DCR or SCR most important when factoring in octane?
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Staging Lane
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From: Overland Park, KS
DCR or SCR most important when factoring in octane? 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 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
Originally Posted by conundrum07
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 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
https://ls1tech.com/forums/showthrea...67#post4982267
Thread Starter
Staging Lane
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Joined: May 2005
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From: Overland Park, KS
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?
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?
TECH Fanatic
Joined: Aug 2005
Posts: 1,691
Likes: 1
From: Sunny London, UK
Originally Posted by conundrum07
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?
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?
I guess you fit into this portion of his comment:
Originally Posted by Tony Mamo @ AFR
Thought I would chime in here quickly...
59cc's with flat top pistons (no valve reliefs) is too much IMO for CA/AZ crap 91 octane. The most I normally recommend for someone pushing it to the max is 62 cc's with an .040 gasket. Assuming valve reliefs and a cam that is 232+ @ .050, then I would recommend 60 cc's in the land of 91 octane....
Tony M.
59cc's with flat top pistons (no valve reliefs) is too much IMO for CA/AZ crap 91 octane. The most I normally recommend for someone pushing it to the max is 62 cc's with an .040 gasket. Assuming valve reliefs and a cam that is 232+ @ .050, then I would recommend 60 cc's in the land of 91 octane....
Tony M.
Good luck! It sounds like you know what you're doing. What heads will you be using?
Thread Starter
Staging Lane
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From: Overland Park, KS
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...
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...
Originally Posted by conundrum07
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...
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...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.
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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 ??
Thread Starter
Staging Lane
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From: Overland Park, KS
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?
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.
Originally Posted by slow trap
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.
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
Bret
