Changing pistons and balance

hoopty5.0 · Mar 20, 2012

I need some advice. I'm still not happy with my compression. For those who haven't read my build thread, my block was built to 9.8 and not the 10.5 that I requested. I realize a point of compression is theoretically only worth about 12 hp, but I had a cam custom ground from Bullet for higher compression and I'm worried it will change the overall performance of the engine.

My question deals with changing pistons and how that will affect the balance of the rotating assembly. Switching to a flat top from dished piston to would change the reciprocating weight and not the rotating weight of the rotating assy. Can the whole package be rebalanced if the crank has already been drilled? Or would I have to buy another crank and start from scratch?

I would think a flat top piston would weigh more than a dished piston but that's probably a false assumption.

Thanks
Collin

RangerJoe · Mar 20, 2012

The machinist who bored my block told me that you could swap pistons as long as they were the same weight, or really close. I would call the guy who ground your cam and see what they say about running a lower compression. Your only .7 points off of the intended ratio.... I would be interested to see if it would make a real difference. 9.8:1 isn't really considered "low" compression, so I wouldn't think it would be a big deal.

Joe

ttopstang · Mar 20, 2012

you will need to get it rebalanced.

you can re-use the crank, if it needs more weight, the machinist will just add some mallory metal to it.

If it was me, I would get the correct pistons for the compression ratio for the cam and have it balanced, you already put this much thought and money into the build, might as well build it like you planned (and don't use the machine shop you originally used, i read your other thread and they shouldn't be getting any more of your money).

CarMichael Angelo · Mar 20, 2012

I think you're obsessing needlessly. It's like Ranger joe says, Even if there is a difference in the end result, It would be negligible IMO.
( at least not worth another 500-600.00 for a new set of pistons, plus the 300 or so to have it re balanced)
Run it,..If it makes 500 HP, would you tear it apart to make 515?

At least call the cam guy, and see what he says.

Boosted92LX · Mar 20, 2012

Yeah man, didn't you say you were going to boost it later anyway? A 9.8:1 408 and gonna be no slouch, plus later if you do boost it that's a nice C.R. to work with. You may even forget about boost after driving it. Just sayin.

hoopty5.0 · Mar 21, 2012

Ok, I'll leave it alone and see how it goes. I am just terrified that I'm going to have all of this money tied up into an engine that runs like a 6 cylinder due to one critical piece of the puzzle being off. madmike1157 and SSeater - Y'all are like the voice of reason on here. "Oh wise ones what shall I do?!?" "Patience, young grasshopper."

T-minus 14 days left on my sentence...errr I mean rotation. First chance I get, I am going on a witch hunt for the build sheet. I still can't believe I never got it.

Changing pistons and balance

hoopty5.0

mechanicus terribilis

RangerJoe

I leave the horn on while driving

ttopstang

Active Member

CarMichael Angelo

my rearend will smell so minty fresh,

Boosted92LX

It's only an inch or two. What's the big deal?

hoopty5.0

mechanicus terribilis

Similar threads

Changing pistons and balance

hoopty5.0

mechanicus terribilis

RangerJoe

I leave the horn on while driving

ttopstang

Active Member

CarMichael Angelo

my rearend will smell so minty fresh,

Boosted92LX

It's only an inch or two. What's the big deal?

hoopty5.0

mechanicus terribilis

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