Compression ratio?

[ireland383]

These calculators are confusing to me. I'm trying to figure out comp. ratio, please help. The build a 71 383,KB400 pistons .040 http://kb-silvolite.com/performance.php?action=details&P_id=327,stealth heads 84cc,felpro 8519 .39 head gasket, comp cams 275DEH 219/235 462/482 closes at 63 ABDC. Thanks

[ireland383]

 

[spitnl]

try this calculator.
http://kb-silvolite.com/calc.php?action=comp

I used your spec below:
Cylinder Head Volume (cc):  84
Piston Head Volume (cc):    -6 (KB400 has Effective Head Volume of -6cc)
Gasket Thickness (in.):      0.039
Gasket Bore (in.):             4.520
Cylinder Bore Diameter:     4.29
Deck Clearance (in.)         ?
Stroke (in.):                   3.375

Rod Length (in.)              6.358
Intake Closing Point         63

With a deck height of 0.040 static compression is 9.182
and dynamic is 7.386

As you can see only unkown is the deck height.
It is really impossible to tell without checking.
Do you know what the deck height is currently and with which pistons?

[ireland383]

I would have to pull the heads to get a deck height. The pistons are the kb400's .040 already in this motor. Thanks

[HP2]

Since every big block heads I've checked for cc tended to be larger than most published specs, I'd tend to think that KB claimed 9.5:1 at 88 ccs may be a bit higher than what you actually have. spitnl's calculation of 9.1 may be pretty close, but it might even be too high.

What is your cranking pressure and elevation of operation? With those two items and the other engine specs, you could maybe back into a figure using this calculator; http://rbracing-rsr.com/comprAdvHD.htm It is for harley engines specifically, but as a four cycle otto engine, the results are the same.

[Moparal]

The cylinder bore size is much bigger than a 4.29  especially since it is  .040 over

[spitnl]

He's building a 383 not a 440, moparal.
The has 4.25-inch bore + 0.040 = 4.29"

[ireland383]

Since every big block heads I've checked for cc tended to be larger than most published specs, I'd tend to think that KB claimed 9.5:1 at 88 ccs may be a bit higher than what you actually have. spitnl's calculation of 9.1 may be pretty close, but it might even be too high.

What is your cranking pressure and elevation of operation? With those two items and the other engine specs, you could maybe back into a figure using this calculator; http://rbracing-rsr.com/comprAdvHD.htm It is for harley engines specifically, but as a four cycle otto engine, the results are the same.

       Haven't done a compression test and live in a flat part of Virginia. The heads were cc'd at 84.

[ireland383]

Finally got around to doing a compression test and found 155 to 160 across the board. Does this help any to determine comp. ratio? Supposedly the pistons are .024 below the deck on a non machined block according to the tech guy at KB.

[ireland383]

Big dummy I am, forgot to have the throttle open all the way. Will report back with new #'s.

[spitnl]

If the pistons are indeed 0.024 below deck, you will have a compression of 8.59:1
Bit low for the cam I would think.

EA predicts a dynamic compression of 164psi, with the ICL at 106degrees(4deg advanced).

The dynamic compression will not help you figure out your actual compression by the way, but it is a good indication of how well the engine will run, how healthy it is and if you have to worry about detonation.

[ireland383]

Spitnl I plugged it into the calc. and came up with 9.512, I think you came up with almost 9.2 last time you calculated with being 0.040 below the deck. By the way, the readings were still all around 160ish with the throttle opened. Thanks

[spitnl]

Ah, I see where I messed up, didn't put a - before the 6, sorry for the confusion.
You are correct, compression is around 9.5:1