ZZ28ZZ

Eng (see sig) had miss at idle.
#4 had 90 psi. Others had 195 psi.
Squirted oil and rechecked. Got 96 PSI.
Toredown eng and here's what #4 looked like.
Top of piston looks fine.
Not a scratch on cyl wall.
Eng has abt 10K miles.
What happened?
Detonation??

92rsbowtie

definatly looks like a detonation, or a preignition problem

Fast355

iTrader: (2)

I've seen that from rings butting togather too. IE, not enough ring gap.

84z28350

Every motor i have blown due to detonation has had damage above the rings unlike yours and did alot more damage to the piston, im not sure it was a preignition problem.

Did you check all your ring gaps when you put the motor together or did you just slap them in and hope for the best?

ZZ28ZZ

No pitting was seen on piston top, so I don't think I had much detonation. Pre-ignition I suppose is possible.
I set the top ring gap to .024" and the second ring gap at .018".
I was thinking I was on the "loose" side if anything. The pistons are hypers.

InsaneIROC

The tune is off its running to lean same thing happened to my motor at 100 miles lost number 3,5 and 6 tore the motor down put in a set of pistons got the tune right and have had no more problems

HalfInchWrench

Running too thin or lean is my guess. Tuning needed to be fattened up.

ede

i'd suspect the end gap closed on a ring. that could of been caused by running lean or not setting it to spec. what are the end gap specs the piston maker said to run? did you have a power adder on it?

ZZ28ZZ

No power adders used.

I've been running a Wide-band 02 on the eng since it was first run. It doesn't run lean and the knock sensor isn't showing any count on the datalogs.
I also have a DIY stand-alone knock sensor and it doesn't indicate detonation either.
I remember reading something on this site abt adding some piston ring clearance when using hypers. I added .005-.006" to what the maint manual called for. I was under the impression I had too much ring gap if anything.


Wondering if I may have cracked the ring land area while installing the piston.

The eng always had a little more blow-by than normal..
I originally thought the rings would seal up and it would go away, but it never got much better.

ede

all the hypers i ever used had something like increse end gap by .xxx per inch of bore or multiply factory end gap by some factor, like 1.xxx. i was thinking you'd end up with about 40% more gap than listed in the manual. you ought to be able to find the info on the makers web site.

Dialed_In

iTrader: (3)

pre-ignition and detonation are the same thing. Look at the upper main bearing halves. What does the wear pattern look like? More wear on the top will point towards detonation.

BUT

It looks like someone didn't file fit they're rings. If they were "drop in" rings, then someone didn't hone the block right. Probalby a tight spot in the bore closed the ring gaps.

mw66nova

iTrader: (13)

looks kinda like the one i just pulled from my 305!

Sonix

iTrader: (1)

hey Nova, think that's due to the 9.5 comp, and 87 octane fuel? (I remember asking you how you did that so successfully... care to retract that statement? )


ZZ: since all the other pistons are ok, and the top of that one is fine, my guess is that it's nothing to do with fuel, as it would effect others as well... (as well as your O2 sensor use...)

Hmm, possible to have faulty rings? like, slightly cracked or something, then installed and shattered later..?
oh well, nice that your cylinder wall is ok, fairly cheap fix...

mw66nova

iTrader: (13)

oh, i know it's got everything to do with my 87octane fuel and 9.5:1 c/r...oh well...live and learn, right?

Cali Z

I have a 305 w/ around 10k miles w/ 9.5:1 comp, and I run 89 octane w/ some advanced timing. Is that safe? I'm wondering after the two pictures I've seen. I don't want my engine to be ruined.

mw66nova

iTrader: (13)

are you fuel injected? that helps alot with avoiding detonation, but i was carbed so...if i were you, and you plan to run your timing advanced, i'd run the 93 octane stuff...only like $.10 more per gallon...

LnealZ28

I don't think so...........Click Here

ZZ28ZZ

On the Federal Mogul/ Speed-pro site I found the following.

1: Their hyper pistons use normal ring gaps (as compared to cast) due to their exclusive alloy.

2: They don't recommend filing gaps for "regular useage".
(But, they don't list "regular useage" gaps)

3: They recommend a larger gap for the second ring as compared to the top ring (that caught me by suprise).

4: The gap I WAS running should have been OK for supercharged or street nitrous operations.

Rob Wade

You may just have a bad piston!? Mass production! I agree also with the post re: the hone in the bore might have been to aggressive but I think I would lean towards quality control. I believe detonation would show the top of the piston to be beat up also.

Dialed_In

iTrader: (3)

I do. Same end result, just taking a different road to get there. Either way the pison/rod is fighting it's way throught the high pressure of the burning charge. Same end result, same damage.

rx7speed

they are a little different
pre-ignition is usually a hot spot that causes the fuel/air to ignite on it's own.
detonation is usually caused by a high pressure combustion leading to an explosion of the gas rather then a controlled burn

rx7speed

so stabbed someone with a .45
or I shot someone with a 12 inch knife
they don't sound right.
sure either wya the person might die but still they are different.

ZZ28ZZ

Looks like most folks here agree my piston probably wasn't damaged by pre-ignition or detonation since the top looks OK.

I installed new Speed Pro rings yesterday and didn't file them at all.
I wound-up with .012-.014" gap on the top rings and .016-.018" gap on second rings.

What is the tightest anyone has gotten away with using Speed Pro hypers?? Won't the gap get bigger over time as the rings/cyl wall wear??