Underdrive crank pullies and harmonics

[mpg99]

There has been some recent discussion on a car forum I frequent about the use of Underdrive crank pullies between some of the engineers. I was curious as to what your opinions are. First, here is a quote from Unorthodox Racing, the manufacteror of the underdrive pullies this particular forums members are using.

"Is my crank pulley a harmonic/torsional/vibration damper or a harmonic balancer?

People are getting their crank pulleys confused with the harmonic dampers found on some V6 / V8 engines. "Harmonic Balancer" is a term that is used loosely in the automotive industry. Technically, this type of device does not exist. The "balancer" part comes from engines that are externally balanced and have a counterweight cast into the damper, hence the merging of the two terms. None of the applications that we offer utilize a counterweight as part of the pulley as these engines are internally balanced.

The pulleys on most of the new import and smaller domestic engines have an elastomer (rubber ring) incorporated into the pulley that looks similar to a harmonic damper. The elastomer in the OEM pulley serves as an isolator, which is there to suppress natural vibration and noise from the engine itself, the A/C compressor, P/S pump, and alternator. This is what the manufacturers call NVH (Noise Vibration & Harshness) when referring to noticeable noise and vibration in the passenger compartment. It is important to note that in these applications, this elastomer is somewhat inadequate in size, as well as life span, to act as an effective torsional damper. If you look at the pulleys on some of the imports there is no rubber to be found at all. We have samples of these, mostly from Acura/Honda, the Nissan Altima, 1.8L Eclipse, 2.3L Fords, Chrysler 2.2L's, and 1.8L VW's, to mention a few. This is not to say that with our pulleys you will hear a ton of noise or feel more vibration from your engine compartment. Most who have installed and driven a vehicle with our pulleys will notice the engine actually feels smoother. This is a natural result of replacing the heavy steel crank pulley with a CNC-machined aluminum pulley. NVH is variable and unique to every car. NVH will increase with the installation of an aftermarket intake and/or exhaust, for example. Think of OEM intake systems in newer cars, they use baffles and resonators in the intake to quiet all the intake noise. Aftermarket intakes eliminate these resonators and create dramatic increases in engine noise from the throttle opening and closing. So to most tuners, certain types of NVH can make the driving experience more enjoyable.

The purpose of a traditional harmonic damper is to protect against crank failure from torsional movement. This is not necessary in most modern engines because of the many advances in engine design and materials. Factors such as stroke, displacement, inline, V configurations, power output, etc., do determine when and how these harmonics and torsional movements occur.

Again, there is a lot of internet hearsay about the pulleys. When engine problems occur, too often people are quick to blame the pulley first, rather than taking the time to look logically into why there was a problem. We hope that after reading this you will understand the crank pulleys better."

 

[mpg99]

Also, instead of linking to the original forum, I will post some of the discussion here:

big toe: "Look at it from the other end.
You can save about 5lbs on the front of the crankshaft with the aluminum pulleys all within a radius of just over six inches. The SR's stroke is 3.38in and the countereight mass is probably another 3 to 4inches off the rod journal centerline. So you have a 5lb weight saving within the radius of the main rotating assembly.
Now you have torsional osscilation at all rpms due to the torsional vibration of the reciprocating assembly, fourth order vibration.
Because the torque is being applied at the flywheel end of the crankshaft the amplitude of osscilation at the flywheel end is significantly lower (ever wonder why Nissan put the crank trigger on the Flywheel end of the new Z). The energy lost at the front of the crank is a function of the frequency of osscilation up front and the mass which is osscilating. Heavier you think is more energy loss, but it's less, because the heavier mass does a better job of cancelling the the osscilation energy and so the total loss is less.
Now the front is flopping back and forth and the rear is more stable. You now put a lighter mass on the rear (light flywheel) and it's mass is now farther outward radially than the mass of the crankshafts counterweights. The effective savings in energy required to spin a lighter mass at a longer radius from the main journal is significantly more than saving it within the raidus of the counterweights.
So save 5 at the front or 5 at the rear, much bigger gain in rate of acceleration of the rotating assembly at the rear. But, because it's less rotating weight the rotaing assembly now has less potential energy and slows quickly when allowed to decellerate.
So light flywheel equals for all intents loss in drivability due to an on off button effect and having to rev the life out of it to get enough torque developed on a big hill with the groceries in the trunk. Builds rpm quickly though and once stable rpm is reached requires less energy to maintain it.
Light front pulley, significantly less energy saved and less potential energy lost, no driveabilty issues.

SO THE MARKETING STRATERGY OF THE ALLUMINUM FRONT PULLEY SETS - ALL GAIN, NO LOSS - PROVEN ON A DYNO GRAPH SHOWING AN ACCELERATION CURVE ONLY.

The other thing with this mod is that the crankshafts torsional vibrations are transmitted directly to the camshafts via the chain drive if not damped out or absorbed at the crank. These vibration frequencies compund the harmonics of the valve train (hence there are timing belts which help reduce the transmission to the cams) and can lead to failure up top if the crank, pulley, timing chain and cam and the valve train all get within multiples of each others orders of vibration and a total harmonic cluster f*** takes place in seconds.
This is the main reason why I do not like crank trigger ignitions either and why Nissan put 360 slots in the SR's distributor trigger disk, the optical trigger effectively looks at the crank every two degrees of crank rotation and therfore does a better job at predicting where the piston is with very high resolution."


Choaderboy2: "Dunno, my race car see's 8000 rpm all day and the motor has many hours with no problems. In fact nearly all of the SE-R cup cars have solid hub UD pulleys and all have very reliable engines. No cup car DNF'ed this year due to engine problems to my knowlege, except me when I missed a shift but that was my fault. Underdriving the waterpump is essential to keep the engine from instantly overheating. My 400 whp turbo SE-R has been pounded hard at 23 psi on the track without failure. The short stiff crank of the SR doesnt seem to have problems although the solid hubed KA24DE does at high rpm."

"the phenomonon you describe as a problem is a bigger problem on real race engines turning very high rpm, like purpose built race engines. My friends at Cosworth and other CART programs struggled with it when the revs went over 13,000 rpm, hence the center crank drives that F1 and CART engines now have.

The scatter phenomina has also been documented by a loss of power in domestic V8's when reved real high.

The SR has a short stiff crank and the block is very stiff. I have recoreded gains of 6-8 hp on both inertial and load type dynos at revs up to 8000 rpm. Ya can't argue with the dyno much. Now if I were to stroke the engine signifcantly, I would use a frictonal damper like a Fisher."

 

[mpg99]

bigtoe: " Nissan put an inertia ring on the crank pulley, because there are harmonics within the rev range of the standard motor that they are trying to diminish.

Does simple physics change when you build a "real race engine"?

3000, 7000, 8000, 13000, or 19,200 on a rear crank power take off F1, regrdless of where it is the frequencies of vibration of the rotating and reciprocating components, if they become multiples of each other at whatever order of vibration you will have failure."


Choaderboy2: "No it doesnt, but a real race engine revs higher and is exposed to more orders of harmonics than a regular engine, hence things like center and rear crank power take off's etc, found in these motors. More revs, the more the destructive energy has to be managed..

I am guessing the damper of the SR is for minor order vibration they are supressing for NVH reasons. There are some interesting white papers on the development of the SR20 avalible from SAE if you want to read more into it.

The power is there and its real, hence its measurabilty in both inertial and load type dynos.

Practical experiance shows that UD pulleys are totaly not a long or short term problem in SR's up to 8000 rpm where my practical long term experiance ends. Practical experiance shows that UD pulley are practicaly manditory for racing an SR to avoid overheating and their is emperical proof to back this up. Not running an underdriven water pump will destroy a SR motor faster than any harmonic issues.

Thats what happens in the real world of racing an SR."

 

[GregLocock]

Would you buy a TV damper from a manufacturer that doesn't understand them? I wouldn't. If listening to the sound of your crank fatiguing turns you on, then fair enough.

As to the 'balancer' part of the name. If you look on the side of an OE pulley you'll see some holes. They are there for balancing reasons.

On the other hand an aluminium pulley could be a good idea, so long as TVs, and balance are sorted out as well.





Cheers

Greg Locock