My 408 317 headed twin 76 blow thru was just sneeking up on running some good numbers went 5.06 140 then 5.05 140 .Then on the 3rd pass on 17lbs right in the lights made a noise went 5.10 136. Got it home took the head off and #2 has a hole on the side down to the first ring. piston is a Mahle powerpak which is forged alum.AFR was 6.6 on E85,timing was 21. Any ideas on this

 

Can u post a picture for the piston? Did you log the run? That would help.

Also, what spark plugs are you using?

 

fuel distribution prob?

 

What was your ring gap?

 

what intake ?

 

blow thru?

 

blow thru = carbed

 

well if that is the case I would bet the problem is fuel. Never heard of a blow thru turbo setup that worked well for long.

 

plugs are Autolite AR 3933. Intake is a Victor Jr. Fuel dis.maybe but it dont look burnt. I would have to look back to found te ring gap

 

I may of just been a bad forging??? Out of curiosity, what type of vehicle is this setup in??

 

Got any pics of the pistons from the side? Just curious as to how thick the crown is.

 

knock.knock.knock.knock

 

A properly setup blowthru is every bit as accurate at fueling as Fuel Injection.

 

Engine is in a 1964 GTO sq tube chassis car on 10.5s. Dont have it out of the car yet. Knock would have showed on the log. AFR was fat and happy all 3 passes

 

Who built the carb?

 

It doesn't look like a melt down failure! Check the other pistons. If it's detonation or fuel issue, it would appear on other pistons.

Could be a metal failure in the forging of the piston as mentioned.

 

Is that a 4032 piston?

If it is I'm surprised it made it that long..

 

Ive heard the ringlands on the dished mahle pistons are pretty crappy to begin with. I ran flat top mahle's with -4cc valve reliefs on my LS6 with 72cc heads which got me about 9.4 scr and i had no problems whatsoever at 15+ psi and up to 20 deg of timing. However for my 370 im building up right now I needed a dished piston, so I went with wiseco pistons. They have a very thick ringland. I know none of this helps what happened, but maybe you can use this info for future, and others reading it can benefit from it as well hopefully... Good luck with it all man

 

I was surprised to see so many posts until KP's about the powerpak pistons...

You have far surpassed the capability of that piston. Its a 4032 alloy that I wouldnt run over about 8-10 psi on (600 rwhp would be my recommended limit for a converter/auto or maybe closer to 700 rwhp on a stick car).

You need to go to a 2618 alloy custom piston /w lowered ring lands, upgraded hell-fire or stainless top ring and upgraded tool steel pins.

 

If it was a detonation failure wouldn't it be evident on the exhaust valve side and not on the intake side? Was there any cool down between runs or were you hot lapping it?