What are the Pros & Cons of a Lightweight Reciprocating Assembly????
10-09-2006, 05:38 PM
#1
Racer
Thread Starter
What are the Pros & Cons of a Lightweight Reciprocating Assembly????
Always was curious, I know they say you "pick up RPM quicker" but did not know exactly what they meant & was too embarrassed to ask. Also, if you run a lighter Crank, do you have to pay special attention to the weight of the rods & pistons? Thanks.
10-09-2006, 05:58 PM
#2
No cons but the higher cost for parts. The lighter weight makes more
power that would normally be waisted trying to spin a heavy crank,
rod , piston assembly.
power that would normally be waisted trying to spin a heavy crank,
rod , piston assembly.
Last edited by Little Mouse; 10-09-2006 at 06:15 PM.
10-09-2006, 06:29 PM
#3
Safety Car
Originally Posted by alnukem
Always was curious, I know they say you "pick up RPM quicker" but did not know exactly what they meant & was too embarrassed to ask. Also, if you run a lighter Crank, do you have to pay special attention to the weight of the rods & pistons? Thanks.
10-09-2006, 06:33 PM
#4
Team Owner
If car "A" weighs 3000 lbs and has a 500 hp 500 FP of tq 396 ci small block. Car "B" weighs 3000 lbs and has a 500 hp 500 FP of tq 396 ci small block.
It's just not the weight of the crank it's the total rotating package. Starting on the front of the motor damper weight, crank, flywheel or flexplate and torq converter weight clear back to wheels and tires.
So lets use a light weight parts on car "B" 5 pound damper instead of big 12 pound. 44 pound crank instead of 58. 22 pound fly wheel like I have instead of 40. I have 37 pounds of less rotating mass.
I don't have all the formulas. But my car "B" with everything else being equal will accelerate like a car that weighs 2700 pounds or something like that. To speed up rotational weight is a big amount of energy.
The gains don't end there. less rotational weight is less bearing friction and ware. Braking is greatly increased. The motor can change speed faster on every up or down shift. That is why cars like RedVetteRCR uses a 5.5 inch or something multidisk clutch. It makes his road racing vette up and down shift better because the rpm can be match relatively instantaniously.
The weight of the pistons and rods is fixed. You alter the crank throw weight to balance any motor
It's just not the weight of the crank it's the total rotating package. Starting on the front of the motor damper weight, crank, flywheel or flexplate and torq converter weight clear back to wheels and tires.
So lets use a light weight parts on car "B" 5 pound damper instead of big 12 pound. 44 pound crank instead of 58. 22 pound fly wheel like I have instead of 40. I have 37 pounds of less rotating mass.
I don't have all the formulas. But my car "B" with everything else being equal will accelerate like a car that weighs 2700 pounds or something like that. To speed up rotational weight is a big amount of energy.
The gains don't end there. less rotational weight is less bearing friction and ware. Braking is greatly increased. The motor can change speed faster on every up or down shift. That is why cars like RedVetteRCR uses a 5.5 inch or something multidisk clutch. It makes his road racing vette up and down shift better because the rpm can be match relatively instantaniously.
The weight of the pistons and rods is fixed. You alter the crank throw weight to balance any motor
10-09-2006, 06:58 PM
#5
Burning Brakes
Originally Posted by gkull
The weight of the pistons and rods is fixed. You alter the crank throw weight to balance any motor
10-09-2006, 07:35 PM
#6
there are plenty of light steel rods lighter piston pins and pistons
availble. You would not want to go with to light of duty stuff for
the amount of RPM and HP you are after. A light weight piston and
pin as long as they are strong enough to do the job makes the rods
life a whole lot easier.
availble. You would not want to go with to light of duty stuff for
the amount of RPM and HP you are after. A light weight piston and
pin as long as they are strong enough to do the job makes the rods
life a whole lot easier.
Last edited by Little Mouse; 10-09-2006 at 07:54 PM.
10-09-2006, 07:54 PM
#7
Race Director
Member Since: Nov 2000
Location: Waterloo ontario Canada
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For street use the heavier rotating assembly is smoother off the line. The heavy flywheel helps pull the car from a dead stop.
You only have 8 cylinders and there is a series of firings. The heavier flywheel smooths out the pluses and makes for a smoother running engine.
You only have 8 cylinders and there is a series of firings. The heavier flywheel smooths out the pluses and makes for a smoother running engine.
10-09-2006, 08:18 PM
#8
Team Owner
Originally Posted by norvalwilhelm
For street use the heavier rotating assembly is smoother off the line. The heavy flywheel helps pull the car from a dead stop.
You only have 8 cylinders and there is a series of firings. The heavier flywheel smooths out the pluses and makes for a smoother running engine.
You only have 8 cylinders and there is a series of firings. The heavier flywheel smooths out the pluses and makes for a smoother running engine.
I buy the big heavy H-beam rods that can handle the big power (685 gram Manley's) Then light weight racing pistons with strong pins. They end up drilling light weight cranks to balance them.
10-09-2006, 09:48 PM
#9
Racer
Thread Starter
Wow...That's why I love this forum, I can post questions I have that no one I know knows & get answers that I understand without Quantum Physics! I appreciate that! So....if a fellow was going to buy a forged crank for a expected 500 hp street motor, he might as well get the 8 pound lighter crank if he had the extra "ching"? Now, longer rods with pistons with a lesser compression height are lighter than shorter rods with a bigger compression height? And, at what hp/rpm level does a "H" beam connecting rod make sense over a "I" beam?????
10-09-2006, 10:07 PM
#10
Team Owner
Member Since: Jul 2004
Location: Redondo Beach, California
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What about internal balancing? I agree that in the best of worlds, having a engine with light weight recriprocal rotating parts is best. But also, I think for performance you need to have the engine internally balanced. To me, balancing the engine by putting weights on the flywheel and the crank damper, is just an expedient. Having an engine wiht unbalanced internal parts puts a lot of stress on crank bearings and also the crank itself. My opinion - yes minimize rotating weights of the engine parts and then.....add mallory weights (tungsten) to the crank throws to internally balance it.
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I've heard of depleated uranium being used to internally balance engines also.
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I've heard of depleated uranium being used to internally balance engines also.
10-09-2006, 10:32 PM
#11
Team Owner
Depleted uranium makes for a great projectile in military weapons....hard, tough, breaks through anything.... But why in the world would I use that to balance a crank. I guess if I were a "huckster" maybe I could charge Big Buck$ for depleted uranium weights. 
10-10-2006, 10:18 AM
#12
Team Owner
Originally Posted by 68/70Vette
What about internal balancing? My opinion - yes minimize rotating weights of the engine parts and then.....add mallory weights (tungsten) to the crank throws to internally balance it.
...........
I've heard of depleated uranium being used to internally balance engines also.
...........
I've heard of depleated uranium being used to internally balance engines also.
