balance or dont balance????

[charger318]

If I where to replace stock 318 pistons with ones that are suppose to be 50 grams lighter would I HAVE to balance the assembly or is that not much change? Over heard going alittle lighter on pistons is OK because u will be over balanced? Do u think if I installed a fluid damper it could take up the difference in piston weight?

 

[Cranky]

Imo, 50 grams is a fairly large difference. Is the motor just for a cruiser or will it see high rpm?

 

[zyzzyx]

Well, . . .

The pistons are not "Reciprocating mass", which means that their weight does not get counterbalanced by the counterweights on the crank, so their lack of 50 grams of weight will not affect

the vibration too much as long as you don't take the engine to 7,000RPM! The "Balancing" of the reciprocating mass means the lower half of the con-rod, and the rod bearing. The piston

just goes up-and-down. I don't think this will be a problem. Luckily you're going lighter, not heavier. Good luck!

 

[Cranky]

Well, . . .

The pistons are not "Reciprocating mass", which means that their weight does not get counterbalanced by the counterweights on the crank, so their lack of 50 grams of weight will not affect

the vibration too much as long as you don't take the engine to 7,000RPM! The "Balancing" of the reciprocating mass means the lower half of the con-rod, and the rod bearing. The piston

just goes up-and-down. I don't think this will be a problem. Luckily you're going lighter, not heavier. Good luck!

I'm thinking you're talking about rotational mass. Recip is back and forth or up and down motion.....

 

[zyzzyx]

Sorry, I sit corrected! R o t a t i o n a l . Got it! Thanks!

I must have compressors on the brain:edgy:

 

[Cranky]

Sorry, I sit corrected! R o t a t i o n a l . Got it! Thanks!

I must have compressors on the brain:edgy:

Worked on a lot of recip compressors during my 'stay' at Exxon lol

 

[charger318]

I only plan on going 6,000-6,500 rpm max, its a street car that will get played with here and there. I was thinking because it is the pistons only which are trapped in the cylinder bores loosing 50 grams would be ok but I don't know that's why im here ;). so do you guys think a fluid damper would be good idea or not need? ive never used fluid damper before.

 

[nm9stheham]

Don't get hung up on 7k rpm being the limit and thank you will be 100% OK under 7000 rpm....6500 limit and little overrev and you will be there.

And this is a 318 where the engine is internally balanced; going lighter on the piston/rod moves you further away from being able to external balance; external balance is done when the piston/rod is heavier than the available crank counterweight.

Sooo you would balance... sorry. Get an online preview of "Hot Rod Small Block Mopar Engines HP1405" and read page 30.

By the way, there will be a (probably very) minor side benefit in some cases... with the lighter piston/rod, the crank counter weights will be lighter and the motor can rev faster. But with an auto trans and its large rotational mass, or a steel flywheel, this advanatage often goes away. Probably not any useful advantage in a street engine.

 

[Garys1969RR]

The only time I would not re balance an engine is if I was going to the same weight piston on the re build. 50 grams seems a bit much, but I am not a professional builder. Maybe some one with more experience can comment on this.

- - - Updated - - -

I built a 451, and the pistons went from 770 grams (stock) to 550 grams (Ross racing). We cut down the crankshaft counter weights, but had to add some mallory metal to get it to balance. But with the much lighter piston, that thing revs quick! Runs real strong.

 

[67 B-body]

I would balance......
The advertised weight matched piston sets have yet to be "Matched" on any set I've ever purchased... I always find them further out from one another than advertised, and not even the weight as advertised....
Take the lightest one,,, get it down to where you want it to be,,, then make the others match.
Set the Bob weights to match rod/piston combo, and spin up the crank....
Read this article, then decide...
http://www.neponsetvalleymachine.com/crankshaft_balancing.html

 

[Bigal75]

If you've got it apart why not just balance the assembly?

 

[snakeoil24]

I would balance......
The advertised weight matched piston sets have yet to be "Matched" on any set I've ever purchased... I always find them further out from one another than advertised, and not even the weight as advertised....
Take the lightest one,,, get it down to where you want it to be,,, then make the others match.
Set the Bob weights to match rod/piston combo, and spin up the crank....
Read this article, then decide...
http://www.neponsetvalleymachine.com/crankshaft_balancing.html

67 B Body that is a great article! Wow!

 

[cryplydog]

318, if its disassembled balance the damn thing. you wont regret spending the coin. good luck

 

[charger318]

If I had it all apart of course I would but the engine is the car was putting a cam, timing gear set, 273 closed (heart shape) heads that are ported to big 340/360 size runners and all, aluminum intake, Holley 750 vac Carb. BUT!!!!!!!! After seeing the pistons being .105 in the hole that will end up being 8.5:1 CR!?!?!?!?!?. So I was going to pull the rods and pistons install pistons with 1.810 CR height so they will be .012 in hole to get CR to 10:1 the solid lifter cam wants 9:1 minimum

 

[nm9stheham]

With this much piston weight change, there will a lot of side forces on the crank trying to bend it. Vibration is one thing, but keep in mind that these side forces are trying to bend/crack/break the crankshaft.... that is the risk you are running. Personally, I think this no-balance plan is heading for trouble with the CR and RPM's being contemplated.

Since the pan will be off, if you pulled back the trannie, (and maybe just the torque converter) and supported it, you could perhaps drop out the crank and take it and the new pistons and rods for balancing.

 

[rumblefish360]

If I where to replace stock 318 pistons with ones that are suppose to be 50 grams lighter would I HAVE to balance the assembly or is that not much change? Over heard going alittle lighter on pistons is OK because u will be over balanced? Do u think if I installed a fluid damper it could take up the difference in piston weight?

OMG, spend the money and make the motor smoother and give it a better chance at a longer life span. Money always well spent.

 

[Cranky]

Does this engine have the factory stock balance? That in itself wasn't that great especially with a passenger car engine that wouldn't see rpm much over 3500 very often. Pistons that are 50 grams less than stock isn't that much but add the factory balance into the mix and what do you have? If it were mine, I'd would go ahead and do it.

 

[charger318]

Hahaha.....that's what I was banking on that the bobweight is maybe alittle to heavy to begin with
What really gets me is how a piston with a taller compression height and a .140 dome could be 50 grams lighter then a factory short piston?!?!? Kb399.

 

[Cranky]

Today's materials are better than what was being used in the 60's/70's. Better materials can produce lighter products....

 

[rumblefish360]

You guys are saying 50 grams isn't much?!?!

LMAO!