this is the 2nd piston we blew the ringlands out of, with a good AFR and timing in the 16deg range.

i think one injector might be bad, or 595rwhp is just too much on 50lbers at 60psi of pressure

any insight guys?

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If your afr didn't go lean, I don't see how the injectors could cause this. Were both pistons the same brand/part #? It looks like it has a weak spot there. Did they break in the same spot?

 

With a good AFR, it's not likely to be the injectors' fault. What was your duty cycle looking like?

 

well i think one might be clogged, or spraying poorly up top its the 2nd piston that did this, same cyl

like 90 ish duty or more on 50lb injectors

 

i think the pistons are rated at 1000rwhp, we have never broken any more than one cyl.

do you think its running lean or do you honestly think the pistons are hitting a weakpoint.

this is our 595rwhp 2bolt 68 mexican block car

 

+90% duty cycle on the injectors is real hard on them and cause them to go "static." If you are unsure about the injectors send them out to get flow tested, a lot cheaper than replacing pistons. At 600rwhp, you may want to consider larger injectors so you aren't pushing the 50's so hard.

Max duty cycle is usually based off 80%. I figure your flywheel HP to be at roughly 700. At 43.5psi pressure and a .65 bsfc you would be right on the verge of maxing out 72's @ 80% duty cycle. Being that you are running higher pressure than that, the 72's are going to flow more like 83's. i would put a set of 72's or 83's in there to be safe and get the car retuned, unless you can only run high imp injectors than get the largest high imp injector you can. Hope this helps!

Anthony

 

thanks, yeah we were at 80% while tuning, hitting 570ish and going over 80%, so we bumped to 60psi and retuned and managed 595rwhp and blew and broke a tbolt clamp, so called it a day at the dyno, then went to the track and busted this piston, not as bad as the last time but the same thing, let me dig up a pic of the last piston

this was the last one that broke and we replaced
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What fuel are you using? How much boost?How is your fuel system setup and what pump are you using? Is it turbo or blower?

 

93 test pumpgas
331ci 1968 mexican 302 block 2 bolt main
14psi is where we broke both pistons
we have it at 14 deg timing at peak tq, and 17 up top towards redline.
aeromotive A1000 fuel pump, 1/2 inch feeds split with a magnafuel Y connector strait to 5/8th inch rails and an aeromotive 1:1 sumped stock tank.
turbonetics 67mm turbo with a .81AR t4 flanged
MSD style 50lb injectors

from my understanding the boost and timing are pretty conservative, and we are hovering around a 11.3 average AFR ratio, just to play safe with that 2bolt block.

 

we can always tell when it breaks a piston, because it smokes grey out the valve cover vent even while the vacuum pump is running.

this time it didnt even take out the head gasket.

 

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Your other piston broke in the same spot. "Piston gives, gasket lives??" Not on an sbf. That piston has a weak point right there. Those are not 1000hp pistons, they are obviously 595hp pistons. You need a better set!!!

Your not the only one to break a piston in that spot. it's a weak design. Read here:http://forums.turbobricks.com/showthread.php?t=145400

That being said, you need more injector to be safe. But the injector didn't kill that piston.

 

that was my suspicion, thanks! no use fucking with the fuel or other things, bring it down to 500-550 and see if she lasts

 

This is interesting. I've seen this happen first hand (nitrous setup). It took a chunck out on the right side of the valve relief when the motor went lean. It also lifted the lip of the piston and pinch the top ring. Before the heads were pulled a compression check was done, all cylinders checked out fine except for the one cylinder that went. The head gaskets were fine when they were pulled.

 

When you look at the cylinder walls can you see witness marks near the top of the bore? The ring will put radial marks in the bore under detonation. I would dial the timing back a bit. What spark plug are you using? I take it with the dish your 8.5ish CR?

 

autolite copper
3924 or whatever that popular plug is. compression is in the 8.2354353 range lol

yeah, i figured it was just too much power, since the head gasket survived and whatnot. not the tune or anything else, thanks for all the help guys!

 

have you ever read the plug after a pass?
im still going to call it to lean

 

A piston should not be breaking at 600rwhp, 2 different times in the same hole. I feel that you are detonating. The pump gas now a days is JUNK. Ask me how I know. Do you have an intercooler? I would put a meth kit on and you shouldn't have anymore problems. It will cool the combustion chamber and also raise the octane of the pump gas.

What pistons are you running BTW?

Are you monitoring FP under boost? Is it dropping at all?