Balance question

[rfoll]

I have a balanced 400 sbc with a flat top piston. We want to drop the compression by using a dished piston, it will net 10.3: cr, well within the range of pump gas. The dished piston is 14 grams lighter. If I understand it correctly, it would give us an overbalance. Will the difference matter? Can we remove weight from the rod balance pad to make up the difference, and if so how much? The crank is factory, the rods are some aftermarket heavy things that I wouldn't mind removing some weight from. The engine will be used for something akin to bracket racing, no street time ever.

[Dave Koehler]

14 grams off the small end of most rods is going to be tough.

[Baprace]

rfoll, if your engine was balanced and ran smooth before, it will be OK with the 14gr lighter piston, when the bobweight is calculated on that engine type I only use 50% of the reciprocating weight anyway. that figures to be 7 grams in your case. Normally if the balance job is done correctly, over or under is not a big issue as long as both ends of the crank have the same change in weight, just change one end only and it will shake you off the seat.

[groberts101]

I have a balanced 400 sbc with a flat top piston. We want to drop the compression by using a dished piston, it will net 10.3: cr, well within the range of pump gas. The dished piston is 14 grams lighter. If I understand it correctly, it would give us an overbalance. Will the difference matter? Can we remove weight from the rod balance pad to make up the difference, and if so how much? The crank is factory, the rods are some aftermarket heavy things that I wouldn't mind removing some weight from. The engine will be used for something akin to bracket racing, no street time ever.

Recipricating weight is calculated at 50% whereas rotating weight is at 100% when figuring bobweights. So you'd only need to remove 7 grams from the big end to achieve proper equalibrium for current crankshaft balance.

Tbh, a dedicated strip engine will never care about that measily 7 gram(your 14 grams @ 50%) overbalance and most will even prefer considerably more than that. 51-52% is very common. The bigger benefit is getting everything matched to one another at higher resolutions which improves bearing life.

[rfoll]

14 grams off the small end of most rods is going to be tough.

I would think the weight would need to be removed from the big end, but my understanding of balancing is slim.

[Dave Koehler]

14 grams off the small end of most rods is going to be tough.

I would think the weight would need to be removed from the big end, but my understanding of balancing is slim.

No, no no. You are working on the reciprocating end, not the rotating big end.

[rfoll]

If the piston is lighter, why would you want to remove weight from the small end of the rod? It would seem that a heavier pin would be in order. I want to say at this point my knowledge on the subject is not slim, but clueless.

[Dave Koehler]

Sorry, I did not mean that. I ASSUMED you knew you were working on the small end and brain faded on what you really meant.
It's a you mean what I know moment.

[rfoll]

Splain please Lucy. I'm now confused. If the piston is lighter where would you make the weight change to restore balance?

[groberts101]

If the piston is lighter, why would you want to remove weight from the small end of the rod? It would seem that a heavier pin would be in order. I want to say at this point my knowledge on the subject is not slim, but clueless.

Taking into account the 50% recip reduction of 7 grams no matter what you do to the rods the cranks counterweights will be overbalanced. And I mispoke earlier.. taking more weight off the big end will only add to the overbalance. To achieve true balance to the new parts you'd need to ADD 7 grams to the big end or would require drilling the counterweights. Don't sweat it and run it.

[rfoll]

OK, basically that's what I needed to know. I had a fixation on the rods when the place needing work was crank counterweight. Thanks to all that responded, Rick.

[Rick360]

Don't worry about re-spinning the crank as long as the new pistons are balanced to each other then it will be fine. Many people balance with 0% overbalance and many balance with several percent overbalance as their target. In my experience it makes no difference.

I've seen engines run with over 100g overbalance and/or underbalance with NO noticeable difference in the engine.

Rick

[user-30257]

I have a balanced 400 sbc with a flat top piston. We want to drop the compression by using a dished piston, it will net 10.3: cr, well within the range of pump gas. The dished piston is 14 grams lighter. If I understand it correctly, it would give us an overbalance. Will the difference matter? Can we remove weight from the rod balance pad to make up the difference, and if so how much? The crank is factory, the rods are some aftermarket heavy things that I wouldn't mind removing some weight from. The engine will be used for something akin to bracket racing, no street time ever.

Recipricating weight is calculated at 50% whereas rotating weight is at 100% when figuring bobweights. So you'd only need to remove 7 grams from the big end to achieve proper equalibrium for current crankshaft balance.

Tbh, a dedicated strip engine will never care about that measily 7 gram(your 14 grams @ 50%) overbalance and most will even prefer considerably more than that. 51-52% is very common. The bigger benefit is getting everything matched to one another at higher resolutions which improves bearing life.

Do you balance your own engines? Asking for a friend.

[Mark O'Neal]

If I might take a moment to cut through all of the "Bill Nye the Science Guy" stuff....


You are fine, Run it.

[user-30257]

If I might take a moment to cut through all of the "Bill Nye the Science Guy" stuff....


You are fine, Run it.

What he said.. lol