Like the title says, is the dynamic or static compression ratio more important when factoring for octane limitations.

I found a good deal on a cam so I decided to go with it instead of a custom grind, but in order to get it back to ~8.5 DCR my static compression is going to be about 11.4:1. I want to make sure this wont kill my timing when I try to tune it since 91 octane is the highest available around here. I understand the concept of dynamic vs static compression, so I think it should be OK, but I wanted opinions from some of the gurus before I start ordering parts.

Thanks guys

 

I don't think that'll work too well with 91.

https://ls1tech.com/forums/showthrea...67#post4982267

 

Well, from what Tony is saying in that post, the combo I want to run should be OK with a good tune.

My planned setup is:

Torquer V2 cam on a 112 LSA
-2cc pistons
60cc heads
.040 headgasket

That gives the 62cc chamber + .040 hg that he mentioned. According to my calculations that will yield a 8.46 DCR and 11.38 SCR.

Anyone else have any input?

 

Have a read of this http://www.popularhotrodding.com/tec...power_squeeze/

 

I see, said the blind man.

I guess you fit into this portion of his comment:

I believe his comment was made in context to the AFR 205's, FWIW.

Good luck! It sounds like you know what you're doing. What heads will you be using?

 

Im actually working that out today. I’m currently looking at either the PRC LS6 CNC heads or a pair of LS6 heads that will ported by a very good shop. I believe the AFR's will have a higher port velocity, will that have a significant effect on suppressing knock? I’m would rather avoid dropping over 2k on the heads if possible. Time to do some more searches...

 

lol

I think the tight quench helps supress knock, and it will be tight as long as you stick with the thin HG.

I have no idea if higher port velocity has any effect.....

Happy hunting.

 

Anyone else have any experience with this type of setup?

 

i don't know about 91 oct and 8.5 dcr but i know i had 9.0 dcr on 93 and only time i had any knock whatsoever was in 5th or 6th below 2k accelerating at 1/2 throttle or more,even then it was only 1.7 * or less.with a good tune i would think 8.4-8.5 should be doable.

 

It seems like when it comes to DCR, there are a lot of opinions. I have heard that for 91 octane you can go up to 8.5 to 1 DCR then other say 8.1 to 1 is max. I wish there was someone who knew for sure. I am sure it depends on set-up also, but I am building a 408 right now with SCR of about 11.27 to 1 and DCR of 8.38 to 1, with a quench of .035 cold. I am hoping to run on 91, I guess we will see what happens ??

 

Ya, I’m in the same boat as you. I’ve read that people have run more than 8.5 and had no knock, and others who have run less and had problems. I think a lot of it is in the tune and things like your quench, but I’m trying to get a general consensus of what can be done safely/what has been done.

And as far as the original question goes - do you look at only DCR or does the SCR play a role in how far you can go on a certain octane? I understand DCR is basically what compression you have left over after you factor in the IVC point, but at high rpms when the cylinder is filling to capacity is SCR going to limit you more than DCR? For example, will a setup that has a low DCR, let’s say 7.5:1, and a high SCR, say 13:1, going to have more knock issues than an 8.5 DCR/10.5 SCR setup on pump gas?

 

i forgot to add that i was also using gm mls gaskets before so my quench definitely wasn't optimum.i would say don't worry about scr even if it is very high and concentrate on dcr.

 

Yeah I agree SCR doesn't mean much. I also think tuning has a lot to do with it. I am actually going to try to make an 87 tune just to be safe. Obviously timing is going to be considerably less than on 91, but I thought I should keep the lower timing table just in case you get some bad fuel or if you fill up in a different area than you usually do and the gas formula is different.

 

DCR and quench are the physical variables that affect how much octane will be required. A tune will also help, but be aware that theres a balancing act, and theres a point where youll have to retard the timing so far to keep from knnocking the added compression does nothing for you.

 

The real key is cylinder pressure and temps. The higher those are the worse of you are with higher DCR's. Cylinder pressure is also influenced by Volumetric Eff, the higher the cylinder fill is the more cylinder pressure that makes so a lower DCR for a certain octane. DCR is what you should be looking at for the most part.

Bret