themealonwheels

iTrader: (20)

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?

RezinTexas

did you use the cam duration specs at .006 lift when calculating DCR? If not, your calculated number is way higher than it really is.

wildcamaro

iTrader: (15)

Yea use the duration at .006

themealonwheels

iTrader: (20)

Alright, the cam card indicated 40* ABDC. I spoke with the Lunati tech service about this, and they mentioned exactly what you two said... the 40* is not at 0.006". He added 15* (seemed kind of like a rule they use) and got a calculated DCR of 9.1:1. I was a little skeptical of his calcs though, here's why:

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

black00ssFL

iTrader: (24)

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.

themealonwheels

iTrader: (20)

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

mark21742

On the cam card there should be an intake valve closing point abdc in degrees....then use that number to find dcr

mark21742

I ran and got 10.62 scr, and 9.81 dcr when I used the 40 abdc and if I add the 15 degrees to get 55 abdc I still get 9.09 drc

black00ssFL

iTrader: (24)

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.

themealonwheels

iTrader: (20)

Well carp, those are almost exactly the numbers Lunati gave me on my call. What are you using to calculate this with my numbers? Is it on a site I could look up? Thanks for calculating...

Thanks for going back and the kind words 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!

mark21742

I use WWW.wallaceracingcalculators.com