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Rebuilt motor w/ broke pistons UPDATE: 11/13/13 1.5yrs later upgrade time!

I'm in for detonation also. Either to much compression or to much timing. That thing should have sounded like a diesel with the amount of detonation required to cause that.

You didn't mention which heads you have on that. 64cc 76cc....ect? Judging that this is the second time this occured I'm going to guess you have to much comression.

4X4HIGH said:
Also keep in mind that a stock ring gap is "usually" on the loose side and unless those are KB pistons (doesn't appear to be by looking at the design) the gaps would need to be slightly larger than normal but you wouldn't butt the rings gaps unless there was severe heat in the cylinders.
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Those aren't KB pistons. The lightning bolt on the side is the give away, those are speed pro pistons.

I can't see the top of those pistons but I suspect they are flat tops. 10.84:1 with 64cc heads and 9.62:1 with 76cc heads
 
Also keep in mind that a stock ring gap is "usually" on the loose side and unless those are KB pistons (doesn't appear to be by looking at the design) the gaps would need to be slightly larger than normal but you wouldn't butt the rings gaps unless there was severe heat in the cylinders.
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nsxxtreme said:
I'm in for detonation also. Either to much compression or to much timing. That thing should have sounded like a diesel with the amount of detonation required to cause that.

You didn't mention which heads you have on that. 64cc 76cc....ect? Judging that this is the second time this occured I'm going to guess you have to much comression.

Those aren't KB pistons. The lightning bolt on the side is the give away, those are speed pro pistons.

I can't see the top of those pistons but I suspect they are flat tops. 10.84:1 with 64cc heads and 9.62:1 with 76cc heads
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That's what I said. I was just explaining about the KB's needing more ring end gap than other pistons.
 
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where was timing set? If you were fighting a huge amount of advance that may have caused the hard cranking also?
 
Ok wow I go to work for alittle bit and I get super behind on responses.:haha:
I will try and respond to some right now, its late im tired and I have a double shift tomorrow so bear with me.

The pistons were as said above speed pro flat top pistons. I did NOT hear knocking. As was said above it wouldve had to sound pretty dang horrible to cause this much damage this fast and I didnt hear anything outve the ordinary. Just the sweet sweet sound of the open headers due to not having a y pipe made yet.(now yes I realize some will be thinking that I might not have heard the knocking due to it having open headers but I was constantly listening to the motor.) Its just what ive always done with all my vehicles when testing/having problems with them. The tops of the piston look fine, other then some darkness from the carbon/oil being burnt. But there doesnt appear to be any damage/dings/pitting/anything.
The heads are stock heads from a 70s 400. If I recall correctly its 76cc? Maybe 74??

If you need anything else answered just ask. Im off to bed, got work again in 9hrs. I will try to respond tomorrow before work if there are responses and then after work tomorrow night. I will also get pictures of the tops of the pistons and cylinders maybe. Thanks for the help so far guys.

Edit to Zimmers response: Timing was set at 6degress base timing. I was running a stock timing(spark advance) table in the tune. So the timing wasnt drastic at all.
 
I'm sorry to say that it's very doubtful that you could hear detonation with open headers.
 
I might be speaking out of turn or knowledge but have you compensated for the additional cubic inches 350 vs 400?

You sound like you are sharp enough to monitor the inputs into the computer and fine tune the program but I understood the base computer in our rigs couldn't handle too much over their base engine be it a 305 or 350.
 
4X4HIGH said:
I'm sorry to say that it's very doubtful that you could hear detonation with open headers.
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The more i think about it, the more im worried that that is true.

The program I am using is called EBL Flash. Dont know if anyone has heard of it or not. But I did make adjustments where I could for my base tune. Then the computer can learn the VE tables(fueling) so it was adding fuel until it got to a good point for the 400. The tune was coming along great, it wasnt running very lean at all. Especially as I added the miles. Every time I drove it I had the laptop hooked up to the ECM and had it doing the learns.
Right now my biggest concern is WHAT was causing this much detonation if that truely is the cause here. The more ive thought about it, the more I am believing it.
 
Without reading thru that yet, I think pre-ign is where the gases are being lit by something before the plug fires. But I will work on reading that before I leave to for work again.
So know my biggest concern is finding what was causing this det. so it doesnt happen again. I am considering going with forged pistons this time, so I DONT want to trash another set.
 
dabomb6608 said:
Without reading thru that yet, I think pre-ign is where the gases are being lit by something before the plug fires. But I will work on reading that before I leave to for work again.
So know my biggest concern is finding what was causing this det. so it doesnt happen again. I am considering going with forged pistons this time, so I DONT want to trash another set.
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Keep in mind that forged pistons require more piston to cylinder wall clearance and they won't stop detonation from damaging them again if you can't or don't find the cause of your issue. The very first thing you NEED to do is get some exhaust installed for two reasons, first so you can hear everything that is happening and also so that any tune is correct. A tune with an open exhaust will be different than one with exhaust. :deal:
 
Yep, part of my reason for thinking it might not be det, was that I could not believe that he could drive it for long with that much det and not wonder what the heck that loud noise was.......

But if he was running open headers.......
 
That is my plan. I will be ordering my ypipe with my piston order. I would really like to figure out what caused this. I mean it wasnt a high compression motor, the timing wasnt super advanced, and the heads are stock heads. Is there something im missing?
On a side note, I will be going to the machine shop tomorrow after class to talk to them about it. I will be bringing one of the pistons along with me.
 
I'm just curious as I'm not a big fuel injection guy, but how well does an O2 sensor work with open headers? I'd think the fuel mapping would be all over the place?:dunno: (just throwing it out there)
 
That is a very good thing to bring up. To me, the readings always seemed like they werent to far off of what I expected. When I first started tuning it was reading lean. Then as the tune progressed it started to come back to the middle(470hz? another words 14.7:1 air fuel) But then again ive been wrong before...
 
dabomb6608 said:
That is my plan. I will be ordering my ypipe with my piston order. I would really like to figure out what caused this. I mean it wasnt a high compression motor, the timing wasnt super advanced, and the heads are stock heads. Is there something im missing?
On a side note, I will be going to the machine shop tomorrow after class to talk to them about it. I will be bringing one of the pistons along with me.
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umm....ya you have pretty dang high compression for a chevy motor sadly anything above 8.5:1 seems to be considered high compression for these old chevy motors. What type of fuel were you running? With the compression numbers I listed previously you would need premium at a minimum. And base timing should have been 0 degrees. Low grade fuel with, high compression, and timing advance I could easily see how things would detroy themselves quickly. Forged pistons won't do you any good. You will just end up with busted rings. Forged pistons just buy you some time they dont solve the problem.

Also running open headers is not a good idea. It will change the tune drastically. I would start with figuring out what heads you have. If you order new pistons try and get a dished set to get the compression down.
 
dabomb6608 said:
that is a very good thing to bring up. To me, the readings always seemed like they werent to far off of what i expected. When i first started tuning it was reading lean. Then as the tune progressed it started to come back to the middle(470hz? Another words 14.7:1 air fuel) but then again ive been wrong before...
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14.7 wot??
 
Where in the heck is the O2 sensor that you can tune (or even run closed loop) with open headers?
 
Well I guess I did have a decently high comp. Ratio then. Which isn't good because I was just running regular gas in it. So now I need to decide how I'm going to get rid of the det. I will look into dished pistons. I will also be buying a new knock sensor once the engine is ready to be put in. When I order my pistons I will be getting my ypipe as well. I will get the numbers off the heads to look them up when I get home later this afternoon. I've done it once before and I know the cc is somewhere in the 70s. And that they are pretty crappy heads.
 
Where in the heck is the O2 sensor that you can tune (or even run closed loop) with open headers?
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I think you just sorta stick it in the end..............
 
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