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Old 09-28-2008, 12:08 AM
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For a good-running 350 street motor, find heads that will flow 220-230 cfm @ 0.400" valve lift on the intake with the smallest runner size. That's what makes the L31 Vortec Chevy heads work so well on a 350. 170cc intake runners and 227 cfm.

The camshaft will be dictated by the static compression ratio of the motor, the desired operating range and the valve lift limit imposed by the valve spring/retainer combination on the heads. If, for instance, you were to build the motor at 9.0:1 with 64 cc heads, you might choose a cam with 0.050" duration figures of between 210 degrees and 225 degrees, depending on lobe separation angle and operating range. It's always best to call your favorite cam grinder and have the tech person spec out a cam for you. The cam should be the LAST component chosen for the motor in most cases. It is possible of course, to build the motor around the cam. But in that case, you have to be flexible in choosing components and building the motor to match the cam so that everything works in harmony.

It's a tragedy, but we see it everyday where some noobie has a 8.5:1 motor and installs a cam that should have been used in a motor with 11.0:1 c.r. Then he shows up on here asking why the motor won't pull the hat off his head.

Although there are numerous compression ratio calculators to be found on the web, here's the way to do it yourself if you want to learn.....

STATIC COMPRESSION RATIO
First, an explanation of what this means. As the piston comes down the bore on the intake stroke, a negative-pressure area is produced in the cylinder. Since nature abhors a vacuum, atmospheric pressure will rush into the cylinder through the intake valve in an attempt to equalize the vacuum to closely match atmospheric pressure. If we introduce fuel into the inrushing air, we have a combustible air/fuel mixture. The inrushing air/fuel mixture fills all available spaces and crevices until the intake valve closes. When the piston reverses direction to ascend up the bore, this mixture is compressed into a smaller volume. It is the total volume of mixture pushed into the cylinder/combustion chamber/piston deck height volume/piston eyebrow or dish volume/head gasket volume compared to the compressed volume of mixture with the piston at top dead center that we are dealing with here.

The ratio involved here is no different than any other ratio. If two million people like white cheese and twenty million people like yellow cheese, then yellow cheese is preferred by a ratio of 10 to 1. (twenty divided by two). If there are 937 rocks in a large container and 90 rocks in a smaller container, then the larger container has more rocks by a ratio of 10.41 to 1. (nine-hundred, thirty seven divided by ninety). By the same token, if 749 cc's of mixture are pushed into the available volume in one cylinder of a motor and the mixture is compressed into an available volume of 86 cc's, then the static compression ratio of the cylinder is 8.70 to 1, expressed as 8.70:1 (seven-hundred, forty nine divided by eighty six). (cubic inches times 16.387 = cubic centimeters).

The volume available to be filled with the intake valve open includes the swept volume of the piston (.7854 X bore in inches X bore in inches X stroke in inches X 16.387 = cylinder cc's), combustion chamber volume (normally given in cc's), piston crown volume with a flat-top or dished piston (normally given in cc's), head gasket volume (.7854 X gasket bore in inches X gasket bore in inches X compressed thickness in inches X 16.387 = gasket cc's) and piston deck height (.7854 X cylinder bore in inches X cylinder bore in inches X distance measured from piston crown to block deck with piston at TDC in inches X 16.387 = piston deck height volume). You must have all 5 of these volumes to correctly determine the static compression ratio of the motor.

The volume available to be compressed into will include 4 of these volumes. You would leave out cylinder volume, because now the piston will be at top dead center and the cylinder volume will be out of play.

Let's build a 383 with Vortec heads so we can plug in some of this knowledge.

A 350 block, bored 0.030" with a 400 stroke crank will be .7854 X 4.030" X 4.030" X 3.750" times 16.387 to equal 783.84 cc's in the cylinder.

Vortec heads have a published volume of 64 cc's.

The piston is down in the bore 0.015" with the piston at top dead center, so the piston deck height will be .7854 X 4.030" X 4.030" X 0.015" X 16.387 to equal 3.13 cc's in the piston deck height.

The pistons are flat-top with 6 cc's of eyebrows in the crown according to information published by the piston manufacturer.

We're using a GM #10105117 head gasket that has a published bore of 4.000" and compressed thickness of 0.028". We chose this gasket thickness, so that when added to the piston deck height, our squish will measure out at 0.043" and should work well with our pump gas motor to prevent detonation. .7854 X 4.000" X 4.000" X 0.028" X 16.387 = 5.76 cc's in the gasket.

Now, we add up all 5 of these values. 783.84 + 64.00 + 3.13 + 6.00 + 5.76 = 862.73 cc's pushed in on the intake stroke with the intake valve open.

Now, we add up 4 of the values, leaving out cylinder volume. 64.00 + 3.13 + 6.00 + 5.76 = 78.89 cc's into which the mixture is compressed.

Now, we divide 862.73 by 78.89 to arrive at a 10.93 static compression ratio.

By breaking all sharp corners in the chamber and on the piston which could glow hot, de-burring the spark plug electrodes and last plug thread in the heads, optimizing the ignition advance curve and choosing a cam with the proper intake valve closing event (36-42 degrees ABDC @ 0.050" tappet lift), this motor should run on premium pump gas without detonating.

Last edited by techinspector1; 09-28-2008 at 12:23 AM.
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