LB9GTA

iTrader: (1)

I am building an engine right now and I can't figure out how to calculate dynamic compression

Here is the specs
383 stroker
10.94 to 1 compression(static)
4.03 bore and 3.75 stroke
cylinder head =65cc(afr 195 elim)
piston head(flat top forged speed pro with 2 valve reliefs)=3.4cc
gasket thickness=0.015
gasket bore=4.10
deck clearance=0.035 piston down the hole
Rod length=5.7(I am using stock 350 rods so is this correct?)

This is the one that stumps me
Intake Closing Point (degrees)
ABDC @ 0.050 lift plus 15 degrees=???
I am using the 280XFi cam, part number CCA-12-367-4
Duration at 050 inch Lift: 236 int./241 exh.
Advertised Duration: 280 int./290 exh.
Valve Lift with Factory Rocker Arm Ratio: 0.552 int./0.547 exh. lift
Lobe Separation (degrees): 113

Can anyone help me to figure out dynamic compression??

Sonix

iTrader: (1)

What a bunch of BS, I can't find the cam card on comps website. If you have the cam in hand, you have the cam card?
I don't use the ABDC @.050 +15, I just use the intake closing point as is. I think that's more accurate.

So if you can find it, use that number
I was going to plug it in for you at
http://kb-silvolite.com/calc.php?action=comp

With 62* (as a pure guesstimate), 10.9SCR came to 9:1DCR.

1989GTATransAm

iTrader: (3)

What is the .050" closing point? Then add 25 degrees.

The cam is advertised as 280 degrees of duration. 230 degrees at .050". That is a difference of 50 degrees. Divide by 2 and you get 25 degrees. That should be pretty darn close.

LB9GTA

iTrader: (1)

I'm sorry I don't follow.

What is the last number I am looking for?

Do I put in 25* for the Intake Closing Point ?

1989GTATransAm

iTrader: (3)

Sorry about that. On the card what does it say the intake valve closing at 050" is. It should be something like 42 ABDC. Thats what you would add the 25 degrees to. So the IVC would be 25 + 42 = 67 degrees.

LB9GTA

iTrader: (1)

I ordered the cam on tues.
So I won't get the cam for another 2-3 weeks.

LB9GTA

iTrader: (1)

If I use the 67* I get a DCR of 8.615 with a SCR of 10.932, with a quench of 0.05.

How would that be with 91 octane fuel??

I will use 93-94 octane at the track tho to run a little more timing and nitrous

1989GTATransAm

iTrader: (3)

Your quench is not that good. Normally with a good quench of say .035" I would say 8.5:1 dcr would be max on 91 octane. Just my opinion but I would shoot for something lower. Maybe an SCR of 10.5:1 and see what the DCR is.

LB9GTA

iTrader: (1)

I called Comp cams and they said that the intake close point for the 280xfi h13 is 69, so that would be 10.93SCR and DCR of 8.47.
Is that better?

1989GTATransAm

iTrader: (3)

Lots better. You can also retard the cam 1 degree and that will lower the DCR a bit more. I installed my XFI268 cam retarded 1 degree fwiw.

LB9GTA

iTrader: (1)

Retarding the cam 1*, is that going to effect power or driveability?

1989GTATransAm

iTrader: (3)

One degree is well within the margin of error that most cams are installed at. I would not give it a thought. So enter 70 degrees in the calculator and see what it comes up with.