10.2:1 dcr?
Thread Starter
Joined: Jan 2011
Posts: 1,921
Likes: 80
From: North Houston
10.2:1 dcr? So I got curious what my DCR was... mostly just because. So I gathered up my part specs and started calculating. First, I confirmed that my static compression ratio is 11.0:1. Engine specs are as follows:
3.903" Piston Bore
4.000" Stroke
3.91" Gasket Bore
0.040" Compressed Gasket Thickness (GM MLS)
64cc Combustion Chamber Volume
-8cc Dished Pistons
-0.008" Piston Deck Clearance
6.125" Rod Length
So that gets me to an SCR of 11.0:1. Now, the cam I have is a Lunati Voodoo 60511 (.567"/.567", 222/228, 113LSA (109+4) and 40* ABDC). That cam, with 0 psi used for boost and 0ft altitude puts me at 10.18:1 Dynamic Compression Ratio and 216.5psi dynamic cranking pressure.
Everything I've read indicates I'm about to blow up my motor on 93 octane and that I should be closer to 7.5-8.5:1 for a street motor. Here's the dyno graph...
So is this the absolute worst cam I could have for longevity or am I calculating something wrong?
3.903" Piston Bore
4.000" Stroke
3.91" Gasket Bore
0.040" Compressed Gasket Thickness (GM MLS)
64cc Combustion Chamber Volume
-8cc Dished Pistons
-0.008" Piston Deck Clearance
6.125" Rod Length
So that gets me to an SCR of 11.0:1. Now, the cam I have is a Lunati Voodoo 60511 (.567"/.567", 222/228, 113LSA (109+4) and 40* ABDC). That cam, with 0 psi used for boost and 0ft altitude puts me at 10.18:1 Dynamic Compression Ratio and 216.5psi dynamic cranking pressure.
Everything I've read indicates I'm about to blow up my motor on 93 octane and that I should be closer to 7.5-8.5:1 for a street motor. Here's the dyno graph...
So is this the absolute worst cam I could have for longevity or am I calculating something wrong?
Thread Starter
Joined: Jan 2011
Posts: 1,921
Likes: 80
From: North Houston
1. Using my motor information (see 1st post), he got a SCR of 10.6:1. Umm, what? I've calc'd 11.0:1 independently and confirmed what the engine builder calc'd, 11.0:1. This was one of my questions though, so could someone confirm my calculations, just to be sure?
2. 15 degrees seems arbitrary, but not completely unreasonable. The duration of the cam at 0.050" is 222/228, while the advertised duration is 272/278. I'm new to this, so I am counting on you guys to correct the next statement:
272-222 would result in 50 degrees total where the cam is between 0.050" and 0.0" lift. If a cam lobe is symmetrical (I'll bet it's not), then that puts 25* on the beginning and the end of the lobe. If 15 degrees is used for the duration from 0.050" to 0.006", then 10 degrees takes it to closed? I was just doing that to get a mental image of what was going on, but can one of you help me figure out where 0.006" is for a cam if it's not explicitly advertised?
Using 11:1 and a 15 degree addition to 40* ABDC, I get 9.43:1 DCR. Even 9.1:1 seems too high for a 93 octane motor from what I've read. Thoughts? Do I need to address the cam because it's resulting in too drastic of a DCR? I live near sea level, which certainly doesn't help DCR... Thanks for the insight, guys
Last edited by themealonwheels; Jan 18, 2012 at 07:27 AM.
With that info I'm getting 11.02:1 scr and 8.81:1 dcr. All looks good to me.
Edit: changed to represent the 4" stroke I didn't take into consideration.
Edit: changed to represent the 4" stroke I didn't take into consideration.
Last edited by black00ssFL; Jan 19, 2012 at 08:32 AM.
Thread Starter
Joined: Jan 2011
Posts: 1,921
Likes: 80
From: North Houston
Guys, can any one tell me how to get the ABDC degree at 0.006" lift if I have the advertised ABDC at 0.050"? This is critical, because it appears it's the last variable that determines whether my DCR is streetable or not
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Verdad Gallardo Thanks for confirming the SCR, did you redo the DCR calc? If you got the same SCR I got, then it's likely you'd get the same DCR... which is too high.
Guys, can any one tell me how to get the ABDC degree at 0.006" lift if I have the advertised ABDC at 0.050"? This is critical, because it appears it's the last variable that determines whether my DCR is streetable or not
Guys, can any one tell me how to get the ABDC degree at 0.006" lift if I have the advertised ABDC at 0.050"? This is critical, because it appears it's the last variable that determines whether my DCR is streetable or not
Thread Starter
Joined: Jan 2011
Posts: 1,921
Likes: 80
From: North Houston
I edited my post with the new numbers with the 4" stroke. I was at the end of class yesterday when I did the original calculation and couldn't redo it until now. Also, do you have the duration at .200 for that cam? I had to guesstimate for that value. But it looks like you're just fine. That's good power for such a "small" cam in a 383.
I just recalculated with 65* ABDC and got 8.80... confirmed your numbers 
Aw man, just saw your comment about the .200"... didn't ask I also called Lunati and spoke with a different tech service guy. He did not mention the 15* rule, he seemed to know exactly what I was asking for when I mentioned DCR, 0.006" vs .050" and the additional angle. He said the following:
"The intake opens at 27 and closes at 65, and the exhaust opens at 76 and closes at 22." I saw the 65 and felt a bit better. Was he likely giving me information about events at .006"?
I get that 65 + 180 + 27 = 272 (advertised intake duration)
and that 22 + 180 + 76 = 278 (advertised exhaust duration)
Thus, advertised duration is at 0.006"?
Math is fun when it means your motor won't detonate!
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