targa72e

You are confusing torque with inertia. The torque out put of an engine has nothing to do with the weight of the internal components. The inertia of an engine has everything to do with the weight of the components. The heavier the engine components the more energy that goes into getting them moving "accelerating" more inertia. You get this energy back when "decelerating". If the engine parts or flywheel are lighter more energy can go into moving the car forward (same goes for wheels car weight etc).

John

Bill

The reason the 944 engine has such a heavy crankshaft, is to add recipricating mass. In order to improve fuel mileage, the Bosch DME strategy shuts off the injectors when the throttle is closed. When the engine management is ready to turn back on the injectors, it tis the heavy recipriating mass of the crankshaft that prevents the engine from stalling.

Yes a heavy damper was installed on the 3 litre S2 to reduce vibrations. On the 968 3 litre engine, Porsche did a number of changes to increase output. With each cylinder's volume at 750cc, increasing volume was not concidered an option. To increase output, it was decided to increase the rpm limit of 5,800rpm that was the maximum on the S2 to 6,200rpm on the 968. The Pistons and rods were forged and were significantly lightened. This weight reduction in turn moved the crankshafts "forth order" natural frequency of vibration outside the engines normal rev range, allowing the heavy crankshaft torsional vibration damper (weighting 5.5lb) to be discarded.

FRporscheman

Suddenly everyone is a physicist. Well not to brag, I actually am a physicist. But that doesn't mean I know the answer either. I'll just add my 2 cents now to keep things churning.

True, less mass means less rotational interia, and that will be "easier" to get spinning. Meaning less force consumed to make it accelerate. The force generated by the combustion, acting as a pressure on pistons to cause them to move in a time interval over the stroke distance, all translates into power. Bore and stroke are fixed but decreasing time (higher rpm) will multiply the power. This is all a simplistic view. The point is a lighter fw or crank will free up some power.

Torque is commonly defined at lever arm times force, but I think the more applicable interpretation is rotational inertia times angular acceleration. Since rotational inertia factor is multiplied, then increasing this will increase the torque. I would think angular acceleration, for an engine, would be inversely proportional to rotating mass, but I think the proportional dependency is greater than the inverse dependency (i.e. the net dependence on mass will be a multiplier, not a divisor).

OK if I proof read that, it probably won't make any sense, so I won't! So please ignore typos.

sawood12

The torque is produced by the engine (combustion force x distance of the crank arm) but soaked up by the roataional masses during accellaration of them so you end up with less torque and therefore less power at the wheels. At a contant speed once the masses have been accellarated then there is no loss of torque apart from through frictional and windage losses. When decellarating the rotational masses increase torque as they give up the energy stored within them to the drive train, which is good in a road car where you want to maintain constant speeds with minimal input of the accellarator, but not so good in a track/race car where you are either accellarating or braking and want your engine to be more responsive to throttle inputs.

A lightened flywheel allows faster accellaration and decellaration of the rotational masses, so better accellaration of the car and more severe engine braking, as less torque is soaked up in accellarating them i.e. more power, however you get lumpier and more difficult running.

That's my understanding at least.

333pg333

I understood everything up to this point:
"Suddenly..."
no seriously, it seems as if creating enough power via the engine to start the propulsion of the car in overcoming the inertial components is...oh don't bother I can't get my mind around this.
So the heavier components are working against you to a certain point and then they start working for you. Whereas the lighter stuff is easier to get going and operate, but it won't carry as much momentum for street driving. What about braking? Seems to me that you would also prefer lighter parts? Does seem like tq and inertia can be confused pretty easily, are they not distinctly related or even evil twins?

Duke

The moving force is created by the ignition of the air/fuel mixture in the cylinder resulting in pushing the piston down to turn the crankshaft.
How could possibly adding more weight for this crankshaft to turn result in more torque?

You are overlooking far too many things to conclude that a lightly revving engine has less torque than a heavy engine!


BTW 333pg333>>

fastmover

I understand that a lighter rotating assembly will have less inertia, but there is a differance between rotaing mass and reciprocating mass. reciprocating mass uses heavier components to keep torque output up. Basically used in boat, aircraft engines and tractors. In car engines, the engine is based on rotating mass, so lightening up that mass is going to increase power output. Driving characteristics will change, but they are usaully beneficial to a performance vehicle. For the stalling aspect of the engine, because the rotating assembly comes to a stop faster, that can be tuned in the dme to slow the rate of deceleration of the engine, by not shutting off the injectors as fast and changing the dashpot decay rate of the idle control valve. That's what i did on my last lightweight engine and had no negative aspects when driving on the street.

In any case if you want to build an engine with heavier rotating components go ahead, I just think from personal expeiriance that the positive benefits of a lighter rotating assembly far outweights the negative aspects, and makes the engine response more intuitive while driving.


Semper fi

George D

In any case if you want to build an engine with heavier rotating components go ahead, I just think from personal expeiriance that the positive benefits of a lighter rotating assembly far outweights the negative aspects, and makes the engine response more intuitive while driving.

Perfect. Nothing more need to be said.

tconn

Another thing to consider about a knife edged crank.
I cuts the oil instead of slapping it. This reduces oil foaming,
which we all know is a good thing.

FRporscheman

Like I said that's just a textbook-style understanding. Also I was only talking about rotating mass not reciprocating mass. I don't know as much about the engineering of motors as I do fundamentals of physics so I'm just offering basic theory.

It's hard to predict the form of the output based solely on how we change one variable like rotating mass. Clearly, slapping on a 500lb flywheel isn't going to result in super-muscle-car torque. There's a practical limit on just how heavy the parts should be to maximize the torque output with everything else left alone. And maybe the torque is maximum at an undesirable point, where too many other factors (like responsiveness and drivability, etc.) have been sacrificed.

I don't think anyone will like a fw any heavier than a stock one. Maybe a DMF but no heavier. I think of the practical range of fw mass to be somewhere between the DMF mass of like 25lbs and a lwfw mass of like, what, 5lbs? There is a zone where torque, manners, and response is in good balance, and it might be somewhere in the middle of the range... I'd trust the master engineers at Porsche who have decided the stock weight is the best overall. As your performance level changes, your goals should be reassessed and if you really want lots of high end hp then you need lighter parts.

I'd love to see dyno sheets of 1 car with like 3 or 4 different flywheel masses. Maybe a 968 where the flywheel can be swapped more easily. Or any car out there... just to get some real data.

333pg333

I pretty much agree with George based on having a knife crank and lite fly and I love the responsivness. I also went to the s2 cwp which shortened the gearing considerably so all this means I can be in 4th gear before the corner shop! I enjoy changing gears and consider it an art form to do it well so that doesn't matter to me that I'm using them more often but back to the topic. I guess what Porsche designed all those years ago were for a fast hwy cruiser with about 210whp so they made everything to suit that car. Now imagine what they would build if Bill Gates offered them an open cheque book to make him the ultimate 951? Just wonder what they'd come up with then?

George D

Funny! Bill would at least get a lightened crank, stronger forged rods, and stronger forged pistons with everything balanced. LOL

333pg333

I still don't get the process of balancing? What actually happens? I have to guess that the way to balance an engine is to design and build it with trial and error until you get it right? If you change all these components that we're talking about apart from putting the engine together and testing harmonics (I guess?) and then changing the balance shafts to compensate, how else do you balance the sum total of all this stuff?

FRporscheman

I think the balance shafts cancel the harmonics of 4 pistons in motion, not the vibrations of various unbalanced engine masses. That's why all variants of the 4 cyl motor have the same balance shafts (they do, right?). Balancing the rotating and reciprocating mass wouldn't affect the effect of the balance shafts, but lightening the recip mass would.

333pg333

Yes but when a builder says he balances these components before assembly, what is he doing? Obviously he's not altering the balance shafts so by merely making sure the pistons are all an equal wgt, or spinning a crank on some measuring device (does that exist) or ensuring that the rods are all equal, wrist pins blah blah blah, this still doesn't ensure that the engine as a whole is balanced in my eyes?