New 408 build cam compatibility/compression question
04-18-2016, 08:39 PM
#1
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New 408 build cam compatibility/compression question I have a 408 short block (4.030 bore, 4.0 stroke, 6.125 rods) with Wiseco 4.030 -3cc K464x3 Forged Pistons. My intent is to run GMPP LS3 CNC'd heads (68.5cc) with an LS9 head gasket (.051 compressed thickness) and I also have a BTR Stage 4 LS3 cam (44 ABDC @ 50). I know that my SCR will be around 11.5 with the head/piston combo. Will this cam work with this set up and be able to run on 93 octane pump gas or do I need to consider selling the cam and getting something different?
Pardon my ignorance but I am struggling with the dcr/scr and getting my head wrapped around all of it properly. I have attempted to use the calculator on the Wallace Racing site but I'm not certain I am doing it correctly. I would really appreciate any assistance so I can get this combo worked out correctly.
Pardon my ignorance but I am struggling with the dcr/scr and getting my head wrapped around all of it properly. I have attempted to use the calculator on the Wallace Racing site but I'm not certain I am doing it correctly. I would really appreciate any assistance so I can get this combo worked out correctly.
Last edited by TimE; 04-18-2016 at 09:01 PM.
04-19-2016, 10:15 AM
#2
Not that I'm a believer in DCR numbers, but they're usually calculated using the IVC point @ .006", not .050". An IVC point of 44 @ .050" is probably closer to 70 @ .006" ABDC depending on the lobe profile.
It should be fine though. Most of the cams out there have a IVC point that falls in the mid to high 40's @ .050" ABDC and most NA engines are built with between 11:1 and 12:1 and they all seem to run just fine.
It should be fine though. Most of the cams out there have a IVC point that falls in the mid to high 40's @ .050" ABDC and most NA engines are built with between 11:1 and 12:1 and they all seem to run just fine.
Last edited by KCS; 04-19-2016 at 11:05 AM.
04-19-2016, 10:28 AM
#3
Not that I'm a believer in DCR numbers
My previous setup had something like 8.9 DCR from a small cam with plenty of advance. Would struggle with KR all over the place (even with a healthy .040 quench) and at WOT could barely muster 18-20* of timing.
Now I went with a slightly bigger cam and less advance, resulting in a DCR of around 8.6. Everything else stayed the same and now I'm able to get something like 25-26* of timing and no issues with KR.
And yes, use the 0.006 duration which most cam vendors can give you the specs of for your calculation.
04-19-2016, 11:14 AM
#4
I am!
My previous setup had something like 8.9 DCR from a small cam with plenty of advance. Would struggle with KR all over the place (even with a healthy .040 quench) and at WOT could barely muster 18-20* of timing.
Now I went with a slightly bigger cam and less advance, resulting in a DCR of around 8.6. Everything else stayed the same and now I'm able to get something like 25-26* of timing and no issues with KR.
And yes, use the 0.006 duration which most cam vendors can give you the specs of for your calculation.
My previous setup had something like 8.9 DCR from a small cam with plenty of advance. Would struggle with KR all over the place (even with a healthy .040 quench) and at WOT could barely muster 18-20* of timing.
Now I went with a slightly bigger cam and less advance, resulting in a DCR of around 8.6. Everything else stayed the same and now I'm able to get something like 25-26* of timing and no issues with KR.
And yes, use the 0.006 duration which most cam vendors can give you the specs of for your calculation.
Out of curiosity, what was your SCR with the old setup?
04-19-2016, 11:39 AM
#5
Out of curiosity, what was your SCR with the old setup?
New setup is 11.43 SCR and 8.62 DCR
So technically, I am actually running slightly more compression now than before if you only look at the static.
But I did go from an IVC of 41.5 to 44.0
I do agree that you shouldn't focus too much on it, but in my case where you kinda push the envelope and you get too crazy it did come back to bite me in the ***, at least in terms of how much timing I could run. Anything under 8.7 on pump gas and yeah it shouldn't matter much.
04-19-2016, 11:54 AM
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Thanks for the info guys. I guess I'm just being a little **** as I'm spending more on this motor than I have on several different cars I have bought in the past (and the wife's looking at me all 
when I start talking about $$, then it turns to 
). Just trying to wrap my head around the combo, on paper, and be a little more informed of what to expect real world.
when I start talking about $$, then it turns to ). Just trying to wrap my head around the combo, on paper, and be a little more informed of what to expect real world.
04-20-2016, 09:32 AM
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DCR is really only relevant at low rpm's and high loads; think turning 6th gear below 2000. Beyond that it's just a guess.
FWIW My combo ran at 26 degrees timing with no KR, even at 28 degrees, but with same power. Granted I had cathedrals with welded chambers, but I had zero issues even with an a/f averaging low 13's.
Jason
FWIW My combo ran at 26 degrees timing with no KR, even at 28 degrees, but with same power. Granted I had cathedrals with welded chambers, but I had zero issues even with an a/f averaging low 13's.
Jason
