Re: Rotational Weight Question
 

You buy the lightweight stuff when you are building to begin with, or you need new parts. Then it is worth the money. If you're replacing perfectly good reasonably light stuff that is working now, with high dollar trick lightweight stuff, most often it is a waste of money better spent elsewhere.

The lower the power of the engine, the more it has to gain from light parts, as it has no "surplus" power.

Too many people go spend meg dollars on trick stuff long before they've mastered the basics, and maximized the rest of their combination. These are the people who have rarely if ever rented a track or spent days at test and tune sessions.

To the original question, the only way to know what it is worth is to test back to back on a given combination, with serious A-B-A testing. Most people who spend the time and money to do that are understandably quite reluctant to share that hard learned data with anyone, save a few people that share with them.

Some of the very fastest cars have none of the trick stuff in them, not even in the engine. One engine builder responsible for the legendary performance of a certain Stock Eliminator car once told me, while we were discussing dyno parts, that his "shop was filled with camshafts and headers the engine and car did not like", and "the secret to it being fast was not so much knowing what the car liked, but more a matter of learning everything it didn't like".

Re: Rotational Weight Question
 

to answer your question look for a book by Herb Adams on chassis--- he explains in great detail the difference between rotational and static weights and the ET/MPH/HP advantages in going to lighter weight items versus a heavier weight item ---performance wise he breaks it down into static weight-- rotational engine weight-- and rear axle rotational weight ---and the performance gains that can be achieved by using lighter weight components ---This should answer any questions you might have--- educational reading--FED 387

Re: Rotational Weight Question
 

They may not have the same effect. Also, is the rotational weight wheels or axles? Brakes or spool? Rotational weight isn't all the same. All the weight can be quantified if you know the moment of inertia.

Re: Rotational Weight Question
 

Very good replies and in my experience
there is more between the fenders than
You will ever find in the diff!! My.02$$

Re: Rotational Weight Question
 

Unfortunately, there's not a hard fast rule for the question you ask.

You need to look at the rotational weight AND the effective gear ratio to truly determine the gain you might see.

Translated, rotational weight removed between the tail shaft of the transmission and the tires is not very effective.

Rotational weight removed in the transmission is very effective. Ask the guys Pro Trans or Scott McClay if reducing transmission rotational weight is effective. You won't see the gain in mile per hour, but the lighter transmission will ET better.

Rotational weight of the engine is even better than the transmission. Ask the quick stick guys if they like a light or heavy flywheel.

Reciprocating weight of the engine is beyond my understanding to say how much better it is. I tried to figure it out once but it was beyond my ability to comprehend. But lighter is better.

Re: Rotational Weight Question
 

The laws of physics says any reduction in rotating weight has to be beneficial. But the variables associated with the comparison runs themselves can vary more than the benefits.