Balancing question for engine builders in here?

Posted By: a408swinger

Balancing question for engine builders in here? - 03/06/09 12:15 AM

I freshened up my motor this winter and bought 8 new pistons. I also went from a set of hand ported 360-1's to a set of 245cc CNC 360-1's which have a 2.15 intake valve instead of a 2.10 so the only changes I had them make in the pistons was to make them for a 4.045 bore instead of a 4.040 bore and to cut the valve relief for the bigger 2.15 intake valve. The pistons are Ross and they also made my last set. I also had them make the pistons the same weight as my old ones so I didn't have to re-balance my rotating assembly. After getting the new pistons and assembling my motor I had piston to valve clearance problems. After calling Ross they realized they cut the valve reliefs too small and I had to pull the motor back apart and send them the pistons back so they could fly cut the reliefs the right size. Now the pistons are 9 grams lighter than my old ones. They are telling me its no problem and I dont need to re-balance my motor. Is this true? Is 9-10 grams not too light? They are telling me its not a problem for the pistons to be lighter. I turn this motor over 7500 rpm's. What should I do?

Posted By: 40ford

Re: Balancing question for engine builders in here? - 03/06/09 12:58 AM

They are correct! All you have is a very slight "overbalance" due to the lighter pistons. You didn't say what your bobweight is---probably in the 1600-1700 gram range. This 9 grams would be only 1/2% overbalance. Many high RPM engines use 1,2 or even 2 1/2% overbalance. Some shops overbalance 1% for everything but street engines.

What you don't want is an "underbalance" situation.

Posted By: Sport440

Re: Balancing question for engine builders in here? - 03/06/09 01:18 AM

Quote:

They are correct! All you have is a very slight "overbalance" due to the lighter pistons. You didn't say what your bobweight is---probably in the 1600-1700 gram range. This 9 grams would be only 1/2% overbalance. Many high RPM engines use 1,2 or even 2 1/2% overbalance. Some shops overbalance 1% for everything but street engines.

What you don't want is an "underbalance" situation.

I dont remember the exact numbers but engine builder/tester David Vizard did some testing with overbalancing and found out there was free HP to be had. I think he found like 15 free HP by adding a slight overbalance on a 350 chevy. So like stated above

mike

Posted By: dOc !

Re: Balancing question for engine builders in here? - 03/06/09 09:15 AM

Has anyone actually checked weights of the pistons to see how close the shop has actually balanced them to "0" ? ....or whatever the shop claimes ?

A Bud of mine checked a well-known shop in the Detroit area ..... they claimed they balanced to 2 tenths. He weighed the pistons and some were off by almost 5 grams.

But some people claim that with all the oil flying around and on the rotating-assembly ....up to 10 grams off really does not matter.

Posted By: MR_P_BODY

Re: Balancing question for engine builders in here? - 03/06/09 10:13 AM

Quote:

Has anyone actually checked weights of the pistons to see how close the shop has actually balanced them to "0" ? ....or whatever the shop claimes ?

A Bud of mine checked a well-known shop in the Detroit area ..... they claimed they balanced to 2 tenths. He weighed the pistons and some were off by almost 5 grams.

But some people claim that with all the oil flying around and on the rotating-assembly ....up to 10 grams off really does not matter.

Chrysler was the worst of the big 3 when it came to
balance on the factory stock engines. Sure they will
stay together but I'm a big believer in a good balance
specially if you take it up in the revs. For you
to feel it in the seat of your pants... its got to
be BAD

Posted By: Sport440

Re: Balancing question for engine builders in here? - 08/16/09 04:43 AM

Quote:

Quote:

They are correct! All you have is a very slight "overbalance" due to the lighter pistons. You didn't say what your bobweight is---probably in the 1600-1700 gram range. This 9 grams would be only 1/2% overbalance. Many high RPM engines use 1,2 or even 2 1/2% overbalance. Some shops overbalance 1% for everything but street engines.

What you don't want is an "underbalance" situation.

I dont remember the exact numbers but engine builder/tester David Vizard did some testing with overbalancing and found out there was free HP to be had. I think he found like 15 free HP by adding a slight overbalance on a 350 chevy. So like stated above mike

Found it for you RP,

mike

Posted By: DemonDust

Re: Balancing question for engine builders in here? - 08/16/09 02:27 PM

Thanks Mike

My engine is only 2% overbalance. 1528 g assembly and 1567 bobweight on my crank.

Posted By: 440Jim

Re: Balancing question for engine builders in here? - 08/16/09 02:52 PM

Don't stick those pistons in your engine, tell Ross to stick them...
You ordered a certain weight and valve relief size, don't pay for anything less.

If it is "OK" or not is not the right question. You didn't get what you ordered. Plain and simple. If you are thinking of accepting the pistons, ask for the cost of rebalancing the crank, plus shipping and handling... LOL

Posted By: 440Jim

Re: Balancing question for engine builders in here? - 08/16/09 02:56 PM

Quote:

Has anyone actually checked weights of the pistons to see how close the shop has actually balanced them to "0" ? ....or whatever the shop claimes ?

I check them before and after they balance them. My scale isn't as good as a good balance shop, but my measurements showed they all matched within the amount I can "read" on my scale (not claiming 1g accuracy).

Posted By: Sport440

Re: Balancing question for engine builders in here? - 08/17/09 12:15 AM

Jim , what does the shop that you use, "as a rule of thumb" balance thier cranks to or otherwise thier accepted standard perimeters in reguards to 50%/even or 50%+ overbalance. 51% being equal to a 1% over balance. mike