Hello,
I'm trying to match a piston with the 3933148 (112cc chamber) big block heads I already have on hand. Running the compression calculators I run into two TRW/Speed-pro piston choices. One is L2240NF60 that calculates out to a compression ratio of 9.00/1 and the other choice is L2328F60 with a ratio of 10.38/1. I guess I'd like to hit a compression ratio of around 9.75. So my first question is can you shave/mill the dome down on the higher compression piston to get where you want to be compression ratio wise? The second question would be do you folks have any experience with running a compression ratio of 10.38 in a street motor? I know I can deal with some of the compression ratio with my cam selection but is 10.38 too high to approach with that thought in mind and using 91 octane?

The engine I'm working with is a 396/402 in a '69 velle with 3.31 rear end and a Turbo 400 trans.

Thanks, Lance

Hmmm....

Just found a Keith Black Hypereutectic piston (KB160060-8) that the price is very attractive on smile.gif

Anybody have any experience with these? There's a warning about top ring clearances and the fact that the top ring land will break off is the required clearances are not present? This is new to me is it old news to everybody else?

Lance

Yes, you can shave the dome of some pistons if you know how thick they are, a machine shop can usually measure and see. I've had it done. The easier thing to do would be use a thicker head gasket.If you want to run 10.1:1 you might use 93 octane insted of 91. I used to run 12.5:1 with 94 and 104 octane boost on the street.

Badrat, we can only get 91 in the pumps out here without going to the race gas. Lance, the pistons in my 496 had a sharp edge around the circumference of the dome. If you went with the forged 10.38 pistons, you'd probably trim the compression down some just by elimintating the sharp edge around the perimeter of the dome. I'm hoping to be able to run on 91 octane gas with my engine at 10.2 and a .560 I/E, 238* @ .050" 286* adv on a 110* LC. HTH

Bad Rat,
Yep like Wes says only 91 out here. Hadn't thought about the thicker gasket though graemlins/clonk.gif . thanks

Wes,
Yep I always smooth/deburr the top of the piston anyways.

Anybody,

The more I look at those Hypereutectic pistons the more interesting I find them. Anybody have any feedback on those for me.

Thanks, Lance

The easier thing to do would be use a thicker head gasket. ]Hadn't thought about the thicker gasket though You DO NOT want to use a thicker gasket to lower your compression ratio. You will ruin whatever quench that you may have which is what helps prevent detonation.

Well if that's true why do you think they make differant thickness head gaskets. graemlins/angry.gif

Different thickness headgaskets are so you have some options when it comes to milling the deck? So you can choose .035 quench over .045 or .050, or the other way around?

Have used K-B Hypers, very pleased with them. Nice advantage is that an ordinary ring set becomes a file-fit ring set because of the wider top gap needed. Don't expect good leakdown results, but everything's fine at operating temp.

I have had issues with cold engine piston knock, but not a reliability problem, just annoying for the first thirty seconds. By "cold", I'm talking 30 degrees ambient and colder, particularly when it's below zero.

i know that some piston domes are solid and some are hollow and you cant cut them down.

im not sure which are hollow and which are solid, hopefully someone will reply with more knowledge than myself.

I'd rather use an inexpensive TRW or Speed Pro piston if you are likely to encounter any detonation from 91 octane gas. My son used 93 octane in a 10.8 Compression 432 Big Block and it held up just fine until he sprayed a 275 H.P. nitrous hit to it. Live and learn. The K.B. Hyperutectic Pistons are similar to any cast piston in that they can't take much of a beating.

The hyper pistons are made of an alloy that's very hard, so they tend to shatter under heavy detonation.

But the ring gap issue, is that their material is also a much better insulator of heat, which improves the motor's "thermal efficiency" somewhat, and lowers oil temps. But, it also makes the top ring run ALOT hotter than a regular cast or a forged piston would; so the ring will expand more. And as we all know, once the ring expands to where it's longer than the circumference of the cylinder, baaad thing begin to happen immediately; either they'll break the top ring land off the piston (if you're lucky), or they'll destroy the cyl wall, or something.

I have a set of them in a small block I use every day, they work great; I built it with about .0065" of ring gap per bore inch, instead of the usual .0045 - .005".

As far as milling your existing pistons, people do it all the time; but you need to check how thick they are where you want to mill. Alot of forged pistons have extra material around the valve relief area, so that's usually one good place to take off some material.