Pistons

flipper35 · #1 (**permalink**) 03-05-2003, 01:03 PM

Keith Black Hyper... vs Diamond Forged. The car is for mostly street with some track time.

I have heard some KB pistons clearanced for the street get too tight on the track and when clearanced for the track are too loose on the street. Any truth to this? Is it worth the extra $200 or so to go with Diamond forged?

The pistons are for a Stroked 360 Mopar.

Mr.Fixit · #2 (**permalink**) 03-05-2003, 01:54 PM

hypereutectics don't expand as much when they heat up, so you can set their clearance tighter during assembly. Forged pistons grow alot when they heat up, so they are loose in the bores when cold. How much clearance you need is one of those things where you will have a hard time getting different engine builders to agree on, to the exact numbers. I have seen enough wasted pistons in my short life to put forged slugs in all my motors, but I don't baby them either.

KobraKarl · #3 (**permalink**) 03-05-2003, 02:02 PM

mr Fixit,

Are you saying a little piston slap is a preferable price to pay for the ruggedness of a forged piston? and if so would this still be the way you would go if you had a block like a 427 that would only take a limited number of "freshenings" ?

thanks Karl

flipper35 · #4 (**permalink**) 03-05-2003, 02:02 PM

So with forged you get a little noise when they are cold and are a little more expensive. But I get from your post they are far better. Unless you talk me out of it, I will save for the forged.

Thanks.

racer24 · #5 (**permalink**) 03-05-2003, 02:34 PM

just finished redoing my 351-C and we used Keith Black- hyper . The engine designer asked if it would be street or strip. We went with 9.5 to 1 flat top and was told they would be fine for the street , I am only putting out 350 HP but thats enough for street.
Good luck

Rich

Mr.Fixit · #6 (**permalink**) 03-05-2003, 02:58 PM

Yep, I'd rather deal with a little piston slap than run the risk of destroying a piston and grenading the motor. But that's just one greasemonkey's opinion, you should discuss your engine components with your engine builder who will be the one repairing your motor. If you aren't gonna exceed one hp per cubic inch, or rev it up to ungodly RPM's than hypers should be fine, I don't have that kind of self control. Besides how many miles are you really planning on putting on your 427, enough to worry about wearing out the bores of your block, unlikely. You are much more likely to rev the hell out of it until something lets go in the first 5000 miles, which is sadly more miles than the average cobra awner puts on his car during his ownership of it.

flipper35 · #7 (**permalink**) 03-05-2003, 03:14 PM

I know lots of street rodders use the KB pistons with no problem. It was the dual use of running hard on a track and putzing around town. I am not shooting for anything extreme either, 485hp ad 535ft/lbs out of a 408cid.

I asked two different builders since I need to have the machining done. I got two different answers. Fixit has given good advice in the past and have no reason to doubt him now. My car doesn't get a ton of miles, a few thousand per year, but they are hard miles. Why have a high performance auto if you can't open the thing up?

I don't have a lot of money to spend on stuff like this so I want to do it right. If I had known guy's like Fixit (the builders I talked to when I built the car didn't know to drill a hole in the butterfly to keep it in the idle circuit) I wouldn't be looking at building a second motor.

Anyway it can't be much worse than the guys putting pistons in the opposite bank to eliminate the preload on the cylinder walls and $200 difference in the whole scheme of things isn't that much especially if they are really yhat much better.

I was also told the cast pistons hold up better in race conditions than the KB pistons by one of the builders.

DAVID GAGNARD · #8 (**permalink**) 03-05-2003, 05:26 PM

I have to go with Mr. Fixit on this one.....All the motors I build for myself have forged pistons in them and if anyone asked me to build them one,I'd insist on forged pistons simply because they are better and last and I have seen too many other non-forged pistons in the bottom of oil pans just to save a few bucks,not to mention the ruined blocks and other parts destroyed in the ensuing mess....

As stated, most Cobras see few miles but they are usaully hard miles...............

David

SFfiredog · #9 (**permalink**) 03-07-2003, 12:28 PM

Most modern piston manufacturers utilize aluminum alloys and piston skirt designs that require .0008" to .001" cylinder wall clearance per inch of bore. For example, a 4" bore 351 would have a cylinder wall clearance of.0038" to .004". This is much better than the older style pistons that required .006" to .008" clearance. Those would slap pretty good on a cold morning.
Rod length and deck height also have a lot to do with how noisey a cold engine is as well. I too agree with Fixit. I put forged pistons in just about everything.
--Mike

Dave Engel · #10 (**permalink**) 03-07-2003, 01:24 PM

If your heading for the 400HP and above go with the forge pistons, Hipers will not live that long and cannot take the detonation that the forge pistons can.

Another note, Keith black pistons need alot of ring gap, around .028" for the top ring, get wrong and the ring butts and the top of the piston comes off in pieces.

I have ran Keith black pistons in my windsor (circle track car)
with great success 11.3 cr 6400 rpm for over 5 years and the pistons look great. but I'm only putting out 368 HP ( two barrel carb. stock exhaust manifolds.)

Just my opinion.

Dave