I just put in new flat top mahle pistons in my gen 6 454. I checked a couple cylinders and got a deck height of +/- .040
I am going to be using new 781 heads,
Bob m says im going to have low compression around 8.19 he says a stock 330 has around 8.75

My ? is how could i have lost compression by not changing anything but new pistons from old stock ones?

What are my alternatives? Shave heads down? and is that risky?
and 2nd option shave heads and use smaller head gasket?
Thanks

im guessing the motor had the small dome pistons in it from the factory and noy changed to flat tops .you never stated what the motor was orig 454 mag .any of the higher h/p motors would have been small domes .just my guess

first question is what is the compression height of the new pistons vrs the old. the new ones may have a pin height that is closer to the piston top.

First off Bob is the expert and he will not point you in the wrong direction. I think the Gen VI engines had dish pistons to compensate for the 100cc vortec heads. So if you went to 118cc heads you will loose comp. Trust what he is telling you, he gave me an education.

You now have the perfect comp ratio to add a supercharger

Maybe I dont understand what you mean, but how can you have + - .040 ? The pistons are either down the hole or sticking up past the deck. If you have both , something is seriously wrong here. In either case, 040 is a good number to have.

the motor was a 330 bravo motor, with flat top pistons......the engine builder who bored out the motor and ordered pistons.. He didnt give me a option on ordering dome but the stock were flat......

I said +/- meaning give or take a couple thousandths.. no pistons are protruding the deck all below the deck .040...

Maybe the magnum motor had domed pistons, but mine had stock flat pistons and we just bored it .04

fml!!

The heads are 781 they are only 113cc from what ive read and heard unless modified

You might want to cut the deck down. The closer you are to zero deck higth the better off you are. Denetnation can hapen with a lot of deck higth.

The 781's are 118cc chambers. I looked it up just to be sure. http://web.archive.org/web/200805130...ec.com/bbc.htm

Now the 820's have a 113cc chamber, thats what I have. I had a little milled off, now they cc at 110.

What heads came with the engine?