TT_Rex_7

Well I was bored last night looking through some old theads and came across a lightweight rotor thread and searched around on the topic and noticed that people were saying if you had lightweight rotors the motor would create less torque then what it already does, but no one ever stated why or wether it was true or not. So my question is why would lightweight rotors create less torque then the factorys?!

-Alex

iluvmy3rdgen

Well lighter weight parts are usually associated with faster acceleration, but it would take more power to keep the rotor spinning at a certain rate. I could see how it reduces torque. Why do you ask anyway? Curiosity?

TT_Rex_7

Well I was thinkin about buying a used motor and just tinkering around with machining lightweight internals down at my shop during my spare time just to, well, see what happens lol. Although it is also out of curiosity. What I dont seem to get here though, is if its lighter, why would it take more power to keep it spinning at a fixed rate?! Less momentum?

-Alex

iluvmy3rdgen

At a fixed rate it would spin easier, but to speed up create compression a lighter rotor will need more force behind it. It's not like a light weight flywheel.

TT_Rex_7

So then would it be correc to assume if the rotor was heavier it would creat less hp but more torque or am I still way off here lol?

-Alex

akiratdk

wouldn't it create more torque???? like a lighter flywheel....... creates great low end but lags on high end...

TT_Rex_7

Thats what I originally thought also, except I thought it would create great high and low end torque and hp. I'm hoping more can chime in on this topic...

-Alex

Mahjik

iTrader: (1)

There is a little more to it than just removing material from the rotors. The whole assembly also needs to be balanced. The only person in the USA who does this is Daryl Drummond. The whole process is $1200 for used rotors if they are within spec and can be used.

I looked into this not too long ago.

TT_Rex_7

I understand that there is more then just removing material from the rotors and so on, but I think I could handle it just fine at my machine shop and do so cheaper then $1200.00 Granted I don't machine automotive parts but I do alot of medical, and military work that is extremly percise and have very small tolerances. The molds that I do for Bridgestone/Firestone also have very tight tolerances. The thing I'm trying to get at is if it will acctually loose torque or not.

-Alex

shawnk

Why dont you ask Daryl Drummond. Since he does it and charges $1200 for it then he would know why someone would want to spend $1200 on it.

bajaman

One needs to consider exactly WHAT it is that 'makes' the torque the engine puts out. In a piston engine, the movement of the connecting rods to drive the crankshaft is what creates torque. Imagine having several very long cheater bars connected to a wrench and you start to get the idea. This is why a long-stroke engine developes more torque....you have that long moment of inertia as the piston is being driven down...down....down.

In a rotary....much, much different. And quite frankly I can't even begin to calculate exactly how the forces are generated. This IS interesting, perhaps someone with true engineering knowledge of how this works can explain it to the rest of us?

TT_Rex_7

Well, 1 Don't know how to get ahold of him and 2 would you be so willing to give someone details on something that your sellin for 1200 dollars?

-Alex

Mahjik

iTrader: (1)

Very few people on this forum are going to be able to give you a concrete "real" answer on that (other than just "guessing"). Lightened rotors are usually for competition race cars like the SCCA boys. Daryl Drummond builds most (if not all) of the rotary engines for those guys. He is the best person to ask about the benefits or problems associated with using lightened rotors.

Try emailing Daryl at: fastrotary@earthlink.net

It will take him a few days to get back to you, but he will.

TT_Rex_7

Thanks, I really appreciate it. I'll shoot him an e-mail and hopefully get the answers I'm looking for.

-Alex

akiratdk

I really think that the lighter rotors will act the same as a lighter flywheel.......

TT_Rex_7

I for one hope so. We'll find out soon, either from Daryl or from me just simply doing it. I just have to find a low mileage block for sale...

-Alex

DamonB

The exact same way they are in a piston motor. The side to side movement of the rotor as it "orbits" is how the power is sent into the eccentric causing it to rotate. This side to side movement is the rotary equivalent of the "throw" on a piston's crankshaft and also explains why the rotors turn at 1/3 of eccentric speed. If there was no side to side movement then you wouldn't be able to make compression either, you'd just have a triangle spinning inside a perfect circle and the volume above each rotor face would never change.

Lighter weight rotors do not give the engine less torque. An engine makes power due to the expansion of burning gases pushing against parts. In the case of a rotary the gases push against the rotor face which pushes against the eccentric and causes it to turn. Some of the engine's power is consumed in just accelerating its own parts. If you make those parts lighter the engine is able to accelerate more quickly. Automobile engines by nature constantly change power levels. If your tach is moving your engine is changing in power. Lighter weight parts make it easier for the engine to accomplish this because it doesn't consume as much power just accelerating itself.

In drag racing many people like to use a heavier flywheel in order to store inertia. Inertia is not power and it is not torque. Since you can sit at the start line and rev the engine before dropping the clutch the engine with the heavier flywheel is able to store more inertia and therefore have slightly more energy available at the moment the clutch is dropped. The downside is that the engine must accelerate that extra flywheel weight all the way down the track. There is a happy medium in this case between how much the flywheel helps the launch at the expense of hurting acceleration down the track. This is very particular to drag racing because it's the only form of racing where you get to wind up the motor and then unleash it to the tires.

In a nutshell lighter weight rotors DO NOT give the engine less torque. The lighter weight rotors will have much less inertia and so the engine will be able to speed up and slow down more easily. If you ever plan on asking your engine to change rpm as rapidly as possible you want the lightest parts possible inside the engine.

TT_Rex_7

How about a gain in torque? Wouldn't the lighter parts allow you to put down more torque at the wheels?

-Alex

dgeesaman

iTrader: (7)

Thanks Damon. I was just typing a response saying that torque in engines is affected by two things: 1) the power of the expanding combustion gases, and 2) the geometry of the crankshaft/connecting rods/eccentric shaft.

In lightening rotors, you aren't changing the geometry that converts combustion pressure into torque, so torque will remain the same. Just like lightening a flywheel.

The two things you need to watch are: removing material that changes the shape of the face of the rotor, which would affect compression ratio and the power of the combustion gases. The other is mechanical stress. As you can imagine, rotors are both critical parts and highly stressed parts. You can decide what material removal is safe by analytical (FEA) modeling, or trial and error. Whichever approach Daryl took, I imagine part of the cost is recovering the time he spent making sure the rotor won't fail due to stress.

Dave

Mahjik

iTrader: (1)

BTW guys, any place here in the US that offers lightened rotors, goes through Daryl (yes, even Racing Beat).

gslse_in_vt

torque = moment of inertia x angular acceleration.

It's all a matter of physics and inertia. An object's moment of inertia is directly proportional to its mass. So the more mass, the more torque. Think of 2 swinging doors, one heavy and one light. Which one is easier to stop? Of course, youl need more hp to get the heavier one moving....

Overall, lighter rotational mass engines may accelerate quicker (good for pure racing), but will also decelerate quicker (not good for highway cruising). A good mix of both should keep your car streetable and fast. Hope my two cents helps.

jimlab

Ding! Ding! Ding! We've got a winner!!

I can guarantee that lightweight internals will not adversely affect torque. I have an ultralight billet crankshaft in my 396 (22 lbs. lighter than stock), ultra light billet rods (505 grams, about 3 lbs. lighter for the set of 8 than aftermarket 6.0" rods), and lightweight pistons (405 grams with pistons). Check out the effect on torque...

http://home.gci.net/~jimlab/images/E...vered/Dyno.jpg

Nada. Same as if I'd used heavier standard-issue components.

However, most people think that it's the length of the "lever" on the crankshaft, or in other words, the crankshaft's "stroke" (actually, the distance from the connecting rod journal centerline to the crankshaft centerline is half of the crankshaft's stroke) that increases or decreases torque. In part it is, but it's not quite as simple as that. Displacement is what ultimately affects torque production.

If I have a V8 with a 4.00" cylinder bore and a 3.48" stroke, I've got 350 CID. (4.00 * 4.00 * 3.48 * 8 * 0.7854 = 349.9 CID). If I increase bore diameter to 4.03" and stroke to 3.75", now I've got 383 CID. (4.03 * 4.03 * 3.75 * 8 * 0.7854 = 382.7 CID) The difference, of course is that one has a longer "lever" on the crankshaft, but as a result, the piston is also traveling farther on each stroke (the distance of the stroke, FWIW), creating more displacement. More displacement means I can burn more air and fuel on each cycle, creating... more torque.

Now, let's say that I create an "all bore" 383 with the shorter 3.48" stroke of the 350. I'd have to use a 4.18" cylinder bore (4.18 * 4.18 * 3.48 * 8 * 0.7854 = 382 CID). Obviously, the 3.48" stroke crankshaft has a shorter "lever" than the 3.75" stroke crankshaft above. Would you expect torque to differ substantially? In fact, given equal intake components and cam profiles, the two engines will produce almost identical torque curves. Why? Because even with a shorter lever, the "all bore" engine is exerting more force on that lever, because it is burning the same amount of fuel and air, but has a larger surface area (piston crown) to "push on". Neat, huh?

If I put 1 pound of pressure on a lever 1 foot long, I have 1 lb-ft. of torque. If I put 2 pounds of pressure on a lever 0.5 foot long, I still have 1 lb-ft. of torque. Pretty simple once you think about it.

Anyway, as far as the rotary is concerned, there are no connecting rods, obviously. Instead, the rotor exerts force directly on the journals of the eccentric shaft, and the "stroke" of the eccentric shaft (offset of the rotor journal centerline to the centerline of the eccentric shaft) is what applies leverage to the eccentric shaft. Once again, it is displacement that dictates the amount of torque produced.

Since we're talking about turbocharged rotary engines here, you can see from a dyno sheet that once the turbo spools and you're creating more "displacement" by pressurizing the intake charge and forcing more air (and fuel) into the engine, torque rises dramatically. Below that point, the engine doesn't make much torque because A) it has very little displacement, and B) the offset of the rotor journal centerline to the eccentric shaft centerline isn't very large. (look at a a picture of an eccentric shaft some time).

Lightening the rotors will not change the amount of torque the engine produces unless material is removed from the rotor face, affecting displacement. Only changing the eccentric shaft offset (impossible without changing the size of the housing and rotors because of the precise pattern the rotor must travel) or combustion chamber size will affect the amount of torque the engine produces, assuming all other factors (port size, etc.) stay the same.

Even with the rotary engine, displacement ultimately determines torque. A rotary engine with lightened rotors will spin up (rev) faster and slow down faster because it takes less energy to spin it up to a given speed and there is less inertia or stored energy in a lighter part when you stop applying power. However, none of that will affect torque in the slightest.

Merry Christmas!

dgeesaman

iTrader: (7)

Actually, you can think of that equation as the portion of torque lost in accelerating the drivetrain.

Reducing the moment of inertia (which is the real point of lightening a flywheel or rotor) will allow more of the torque to be used on the wheels, and the angular acceleration will go up a bit.

Dave

TT_Rex_7

This is starting to get me quite intreged. I think I'm going to go ahead with this as soon as I can find a low mileage block and a few bad rotors to tinker with. Hopefully all will go well and I can atleast be the second in the US capable of doing this. What had got me all confused was people saying you'd have virtually no torque with lightweight rotors, yet I have never seen any ill effects with lightweight internals with a piston motor as Jim stated.

-Alex

gslse_in_vt

Gotcha Dave, you're right.

jimlab, consider me schooled