Thread: need some help on my 350 chevy build

Tweet

Hey Mike, welcome to the board. There are many, many talented and helpful people on here and the nice thing is, nobody will flame you for not knowing. We're all here to help, because we all started off not knowing anything too.

The first thing you need to know is DO NOT BUY A CAM YET !!!!!!!
Most beginners run right out and buy the biggest, nastiest cam they can find and stuff it in the motor with the stock compression ratio and then don't understand why the motor is a pig and slower than it was before the cam change.

Let's begin your education with compression ratio. You'll need to follow along by doing some math, but it's easy. I'll show you how.

Static compression ratio is the relationship to fuel/air mixture that is drawn into a cylinder during the intake stroke compared to the space it is compressed into on the compression stroke. A 4-stroke motor is called that because it has 4 cycles in which it operates. Intake, compression, power and exhaust. Each one represents roughly a 180 degree turn (one-half turn) of the crankshaft, so for a complete cycle, the crankshaft turns 720 degrees. (two complete turns) If a motor draws in 9 parts and compresses them into 1 part, then is it said to be a 9:1 (nine to one) motor.

Let's take just one cylinder of your 350 for instance. If we use the formula (.7854 times the bore, times the bore again, times the stroke), we can plug in some numbers and find the displacement of that cylinder, then by multiplying the resulting cubic inches by 16.387, we can find the cubic centimeters (or cc's ) of the cylinder. .7854 times 4. times 4. times 3.48 equals 43.73 cubic inches in the cylinder. Multiplying that times 16.387 reveals the cc's in that cylinder to be 716.6

Now we need to know the cc's above the piston with it at top dead center. Most stock small block Chevys will have the piston down in the bore about 0.035" (thirty five thousandths of an inch). If we multiply .7854 times 4. times 4. times 0.035", we get 0.439 cubic inches. Converting this to cc's by multiplying it by 16.387 yields 7.19 cc's in the piston deck height.

Same way with the head gasket. Let's say the bore of the gasket is 4.100" (four inches and one hundred thousandths of an inch) and the compressed thickness is 0.040" (forty thousandths of an inch). Multiply .7854 times 4.1 times 4.1 times .04 times 16.387 yields 8.65 cc's in the head gasket.

We'll say that the combustion chamber is 76 cc's, so we have no calculating to figure it out.

If using flat-tops, they usually have between 5 and 7 cc's in the valve reliefs. Just use 6 if you don't know.

Now, add the cylinder cc's, 716.6, the piston deck height cc's, 7.19, the gasket cc's, 8.65, the eyebrow cc's, 6 and the chamber cc's, 76.

The total cc's of fuel/air mixture drawn into the cylinder, piston deck height volume, head gasket volume and combustion chamber volume is 814.44

Now, when the piston comes up to compress the mixture, it will compress it into the combustion chamber, piston deck height volume, piston eyebrows and head gasket volume. Let's go back, pick up those numbers and add them together. 7.19 plus 8.65 plus 6 plus 76 equals 97.84 cc's.

Now we know that we will be drawing in 814.44 cc's and compressing it into 97.84 cc's, so to find the static compression ratio, we'll divide 814.44 by 97.84 and find that the c.r. of the motor is 8.32:1 (eight point three two to one).

If using dished pistons, just substitute the dish volume for the eyebrow volume.

If you'll work with me and calculate through this a few times so that you begin to understand it, we'll talk some more.

If I never hear from you again, I'll know you were not serious about learning anything and I'm only out about 15 minutes of time. But at least I tried.