[fasteddy]

I have a fiend who is having 2, 3 and 4 main bearing problems. This is a Mopar 500 inch motor but I would like to know if under balancing the rotating mass is something worth doing for a 7000 to 7500 RPM motor? I heard about this from a very reliable source. He said that nascar does it to make the motors live at 9000 to 10000 rpm. Any Thoughts?

[fasteddy]

I have a fiend who is having 2, 3 and 4 main bearing problems. This is a Mopar 500 inch motor but I would like to know if under balancing the rotating mass is something worth doing for a 7000 to 7500 RPM motor? I heard about this from a very reliable source. He said that nascar does it to make the motors live at 9000 to 10000 rpm. Any Thoughts?

[Jim Robertson]

The Sunnen operation manual discusses this briefly and uses the figure of 5000RPM as the magic number. If engine is intended to operate continuosly over 5000 then under balancing becomes an option. Smokey and Grumpy have both discussed the under balance idea in books and interviews but know that vibration will affect other important things like upsetting carb delivery of the correct air/fuel mix. So it is a double edged sword.

[Half-Inch Stud]

Is this the same process as:

Balance for dry.
Balanced with effect of oil.

? HIS

[Rob]

Investigate the oiling system first. You're not going to be smoking bearings because you've balanced a motor to zero.

-Rob

[fasteddy]

Rob, do you think that a faulty fluid damper could cause harmonics and destroy the mains? The front and back bearings look fine.

[Rob]

If all your bolts were coming loose on the motor you could have something shaking the hell out of it. I've seen the wrong flywheel shake stuff around, but it was caught early enough there was no bearing damage.

I would run the oil pump with the pan off, see where there's too much oil and not enough. It can get messy trying to catch it all but good thing to look over with any engine.

-Rob

[74t/a]

The center 3 mains are the ones that take the most abuse. They each have 2 rods firing on them versus the front and rear only have 1.

What is happening to the bearings?

That is the first thing you need to check. How much clearance do you have and how are you coming up with the measurement?

Also, you would not underbalance a high rpm engine, you would overbalance the assy. Overbalancing is making the bob weight slightly heavier than it is supposed to be. This in turn make the crank weights heavier to compensate for the moment of inertia that occurs in a high speed engine.

7500 rpms is not enough to have to do this. A BB mopar is a great high speed engine with the 2.75" main brg size. That engine needs at least .003 main brg clearance to live at high speeds......I have built many of them that turn that high.

[fasteddy]

Thanks for the replies guys. I will get the answers and questions to my friend.

[Half-Inch Stud]

21 84 73 65 7

21 have consecutive hits to wind the crank up.

New thread is warranted to ask: Experiences with a Fluiddamper on a cast Pontiac 455 crank in the 1/4mile.

[Mr. P-Body]

74/TA is correct. Over-balance is called for in a high RPM application, not under-balance.

Yes, FluiDamper can cause BIG problems. We recommend them ONLY for race engines. They (FluiDamper) must be removed and stored "flat" when the engine will sit for extended periods. The silicone will migrate to the bottom, causing an out-of-balance condition. FluiDamper's "advantage" coes from high revs. Most Pontiacs never see the RPM range we're talking about (above 7,000). There are plenty of elastometer types available, which are most suited to street use and the lower engine speeds the Pontiac is used for.

[Half-Inch Stud]

Mr P-body:

YESH, in consideration that the 455 crank is cast, stiff, and plenty, and it's "all-over' after 5200RPM or so for MOST 455 engines.

In those considrations, I feel the factory technique or Rattler technique are superior in the low-RPM Highest torque&twist band. HIS

[fasteddy]

Here is a short blurb from a guy at SCAT:

[Half-Inch Stud]

well said.

Sooooo, the counterweight shape help to round-out the true balance vs RPM.

[74t/a]

Wow, I am never afraid to learn something new!

I am a bit skeptical however, as this is something that does not sound right to me. I will do some research on this. I have some friends that do some cup stuff.

Bryan

[Mr. P-Body]

We've heard of this, with lighter rotating masses spinning at high revs. Cup cars' engines are designed and used for completely different purposes than drag engines and street engines. True enough, we get new info and technology from them, so don't take this as a "diss".
IMO, a more apples-to-apples comparison is in order. Maybe Pro-Stock type engines. Big cubes, big crank, lower revs... Kinda like a Pontiac. I would be interested in Grumpy's thoughts on this one. Anyone know him enough to ask? His stuff is running pretty good these days...

[Rob]

So say a bob weight of 1850 grams thats 35 to 95 grams. Would be neat to balance a crank light or heavy then put the actual bob weight on and then spin it again see what it does on the balancer. Lot of lead shot setting but would be interesting to see how much it shakes the machine, if at all...

-Rob

[Tiger Paw]

I used to have that same problem, beating up main bearings and having to replace them at 12-15 runs. #3 was the worst followed by #2 & #4. Switched to a 4340 crank with center counterweights. Now the main bearing last years, 3 so far. Callies sells cranks with the center counterweights.

[Half-Inch Stud]

Jenkin's Automotive, Malvern PA...don't be shy.