James B

I have not bought the pistons yet for my 355 because I wasnt exactly sure which compression ratio to go with. I calculated it out on paper with two different piston sizes which I found availible through summit. Here are the numbers I used when calculating it(using a formula found in my engine blueprinting book). Vortec heads 62cc. .041 quench level, 3.48 stroke, 4.03 inc bore. The first compression ratio I got was 10.4:1 which was when I Calculated it with a -6.1cc piston dome (4 valve relief flat top). The second compression ratio of 9.8:1 was achieved after calculaing a -12cc dish piston.

My question is...will 10.4: compression ratio be too much compression for pump gas. I have read that most of the time anything over 10:1 you will not be able to run full 36 degree advance without detonation.

Im not sure how to calculate my dynamic compression ratio. The cam I will be using is the Crane powermax 272.

Thanks for the help

James

StickShiftCorvette

I run 10.67:1 with a Comp Cams 294S and can run on 91 octane, however, I have aluminum heads. If you have iron heads the most I'd risk without water injection or retarded timing is 9.5:1 and that would require 92 to 93 octane. Iron heads are much more prone to knock.

63mako

Hi James. Aluminum heads help. 10.4 to 1 is definitly on the high end with the cam you have and the iron heads. I would go With the 12 CC dish even though I like flat tops better. I don't know how to figure your dynamic compression ratio either.

zwede

I'd go for the dished piston.

Dynamic CR is when you consider the intake valve closing point. The valve will close on the upstroke of the piston. The later the valve closes, the less will the ultimate pressure be at TDC. The full stroke is 180 dgr. If the intake valve closes 10 dgr after BDC you then calculate where in the cylinder the piston is at 170 dgr BTDC and then use that swept volume. I would think there should be webpages that do the math for you.

VintageMuscle

Calculate DCR

Here is a calculator. According to the crane cams website, that cam has an intake valve closing point of 65 ABDC. With the iron heads, I constantly read that a DCR of 8.0 is about the max DCR you want to run with iron heads on pump gas. Using the 12 cc piston and the other data you supplied, DCR comes out to be 7.83. That should work for you.

Also, in the intake valve closing point column, don't let the +15deg statement confuse you. That's only used when the valve timing data is reported at .050 valve lift. Crane supplies the data at .004 valve lift, which, in essence, is the closing point. So use 65 for IVC.

With aluminum heads and carb, you get run up to about 8.5 DCR, and with fuel injection around 9.0 max.

There are a lot of factors folding into this, but a little below the maximums would be a pretty safe bet and provide good power.

Dynamic compression ratio is really just a guide to help set up an effective steet motor, and seems to work pretty well in controlling cylinder pressure for pump gas street engines.

jackson

James:
Are you sure about vortecs having 62cc chambers? ... I thought most Iron GM vortecs average 64cc while most Aluminum GM fastburns average 62cc ... maybe I'm mistaken?

Your 0.041" quench ... does that include both the compressed gasket thickness ... AND ... any distance the piston may be down in hole? Dunno if you will zero deck block to piston height?

-edit- If you choose a dish ... please note: Because they have a dish roughly shaped like a "D" with a high flat ... Inverse Dome aka Reverse Dome work better with vortec/fastburn than a simple round dish ...inverse-reverse helps promote the quench effect and helps interupt & prevent pinging.

James B

Jackson, thanks for the info about the D shaped piston, that is good to know! The motor that I am rebuilding has flat tops in it and when i measured the distance from the top of the piston to the top of the deck it was .025. If i use a head gasket with a compressed thickness of .016 then i should be good for .041. The vortec heads are advertised at 64cc but I believe from the people who have CCed them at the shop I go to say most of them are actualy around 62cc.

I appreciate the help guys. I believe I will be going with a -12 dish piston. Id rather sacrifice a little power for drivability.

James

jackson

James:
Take a look at kb-silvolite (keith black) -12cc D-cup hypereutectic piston P/N KB193.

With what you've stated about your bore, deck height & gasket ... KB193 should yield about 9.83:1 CR w/ 62cc ... about 9.62:1 CR w/ 64cc.
-edit- Heads that have been lightly milled to clean-straighten will lose 1-2cc.