Serie 2 piston oversizing ...benefit?? - GM Forum - Buick, Cadillac, Chev, Olds, GMC & Pontiac chat


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Old 03-04-2006, 03:29 PM   #1
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Default Serie 2 piston oversizing ...benefit??

Any benefit or disadvantage to oversize the piston in a Serie II L67 ? like 3.8 to 3.9
I was wondering that, and never seen one on the forum.
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Old 03-04-2006, 03:33 PM   #2
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u mean Bore the Block????

i dont quite follow
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Old 03-04-2006, 05:11 PM   #3
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Yeah I think he means going with an overbore or punching out the block to achieve a 3.9 L displacement... Going from a 3.8 to 3.9 doesn't seem like its worth the $$$$ as a good PCM, ported heads, headers and rockers will net some decent gains... Just my thoughts..

In the long run I'd be more worried about the trans than the engine

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Old 03-04-2006, 05:54 PM   #4
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ok, yes i meant overbore, (forgot the word in english)

because i will get a engine rebuild and i can get a 30 mm overbore for 400$ with piston , ring... i have to meet the guy to get more info, im just looking at my options now.

Thx
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Old 03-04-2006, 06:00 PM   #5
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30mm overbore! Thats over an inch....

Maybe you mean .030". That will only bump up your displacement a few C.I. and isn't enough to achieve 3.9L. If you plan on running boost there isn't enough cylinder wall to bore it out that much. Not to mention the gains don't come close to being worth the $. There are plenty of GP owners who have done .030" overbores on their engines, but thats usually to fix them when they lose a piston. The only thing that has been done other than that is a stroker kit. (in the US market anyways)

Personally I'd spend the money on something else, because the gains will be less (probably significantly less) than 10hp and 10ft.lbs.

Shawn
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Old 03-04-2006, 06:42 PM   #6
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ok, , and yes 0.30 is more realistic i type the wrong number...


I think i will spend money elsewhere
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Old 03-04-2006, 06:59 PM   #7
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The 3800 has been around for so long now, I bet if it hasnt been done yet, it wont really yield many benefits.
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Old 03-04-2006, 07:26 PM   #8
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If you look at the cylinder walls of a 3800 and say a Chevrolet 305( 5.0 ) Small Block V8 you will see that the 305 has a lot of meet between the cylinders to bore it out... The 350 is simmlar... But by the time you get to a 400 Small block the cylinder walls are very thin, and have a couple of small passages drilled into both the block and heads to help aid in keeping the cylinder walls cool as they get very hot... If those small passages were not there you could overheat and actually crack the block between the cylinder walls... So you really do not want the cylinder walls to be too thin...

If I was to build a chevy V8 for power and driveability and it was to be a small block my choice would be a 350 V8.. With it you would not have some of the concerns that you would have with a 400 Small block..

If I was to rebuild a 3.8 and the wear was minimal or next to nothing I wouldn't bore it unless it was required or close to be ing required... And even then I may only go .010 over depending on the wear of the cylinder bore.. Have I made any sense at all?

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Old 03-04-2006, 11:42 PM   #9
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There are high performance Hyperteutic pistons available in various over sizes: 0.030" (0.50mm), 0.030" (0.75mm) and 0.040" (1.00mm). For any high mileage block, I always recommend that you go with the next overbore size. If you have a high mileage block, even if the block is honed, the engine will not have a good seal. on the rings and the increased piston-to-wall clearance can induce piston wall scuffing. Cylinder bores will loose it'* roundness due to wear and honing alone will not compensate.
For our engines, specially for the SC engines, if you are going to overhaul, make sure to find a good machine, and if possible, with expereince on high performance/race engines.
Also, when selecting piston rings, stay away from the Total Seal Gapless rings; a Moly faced top ring, with a cast iron second ring will do an excellent job. The Gapless ring will flutter at high rpm operation. Also, they create additional problems since they build more pressure in the crankcase, will require a restrictor on the PCV..
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Old 03-05-2006, 09:46 AM   #10
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If you bore a 350 .030 over, you wind up with 355c.i. so with the 3.8L, you might pick up 3 or 4 cubic inches or in metric terms, 5 or 6cc.
The primary reason for boring is as mentioned, to maintain sealing and also to prevent piston slap. Blocks will typically wear more at the top than towards the bottom, leaving a tapered cylinder. In a new engine, the piston skirt to bore clearance is typically in the .0015" range. Numbers exceeding that such as in an engine with a tapered bore will result in an engine that is considerably noisier, especially when cold.
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