Engine guys. Need help.

[bullittwrench]

Looking at an engine to buy. Rules state 11-1 max compression. It's an all steel 355, flat top pistons level with deck, .015 head gasket, 49cc dart platinums untouched. Knowing this can compression ratio be figured? Thank You

[Basecircle]

Do you know the volume of the valve reliefs in the pistons? You'll be able to figure it out pretty close with that info, but still not exact. There are a lot of variables if you need to be exact.

[bullittwrench]

Found some info on the pistons. It says 10.4-1 with 64cc chamber, 10.7-1 with 58cc chamber. Heads are 49cc

[Racermb]

Assuming a 5cc valve relief and a 0.070 gasket with a 4.100 bore that leaves you at 11.5:1. Minimum combustion chamber you need is 53cc to work out

[Racermb]

The engine you are looking at is 13.7:1 roughly.

[bullittwrench]

Oh crap! That won't pass the pump. Dang it.

[billetbirdcage]

Do you need to be legal or just pass the pump test, big difference? LOL.

There are several ways to beat the pump test but depends on how smart your tech guy is to which one will work.

http://www.wallaceracing.com/cr_test2.php

4.03 x 3.48 with -6 cc valve relief, 49cc head and .015 gasket (I used a 4.200 diameter on the gasket but for a steel shim I think that is large as I don't remember the diameter now) and zero deck is around 13.33 to 1


Forget that MY BAD, was thinking beating the CID pump not a whistler to check compression.

[bullittwrench]

It's legal pistons are in the hole.

[fast_crew]

Zero deck height and .015 head gasket.??.. something doesn't jive here...pistons are sure to hit the heads. Much tighter than .035 and they will hit.

[CCHIEF]

Try your computation with the actual volume of the head gasket, most are not round! The exact # for valve relief for pistons, add gasket volume to that, and add 1.5cc for ring land back volume. The online calculators don't allow for some of the variables (gasket and space from piston top to rings. Then you will have some ring and valve leakage after your computed #. You may pass a whistler, or you need a thicker gasket if they truly are 0.015". Agree that at 0.015" and zero deck, you'd be running over your rods right quick! What piston # does it have? What gasket #?

[Racermb]

My engine whistled 10.9 with 53cc heads, zero deck, .070 x 4.100 gasket, 5cc valve relief pistons. 358 3.5 stroke 4.030 bore.

[Racermb]

With 49cc heads the pistons would have to be way down in the hole to be 11:1 or under.

[CCHIEF]

https://www.google.com/search?q=quench+combustion+chamber+racing&ie=utf-8&oe=utf-8 , you'll find why deck clearance and gasket thickness are very important for making power and avoiding detonation

[over4T]

It would take 64 cc heads and a .041 gasket to get that engine close to 11-1, even not figuring all the variables. Whoever built that engine didn't do it to meet a low compression rule.

[bullittwrench]

Correct. Engine had a 515 lift rule where it was run. I bought it need to meet 11-1 rule.

[powerslide]

Take heads to a head porting guy and ask him to soften the chambers a little, then thicker gaskets will get you close.