pump gas.

By building the block with zero deck and 0.035" to 0.040" squish and using a cam with an advanced intake closing point that is compatible with the static compression ratio, you can run up to 11.0 with iron heads and good gas, assuming reasonable ignition lead. You'd want to eliminate any sharp corners in the combustion chamber and on the piston top and spark plug electrode. You'd also want to use a gasket with sufficient bore diameter to prevent overhang into the cylinder bore.

However, this is a double edged sword. If you increase the compression and use a cam with advanced intake closing point, the dictates that the duration of the cam will be increased over the duration of a cam that you would use with a lower compression ratio, so with the "larger" cam, the window of operation will be moved up in the rpm range of the motor, away from low-rpm efficiency and moving toward mid-range efficiency. The larger cam may require a looser torque converter and stiffer gears in the rearend.

So, you see, 9.0 with iron heads isn't all bad for a street motor.

Again Techinspector1 is correct. Several years ago Bruce Crower
built a SB Chevy with 14.8:1 compression ratio and ran it on 87 octane fuel. The engine ran ok but he didn't achieve any gains in horsepower of fuel economy. Increasing compression ratio yields a diminishing return: The higher you go, the less percentage increase in performance you will get.

Ok, so, Since I got a good deal on a set of hypereutectics, that are flat tops, and 9.8:1 with 64cc heads, I can pretty much do what I want with the compression with various head and gasket combonations.

I am looking for over 400 hp out of a streetable 383 stroker, with oober amounts of torque.

I have 3.73 12 bolt posi, built 700R4, and a stock lock up converter from a V6 vehicle (higher stall, maybe, like 500 to 750 rpm higher than stock)

Basically, can I get power I "need" (want, actually) with a CR around 9.0 to 9.5 with out going crazy with the cam. Like a CompCam 268, or something like that, lumpy, but plenty of grunt around 2k rpm? Maybe a set of Sportsman IIs or Darts.

First off, you don't want horsepower, you want torque.
Secondly, you don't need lumpy, lumpy is for posers who don't know any better. Lumpy is the sound of a motor being inefficient because the intake valve is opening too soon for the rpm where it's lumpy and the burned exhaust charge that is supposed to be pushed out the exhaust valve is instead being pushed up the inlet tract, disrupting the signal to the carb and screwing up manifold vacuum. Lumpy is for the guy at the drag strip who has a hot motor that makes high-rpm horsepower, but won't pull the hat off your head at low r's. Lumpy is for guys who have to make other arrangements to operate their power brakes because their cam won't make sufficient manifold vacuum to operate the brakes and other vacuum-operated accessories. Screw lumpy.
Thirdly, to make decent torque at 2K rpm's, you need a stock cam, but you won't make 400 ft/lbs of torque with a stock cam.

Now that I have that off my chest, if you want a high-torque 350 (it doesn't even take a 383 to do it), go to this Air Flow Research site, http://www.airflowresearch.com/ ,click dyno tests at the top of the page, then click Chevy dyno tests. The first test shows a 350 with 180cc heads that makes over 400 ft/lbs of torque from 2,700 rpm's through 5,000 rpm's.
Buy their 74 cc heads and with your flat-tops (7 cc's of valve relief) and a 9 cc gasket (0.039" compressed) and zero deck, you'll be at 9.07:1 and can just plug in the components they used to make power.

One last thought, you don't adjust compression ratio with the head gasket. You do it with the piston and the chamber volume, then you use the correct thickness gasket to set your squish to between 0.035" and 0.040".

I'm shooting for 9.6:1 on 89 octane in a 350. I've got Vortec heads that I've deburred a little, KB hyper pistons 4.020" with a 1.561" compression hight and 12cc dish. I will be running as close to stock deck height as possible and use a .015" head gasket to give me a squish of .039". The cam will be a Comp Xtreme Energy XE256H. I plan to run 91 octane until I get a knock sensor, after which, I will experiment with lower grades of fuel. HP and torque guestimates are in the 320-350hp range with 410-430 ft lbs of torque. Gas milage, 15 mpg+ I hope.

I had a 383 with a 268 comp cam, didn't even make a sound. Extra cubes makes it more of a stock cam.

Kodiak