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Old 03-22-2012, 03:21 PM
BogiesAnnex1 BogiesAnnex1 is offline
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Originally Posted by brad33478
had a 383 stoker built for me a few weeks ago, iron vortec heads, 10:1 speed pro pistons and eagle crank etc. Well swapped it into my truck and put about 90 miles of driving here and there and started blowing oil out of the breathers severely. Checked compression and had two cylinders at normal around 175 and the rest were pretty much junk, around 30-80. Well sent the motor back under warranty and heres some pics of the damage. He first accused me of using NOS which I never have and probably never will. He believed me and is covering all the parts/labor but my question is what could cause this much damage?
engine never overheated
had consistent oil pressure
my spark plugs read normal
headers never got excessively hot
and I have a brand new holley 750 on top with 7psi supply giving it plenty of fuel the only thing I did before it started blowing oil out the breathers was mess with the timing with a different timing light, cause ignition timing cause this much damage in such a short time?

The could be's from the pictures are:

- Detonation/preignition; but the spark plugs usually either have metal deposits on the insulator or are beat up like the pistons. Detonation tends to pound the piston apart where preignition tends to melt it. Like the piston detonation busts chunks off the insualtor, preignition tends to put melted metal blobs on the insualtor and rounds the edges of the electrodes.

- Running hot. The scorched oil and heavy carbon deposits indicate this. The combustion temps or piston temps can get out of line without the cooling system showing anything so gauge temp is not a clear indicator unless the engine runs low or out of coolant. This can also be a function of detonation or preignition.

- Inadequate bore clearance; this can be a cause of what you're seeing or a result. However, most damage seems to be in the ring lands and up to the crown rather than the thrust faces which leads be back to cooking the combustion chamber side of the piston or see below, inadequate ring gap or ring to groove clearance.

- Inadequate ring gap, I notice what appears to be a crack between the first and second ring land of one of the pistons in the first picture which leads me to this thought. Of course massive overheating of the crown could run the rings out of gap. when out of gap for whatever reason then they buckle and pop the piston ring lands apart. A similar situation can be associated with insufficient top to bottom clearance with the ring to ring groove.

Over-all for 90 miles these look more like what you'd see at a hard run 90,000 miles.

A question arises in my mind as to how dry you keep the sump and crankcase. The piston does depend upon oil throw-off from the crankshaft for lubrication and cooling. When you use a windage tray with a crank scraper you can contribute to pulling too much oil out of the windage, it gets to be a tricky line to walk in some cases. Keep in mind that many competition and industrial engines go to some lengths to put oil on the underside of the crown for cooling purposes. The Chevy, as well as many others, used to have a groove through the rod split to pass pressure oil off the journal/bearing to spray onto the cylinder wall and underside of the piston for lubrication and cooling. Since the advent of emissions restrictions this practice has been discontinued to reduce the oil pull over into the combustion chambers to reduce the octane contamination of unleaded fuels and reduce contamination of catalytic converters. The hypereutectic piston is also a response to this requirement, it is an attempt to get a more temperature stable piston that can be run at higher temperatures with tighter clearances and is cheaper than going to a forging for such a solution.

I rather think that 10 to one is a bit high for a cast iron Vortec head. This would especially be true with a fairly mild cam which will jack the DCR up and will be more susceptible to the problems you're seeing when in a heavy vehicle that has high gearing. This brings the engine into the lugging RPM range and dumps a lot of weight on it at the same time. The driver's response is to open more throttle which raises the temps in the combustion chamber which makes for detonation, preignition and other localized temperature related issues. You're running a carb, this has fueling problems that EFI doesn't have (there is no way around this), you need to consider that when using high compression EFI heads on a carb'ed engine. You just can't push these combo's as close to the edge, especially when an EFI engine is listening for detonation/preignition and will take steps to suppress it when it's present; it does not tell the driver it did that so the typical driver of a L31 pickup would be unaware that there was a problem as it's taken care of for him, however, not in your case with a carb unless you add such a circuit to the ignition which is done a lot with these things to head off these kinds of operating issues. MSD and others sell black boxes that do this for carbed engines running on the edge.

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