how to get comp ratio?

HighPlainsDrifter · #1 (**permalink**) 01-05-2004, 02:12 PM

Hi,
Does anybody know how to get the new compression ratio if I switch my open chamber iron 460 heads 98 CC and 8.8 : 1 compression to the new SCJ Kaase aluminum heads with 72 CC ?
Perry.

blykins · #2 (**permalink**) 01-05-2004, 02:38 PM

Sure....

You need some other numbers first....

1. Piston cc dish or dome
2. Head gasket thickness and bore
3. Piston stroke
4. Deck height (distance piston is installed into bore at TDC)
5. Head cc

That's about all you need...now there are some websites that will calculate it based on the above info....

However, you can do it yourself...

CR is basically the the volume of all the above things when the piston is at BDC divided by the volume of all the above things when the piston is at TDC.

If you want, provide me with the above info...I'll be glad to do it for you.

MJN · #3 (**permalink**) 01-05-2004, 02:42 PM

Perry

Perry I calculate you would have approx 10.95:1 compression ratio just by changing the heads assuming every thing else remained the same. Sounds like it could be a good swap to me depending on the cam duration you are running. You would definitely need to run premium gas.

The detailed formulas can be found by searching on line but basically compression ratio is the total cylinder volume at bottom dead center divided by the total cylinder volume at top dead center. Once you have figured out the relative volumes you can just subtract 26cc (the difference in the chamber volumes) from the volume at top dead center and back calculate the new compression ratio.

The single cylinder swept volume on a 460 (7538cc) is 942cc
so to get an 8.8:1 compression ratio

volume at bottom dead center / volume at top dead center = 8.8

volume at BDC = swept volume plus "chamber volume" / "Chamber volume" (Where the chamber volume is the volume at TDC.)

The chamber volume is the cylinder head chamber volume plus the other volumes above the piston such as valve reliefs, head gasket etc.

So to get an 8.8:1 CR your TDC volume must be 120.75cc

(942+120.75)/120.75 = 8.8

Now to calculate your CR with the new heads just subtract 26 cc from the 120.75 number and re calculate

(942+120.75-26) / (120.75-26) = 10.95

blykins · #4 (**permalink**) 01-05-2004, 02:43 PM

nevermind....

HighPlainsDrifter · #5 (**permalink**) 01-05-2004, 09:18 PM

Mike,
That's one area where I need help.
I kept the compression and cam mild to get through the smog test here but am working on getting higher compression cars through the smog test with acceptable NOX. If it still comes in under 11 to 1, I may go for it. J Kaase website says the heads are good for 40 HP more than the old CJ heads which are 100 HP better than my smog heads, would be a big jump.
Thanks,
Perry.

MJN · #6 (**permalink**) 01-06-2004, 05:39 AM

Perry

You may want to call the head manufacturer and ask if they think that compression ratio is too high for their heads with your cam. Some heads are very tolerant of higher compression ratios and some are not. Different quench area designs and also the amount of swirl or tumble that is imparted to the incoming mixture all enter into it. If you have a cam with zero or minimal overlap then you may find that the compression ratio is a little too high to run on pump gas.

cobrashoch · #7 (**permalink**) 01-06-2004, 10:41 AM

Hmmmmm....

Hope this works,
http://www.clubcobra.com/forums/show...threadid=21828
cobrashock

MJN · #8 (**permalink**) 01-06-2004, 11:14 AM

So Ron, do you think his compression ratio will be too high? After reading the other thread I get the impression that you do, but you didn't say.

BTW I read the articles on the link you posted and found it very interesting. I had never heard of dynamic compression before reading that. Sounds like a great tool for anlyzing the performance of a motor. Thanks....

HighPlainsDrifter · #9 (**permalink**) 01-06-2004, 12:38 PM

Mike,
I have .020 flat tops so if all I change is the heads, the only thing would be the static compression ratio change?
I also run a comp cams 280 H street cam, it has .530 lift and 230* duration @ .050 . Would that have enough duration to work with the high compression heads?
Perry.

MJN · #10 (**permalink**) 01-06-2004, 01:11 PM

Perry.

I have no direct experience with 460's so I'll defer answering that question to someone like Ron who has a lot more experience in that area. If it was a small block I would say it could be made to work fine. For a big block the cam is probably (?) too mild. In addition to the cam specs you gave it is important to know the overlap. What is the lobe separation angle? The overlap will bleed some of the cylinder pressure off allowing the use of more compression.

Just out of interest, and this has little bearing on your 460, the LS6 (Z06) Vette motor has 10.5:1 compression, aluminum heads and only 5.5 degrees of overlap at 0.050" Duration on the '02's was 204 intake @ 0.050" (.551" lift) and 218 exhaust @0.050" (0.547"). It is also a very wide lobe center cam, 117.5 degrees. There is a ton of technology in this motor but high compression motors with not so radical cams can be made to work.

cobrashoch · #11 (**permalink**) 01-07-2004, 09:02 AM

Don't know anything about the new Kaase heads or the new twisted wedge heads for 460's. The only applications I have seen on the Kaase head was all on 500 cubic inch or less engines. And that was only info I've gleemed from the internet.
From the way you described things I would say that you will be O.K. though. In the past all the 385 - 10 to 1 motors I've seen will run 93 unleaded with aluminum heads.
There is some things that you can do to help yourself though. Polish the pocket domes of those heads till they look like mirrors. This will eliminate hot spots and allow you to run higher compressions. Takes only a couple of hours work to do this, using steel wool, Mothers polish and a rag. Retard your ignition to start with when you first fire the engine up. You can always advance it out a later.
I wouldn't run any more than a .65 lift hyd. cam on the street, .67 on a roller with 10 to 1 slugs. So have those heads set up in advance for those lifts or better, because you may want to change cams later. On the heads- use a double spring setup, not a triple spring setup on the street. Triple spring setups usially accelerate valve seat wear.
I also openly question the costs of those Kasse heads. I wonder out loud if you will get the most return for the performance dollar spent going that way. That's something only you can determine though. Lots of luck.
cobrashock