Thread: compression......

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OK, time for some math.
.7854 x 4.310 x 4.310 x 4.000 x 16.387 = 956 cc's in the cylinder.
For this exercise, I'll use a Speed Pro L2399NF60 forged piston that has a 13.8 cc dome.....
http://www.summitracing.com/parts/sl...make/chevrolet
The heads can be anywhere from ~112 cc's to ~124 cc's. You have no idea and neither do we, but for this exercise, we'll call them 118.
We'll theoretically cut the dome off the piston and stuff it in the combustion chamber to make it easy to calculate. That gives us a zero value piston because it's now a flat-top and adding 13.8 cc's of volume to the chamber makes the chamber 104.2 cc's.

Let's figure out the stack before we go further. The pistons are 1.645" tall, the rods are 6.135" and the stroke radius is 2.000". Add these up and find a stack of 9.780". Since the block deck height is ~9.800", if we subtract the stack from the block deck height, we are left with a piston deck height of 0.020". If it were my block, Jigging the block up on the mains, I'd take at least a slight cut on the decks to insure they are parallel with the main bearing bore. Let's say we took a 0.010" cut, leaving the piston deck height at 0.010" and the squish at 0.050" if using a 0.040" gasket. Now, understand, Pat McCarthy is the BBC guru, so if he says to do something different than what I'm saying here, then do it. I'm simply trying to arrive at a specific static compression ratio and a specific squish for you.

Let's figure the volume in the piston deck height. It's at 0.010".
.7854 x 4.310 x 4.310 x .010 x 16.387 = 2.4 cc's.

The piston will be a zero value since we cut the dome off and stuffed it into the chamber.

I just reached in and picked a head gasket from Summit. Again, Pat is the man. Use the head gasket he tells you to use. This particular gasket is 10.6 cc's in volume......
http://www.summitracing.com/parts/fe...make/chevrolet

So, 956 + 104.2 + 2.4 + 10.6 = 1,073.2 total cc's
Deduct the cylinder cc's (956) from the total cc's (1,073.2) and find 117.2 cc's.
Divide the total cc's (1,073.2) by 117.2 and find a static compression ratio of 9.15:1

Now, if the heads were shaved a little to make the chambers 113 instead of 118, the math would look like this......
Remember, we're stuffing the piston dome into the chamber, so the chamber will be 99.2 cc's......
956 + 99.2 + 2.4 + 10.6 = 1,068.2 total cc's.
Deduct the cylinder cc's (956) from the total cc's (1,068.2) and find 112.2 cc's.
Divide the total cc's (1,068.2) by 112.2 and find a static compression ratio of 9.52:1 with a 0.051" squish and a dynamic compression ratio of 8.18:1 with the Crane cam listed below. Running on cat piss pump gas without detonation should be a snap with this combo.

Now, the key to figuring all this out is knowing the chamber volume. You cannot guess at this, you must pour the heads and get the chambers down to 113 cc's. Plan on investing in a cc'ing kit and learning how to use it. If you don't listen to me, the static compression ratio will be unknown and you will have built a mystery motor, not knowing how much cam to use because the cam must be matched to the static compression ratio.
Here's an affordable cc kit from Summit....
http://www.summitracing.com/parts/sum-911581/overview/
Stop by the grocery and pick up a quart of Isopropyl rubbing alcohol and a bottle of food coloring (your choice of color, but use red, green or blue so you can see the alcohol in the burette.

There is something else I want to ATTACK here and that's the fosdick XE cam that you have chosen. Best thing you can do is to sling that junk over the fence because you have about a 90% chance that it will frag and cost you another complete teardown to clean out the shrapnel. Bite the bullet and install a roller cam from the git-go. No excuses, no whining, just bite the bullet and do it.
Flat tappet cams will not last in a BBC. XE flat tappet cams don't have a snowball's chance in hell.

Here's an example of a cam I might use in this motor. Pat would have a better choice I'm sure, but this will get you in the ballpark. Take note that the Crane engineers have specified that this is the most cam that can be used with 9.5:1 static compression ratio. More cam than this will turn the motor into dog doo. You could use a little less cam if you wanted to, but this cam is the max timing that can be used with 9.5:1.

Crane Hydraulic roller, retro-fit, part number 139011
Grind number ZHR-288-2S-12-IG
Operating range 1800-5600 rpm's.
Excellent midrange torque and HP, fair idle, moderate performance usage, mild bracket racing, autotrans w/2500+ converter, good w/plate or manifold nitrous
system, marine applications: for 454-502 cu. in. modified engines in performance applications with aftermarket high flow above water exhaust systems.
9.5 to 11.0 static compression ratio advised
3000-3400 cruise RPM.
Advertised duration 288/296
0.050" tappet lift duration 226/234
Lobe separation angle 112
Intake valve opens 6 degrees before top dead center @0.050"
Intake valve closes 40 degrees after bottom dead center @0.050"
Exhaust valve opens 54 degrees before bottom dead center @0.050"
Exhaust valve closes 0 degrees after top dead center @0.050"
Valve lift 0.587"/0.610"


I´d use a set of long-tube, tuned headers with a minimum 3/8" flange. Thinner flanges will curl up and spit out the gasket. Make an H or X pipe after the collectors, before the mufflers.
Use an 850 carburetor on an Edelbrock Performer RPM intake manifold, part number 7161. This manifold will make max hp and torque throughout the range. Use the Air Gap model if you want to. I don´t place much faith in them, considering the air gap feature to be just more marketing hype. Top the carb off with a minimum 14¨ x 4¨ air filter assembly. The motor must be able to breathe.
The motor will want a ton of ignition timing. I might lock out the centrifugal and run 38 degrees at the crank, using an ignition interrupt switch to crank the motor.