Crazyboy

What is the highest Static compression ratio in a 383 LT1 you are successfully using on 92 octane without detontation or preignition?

TQdrivenws6

That really depends on the DCR, not so much the SCR. What is the cam?

rskrause

Ditto.

Rich

Crazyboy

Ok guys,
My next question would have been "then what cam are you using"
Since your jumping to more questions, Why not just help out by stating some proven 383 combinations(ie Static compression, dynamic compression and cam used) that work on 92 octane without detonation.

TQdrivenws6

God forbid we ask for more info to give you the correct answer. If you want a common combo, how about a -16cc piston, pistons .020 in the hole, 6" rod, 1.125 compression height, 3.75 crank, 4.03 pistons, .029 gasket, cam that is .050 from close at 46 ABDC gives 10.9 SCR, and 9.02 DCR. The general concensus is 9.0 DCR is the limit on a LTx based motor. Do a search in advanced tech as its been beaten to death there.

The point that I was trying to make is that there is no 'common' 383 combo. We just plain need more info to help you out.

Crazyboy

Thx for the reply TQdrivenws6.
The first question was intended to get a discussion started on the subject and not with regard to specific cam I had.

Would you happen to know the intake closing point (in degrees ABDC) for the stock LT1 cam in a '95 or '96-97" F-Body?
I'm trying to calculate the dynamic compression for the stock LT1 for comparison. I can't seem to find it on the net.
The stock engine has 10:1 CR with a pretty small cam so I was thinking one must be able to get the static CR to much higher with an aftermarket cam.

Crazyboy

I tried plugging the 383 combo you mentioned and I get a dcr of about 9.8:1.
10.9 SCR
3.75" stoke
6" rods
4.03" bore
46 ABDC intake closing at .050

Any ideas? Did one of use overlook something?

IrocSS85

hey guys, I dont know how to determine dynamic CR, but I just wondered about something here. maybe you guys can tell me. here's my combination, and combomotorsports told me it would be around 11.0:1. but not according to your above recipe.

3.75 crank, 6.0 rods, -16cc srp pistons, .029 head gasket(imp. gasket), stock head cc (maybe slightly oversized cause I did clean it up w/the sanding drum), and the block has been decked about .15". I have .050 deck height.

now the cam I have is the CC 233/239 cam. here's the specs

open close
intake 7btdc 45abdc

exh. 54bbdc 4atdc

I run 92octane. I only have a coule hundred mi.s on this combo w/no time's or HP #s, but there is no audible knock, and it sure pulls great, i have a hard time beleiving it's detonating. and if I had 11.5:1static seems like it would be. when I get back from vaca., I'll hook up my scanner to it and make SURE. I'll post as soon as I have a time ticket.

Crazyboy

irocss85,
Are you sure the block has been decked down by .150"? or is it .015"?
If it is .150" then you are about 10.0 SCR (estimate). But that would mean you have pistons with a Compression height(CH) of 0.935". thats a really small CH.

IrocSS85

10.0:1! huh? must be .015. he took as much out as he could w/o havin to machine the intake to match the lower heads/angle. and, when I preassembled it, the new deck height at~tdc is .005 now. one side read .005 and the other act. read .007. but there were no rings in it at that point, so I fig. the pistons could have been rocking a tad. and I was using a flat edge across and using a feeler gauge under that. I know thas not the right way, but thats why I said ~ isnt stock height between .020 and .029"? what is the formula? I know someone had it linked from some site before.

TQdrivenws6

I just did the calcs again, and got 10.895 SCR, 8.948 DCR. I believe that you did not add 15 degrees to the Intake closing point. Here is the easiest calculator that I have seen out there: http://kb-silvolite.com/calc.php?action=comp and when I put in the 46ABDC I get 9.789 which is obviously wrong.

TQdrivenws6

I want to know how you got your deck height of .050? Pistons are in the hole about .025 +/- .004 from the factory so the closest number that I can see is that they took off .015 and you are somewhere in the area of .005-.010 in the hole.

I just ran your numbers through that calculator, and it gave 11.302 SCR, and 9.340 DCR which isn't going to like very much timing.

Crazyboy

TQdrivenws6,
So in the calculation you are taking the closing time (in degrees) at .050" and adding 15 degrees. Is that a general rule that is excepted among engine builders?
Depending on the cam lobe profile I would suspect the 15 dgerees would be quite a variable but I haven't realling had enough cam data to say it is so.

"I just did the calcs again, and got 10.895 SCR, 8.948 DCR. I believe that you did not add 15 degrees to the Intake closing point. Here is the easiest calculator that I have seen out there: http://kb-silvolite.com/calc.php?action=comp and when I put in the 46ABDC I get 9.789 which is obviously wrong."

By obviously wrong do you mean the calculator is not correct or are you saying you need to add 15 degrees to the 46ABDC?

WS Sick

I'm running 10.5 and the GM 847 on 91 octane all day long.

Crazyboy

IrocSS85 ,
Do you happen to know what timing your running at WOT?