TonyG

The differences are big in first and second gear. By third it's barely noticable, and in fourth and fifth, it makes no difference as the engine is accelerating pretty slowly.


The more power your car makes, the harder the turbo hits, the bigger the difference (in first & second).

TonyG

AlexE

So with this reasoning...

...if I have a car that I can barely keep the wheels from lighting up in 1st and 2nd then I would imagine a lightened crank is probably the last thing I would want to do in this car. No point in having it rev any quicker than it already does in the lower gears...

So I guess I will keep my crank just the way it is.

Russ Murphy

Reducing mass rotating at engine speed will absolutely help with top end acceleration.
Not to the degree mentioned, but significantly. Turbo Tommy is right about the factors but glossing over rotational inertia of the driveline components. Reducing mass there has a fifteen times greater affect than reducing weight of the chassis. The text I'm pilfering from gives an example of a 2985lb car with a 15lb reduction in driveline mass with a corner exit speed of 64 mph and accelerating for 5 seconds reaching 115.7 mph vs. 112.74 mph for the baseline 3000 lb car. This is equivalent to a 32 hp gain.

TurboTim

We can knife-edge your stock crank for $750.If we can get 5 or more cranks at a time we can do it for $675.This includes cross/perp-drilling,allen plugging and balancing as well as polishing the journals.We can lighten your stock flywheel for $250.

m42racer

It should be noted, that Knife edging is not done for weight reduction reasons. There seems to be 2 points been discussed here.

Reducing rotating mass is done for engine acceralation purposes, and knife edging is done for windage purposes. Both are typically done on race engines where gearing can be changed to make up for the loss of Flywheel effect, and the need for better windage is required. To do either on a street car is ok, if the reasons for doing such things is understood. To say its necessary is up to the user.

To lower rotating mass changes the time it takes to accelerate the engine. This makes the time to torque quicker, thus giving the feeling of having something to do with engine performance. Once in the higher RPM's, the time to accelerate and the work done, or power loss to accerlerate the engine is less.

Knife edging the crank must lower the mass, but it is done not for this reason, but to help the counterweights go thro the oil. The amount of drag on these parts at high RPM's is well documented. To do this without knowing how much is removed from the counterweight and the effect on balance is absolutely the wrong thing to do.

I'm told that indiscrimate lowering of the couterweights is very dangerous. This is why others have told some here not to do it. Many factors have to be known. What is the weight of the reciprocating masses, the engines RPM upper limit, the rotating masses, the amount of original conterweight, and the type of engine, and therefore the type or number of torsional stresses in effect. In line 4 cylinder engines have the worse balancing and torsional stresses. I think maybe into the 3rd or 4th order. Add very heavy Pistons and Rods to the equatation, and its no wonder the engine comes with balance shafts. To lower the counterweights without first knowing the amount (%) of counterweight is crazy. I do not know this number, however I do know that it can be calculated by those who know the correct formula's. To lower the % of counterweight moves the torsional orders around, or into further orders I think. Seems to me it should be left to those who really know, and can offer insight into this area of engine engineering. Many offer this service, but fail to provide the necessary information as to why this is been done, other than to offer a service to those who know no better.

Matt H

Tim - good price on the crank.

As far as the flywheel, any machine shop can lighten and balance them pretty darn cheap. I want to say I had two quotes of less than 90 dollars. I am sure it is not apples to apples with what Tim is offering but it is one way to go.

Since the last shop air gunned the 12pt bolts into the clutch I decided to wait, they werent coming out!
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TurboTommy

Yes, m42racer, would agree not to mess with the balancing, and couldn't even comment on it. And yeah, the knifedged crank is a good thing as far as windage is concerned; or lack of it. You want to get less airation of the oil so it's better for the crank to cut cleanly through it (I think that's the theory).


I do know that less engine rotating mass has nothing to do with how much power you will make. One of the functions of horsepower is the turning force of the engine (torque), and having different weights of crankshaft or flywheel assembly will not change that. The ability of a car to accelerate is opposed by load like for example: weight of vehicle, taller gearing, hills, aerodynamic drag. The mass of the rotating flywheel, etc is part of this load but minimal compared to the others. Therefore, you really only notice the lighter rotating mass when the other loads are almost non exsistant (like matching revs on a down shift, which is still a pleasant feeling; or maybe accelerating out of a very low speed corner).

It's all about engine inertia. Something to illustrate the point: you're at full power in a high gear flying down the road/track. You come to a fairly steep uphill section and you keep it floored. It's an absolute fact that if you had a heavier flywheel, you will be traveling at a higher rate of speed by the time you get to the top of hill than if you had a lower weight flywheel. So what now; are you going to say you have more power with the heavier flywheel? I don't think so.

TonyG

AlexE
>>>...if I have a car that I can barely keep the wheels from lighting up in 1st and 2nd then I would imagine a lightened crank is probably the last thing I would want to do in this <<<


Get better rubber and you won't be spinning the tires. The differences are huge with respect to adhesion, and if you're running a set of Hoosiers on the track.... well.... you won't have that problem!


TonyG

adrial

I'd be curious to actually figure out exactly the differences using physics....the formula's you need are pretty basic, problem is find the center of Inertia of all the rotating masses. Other than that its just energy equations for work, linear KE and rotating KE...

m42racer

We should not forget we are rebuilding a 15 year old engine, used either for street or amatuer racing. We are not building the next FI engine. Some perspective should be kept.

Tomas L

When the piston is accelerated up and down in the cylinder there is an opposite force up and down on the main bearings. To counteract this the crankshafts has counterweights that will transform some of this force to a sideways directed force. The idea is that it's better to have half the force working sideways and half in vertical direction than it's to have the full force in vertical direction. The idea gets even better since the separated forces doesn't work at the same time, they are separated 90° crankshaft rotation. The weight of the crankshaft counterweights determines the amount of vertical force that is transformed to horizontal force. This can be expressed in % as the engines balance factor.
In an ideal engine these counterweights would not be necessary as the force from one piston going down is (almost) counteracted by another piston going up. Since no engine is ideal (?) the absence of counterweights would mean large stress on the crankshaft that has to transfer the force from one piston to the other. And since the increased crankshaft stress would mean flex, it would also mean increased stress on the main bearings and journals.

Lightening the counterweights will increase the stress on the crank, the main bearings and journals. The lighter the counterweights, the more stress.

The purpose of the balance shafts is completely different and is to counteract the difference in acceleration between the pistons moving up and the pistons moving down. It may seem that when piston #1 is moving down it's counteracted by piston #2 moving up in the same speed, however this is not the case as there is a slight difference in their acceleration and speed. This fenomen occurs in engines with 180° cylinder separation (and perhaps less noticable in 90° separation engines), engines with 120° separation (3, 6 and 12 cyl) doesn't have this problem and therefore needs no balance shafts.


How steep is the hill? If you loose speed going uphill, you will loose less speed with a heavy flywheel. If you still accelerate up the hill you will accelerate i little harder with the lighter flywheel. It's all about energy storage.
It takes a certain amount of power to accelerate a flywheel at a certain rate. If for instance it takes 5 hp to accelerate a flywheel from 1000 rpm to 6000 rpm in 10 seconds, then it will take 10 hp to do it in 5 seconds and it will take 25 hp to do it in 2 seconds. In first gear the acceleration rate is high and therefore more power is needed to accelerate the flywheel. The amount of energy stored in the flywheel at a certain rpm is always constant. It's just that when you need to accelerate the flywheel quickly up to that rpm level you'll need to put that energy in to the flywheel over a shorter time period.
In higher gears the acceleration rate is lower so the effects of a lighter flywheel diminishes. Since the flywheel is only on component in the drivetrain we also have to take other parts into account. For instance the effects of the tires and rims. Since these are located after the gearbox their effect is dependant on which gear is selected. The higher gear, the greater the inertia effect from the tires.

Tomas

TonyG

>>>It's an absolute fact that if you had a heavier flywheel, you will be traveling at a higher rate of speed by the time you get to the top of hill than if you had a lower weight flywheel<<<

That is unless you have enough HP to be accelerating up the hill. Your statement assumes that the hill would slow you down, thus the larger flywheel engergy stored would help keep you from decelerating.

The opposite is true, if the opposite is true. :-)

TonyG

NZ951

Is there an art to lightening a flywheel? Is there a pattern one should follow? I am thinking of having this done.

TonyG

NZ951

Know that a lightened factory flywheel is no where near as light as an aluminum flywheel at 5.5lbs.

Factor in all the costs before you make your decision.

Good luck,

TonyG

David Floyd

If you install lighter pistons and rods then a knifed (lighter)crank would be in order ?