Pontiac GTO Forum banner

1 - 13 of 13 Posts

·
Registered
Joined
·
40 Posts
Discussion Starter #1
I'm starting the build on my 1st ever Pontiac engine. I have a 455 with a set of 62 heads. My plan is to do a +0.030 piston and zero deck the block. I am going to reuse the crank and replace the connecting rods with either h or I beam forged rods. I am going to leave the 2-bolt mains but will stud them.

I have used the CR calculator at the wallace racing website and it looks like I'm going to need pistons with a 30cc dish to bring me in at 9.5:1. The problem is, I don't see anyone list these on the web.

Is this an 'off the shelf' item or will I have to have them custom manufactured?
 

·
Super Moderator
Joined
·
5,782 Posts
You're probably not going to find them unless you have them custom made, and maybe not even then. 30 cc's is a lot. Are you dead set on using those heads? What static compression ratio are you shooting for?

Bear
 

·
Registered
Joined
·
4,209 Posts
Keith Black offers a 30.5cc dish Hypereutectic .30 piston. Part # KB371-030, at Summit Racing for $453.97 a set. Butler Performance offers Ross custom made forged pistons which are called reverse dome pistons, but at a much higher price.
 

·
Registered
Joined
·
365 Posts
For forged, it's worth the cost. Personally, I'd never put hypereutectics in any engine I cared about.

Bear
I am not for or against them, curious what is your objection? As long as silicon is held low?? or is it just cast vrs forged?
 

·
Super Moderator
Joined
·
5,782 Posts
I am not for or against them, curious what is your objection? As long as silicon is held low?? or is it just cast vrs forged?
Hyper's are the middle ground between conventional cast and forged pistons. They are a little stronger than conventional cast pistons, but they're not nearly as strong as forged ones. Their slight increase in strength comes from manufacturing tricks to get the silicon content up to around 16% to 19% as opposed to around 12% for regular cast --- the increased silicon content is the definition of hypereutectic.

What is touted as the "advantage" of hyper's is that their dimensions don't change with temperature as much as those of forged pistons, so when running hyper's you can build to tighter clearances and avoid the noise ("piston slap") in a cold engine that you sometimes get with forged pistons (because they require looser clearances). Once an engine with forged pistons warms up to operating temps, that problem goes away.

However, hyper's aren't as strong as forged and almost as important: they don't "bend" - they break.

So, when you run hyper's what you're doing is gaining a few minutes of slightly quieter operation while the engine is warming up and you're also saving on some cost, but you're also making a significant sacrifice in strength and toughness, and you're also taking a risk with what can happen in "unusual circumstanced". (Remember, cast and hyper's break, they don't "bend" like a forged piston can.)

I dunno about ya'll, but having an engine that is "quieter during warmup" isn't exactly high on my list of prioritiies.. :D
However, strength, toughness, and longevity are VERY important to me so that's why I said..

"Personally, I'd never put hypereutectics in any engine I cared about."

:cheers

Bear
 

·
64-67 Expert
Joined
·
8,569 Posts
I agree with Bear and Rukee. From what I've read and experienced, forged pistons are my only choice, period. Hard= brittle, and brittle breaks.
 

·
Registered
Joined
·
40 Posts
Discussion Starter #9
Sorry for the break; hectic work week followed by a day without internet.....

Bear:
Shooting for 9.5:1. Yes, I'd like to use the heads since 1) I already have them and 2) I already have them back from the machine shop.

PontiacJim:
I saw the Ross pistons and will most likely go that route. I'll just have to put in a call to Butler Performance and see what they have to say.

I started out wanting to build a 400 but had the 'worst' block for a rebuild (557) and after talking to a few people it was apparent I wouldn't be spending that much more money building a 455 over the 400.

I appreciate all the feedback and I'll let you know how things work out. I'm still seeking out a machine shop locally so I'll be spending tomorrow tearing down the 455 (got it today) and getting it ready to send off.
 

·
Registered
Joined
·
4 Posts
First time here and not sure if proper to revive this thread, but it describes where I am now... down to the 62 heads (which I learned I have to check to see if they're 72 or 75 cc).

This thread says no hypereutectic pistons, about which I agree, but says forged pistons will slap when cold - but slapping only happens with Ross/other pistons that are made from 2618 aluminum, right? but not if they're made from 4032. Butler says the Ross pistons are 2618 T6 but I don't know what T6 means (less expansion?). I assume Butler sells a LOT of them, and I can't imagine so many people are out there with engines that slap when cold - there's also cylinder and piston wear that comes from slapping during each heat cycle.

Butler talks about Mehle dished pistons on the rotating assembly page - 4032 aluminum with 22 cc dish. I used Summit's calculator and with 4.155" bore, 4.25" stroke, 72 cc chamber, 22 cc dish, (assuming) .02" deck clearance, and (need to get) a .06 thick gasket yields 9.44:1 CR.

Does this CR sound reasonable and are forged 4032 aluminum pistons the best choice? My machine shop said 2618 is not good for a street engine.
This Mehle article describes differences.
Which Mahle Piston Is Right For Your Engine? - High Performance Pontiac Magazine

I assume 4032 was available when this thread started.
It's odd that I don't see any dished Pontiac piston in Mehle's catalog.
 

·
Super Moderator
Joined
·
5,782 Posts
It's up to you of course, but I wouldn't worry at all about piston slap. It only happens while the engine is cold, and chances are that if you build it "healthy" you'll never hear it anyway. Just keep your foot out of it until it warms up - which you should do anyway for all kinds of other reasons.

I don't recommend thick gaskets unless you have no other options available in order to "correct" compression. They have a negative effect on cylinder turbulence / "quench" area in a way that actually tends to promote detonation, and also has a bad effect on combustion efficiency. That costs you power.

Running the numbers using your measurements, I get a compression ratio of 9.715:1 using standard 0.045" head gaskets. Using pistons with 26 cc's of dish brings that down to 9.405:1. So you see just a few cc's of volume makes a pretty big difference.

First order of business: actually measure those heads to find out their exact chamber volume (I assumed 72 cc's for #62's, but they can and do vary). If they really are 72 cc's, then machining another 4 cc's out of each chambe ought to be doable but make sure that you don't touch the "flat" part of the chamber to get it. That has the same effect as running fat gaskets and messes up the quench pad area. (Also make sure the pistons you get have D-shaped, not round, dishes --- for the same reason.) You could get the same results by using 26 cc dishes - if you can find them or afford to have them custom made.

Bear
 

·
Registered
Joined
·
58 Posts
Im running the Ross/Butler pistons. No slap when cold. Block was plate honed. Of course i drive like grandma till warmed up.
BE sure to double check chamber cc's after valve work. Could be several cc difference before and after.

Once the piston and rod combo is installed, check piston to deck clearance again.
Then if a final compression adjustment is needed the heads can be milled or gasket thickness can be adjusted.

There are pressure losses in a running engine. I shoot for 0.1 or 0.2 higher than target compression.
 

·
Registered
Joined
·
4,209 Posts
Summit shows the Keith Black Hyperteutectic 455 30cc pistons.
http://www.summitracing.com/parts/uem-kb371ktm-030/overview/make/pontiac

When reading about the Hyperteutectic pistons, I found a very mixed bag of information and reviews. Some love them, others say they are junk(and some of the negative is most likely being generated from the competition who wants to promote/sell their brand). I did a lot of reading and came to the conclusion that the pistons were better than cast, but not as good as forged. They have a specific fitment, just like any cast or forged piston. What I found were a lot of comments that they are "brittle" and won't take a lot of detonation and will shatter. BUT, it doesn't seem to be a piston initiated problem, but rather an improper ring gap setting, leaned out engine on nitrous, or other. Here is an interesting read. Hypereutectic Pistons not so bad? - Third Generation F-Body Message Boards

I chose the Hyper's for my brother's 360 Six Pack build after reading a lot of material. I saw more advantages than disadvantages in using them. He wasn't going to use nitrous, although they seem to hold up fine without going ridiculous with the nitrous -which then needs forged pistons. So far, no problems with his 360 and he was having severe run-on/dieseling after he would shut the engine off. He finally got it fixed so it doesn't. They didn't come apart on him, so how brittle is brittle?

I was going to use them myself on my 455 build, but did not need the 30cc dish with my 7K3 96 cc heads and decided I want to add a shot of nitrous -so I went with forged pistons.

Not endorsing the Hyper's, but I really don't think they are as bad as seems to be found on the web. If they were, they would not be selling and would have disappeared from the offerings of Summit, Jegs, and other suppliers, to include the manufacturer.:thumbsup:
 
1 - 13 of 13 Posts
Top