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Rebuilt motor w/ broke pistons UPDATE: 11/13/13 1.5yrs later upgrade time!
- Thread starter dabomb6608
- Start date
GMCSuburban2500
Registered Member
Did you compensate for your 6 degrees of base timing in the ECM? IDK about TPI but TBI runs 0 degrees base timing. 6 is alot for a 400sbc anyway. in the olden days 400s ran 4deg of base timing. and had 8.2/1 compression ratio.
I have a tbi'd 400 in my suburban and they are delicate compared to a 350 or 383 or big block. better know a little bit of what you're doing.
I'd say it was detonating like hell and you didn't know it for your headers. Also running even slightly lean at that timing and compression will melt any engine, especially a 400, in short order.
I have a tbi'd 400 in my suburban and they are delicate compared to a 350 or 383 or big block. better know a little bit of what you're doing.
I'd say it was detonating like hell and you didn't know it for your headers. Also running even slightly lean at that timing and compression will melt any engine, especially a 400, in short order.
It ran fine, and went right into Closed Loop with no problems. The O2 sensor readout in my tuning program did the up and down movements just as it was supposed to do.
Here are some pictures of the #1 cylinder before the pistons were pulled. That way you can kindve see both the top of the piston and the cylinder walls.
Now the dark vertical marks in the cylinder looks worse then it is I think. It isnt a low spot that I can feel. There are only a few tiny spots that I can feel a little bitty hair like low spot.
I went and talked to the machine shop today and showed them a piston. The guy at the desk took it back to show their engine guy and he said it was definitely detonation as well. At this point I want to say something to Scott. Im sorry for even remotely doubting you sir.
SO, now the question arises. Should I go with the hyper pistons again? Or try and puke out the cash for forged? I dont REALLY have the money to be paying 700ish dollars for this motor again. I already spent more then that on the rebuild.
I guess I should give you guys my future plans for this car since that plays a big part in the piston choosing. I dont plan on running NOs on this car, or atleast on this motor. I also dont plan on going boosted at all on this motor. I just plan on getting better heads/intake down the line. THEN maybe building a motor that can handle either boost or NOs.
Here are some pictures of the #1 cylinder before the pistons were pulled. That way you can kindve see both the top of the piston and the cylinder walls.
Now the dark vertical marks in the cylinder looks worse then it is I think. It isnt a low spot that I can feel. There are only a few tiny spots that I can feel a little bitty hair like low spot.
I went and talked to the machine shop today and showed them a piston. The guy at the desk took it back to show their engine guy and he said it was definitely detonation as well. At this point I want to say something to Scott. Im sorry for even remotely doubting you sir.
SO, now the question arises. Should I go with the hyper pistons again? Or try and puke out the cash for forged? I dont REALLY have the money to be paying 700ish dollars for this motor again. I already spent more then that on the rebuild.
I guess I should give you guys my future plans for this car since that plays a big part in the piston choosing. I dont plan on running NOs on this car, or atleast on this motor. I also dont plan on going boosted at all on this motor. I just plan on getting better heads/intake down the line. THEN maybe building a motor that can handle either boost or NOs.
Yes, I did tell the ecm that it had 6degrees of base timing. Most things I have read say that the tpi's stock were at 6degrees Base timing. If my dad was here with his corvette I would check to be sure.
At this point im putting to blame the detonation on two things. Me needing the base timing to be lower for the 400(Most likely 0). And me needing to run higher octane fuel. Regular is 87 in my area with 91 being the highest I believe. I mean no matter what the pistons need ordered along with the ypipe.
Could you explain the different cylinder sizing? Ive never came across hearing or reading that before. From what I recall from my readings, I just thought that different types of pistons need different ring gaps?
Could you explain the different cylinder sizing? Ive never came across hearing or reading that before. From what I recall from my readings, I just thought that different types of pistons need different ring gaps?
hyper pistons are fine assuming you get the compression ratio right. As I've been saying its way to high. You need yo know which heads you have and the that will tell you how much of a dished piston you need. You need to be around the 8.5:1 range
Ok, heres some pictures of the heads. Casting number looks to be #462624. Quick search on google came up with 76cc.
I dont want to be the bearer of more bad news but you might want to take those heads to the machine shop and have them inspected. You see those dark black spots between the cylinders near the exhaust valves? That is cause by detonation and running straight headers. Those areas got extremely hot and could have comprimised the metal in that area. I would clean them up and inspect them really well. If that thing would have ran any longer you would have blown a hole straight threw those heads beteen the cylinders.
Yeah, I noticed those during the tear down. I will call the machine shop tomorrow to see what they will charge to check the heads out. Im trying to figure out what cc of pistons I need to get the CR down. The ones that came with the rebuild kit were 6.00CC. So I am thinking something in the 10+ range?
I think your going to need -12cc to get the compression to 8.64:1 or -18cc to get it to 8.24:1
You might want to run that buy 4x4high as I'm no sbc expert.
You shouldn't have to tear those heads apart maybe a good wire brush to those areas.
But you should check the integrity of the metal in that area. You will also want to check the top of the block in the same area.
I'm thinking of just going with hypers again if possible. It looks like the dished pistons above 10CC are Keith black. I've heard they are just as good as speed pros. That way I'm not spending money I don't have on something that's going to be overkill. Plus I can spend more on the heads then if needed.
How should I check the integredy of the metal there? I will try wire wheeling them tomorrow to see if it cleans up.
How should I check the integredy of the metal there? I will try wire wheeling them tomorrow to see if it cleans up.
I would look to see if its pitted at all then you might try poking at it a little and see it breaks or mahrs easily. Others might have a better idea.
Given a .039" compressed head gasket thickness, .025" deck height, 76cc head chamber and an 18cc dish piston would net you right at 8.593 CR
Ok, and that's a good typical compression ratio? Im guessing the .025 deck clearance number is a stock number? As far as I know, the block hasn't been decked or anything.
From my 22 years experience that is about the average that i've seen for deck height on a SBC. Keep in mind those heads are "supposedly" 76cc chambers but i've also seen them larger and smaller in size. Obviously if the heads get surfaced the number gets smaller.
Alright, thanks so much fir the help everyone. And thank you Scott. I will update everyone tomorrow on how those spots look on the block and heads. As of right now the only pistons I've found on summit that are 18cc are Keith black hypers. Which is fine, but they are almost double what the pistons that came in the rebuild kit. I guess that's because of the name and them being dished.
Cleaned up the block and heads between the cylinders. It cleaned up pretty good and I cant see anything outve the ordinary at those spots. Heres some pics.
So, the pistons I am looking at are http://www.summitracing.com/parts/UEM-KB147KTM-030/.
That brand is really the only brand available as dished.
http://www.summitracing.com/parts/UEM-KB147KTM-030/
So, the pistons I am looking at are http://www.summitracing.com/parts/UEM-KB147KTM-030/.
That brand is really the only brand available as dished.
http://www.summitracing.com/parts/UEM-KB147KTM-030/
