rfoll wrote: ↑Fri Oct 19, 2018 8:16 pm
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.
Lol.. you have friends? I generally balance and weight match everything to 1/10th gram or less. Then have the cranks spun to around 1/8th oz resolution. prefer to have counterweights shaved in a lathe vs drilling holes. Also prefer to hand polish nearly every square inch and double knife edge leading and trailing edges on my own stuff.
What about you? Do your own building and balancing too or just prefer to mess with heads?
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.
Lol.. you have friends? I generally balance and weight match everything to 1/10th gram or less. Then have the cranks spun to around 1/8th oz resolution. prefer to have counterweights shaved in a lathe vs drilling holes. Also prefer to hand polish nearly every square inch and double knife edge leading and trailing edges on my own stuff.
What about you? Do your own building and balancing too or just prefer to mess with heads?
You are right, for the last 20 years I haven't drilled a hole. I'm a loser I send Chambon, MCT, Winberg my bobweight and forget it....
Headguy wrote: ↑Fri Oct 19, 2018 11:54 pm
You are right, for the last 20 years I haven't drilled a hole. I'm a loser I send Chambon, MCT, Winberg my bobweight and forget it....
Don't be so hard on yourself and count your blessings to be able to pass the workload to others on someone else's dime. Very few of us have that same luxury and have to do as much as possible and spend hours upon hours just to emulate the look of a microfinished winberg crank.
Seems like your way might be better anyways since it gives you more time to mess with heads.
Holy cow, alot of misinformation here. 14 grams lighter on the piston than what you had before. General rule. Up to a 10 gram difference in piston weight in a full race engine and most of the time no rebalance needed. If your crank is balanced at 50% you should be fine in a street strip or street engine. Why are we talking about removing weight from the big end balance pad of the rod? If you would rebalance you would remove from the counter weights of the crankshaft.
This started out about how to avoid having to spin the crank up...which is the correct thing to do...but it got convoluted in different directions from that point on.
We don't have all his weights but just taking 14 gms from the pistons on the last SB I did comes out to 0.275% over balance. Not a big deal to run it that way.
Last edited by Dave Koehler on Sat Oct 20, 2018 9:50 am, edited 1 time in total.
Dave Koehler - Koehler Injection Enderle Fuel Injection - Nitrous Charger - Balancing - Nitrous Master software http://www.koehlerinjection.com
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houser45 wrote: ↑Sat Oct 20, 2018 9:02 am
Holy cow, alot of misinformation here. 14 grams lighter on the piston than what you had before. General rule. Up to a 10 gram difference in piston weight in a full race engine and most of the time no rebalance needed. If your crank is balanced at 50% you should be fine in a street strip or street engine. Why are we talking about removing weight from the big end balance pad of the rod? If you would rebalance you would remove from the counter weights of the crankshaft.
Lol.. in my defense.. I hit 83 hour workweek as of 4pm yesterday. Sometimes helping is hurting but intentions were genuine.
