I'm trying to do an accurate dyno file for my GTA and I have all of the specs perfect so far but compression is giving me a problem!!

I know the factory compression is 9.3:1 and the factory heads have a combustion chamber size os 58cc but when I enter 9.3:1 into desktop dyno it says my combustion chamber size is 75.48cc!!! I have to change the compression all the way to 11.8:1 for it to say the combustion chamber size is the correct 58cc.

What gives???? Why can't I change the combustion chamber size manually so I can put into 9.3:1 AND 58cc ????

Thanks!

BTW, I was told that for TPI I should enter 500 cfm for the induction cfm flow, is this correct? How much would the SLP runners add?

 

Don't confuse Total Combustion Volume with Head Chamber Volume

Head Chamber Volume is just that.... it's the size of the combusion chamber in the head.

Total Combusion Volume adds all the variables together. There's the combustion chamber, the deck height (i.e. the area between the piston and the top of the block while the piston's at TDC), and the area between the head and the block where the gasket sits.

There's also the area between the top ring land, and the top of the piston, which that program doesn't calculate variably. It uses a fixed amount on every combo. And if you have dished pistons or pistons with valve reliefs, those have to be added into the equation also.

Just concentrate on the 9.3:1 area. If you get it there, then ONLY change one variable at a time, then you will have successful results.

The program is actually made to use fluid and a beurre (spelling?) to measure each volume separately so you can get a precise measurement. Rounding off like what you're doing should only be done to get a "ball park" idea. Don't expect accurate calculations from inaccurate measurements.

 

DD2000 adds the the cc of the gasket ,the cc of the piston being below the deck,and the cc of the dish of the piston. Thats why its so much higher.You can use the compression calculator at the top if you know the all the specs.

 

Now I understand, wow that's makes total sense now!

Thanks guys!

Now is the 500 CFM correct for Desktop Dyno for TPI induction flow?

1 last question, should I select "Large-Tube Headers With Mufflers" or "Small-Tube Headers With Mufflers" ??

I have "Large-Tube Headers With Mufflers" selected because I have SLP Tri-Y's which are headers made for 350's, I have them in my Firehawk too. Is this a correct asumption or should I select "Small-Tube Headers With Mufflers" since I have SBC headers and not BBC headers???