Dynamic compression ratio help

Hey guys, from what I have read here, it seems the ideal number for dynamic compression is about 9 to 1. I entered in my engine specs into some dynamic compression ratio calculators, and I’m coming up with 9.515:1. I am little concerned as most would feel that number is a little too high to run on pump gas. I am currently running a PCM4LESS mail order tune, and I haven’t had any problems with knocking at any RPM, or at least anything audible. I will have to log some driving in LT1 edit to verify this. Here are the specs I plugged in:

# of cylinders 8
bore 4.010
stroke 3.875
connecting rod length 6.0
cylinder head volume 57 cc
deck height .013
head gasket bore 4.100
head gasket thickness .036
piston to wall clearance .004
top ring land height .225
piston done volume -14 cc
IVC (*ABDC) 51
results:

static compression ratio: 10.853:1
dynamic compression ratio: 9.515:1

This calculation came from SMOKEumUP.com. I had to pay 5 bucks for 3 months of use of the calculators on that board, but there are free ones, like from KB Pistons. Any thoughts or comments on this?



Nick

use your seat to seat (advertised duration) intake valve closing point, not the @ .050" IVC. if you cant find it take the advertised duration and subtract the .050" duration from it. take that number and divide it by two. add that to the .050" IVC and that would be your seat to seat IVC.

I did the exact same thing when I calculated my 383's DCR haha.

Yes, you have to use the advertised duration to find the proper IVC. Should be high 60's or low 70's.

You should have searched here (or google) for a calculator first. ;)

http://www.empirenet.com/pkelley2/DynamicCR.html

What are your cam specs? That DCR seems awfully high if the SCR is 10.8:1?

Rich

What are your cam specs? That DCR seems awfully high if the SCR is 10.8:1?

Rich

We agree again! Super, super doubtfull the dynamic can be that high. Only in the 8.4 to 8.6 range with just less than 13 to 1 compression. The reason for the range is it dawned on me I may be using the wrong head cc volume. This getting old stuff is a bitch....don't remember if my heads are 56cc or 54 cc or if I've been assumming all the time they were 56 cc. What day was it again?

Have a .015 gasket leaking so pulling the head next week. May be a good time to have it checked to make sure. Ya think??

OK, after following the formula Zigroid posted, my dynamic now calculates to 8.0:1. I think that’s a more accurate number.

Rich, as far as the cam specs, here goes: 242/252 @ .050, .576/.576 with 1.6 rockers. Lobe separation 114.

Denny: .015 thick head gasket? Wow, that’s super thin. Who makes a head gasket that thin?



Nick

OK, after following the formula Zigroid posted, my dynamic now calculates to 8.0:1. I think that’s a more accurate number.

Rich, as far as the cam specs, here goes: 242/252 @ .050, .576/.576 with 1.6 rockers. Lobe separation 114.

Denny: .015 thick head gasket? Wow, that’s super thin. Who makes a head gasket that thin?



Nick

8:1 sounds more like it.

Rich

OK,

Denny: .015 thick head gasket? Wow, that’s super thin. Who makes a head gasket that thin?

Nick

Felpro 1094. One gasket sealed perfect but one side leaks. Think it may have been drivers error as you have enlarge the openings for the water ports and I put super tiny bit of silicone around the actual port itself. The silicone may not have compressed enough. Try it again with just copper spray and see what happens.

Yes, you have to use the advertised duration to find the proper IVC. Should be high 60's or low 70's.

You should have searched here (or google) for a calculator first. ;)

http://www.empirenet.com/pkelley2/DynamicCR.html

Thanks.

Felpro 1094. One gasket sealed perfect but one side leaks. Think it may have been drivers error as you have enlarge the openings for the water ports and I put super tiny bit of silicone around the actual port itself. The silicone may not have compressed enough. Try it again with just copper spray and see what happens.

Hope you get if worked out. I am doing the same in a month so any input on getting those to seal would be great. I know cantonracer has had luck with those gaskets.. I would shoot him a pm.

Hope you get if worked out. I am doing the same in a month so any input on getting those to seal would be great. I know cantonracer has had luck with those gaskets.. I would shoot him a pm.

If you place the gasket on the block it appears all you need to do is hog out the gasket to match the cooling ports. However... when I matched it up to the head it’s very easy to enlarge the port too much and interfere with the oil drain hole on the head. There is a hole in the gasket right next to the oil drain and a very thin contact area separating oil from the water. Ho, Ho, Hoooo. The tricky part that’s easy to miss and probably why it didn’t work last time.

Anyway I was more careful cutting out the water ports and very careful to just put a tiny damn of silicone between the water and oil openings. And….gooped the crap out of the gasket using the copper spray and seems to be working fine.

We’ll see but so far so good.