Fred or FastTA would you mind stepping in?

[Trash man]

i think i found a set of pistons on my 400, but heres the delimna(sp?) i want bout mid 10 for compression ratio(i have 76 CC head, no mill work done) ? is can i use a piston made for a 5.7 rod(it has the higher comp ratio) and put it on my 5.565 rod to get my desired results? this is the piston specs im thinking off and yes i kno i need to get the matchin right side

400ci Left-Side Race Piston, .030'' Overbore
Piston Dia.: 4.155''
Piston Top: -0.5cc Dome, 2-Valve Reliefs
Comp Ratio: 13.6:1 w/58cc Heads, 10.9:1 w/76cc Heads
Rings: 1/16'', 1/16'', 3/16''
Sold as Each
648-KB140.030.11

[fastTa]

Generally speaking if you utilize a piston that is designed for the same motor and is intended for say a 5.7" rod, then you use it with a shorter rod, you are going to usually lower the static compression ratio.

Considering everything remains the same in a motor build, when you use a longer rod, the wrist pin bores typically get moved up higher away from the skirt in order to keep the piston to deck clearance relatively the same.

Normally it is accepted that the longer the rod possible in accordance with the application the better. Longer rods keep the piston speed down, subject the piston to less cylinder wall loading, and yield longer dwell....all which equate to more power. However the differences between a 5.565" rod and a 5.7" rod are not as significant as the difference between a 6" and 5.7". We're talking a 2.37% difference.

Since you are looking at 4.155" pistons, I'm assuming that you are just going .030" over.

Suffice it to say, I think you will be closer to the upper 10's for SCR with a 76cc CC and only a -0.5cc dome. The 400's I have had experience with, we had to use more of a 5-7cc dish in order to get the SCR close to mid 10's.

If you are looking towards the SCR as an indicator of how well the motor will run omn pump gas, you should really be looking at the DCR(dynamic compression ratio) to determine how pump gas friendly it will be. DCR takes into account the IVC(intake valve closing point) and therefore gives you a much better indicator of how well a motor will run on the pump gas.

[Trash man]

thanx, i got the motor for free and was already bored 30 over. how would i determine DCR? would i need to figureout the cam i would like to use? id like to make 400hp n/a on 93 if possible

[fastTa]

thanx, i got the motor for free and was already bored 30 over. how would i determine DCR? would i need to figureout the cam i would like to use? id like to make 400hp n/a on 93 if possible

I can calculate your SCR and DCR for you if you know the specs or once you have the shortblock together and can measure the piston to deck clearance. I already know the rod length, bore, and CC volume, so I just need the following:

1. IVC ABDC, or if you can just give me the advertised(not @.050") intake duration, exhaust duration, LSA, ILC. These will all be on the cam card.
2. Crank stroke.
3. Compressed(installed) head gasket thickness
4. Head gasket bore
5. Piston to deck clearance

[Trash man]

ok thanx, ill get the rest of the info for ya