Balancing Act

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Balancing Act

Postby Weatherhound » Sun Mar 21, 2010 12:33 pm

I have been running my 540 BBC methanol injected engine so long that it has been honed for new sets of rings 3 times since 1998 from the original build. Since then the engine started to feel a resistance to being able to spin above 8000 rpm so I settled from dropping the shift points for a while. Finally I decided to go through the engine and low and behold I all the pistons were slap hitting the heads due to a huge piston to cylinder wall clearance. No damage, but the piston slap was effectively acting like a wagon wheel that wiggles when you spin it really fast. Turns out that I was essentially running a standard piston with almost a .015" over bore! I know, shame on me, but hey it was working well up until now.

Now I have bored the engine .060" to make a 4.56" bore. The new Wiseco pistons are 10 grams (~ .35 ounces) heavier than the 4.50" pistons. I am running a wet sump oil system so there is plenty of windage no matter what I do. Does anyone out there feel that I should take the time to re-balance the engine (already internally balanced with heavy metal slugs) for only 10 grams of overweight on the piston? Or just shave a little off the underside of the piston to drop the mass delta to 5 grams and call that good? Finally, will the engine ever notice 10 gram heavier pistons in a wet sump application?

It has a 4.25" stroke and I plan on shifting around 7800 RPM.

Any advise would be greatly appreciated.

Regards,
Brian
Brian "Weatherhound"
1969 Z/28 Camaro
540 Injected Enderle Bird
Lake Jackson, TX
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Re: Balancing Act

Postby Billy Mac » Mon Mar 22, 2010 10:36 am

If ALL the pistons weigh the exact same amount, (To each other) then "I" think you would still be in the ball park with the current balance job.....but if there is ANY variation in weights then you would need to re-balance.

7800 Shift point :shock: ...Why so high?? (then again I was shifting at 76 in my junk :roll: )

Is the cam falling on its face at 8000??

I know, some questions just lead to more questions...LOL
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Re: Balancing Act

Postby Weatherhound » Wed Mar 24, 2010 8:55 pm

Hey Bill, I did weigh all the pistons that were removed and the new ones I went ahead and installed. All the new ones were within 0.23 grams of each other. The old set had a little more spread to them by 0.67 gm. The difference between the old set and the new set per each piston was greatest at 10.6 grams and least and 9.9 grams. I found some articles that stated no more than grams and another said 30 grams depending on the level of performance.

I believe the camshaft that Reher & Morrison custom made for my application calls for 8100 RPM shift points. The Erson grind has .800"/.737" (I/E) and a healthy 050 duration of 292/299 and the LCA of 113 degrees. With that LCA it has a lot of top end. So far it has been the best cam profile I have tried with alcohol injection with a broad torque curve.

No the cam wasn't falling in it's face. When the engine was fresh it zoomed to 8100 and still seemed to want more, but I remained sane enough to stay away from higher revs. But after 3 hone jobs from 1998 to 2004, and still using the same pistons with new ring sets, the piston to cylinder wall clearance was so big that the pistons were wobbling at the top of the stroke, enough that the edges opposite of the pin were contacting the heads. All the pistons were making contact. This is what was making the engine seem to fall on its face around 7000 RPM.

I am expecting it to be much better sailing from here all the way to 8100 again. Plus I made some oiling system improvements by get a competition full sump full kick out oil pan fitted with all sorts of windage reducing devices. Compared to the street pan it had without a windage tray, I am expecting 20+ HP to be freed up above 7000 RPM, and most of the oil should not get whipped up in the crank circulation. Not to mention it's now a 552 CID engine, and a little more compression since the bore is a little larger in diameter.

It will be interesting none the less. At the height of the engine life, it made just at 1000 HP. Hoping to see 1075 max HP with all the extras.

Thanks for the reply Bill. Good luck to you this season. Regards, Brian.
Brian "Weatherhound"
1969 Z/28 Camaro
540 Injected Enderle Bird
Lake Jackson, TX
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Re: Balancing Act

Postby Billy Mac » Thu Mar 25, 2010 1:03 pm

Sounds like a pretty good kick in the pants is waiting for your first launch...LOL....

Without putting mine on a dyno, I really don't know where my hp is numbers wise...but "I" estimate somewhere between 850 and 950...(prolly closer to the middle of those figures)...still, not too shabby for an iron headed Ford with a stock block :wink:
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