JET951

Hey Pat,
I have found blue tac works extremely well in balancing lightened cranks , all we do is whip off the sump on a regular basis and apply more blue tac , it has a tendancy to wash off ibn the oil at times. The wife also likes to use it to hang pictures on the wall at home. regards . Bruce

Diver944

My 3.2 engine used a balanced but not lightened crankshaft and the standard 951 flywheel and is as smooth as smooth can be. I didn't want to risk any driveability in what is a road/track car by overlightening.

George D

Interesting discussion. There are benefits to having a factory crank and flywheel, this is especially true when you are using stock rods. They are much heavier than many forged rod options for our cars. If you are using stock rods, you should probably keep a stock HEAVY crank. You don't want the crank to want to wander under high hp and tq loads.

Someone stated that you will loose tq when lightening the crank. This is true as a heavier crank will allow better momentum when spinning. But the better option is to have a HEAVY flywheel. Once moving you will be a tq monster, but what about the initial response and deceleration? If a simple steady state RPM is what you are after, a heavier crank and flywheel is optimum. Think of an airplane piston motor in this case. You want heavy mass for consistent low rpm SAFE power.

Point here is that when you are using modern forged LIGHT rods, you can lighten the crank as long as you balance the crank. If the rods are balanced, pistons are balanced, and your crank is balanced once you put things together, they WILL still be balanced. What is heavier, a ton of feathers or a ton of gold?

Another huge benefit to a lightened bull nosed crank is the reduction of windage. That heavy weight is spinning in OIL. Heavy, sticky, oil. When oil starts to wind on the crank at high RPM’s, the less mass whipping through the oil, the more power is transferred to where you want it. A properly bull nosed or edged crank will spin in the oil much easier. The motor isn’t really making any more power, but more of the power is transferred to your wheels as the crank is not overcoming as much resistance.

Porsche used lightened cranks on the 944 GTR. They use lightened cranks on all of their racing cars.

JET951

sounds like your talking about race cars george, porsche did use lightened cranks on all thier race cars, but there was a purpose to these race cars, they were made for high revs all the time, ever driven a 993 RSR, they had so little reciprocating mass they are painful to drive unless past 5000rpm but they rev like a 200cc 2 stroke dirt bike engine, they only produce 350nm at a rediculus 6250rmp from a 3.8ltr very little torque. street cars dont do high revs all the time. by removing weight you will decrease drivability compared with the stock crank and flywheel, everything is a compromise

George D

I am in agrreement with you. If you want a very reliable 3.0+ motor, sticking with the stock crank and flywheel is the best option. The point in my post was that you can have the best compromise based on your goals with a proper build. You will not have issues when shaving mass from your crank as long as what you take off is equal everywhere and properly balanced.

The only reason we machined this crank was to lower windage and have less restriction while bathing at high RPM in oil. We left a bit on this crank as shown in the pictures.

This motor is a street/track build. I hope to surprise many of you with the dyno numbers.

RajDatta

This could also be a case where the flywheel is just too light. I swapped out my DMF in the 968 cab for a Porsche Motorsports single piece flywheel. Guess what, no stalling or tricky behavior at low RPM. Its no coincidence that it weighs what it does. Porsche did their math to come up with a setup which provides driveability and also allows you to lower some rotating mass. This setup was built for 968 club racing in Europe.
I run my setup with stock Motronics and anyone that has driven my car has never ever stalled it. It drives like a 951, but with some transaxle noise and small vibes at certain rpm. Pulls a lot faster than a stock 968 from the weight loss. I did add a vibration damper to this setup as well.
Raj

George D

. I did add a vibration damper to this setup as well.
Raj[/QUOTE]

Did you add this to the clutch disc?

RajDatta

Its the crank pulley that runs the alternator belt. 951's have a very lightweight aluminum one, S2's have a much heavier pulley, which is the vibration damper. It was on all 3.0's that didn't have DMF, ie S2 and turbo S. I felt safer running it on my 968 cab since I run a Porsche Motorsports 1 piece flywheel.
Raj

FRporscheman

Raj, is that just a heavier version of the same pulley? Is it a wear item?


I wonder how the 2.8L motors are doing. I assume a lot of these motors have lightened cranks and/or flywheels as well, I wonder if they're smooth or not and what kinds of issues they might have?

333pg333

I'd prefer Big Charlie bubble gum if you can still get it. hahaha

RajDatta

Arash, that is correct. In general, its the same piece. Too bad, both of mine are installed or I could have posted a picture of the differences. It does weigh quite a bit. I could feel the difference in my cab after I installed it. The car revved quicker with just the lighter flywheel and without the vibration damper.
My feeling is, they used it on an S2. With my car no longer using DMF, it is essentially an S2.
Raj

Chris Prack

How was it balanced? Who did the balancing? What did they use to achieve this balance?

fastmover

Correct me if I'm wrong because I don't understand how a theory of a lightened
rotating assembly will reduced torque output. I alway's understood the torque output of an engine is due to the length of the throw of the crankshaft.
Since the distance between centerline of the crank main journal to the centerline of the connecting rod journal is effectively a lever (longer lever = more torque, shorter lever = less torque) and thus responsible for the torque characteristics of a engine. So the weight of the lever that is multiplying the torque output is not relevant, especially when the applied force to the lever is coming from the combustion process acting against the piston. So I don't understand how weight will reduce torque, considering most modded 951's if built and tuned correctly will have a more powerful combustion event taken place in the combustion chamber. So the more weight in the assembly will consume more energy to overcome the weight of the rotating assembly in order for the engine to begin to produce torque.

As for the balance issue of our engines, if you go lightweight on your reciprocating assy, as long as the crank, pistons, connecting rods, flywheel and crank pulley/harmonic balancer is balanced together, and balanced to neutral/zero balance it should be good to go for a long while, and the lighter you make it the higher you can set your redline. the only question I have is the balance shafts, since they are a certain weight meant to create a certain frequency to cancel out the frequency created by the crankshaft/connecting rods/pistons in order to balance the engine (to make it feel smoother) , would you scale the weight of the balance shafts to the stock rotating components, and then use that scale to determine how much weight to take off the balance shafts to create the proper frequency? I'm not sure on that one. Would the balance shafts even be nescessary? Also a harmonic balancer is also a very good thing to run in any engine, even if it is nuetral balanced.



Semper fi.

fastmover

Also another aspect of torque output of an engine is if it is sqaure, undersquare, or oversquare.
Square meaning piston diameter and stroke (crank throw) are equal, this would result in an engine that has close or equal hp and tq outputs.

Under square ( I think , might be backwards) would be smaller piston diameter compared to throw/stroke, which would be a high torque output, low reving , low horsepower engine.

Over square would be larger piston compared to throw/stroke, which would be a high reving' higher horsepower, lower torque output engine.


Semper fi

JET951

Fastmover,
you are correct with the aspect of the longer the stroke the more torque that is produced and mixed in with thatis the large(read heavy) reciprocating mass. like on a farm tractorthat have a massive flywheel purly for torque reasons. conversly a small very thin/light flywheelhas an effect on the same engineof much less torque and much easier to stall.
thats why road cars that have a lighter flywheel fitted and then mix that with a lighter crankshaft will allow quicker response and the equal amount of less torque. a flywheel is fitted for urban conditions to make the vehicle more consumer friendly.