What is ideal, and why?

Lower SCR with earlier IVC to reach DCR - 9.0:1

Higher SCR with later IVC to reach DCR - 9.0:1.

Lets assume the motor must run on pump gas (93 octane), and let's use the range from say 10.5:1 SCR vs. 13:1 SCR.

Any input?
Ryan

 

This is really going to depend on the situation. Since you want it to run on 93 octane, I'm assuming this is a street engine. In that case depending on the power goals If you arent goign too radical I would probably go for a moderate SCR and use the proper cam with a early IVC to keep from bleeding off the compression. But really you should pick your compression ratio to suit your cam, not pick your cam to suit your compression ratio.

 

Depends also on the revs ya want to turn. Larger duration means later IVC most of the time(unless the closing ramp is like falling off a cliff). IMO ya select the cam for the HP your heads will provide and bring the static as close to 9.0DCR as possible.
Remember to deduct the 5-15%(depending on intake) your intake will take off the head flow. I would try to do an intake with 4.5-5" runners to bring the torque up and not especially a carb intake that has 4 one length and 4 shorter. Something like a Holley Stelth Ram or similar.
If ya already got the compression set in stone(parts bought or built) ya have to pick a cam to keep the DCR at or below 9.0.

 

'Dude: You are basically saying to use think of SCR as (one of) the dependent variable. Makes sense. I'm at 12.5:1 static ratio, mediocre heads, single plane (Jr) and I'm not sure if having a cam ground to make it work on pump gas is a good idea or not. Ain't skeered of revs...

I'm not so interested in my own situation as what a good practice to follow is...in regards to building the thing from the ground up.

Thanks for the input.
Ryan

 

Get the best set of heads ya can afford and build the engine around them.Stroke,intake cam,RPM's,compression,etc.

I would start figuring what duration would keep my 12.5 static in the 9.0DCR range and what the ICL would have to be for the most HP-TQ. Then call a cam grinder and see what they recommend. Just don't go too big.

 

Isn't it the IVC point that would regulate the 9.0:1 DCR? I'm using these figures and I come up with

4.04
6.0
3.75
12.5
X = 76º
0
0

http://www.rbracing-rsr.com/comprAdvHD.htm

...to get the right DCR.

How does total duration affect DCR?

Ryan

 

Normally the bigger the duration the later the closing.
For a 76* IVC it would be 296*advertised on a 112+4 ICL or 304 advertised on a 108+4 ICL
This will be in the 260* range@50 262/269 that's a pretty big cam.Ya may get a special grind with quick closing ramps to do it with less duration but ya will have to see what's available.

 

GM 847 cam has a 79* IVC with significantly less duration.

http://www.cranecams.com/?show=brows...tType=camshaft

 

Two ways to make a later ivc, Advance the cam so that the icl is > 110 or to run more duration. Hence why most people are saying more duration == later ivc

 

Every ported Single Plane I tested flowed better than a HSR that was ported.

The runner length difference in a SBC single plane is not that bad at all and the HSR is a little to long to start with.

Bret

 

That runner length thing is an easy fix-- HACKSAW

Let Wilson do the porting it might not come out that lopsided.