vimzzz

Hi.

Im building up my 1968 Corvette 327 CID engine, and Im having a bit of a struggle finding out what my comp ratio will be with my new setup.

The block, crankshaft and pistons are all original.
Im putting in these cylinder heads:
http://www.summitracing.com/int/part...-m72/overview/

and this camshaft (if that matters):
http://www.summitracing.com/int/part...1001/overview/

So what I've already figured is that my bore is 4inches(?) and my stroke is 3.250inches(?).

I had a look at the summit racing calculator, but figuring out the last numbers has turned out tricky for a newbie like myself.
http://www.summitracing.com/popup/ca...ion-calculator

I have no idea of the dome on the stock pistons for my engine.

Oh yea, i also know my gasket thicknes is 0.045

Would appreciate any help.

ddawson

I use this one but you need to know the dome and desk height.

http://www.projectpontiac.com/ppsite...tio-calculator

doorgunner

I entered the value of -6 (standard dome piston) and came up with at least a 9.5:1 compression ratio....it probably will be higher.

Using a .030 thick head gasket will gain you a .3 increase in compression (9.8:1)...BUT YOU HAVE TO BE SURE THERE IS SUFFICIENT VALVE-TO-PISTON-CLEARANCE AT TOP DEAD CENTER BEFORE INSTALLING THINNER HEAD GASKETS TO PREVENT BENT VALVES!

speedreed8

i came up with approx. 8.4 static cr. of course i'm just approx. the piston volume. assuming 4 valve reliefs. and deck height.

hamck

Without knowing what pistons you have its a guess, but I used jeep strokers calc. and came up with a flat top piston, +5cc ?? scr = 8.28 / dcr = 6.26 with that cam. If the pistons are dome -5cc ?? scr = 9.17 / dcr = 6.93 Your cam requires a comp. of at least 9.5 scr.

vimzzz

Thanks for the help, but I understand there are to many unknown variables. So I bought myself a compression tester and will check it later on

c3_dk

More info needed to tell you, what you have.

But if your are 0.025 down the hole.
0.045 x 4.1" head gasket.

Then with 5cc pistons you have: 8.07:1 and Quench 0.069(not good)
10cc : 7.71:1
15cc : 7.39:1

If you buy fel pro 1094 head gasket (very cheap gasket) 0.015x4.1" then you will get 0.5 more in compression. I would do this, if it was me, just for the improvement of the Quench (and of cause 0.5 in compression)

If you buy the Fel pro 1094 AND 65cc heads you will have:
5cc pistons 9.54:1 AND Quench 0.040 (very good)
10cc pistons 9.03:1
15cc pistons 8.57:1

REELAV8R

Hmmm, I don't think I would go about it that way. With that cam you may find that you have zilch for power below 3000 rpm. That's if you get the CR up to 9.5.
The specs for the cam are always based on a 350 CID engine. Yours being a 327 means that the RPM ranges are going to be higher, at least the point where you start to get on the cam. This is because a 327 is moving less air for every stroke. A 3.25" vs a 3.48" stroke.
If you want this engine for street use then you're going to want more bottom end power. That cam is bigish even for a street driven 350.
If you don't have the heads yet, take the current ones off and cc the piston in the block like shown here starting at post #31;
https://www.corvetteforum.com/forums...er-size-2.html

Then you can compute the volume of your piston and know how far down the hole it is.

Knowing those two items is key to getting the CR right. Picking the right head gasket to keep the squish distance around .040 is also key to making good cylinder pressure for good torque. Torque is what is going to give you power off the line at lower RPMs.
Once you figure out all the specifics on your CR and what is needed for the cam, then order the heads, gasket and the cam to match the desired results for your top end.
That cam is not going to produce good low RPM torque based on your current info.

Figuring out after the top end is complete that you have insufficient CR for the performance you want is a less desirable way and more expensive one of figuring out your CR.
A compression check is good for diagnosing problems with a cylinder but won't neccesarily give you usable info on your compression ratio.

TheSkunkWorks

Some good advice has already been shared here, particularly in REEVAV8R's last post, so I won't bother repeating it. But, I will add that IMCO the extra work put into going about this the right way will be well worth the effort. Knowing what you have is the right way. Guessing isn't.


That said, before you crack it open I'd still do a compression test on all 8 cylinders to get an idea of their health (a leak down would be even better). And, assuming no significant differences between cylinders, averaging the 8 resulting cranking pressures will also give a read on just how decent or how poor may be your current CR/cam combination. In any event, informative as this diagnostic process might be, it shouldn't be viewed as a replacement for due diligence. My $.02





.