Rotating weight.....crankshaft....bobweight

[358T]

How much performance could a person expect to gain from taking 9lbs out of the crank (54lbs down to 45lbs) and reducing the bobweight 60-70 grams?

This is a 1/4 mile drag race application


Thanks,

Scott

[bigjoe1]

I really go after the light wt rotating crank assembly. I could see it to be worth .15 to .30 in ET on the drag strip. This would be very hard to get a real back to back test, but based on hundreds od engines I have done, this seems reasonable. JOE SHERMAN RACING

[358T]

I really go after the light wt rotating crank assembly. I could see it to be worth .15 to .30 in ET on the drag strip. This would be very hard to get a real back to back test, but based on hundreds od engines I have done, this seems reasonable. JOE SHERMAN RACING

WOW! I'd be extremely pleased with that.

I know for sure the crank will be 9lbs lighter. I'm guessing a little on the bobweight but it should be at least 50 grams less. I'll know for sure when I pick up the stuff from my machinist. Does crank weight changes and bobweight changes have equal amounts of effect on acceleration?

This won't be a true apples to apples test as the engine is being changed from a 4.125"bore x 4.000"stroke combo to a 4.155"bore x 3.875"stroke combo at the same time as the rotating assembly weight change. We are also giving the car more gear to accomodate more rpm's.

Thanks for the reply

Scott

[Wolfplace]

I really go after the light wt rotating crank assembly. I could see it to be worth .15 to .30 in ET on the drag strip. This would be very hard to get a real back to back test, but based on hundreds od engines I have done, this seems reasonable. JOE SHERMAN RACING

WOW! I'd be extremely pleased with that.

I know for sure the crank will be 9lbs lighter. I'm guessing a little on the bobweight but it should be at least 50 grams less. I'll know for sure when I pick up the stuff from my machinist. Does crank weight changes and bobweight changes have equal amounts of effect on acceleration?

This won't be a true apples to apples test as the engine is being changed from a 4.125"bore x 4.000"stroke combo to a 4.155"bore x 3.875"stroke combo at the same time as the rotating assembly weight change. We are also giving the car more gear to accommodate more rpm's.

Thanks for the reply

Scott

=

As piston & rod weight go down the counterweight on the crank will need to be lighter so they sort of go hand in hand.
If you go from 1600gms to 1400 gms of bob weight which is what you put on the crank throw to simulate the reciprocating & rotating weight of the piston & rod assembly you are going to have to remove an appropriate amount of counter weight to return to a balanced condition.

[358T]

I really go after the light wt rotating crank assembly. I could see it to be worth .15 to .30 in ET on the drag strip. This would be very hard to get a real back to back test, but based on hundreds od engines I have done, this seems reasonable. JOE SHERMAN RACING

WOW! I'd be extremely pleased with that.

I know for sure the crank will be 9lbs lighter. I'm guessing a little on the bobweight but it should be at least 50 grams less. I'll know for sure when I pick up the stuff from my machinist. Does crank weight changes and bobweight changes have equal amounts of effect on acceleration?

This won't be a true apples to apples test as the engine is being changed from a 4.125"bore x 4.000"stroke combo to a 4.155"bore x 3.875"stroke combo at the same time as the rotating assembly weight change. We are also giving the car more gear to accommodate more rpm's.

Thanks for the reply

Scott

=

As piston & rod weight go down the counterweight on the crank will need to be lighter so they sort of go hand in hand.
If you go from 1600gms to 1400 gms of bob weight which is what you put on the crank throw to simulate the reciprocating & rotating weight of the piston & rod assembly you are going to have to remove an appropriate amount of counter weight to return to a balanced condition.

That makes sense. But my question in the post quoted is more related to how a 54lb crank and a 45lb crank both have the same relative target bobweights. How much effect does the lighter crank have on acceleration in comparison to lowering just the bobweight.

Scott

[Wolfplace]

I really go after the light wt rotating crank assembly. I could see it to be worth .15 to .30 in ET on the drag strip. This would be very hard to get a real back to back test, but based on hundreds od engines I have done, this seems reasonable. JOE SHERMAN RACING

WOW! I'd be extremely pleased with that.

I know for sure the crank will be 9lbs lighter. I'm guessing a little on the bobweight but it should be at least 50 grams less. I'll know for sure when I pick up the stuff from my machinist. Does crank weight changes and bobweight changes have equal amounts of effect on acceleration?

This won't be a true apples to apples test as the engine is being changed from a 4.125"bore x 4.000"stroke combo to a 4.155"bore x 3.875"stroke combo at the same time as the rotating assembly weight change. We are also giving the car more gear to accommodate more rpm's.

Thanks for the reply

Scott

=

As piston & rod weight go down the counterweight on the crank will need to be lighter so they sort of go hand in hand.
If you go from 1600gms to 1400 gms of bob weight which is what you put on the crank throw to simulate the reciprocating & rotating weight of the piston & rod assembly you are going to have to remove an appropriate amount of counter weight to return to a balanced condition.

That makes sense. But my question in the post quoted is more related to how a 54lb crank and a 45lb crank both have the same relative target bobweights. How much effect does the lighter crank have on acceleration in comparison to lowering just the bobweight.

Scott

=
I don't know how to answer that, the bob & counter weights are going to go hand in hand.
You cannot just make the crank lighter but you can move the weight towards the center.
The closer to center the weight removed is the less effect it will have on acceleration. (also on balancing)

This is the reason for the "pendulum undercut" crank.
You are removing the weight as far out as you can while still leaving the heavy part at the peripheral of the counterweight for the most effect in balancing.
I think as you move the counterweight closer to center it should accelerate better but you are now going to have to compensate by adding Tungsten to rebalance or you are going to have to run lighter parts so I don't see how you can really separate the two?

[SchmidtMotorWorks]

It depends, what do you have for a flywheel?

You can get a lot of weight out of some cranks, especially if you remove center counterweights, lighten rod throws, drill holes through the mains and rods, scallop cut the flange and pendulum cut the cws.

[FordManVT]

I've seen cranks without center counter weights. Betcha that would save 8-10 lbs easy.

[rskrause]

I think that there are too many variables to make anything other than a WAG. Some of the more obvious variables: car weight and hp, tranmission type, converter used (if it's an automatic), rpm range, etc. Just one for instance: if the car is an automatic with a high stall converter, the revs will change very little from launch to the traps. So, in that case I wouldn't anticipate much effect. Also, in an "underpowered" car, more rotating mass can help the car launch, which would tend to negate the beneficial effect of lightening the rotating assembly. I'd look elsewhere for significant improvements in the typical drag car setup. But the details probably matter and under the right circumstances it might be worth it. If you are in a very competitive heads up class, where hundredths count, that might be a time to consider it.

Another for instance: Years ago I used to road race. On certain tracks there was a wide rpm range that was needed and the engine was accelerating/decelerating many times in a lap. My whole setup was far from optimized and my driving needed improvement to! But I was looking to optimze the car as much as I could and I did make a lot of effort to take weight out of it, including rotating weight. Because I never made any single change at one time I can't say for sure. But the efforts did net improvement in lap times and the car felt more responsive and was easier to shift with a lighter rotating assembly. The differences in the physics of that situation and the typical drag race setup should be obvious.

Richard

[358T]

Well I picked up my stuff. I was way off on my guess of bobweight change... long story.

Here are the actual figures.

The new crank is 45lbs. The new bobweight is 1838 grams.
The old crank was 54lbs. The old bobweight was 1850 grams.

That is a difference of 9lbs and 12 grams respectively. Can this alone make any difference in performance on a dragstrip.

Thanks,
Scott

[bigjoe1]

probably NOT. It is more important to reduce the weight of the rods and pistons than only the crank. Too bad. JOE SHERMAN RACING

[Rick360]

To remove over 9lbs of weight from a crank and still be balanced to the same (or similar) bobweight, weight would have been removed from counterweights and rod-journals alike. This WILL make a difference in the MOI of the total crank/rod/piston assembly and therefore a difference in the power it requires to accellerate.

Any weight removed by drilling the center mains will have minimal impact, and undercutting counterweights also has a lesser effect but drilling rod journals or removing weight around the rod journals is just as good as the same amount of weight removed from the big end of the rod.

The amount of ET it will gain depends on where the weight was removed from the crankshaft and how quickly your engine accellerates (car weight, gearing and power) during a run since this determines how much torque and power is taken from the engine to accellerate the internal components.

Rick

[358T]

The old crank was a standard 4340 Eagle crank.

The new crank is a 4340 Scat "Superlight". Pictures on the internet showed this crank having the undercut counterwieghts and such. But mine does not have that.

I was sceptical. So I verified the weights of both cranks on a scale myself.

Rick, What does MOI mean?

Scott

[Rick360]

Rick, What does MOI mean?

MOI = Moment of Inertia. It is a term used to describe the resistance to rotation of something. It is a rotational equivalent to mass.

Rick

[stock z/28]

Hi,

Im guess his MOI, is moment of inertia?

Kinda like smaller diameter flywheel of the same weight as a larger diameter flywheel should accelerate faster.