What dynamic compression ratio should I aim for on a 355 gen 1 small block with iron heads? What about aluminum heads?

The setup will be 355 cubic inches, protopline iron heads (200cc intake ports, 64cc chambers), modified LT1 intake manifold, and maybe flat top pistons with 6-8cc valve reliefs. I estimate the cam will be around 224-228 intake duration at .050" lift and 228-234 exhaust duration at .050 lift.

This will be run on 92-93 octane gas and I'm in Houston if altitude is taken into account.

 

Ummm, you don't pick the DCR with the cam, you should get the right cam and then match the static compression to it. So it sounds like you already have the SCR picked out.

Bret

 

Thank you for the reply.

What do you think of this setup?

Nothing is set in stone as far as the parts besides the intake, block, crank, and rods. The rest of the stuff hasn't been bought yet so I am open to suggestions on what might work better.

I want a cam around 224-228 intake duration because I want a lopey idle but not too loud so that is why I put those specs. Any suggestions on a particular cam that would work well.

What is a good range of static and dynamic compression ratios so that when I type my setup into one of those calculators I have a reference so that I am not way off on anything.

 

http://members.uia.net/pkelley2/DynamicCR.html

I hope the above link works!!

It will give you a good understanding of Static and Dynamic Compression ratios.
And a good calculator to work with too.

You already have all the parts for your rebuild that will give a static compression ratio if you use the flat top pistons with the valve reliefs mentioned.

Your cam selection will definitly determine Dynamic Compression.
It's all about when the intake valve closes.

I believe your question was what dcr to shoot for. It's not a cut and dry answer, variables being temps, cumbustion chamber shapes, fuel quality, etc...
but read the article in the link above and make your own calculation.

I am building up a 383, and am shooting for 8.5 dcr.

Hope this helps.

Keith

 

I will give it a shot. With aluminum heads I would not go over 8.5:1 DCR for a first gen. With iron heads I would not go over 8:1 DCR. Good tuning will be required and other supporting mods such as a good cooling system and proper quench just to name a couple. There was like 10 items on the list by David Vizard to keep knock in check.

 

Thanks for the replies guys.

 

Ok, I have some numbers. The pistons will be flat tops, 5cc valve reliefs, left .010" in the hole, 0.038" head gasket. Cam is the comp cams XE 224/230, .503 in. .510 ex. lift, and the rest will be the same as mentioned above. Hooked to a t56 and 3.73:1 gears.

This puts static compression at 10.2:1 and dynamic compression at 8.06:1.
Am I pushing it with these compression numbers on pump gas and iron heads?

Exhaust will be slp coated shorties 1-3/4" with true dual 2-1/2" with h-pipe and mufflers.

What do you guys think of this setup? Am I headed in the right direction? How fast/how much power you think it will do?

 

There is a car here locally running iron Vortec heads, 10.5:1 compression with a ZZ4 cam no less. It is on a 383 SBC L98 motor. I don't know his DCR. We only have 91 octane here in SoCal. Nautuarlly when it was first fired up it would ping. Well after a lot of tuning he finally got it so it would not ping and had very good performance out of the motor to my surprise.

Did a search and his car put down 442RWTQ and 389RWHP. He was running an Accel Superram. So to answer your question it looks like it can be done with good tuning.

 

There is NO problem with 9.0DCR on 92 octane, if the engine is built for it.
Takes a lot more than just saying 8.5-9.0DCR is right and won't ping.

 

Like what? Let me know, this is my first engine build and I want to do it right.

 

Well like a tight quinch,IVC,chamber design,piston design,rod length,tune,etc.
It takes a lot of research and understanding to get it done. Ya need the tools to go along with it and a lot of time doing it the first time.
I would be glad to build it for ya for a nominal fee.

 

Well this is my first build, but definately not the last. I have been researching this stuff for a long time. It is a learning experience also and I would like to gain knowledge in doing this. Thanks for the offer though and out of curiosity, what is the nominal fee?

 

Depinds on the parts ya already have.
Machine shop will be about $1000.00
Labor would be $5-800.00 for a blueprinted assy, depending on what ya want. This doesn't include any head work.
Parts are unlimited.

 

A small price to pay for an experienced builder

Wanna do my 4valve head conversion for me?

(just kidding, you probably don't want to commit the time to a project like that, and I definately don't want to commit the money )

 

I think Bret has the corner on that market.