Speed Pro Pistons
 

So I took the heads off my 400. I see that the pistons are Speed Pro Forged and they are 030 over and have 4 relief cuts on the top. The guy I bought the car from knew nothing about the car. So now I'm trying to figure out the compression I'll be at using 670 heads. I've used the calculator on wallace racing and it's going to be high. I'm just hoping that the relief cuts give me a little drop and maybe throw in a Cosmetic gasket. Do they????? If not I'm going to have to get a new set of dished pistons or have some head work done to open up the heads. I'm using a XE 284 cam and it needs 9:1 compression to work.

Speed Pro sez that the .030 over 400 pistons will net 9.94:1 compression with 72cc heads.

Perfect.

You need to know deck height also, which you can measure with the heads off. You will need to cc head chambers to get an accurate picture of what your compression is...

Since you are going to be primarily concerned with dynamic compression, you could also do a compression test. I think you want to be in the 160-170 PSI range if I recall correctly. Someone may set me straight on this...

Lastly, I think the valve reliefs are worth a reduction of nearly 7 cc's.

Dens, where did you get that info from???????

GTO Greg is right,you need to measure the deck height and CC the heads to get good CR numbers,otherwise it all amounts to guessing.

Most Speed-Pro pistons are appx. .020" in the hole on uncut decks,and occasionally more.

And most iron heads are actually bigger than their "nominal" CC specs most sources quote,so the only way to know this for sure is to measure it.

All this needs to be known to get reliable numbers.

Not to mention the fact that there are a couple of different style of those pistons,some have a "bevel" around their perimeter that adds some volume as well,so that needs to be known as well.

HTH.