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whats my compression ratio and did I get wrong cam.

5K views 56 replies 8 participants last post by  techinspector1 
#1 ·
My block casting number is 3970010. My head casting number is 3917293...75cc chambers. Im wondering what the stock compression ratio is. I also bought a cam kit comp cam 279t. And was looking at some edelbrock heads thati could pickup used for cheap but they are 64cc chambers and idk what that will do compression wise cause I dont know how to figure out stock compression. Any help will be appreciated.
 
#4 ·
In simple terms the Static Compression Ratio is with the piston at Bottom Dead Center (BDC) or full stroke, it's all the volumes under and including the combustion chamber to the piston crown and including any volumes created by the shape of that crown divided by the volumes under and including the combustion chamber and piston crown with the piston at Top Dead Center (TDC) all the stroke used up.

To get to the total volume calculates the following then add them together:

1) The swept volume of the cylinder simply the stroke times the formula for the area of the piston/bore diameter.

2) The clearance volume measured from the outer edge of the piston diameter to the top of the block called the deck, or head deck.

3) The volume of the piston crown, flat tops add nothing except a small amount for valve clearance reliefs present, dishes add volume, and domes subtract volumes. One can include what’s called the crevice volume from the very edge of the piston diameter to the top ring, but given it's pretty consistent unless you're anal about numbers it's usually ignored.

4) The volume of the head gasket which can be thought of as bore area based on gasket bore diameter times the thickness of the gasket. Note that gaskets usually aren't exactly round but if you don't have the manufacturer's volume or your calculus skills for areas of weird shapes are rusty; area of the round bore times thickness of the gasket is plenty close enough.

5) Last is the volume of the combustion chamber itself. If you can't measure it directly you're stuck with the manufacturer's stated volume. This is usually small compared to real volume and of course the actual volume will vary is the head has been milled reducing volume as a result; and or a valve job (has been performed) which sinks the valves deeper which adds volume back.


Adding 1 through 5 gives total cylinder volume. Adding 2 through 5 gives the volume into which all the volumes are compressed at TDC. Dividing the sum of 1 thru 5 by the sum of 2 thru 5 gives the Static Compression Ratio or (SCR) Where this gets complicated is when the engine is assembled and you have to make some assumptions about what's inside.

You also need to switch back and forth between English and metric measurements for cubic inches and cubic centimeters. There are 25.4 millimeters to the inch and 16.4 cubic centimeters to the cubic inch, from those we can hack what we need.

So let's peel this baby apart with reference to the process listed above.

1) A 350 uses 3.48 inch stroke and 4 inch bore. The long way around to the bore area is 1/2 diameter squared times pi (3.1416). So half of 4 is 2 times itself which happens to be 4 again, times pi which nets 12.5664 square inches of bore area. This times the 3.48 inch stroke is 43.7310 cubic inches. That times 8 equals 349.8486 cubic inches which Chevy rounds to 350. We can use the cylinder volume in cubic centimeters later so 43.7310 times 16.4 cc's to the cubic inch makes 717.1884, I'll round that to 717.2 ccs to reduce the math load.

2) The GM states the Clearance Volume from piston crown to head deck is .025 inch that times the bore area of 12.6 sq inches (rounding) equals .315 cubic inch which times 16.4 cc/in is 5.166 cubic centimeter.

3) Here's something you've got to guess at as GM uses several different pistons with round dishes of different volumes but a pretty common one is 13 ccs so we'll grab that. There's bigger!

4) Head gasket volume, well the bore of these is larger than the bore's diameter; a 4.1 inch gasket bore on a 350 is pretty common. We also have to guess at the factory gasket thickness, these range from about .020 inch for a shim to .053 for a composite. Most trucks came with the shim. If we use the same formula as figured for the bore the larger 4.1 inch gasket generates an area of 13.203 square inches times a thickness of .020 equals .264 cubic inches times 16.4 equals 4.33 cubic centimeters.

5) Combustion chamber well since it's on the motor we'll have to take the factory at its word and use 75 ccs.

So if we add 1 through 5 for cubic centimeters we get 814.69 (rounding as I go). Adding lines 2 thru 5 gets 97.5. Dividing 814.69 by 97.5 results in a compression ratio of 8.355 to 1. That's the process to get to the Static Compression ratio (SCR). In reality having measure more than a few of these engines the dimensions not including the bore and stroke but especially of the combustion chamber tend to be a bit larger than published. You find the actual measured SCR runs more like 7.6 to 7.8 rather than in this case the calculated 8.355 which is very close to GM's advertised ratio.

I've given you enough ammo to compute the 64 cc head compression for yourself, it will go up. Raising the compression is something the 279T cam will need to get all it has. This gets into the field of the Dynamic Compression Ratio or (DCR). The DCR adjusts downward the SCR for that point where the cam closes the intake valve in crankshaft degrees. The arithmetic for this most defiantly gets into trigonometry but there are a lot of DCR calculators on the web to make life simpler. You'll need the rod length which for the 350 is 5.7 inches. You'll need the cam card to get when the intake gets seated in crank degrees. When you get that data here's a place to go play with numbers <<< http://www.kb-silvolite.com/calc.php?action=comp2 >>>. You want to calculate an SCR that provides a DCR pretty close to 8 to 1.

I've got to go for awhile, have fun.

Bogie
 
#11 ·
I'm not even on the same planet as these guys apparantly, but I'm going to make some assumptions and give you a less confusing answer before you eat a bullet out of frustration.
IF the short block is stock and untouched, assuming its low mileage and in excellent running condition....
Your cam choice sucks, and your not going to see the potential this could have unless you ditch the thumpr
There...now that that's out of the way.
Likely a 12cc dish stock piston,
Use FelPro pn#1094 head gasket
Buy your 64cc heads
Quench is 0.040
Compression is 9.5:1

Its going to want alot of initial timing with that cam, 20+ degrees, 36* total
Add 10* vacuum advance from a manifold source.
Search on here for limiting mechanical advance. You want a 14-16* curve
Its now not going to want to start when hot,
Search here for a ignition interrupt switch
Its dead simple.

That should get you up and running as well as you can expect.
There are a lot better cam choices
Is that a kit with a gear drive btw?
 
#12 ·
I watched a video on compression ratio. Way easier to follow still tuff to do with the converting. Ill take the heads off tomorrow.

Thanks bygddy. What cam u recommend. Ill check on pistons tomorrow. Yes its the kit with hydroulic lifters matched valve springs and new timing gears n chain. You lost me on the timing. And degrees and stuff. But when u mentioned hard to sart when warmed up. Can u tell me what causes that and what I might do to fix it. I also have a 61 thunderbird with original rebuilt 490 and after its warm itll die once or twice I have to give it gas for a bit then it goes backtorunning like a champ. I know off my topic but now interested.
 
#13 ·
Yah, get the heads off and see what pistons are in it. Are you looking at rebuilding the short block(bottom end) or just adding cam and heads to what you already have?
Cam choice is very difficult, it completely depends on what your intended usage is. What's the car, what transmission, what gear, street car? Daily driver? Weekend toy?
Answer what you can, and let's get the heads off and see what you have. Was it running and running well? Was it abused? Do you know the history of the motor? If its a budget build using a stock short block that you not really concerned with the you can buy your used heads, and have them if something goes sideways with the current motor. So really, worst case your out a few hundred bucks in cam and gaskets. I have done a couple dirt cheap builds like this and loved it. Very little invested, and if it breaks its not the end of the world, but generally they live long and happy. Its actually pretty amazing what a SBC will take.
As for your ford, in assume you meant a rebuilt 390? If its the stock autolite 2brl Carb then they are common for boiling the fuel out of the bowl causing flooding. On mine the float was too high at first and it would cause a stumble and occasionally stall when hot. I adjusted the float down ("youtube" this its easy) and that fixed that problem. Then I had a hot start issue, when warm it would sit and boil the fuel out and be very hard to restart, I pulled my Carb and installed a spacer, I also bypassed the coolant line that ran through the base and this has cured any hot start issue I had.
 
#14 ·
#19 ·
Sorry I didnt know you were under the impression it was a 350 it is a 327. I plan on useing stock bottom end unless pistons or cylinders show scaring when I remove the heads. It ran perfect before I started taking it apart but its in my 1928 ratrod and I couldnt do good enough burnouts!!! I drive it when im in and on weekends. I have a th400 behind it with a stock th350 stall. Gears im not sure of but the rear end is from a 85 chevy van so im guessing 3.08s I wanna go to 373s I know this is holding the car back some but the engine was too just decided to start at engine first

as far as the tbird thanks for the info. Yes I did mean 390. Ill check on carb. Prolly right with the fuel boiling over
 
#23 ·
Pics?
If its a 2 valve relief dish piston then its an early truck 327
Your still fine with everything I reccomended.
Change your math in the comp calculator to +10 for piston volume
Drops you to 9.1:1 which will still get the job done
Or if the heads are used you could have them machined to clean them up (a good idea with the felpro 1094) and get them cut down to 60cc bringing your comp up to 9.5:1
 
#22 ·
Your still going to be fine.
The only change is going to be the stroke and piston dish
Stroke changes to 3.250
Dish is now +4
(Assuming no rebuild)
This puts you right around 9.8:1
Alum heads and a good tune it will run on junk gas.
If its going in a model a then it likely weighs nothing lol.
Model A rat rod? Sidepipes per chance?
Hell I would leave the thumpr cam in it at that point.
As a "rat" I assume its a rolling burnout machine and just something to piss around at cruise nights with.
Hell yah, buy your heads, use the felpro #1094
Performer rpm intake
Holley 650DP
Use the guide I gave you for timing.
Its (hopefully) a sub 2800# car, leave the 308 gear in it, it will cruise nice. It will make plenty of power to be as silly as you want, it will sound like an animal (again, sidepipes? Love sidepipes) and be tons of fun.
 
#27 ·
I would start by measuring the bore to see if its still 4.0. The only info I can find shows a 2 valve relief piston with a dish, or a 4 valve relief flat top for a 327....
Maybe the motor has been apart,or maybe its actually a 350 that's simply had a head swap.
Either way, if your doing a quick and dirty budget deal, use the felpro gaskets, slap the 64cc heads on it and call it whatever you like, the end result is going to be the same. It will still make good power, and still sound nasty. Sometimes things get over thought. Is it a race car? Is it going to be a daily driver? Or is it a cruiser to do burnouts, cruise nights, and maybe a trip to the drag strip once in a blue moon.
 
#30 ·
Then your options are, pull it completely apart to see what you have, or use the gasket i mentioned, bolt your 64cc heads on and hope for the best.
Would guess its either a 327 that's been rebuilt, which isn't great due to the unknown pistons, likely a rebuilder with a poor compression height so this could actually give you sub 9.1:1 which is going to suck.
Or if your positive its never been apart I would guess its a 350 that someones put 327 heads on, you were told its a 327 because someone checked the casting number on the heads only.
How lucky do you feel?
 
#32 ·
Those dish pistons with the 45* chamfer on the outside are some of worst pistons you could use.I'd definitely try to get a different type.i don't think the 327 ever came with that type of piston.measure your deck hgt with a piston @ TDC to see how far in the hole they are.
 
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