I'd want to be within 10 or 12 grams of the old pistons.

So I'm looking at putting a supercharger or turbo setup on my 406. My question is simple, is lowering the compression as easy as taking out the pistons and putting in a set of low compression pistons, forged this time into the motor? As long as they are balanced I wouldn't think it would be a problem as my motor has only been together a few thousand miles, not even. Then I heard that my forged pistons would have to weigh the same as the pistons that are in the motor or it would throw it off.... I thought it was balanced separately? :smash:

IMHO////If you are sure you have NO ring wear ridge of any kind at all, you could get a set of "D" cup pistons and not only reduce the CR but maintain whatever Quench you have. But I would still blade hone pretty well to be sure there is no wear shadow because of probable difference in top ring height.

As far as weight, I'd make sure the new ones all weigh the same and that they are close to what you have. The weights are separate/// There is a Rotating weight and a recip weight. It won't change the rotating part, but the recip will be affected.. When we balance pistons we often taper inside the ends of the pins if we have more than a gram or so to lighten.
I'd want to be within 10 or 12 grams of the old pistons. I'm sure many will say it takes more than that to throw off the harmonics of the crank. And they are probably right but I've never taken the chance to find out.

Keep in mind though, that when we start to balance a typical factory small block, it's not unusual for rod/piston combos to be off 10 grams from light to heavy So....wait for some other comments make your own decision.
Duntov

 

I feel low rpm engines don't "need" to be in perfect balanced.

as long as the pistons are from a set and the rpms are kept under 5500, then i wouldn't worry about a balance job.

 

If you can afford the supercharger than I would go ahead and spring for the balance job and do it right. my .02

 

If you are going to balance the rotating assembly, hone the cylinders, and purchase pistons you could probably get a nice set of heads for the same money.

I don't know what CC the chambers on your heads are, but I would leave the bottom end alone and go with a larger chamber head. If you have 64cc heads, and you aren't running that much boost, then a set of 72cc heads would (generally speaking) lower the Compression significantly. Also, the heads you have on the motor probably are worth some money on Ebay or Craigslist. Used pistons are generally good for paperweights at best.

Flat top pistons contribute to better combustion efficiency due to larger quench areas and better flame propagation. So you get the nice side effect of not having to reduce the compression ratio as much to make it run on pump gas.

 

Well the problem I have is the pistons that are in the motor are hypers. I don't know if I can get forged pistons close to their weight (within 10 grams) can I? Also to who posted about the low rpms... I stated in the original post I will be turning it to at least 6 grand as I turn it to 6300 now. I planned on since the motor is so new pulling the pistons out... running a small hone with a drill to clean up anything that MIGHT be there...but I highly doubt it w/the mileage, balance the new pistons put in some new rings and rock out.

Here are the specs on the motor

400 .030
5.7 rods
hydraulic retro fit voodoo 231/239 @ .050
565/550 lift
KB hyper pistons
sportsman 2 heads 72cc chambers.
Block filled to bottom of freeze plugs

 

Mainly "Have Fun"

I'll stick with my first post pretty much.. 10 or 12 grams would without fear at 6300 and you could go more but you don't have to because you can select pistons that close. You can adjust the new set of pistons to inside a gram yourself which is I imagine, closer than the balance guy had it. And you can buy a scale less than fifty bucks to do it yourself. A porting tool or drill will do it. And the first few times you do it it's very satisfying. I don't think you have a problem. You have a good combo now and are only going to improve on it. I'm a forged piston guy myself and understand how you feel about that. No you could be out more than 12 grams if they are all right on each other, just said it would be nice to be close. If you balance your own set of pistons, take time to weigh the ones coming off and see if they are perfect. Bet not. Makes it more fun to do what you can yourself. And only then do you gradually realize how "less than perfect" even good running engines are.
Mainly "Have Fun"

Duntov :drool:

 

I really like that idea of just using different heads. The way I see it, with a blower it will matter less if the heads don't flow that great so you could use some cheap used low compression smog heads from the junkyard and a thick head gasket and get the compression down pretty low I would think. I never had a blower tho, so maybe somebody might know better. I always wanted a supercharger my whole entire life and love thinking about this stuff. I know one thing for sure, don't run too much ignition advance!! An exhaust gas temp gauge would be nice along with a knock sensor rig up. If your running too retarded on the ignition advance to keep from pinging you will ruin the exhaust valves with too much heat, so egt gauge would be nice

 

Seeing what his heads are already I would have to say that the pistons are the way to go anyways. If you got a set of 76cc heads you would still need a really thick head gasket to lower the compression significantly. All that is going to do is move the spark plug away from the piston and create a dish anyways. Plus I forgot that he said he had non forged pistons and I would definately go with a good quality forged piston. Even though most KB pistons include piston side clearance specs and ring gap specs for forced induction applications I just feel better knowing that my pistons are better than what they need to be.

 

HUMOR " ARK ARK ARK" Mork of Ork 1976

with a blower it will matter less if the heads don't flow that great so you could use some cheap used low compression smog heads from the junkyard and a thick head gasket and get the compression down pretty low

TRANSLATION>> "Go get some really shi**y heads and then go to some effort to KILL your quench""
Little humor goes a long way on the forum. I almost missed that one!

 

I would have to say that the pistons are the way to go

Seeing what his heads are already I would have to say that the pistons are the way to go anyways. If you got a set of 76cc heads you would still need a really thick head gasket to lower the compression significantly. All that is going to do is move the spark plug away from the piston and create a dish anyways. Plus I forgot that he said he had non forged pistons and I would definitely go with a good quality forged piston. Even though most KB pistons include piston side clearance specs and ring gap specs for forced induction applications I just feel better knowing that my pistons are better than what they need to be.

ME2 ME2

 

Phewww sorry to bring this back from the dead. I've started collecting turbo parts here and there. Duntov, so you are saying get the new pistons to w/in 10-12 grams of each other? Meaning one of my new forged pistons could way 10 grams more than one of my old hypers, as long as the finished product is all 8 of the new forged pistons being w/in a gram of each other if possible? And I can leave the rest of the bottom end alone as far as balancing? Thanks for the help guys, I'm pretty excited to get this build going.. :thumbup:

 

It's doable..

so you are saying get the new pistons to w/in 10-12 grams of each other? Meaning one of my new forged pistons could way 10 grams more than one of my old hypers, as long as the finished product is all 8 of the new forged pistons being w/in a gram of each other if possible? And I can leave the rest of the bottom end alone as far as balancing? Thanks for the help guys, I'm pretty excited to get this build going.. :thumbup:

Well, ......... I would try to find a set of forged pistons to adjust my CR, which were no more than 10 or 15 grams heavier than your KB ceramics, and them lighten them as much as possible in the process of righting them to each other. You might be able to start out heavier than that if your wrist pins are on the heavy side, like 142 grams or so, because you can find lighter pins and you can also lighten those pins by tapering the inside of each end of the pin by a couple more grams. By the same token, you can get heavier pins if your new pistons are lighter...

You might also have the rods lightened a few grams while they are off. That's the quick and easy part of the balance, so that adjustment wouldn't cost much. Hope your rods are good enough for the power increase...

When it's all said and done, if you can be less than 10 grams heavier than you were, and all the rods/(pistons, including pin weights with the pistons) as precision as possible to each other, then your harmonic should be about as good as it was before. The actual balance might even be better.... given that I shoot for a tenth of a gram top to top, bottom to bottom, and overall rod weight. Then I shoot for a tenth gram on overall piston weight.. Then I want the assembled weights that close if possible.

Most will say that being that close is not necessary, but it makes you feel really good about your engine and it can take hours more than shooting to a half gram...... But you could be watching TV all that time, or playing a board game.... I lose lots of time doing that kind of balance but I enjoy it more than TV.. I've think I've seen all the good movies at least once.