Late Model Vette Pistons?

I just agreed to buy a fairly fresh 355 from a buddy of mine. Its the later model SBC with one piece Rear Main, Center Bolt Heads, etc. It has the same Roller cam thats in the ZZ4 but has 64cc chambered heads and late model Vette Lightweight Flat-top Pistons. Compression is around 10:1.

The guy built it for his dirt track car and is estimating horsepower to be in the neighborhood of 325. It doesn't sound anywhere near as good as my 292 cammed 355, but it will have a hell of a lot more low end torque. In that heavy assed '65 F100, I need the torque.

The question I have is about the Lightweight Vette pistons. He says they don't have skirts like a regular piston, and said to make damn sure the engine is at operating temp before I start wailing on it. Something about the way the piston fits the bore when its cold. I can see what he is saying, but if these pistons were designed for a factory application that really doesn't make much sense to me.

Anybody else heard this about Lightweight pistons? Its only going to cost me $950, plus the cost of a new style intake so I'm not too worried about the pistons. I can live with a little rattle when its cold.

 

He's probably talking about hypereutectic pistons. My 300 has em, and so did my Silverado. GM is in a world of shit with them right now, in that 4.8/5.3/6.0/8.4 lineup. Now, there is one hell of a cold start knock mess going on. GM says that the knock is normal and isn't detrimental, but alot of guys with the newer trucks are blowing up engines and GM is saying "oh well, not our fault." The ones in the Chrysler are KB's and have given me no problems. You shouldn't really worry, if your bud built the engine and knew what he was doing.

I wouldn' t worry, but you may get a slight knock when you start it, but after a few seconds, it'll go away. That is from the fit to the bore. It's if it continues to knock after it is warm that the worrying should start!

Jay

 

He did mention the knocking, but said it had something to do with the lack of skirts on these particular pistons. He said he hasn't heard it doing it yet, but he only ran it in about 3 races before he decided it was underpowered so its not even fully broken in. He knows what he's doing as far as SBC's go, I just had not heard that about the Skirts before.

I was told the same thing about the Hyperutectic pistons when I asked about the cold knocking in my last 4.3 powered Blazer so maybe it is the Material instead of the skirts.

 

I had 2 dirt track 350s in which those pistons were used and did not have any problems with them. My machinist (Wayne Calvert) recomended them for a class that specified flat tops in the rules. Ran those engines on 1/2 oval mile up to 7000 RPM. I never heard any knocking and on one of the engines, I installed them backwards to get a little extra kick. I liked them, sadly I lost one set when an exhaust valve lost its head at high RPM and pretty much everthing south of the intake was trash on that deal. The other set I'm still running in my mulletmobile. The main disadvantage that I know of is that rings are $90 bucks a set for those puppies.

 

I have a couple of the LS series engines, LS1 and LS6s. The newer design hypereutetic piston has much smaller ring lands which are located closer to the tops of the pistons. The sidewall tension on these rings is lower to reduce friction. Couple that with the thinner rings and you get more effecient operation, less friction equals more efficiency. One of the changes GM incorporated in the later model LS engines was to coat the skirts with a dry lubricant to minimize friction even more...not sure that it reduces noise though. Here is a pic of a set of LS1 pistons showing the skirt coating...these are stock GM units.

 

I wonder if the 0W20 oil spec on many new engines has anything to do with it? My LT1 is supposed to use 5W30 but I run 20W50 and it sounds a bit quieter.

Just got thru with a hypereutetic piston Y Block build and after the break-in that will also get 20W50 based on the vendors advice.

What I have heard from several sources is that the oil specified by car makers is to meet the EPA mpg mandates, and not because it is the best stuff to use.

 

So whats the deal with the new-style intake? I had heard that older intakes can be made to fit, but he said it wasn't worth messing with. Wouldn't tell me why, just told me to do it right and buy a new intake.

I haven't had any experience at all with the newer style SBC's, but he likes them enough that he doesn't even bother building the older blocks anymore. I guess i'll find out.

 

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What I have heard from several sources is that the oil specified by car makers is to meet the EPA mpg mandates, and not because it is the best stuff to use.

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That is exactly what I have heard...lower viscosity oil means less resistance and better fuel efficiency. GM has an updated second ring and oil ring set to address the oil consumption issue that results from low tension rings and low viscosity oils. This was one of the main complaints in the Northstar engines and it carried over into the Gen III motors as well. Looks like GM may have gotten it right now...

 

That was confirmed when I was a college student by our Mobil rep... that's why they RECOMMEND the lighter weights, but in the owners manual (who reads those anyways ) they tell all the different oils that can ALSO be used. Funny, if you look close, the 5w30 and 10w30 charts overlap. Wonder why?

Jay