Heavier flywheel = less rwhp??

[Guest bastaad525]

I didn't think about this when I was having the clutch and flywheel replaced. My old flywheel was a stocker (turbo/2+2 unit) that had been lightened to somewhere in the 16-18 lb range... I forget what they weigh stock, but it was only a few lbs difference if I remember.

 

Anyways, I just replaced it with a regular stock flywheel, and someone told me yesterday, that was going to cost me hp on the dyno when/if I went again. Is that so??? Any idea how much of a difference we're talking???

[johnc]

Steady state horsepower (let's say 200hp at 6,000rpm) is unaffected by driveline mass, MOI, or vehicle weight. So, your car hasn't lost any horsepower by increasing the weight of the flywheel. But, you car's rate of acceleration has been decreased because of the additional weight and by the increased MOI. Is it a significant decrease, probably not. You'll just notice the car revs a bit slower.

[ecp48]

Sure it will cost you a "little" HP. I can't give you a number. It takes horsepower to overcome inertia and weight. If you are racing, it is a big deal, as evey little bit helps, but for a street driven turbo, it should be smoother as you won't have as big an RPM drop between shifts.

 

I will be installing one of the Maxima Flywheels on my NA 280Z, which will make it a little sharper (quicker reving).

 

I and every other Formula V racer ran a lightened fly wheel. Low HP engine (race trim 59 to 62 HP), everything was fractional HP gains and ability to rev freely.

 

But again, if maximum HP is your thing, you wil lose a small amount.

 

ecp48

[Guest bastaad525]

oh yeah I DEFINATELY notice the revs reacting slower, dropping slower between shifts and such...

 

but would this show up as lower hp on the dyno?

[ecp48]

bastaad525,

 

Short answer, YES. It takes HP to move/offset weight and the increased flywheel weight will cost some power. It will be interesting to see how much after you dyno it. Please post.

 

Ed/ecp48

[Guest JAMIE T]

I'm not sure if it will show up on the dyno. BUT, I do know that a DynoJet type Chassis dyno measures how quickly you accelerate the rollers. I like a light flywheel for the reasons given. I don't care what effect the flywheel weight has on "Dyno Horse Power". I drive my car, I don't "dyno it". A chassis dyno is great for testing if you have something to test against and of course tuning.

[johnc]

but would this show up as lower hp on the dyno?

 

To elaborate on what Jamie said: Chassis dynos and engine dynos do not measure horsepower.

 

Engine dynos (load cells) measure the ability of an engine to work against a load at some rpm. From that load number a calculation is performed that gives a horsepower number as its product. The basic calculation uses rpm and load.

 

Chassis dyno measure the ability of a vehicle to accelerate a mass. From that rate of acceleration, a calculation is performed that gives a horsepower number as its product. The basic calculaiton uses rpm, mass, and time.

 

If you took your engine out of the car and put it on an engine dyno, the horsepower numbers would be the same because mass and time are not part of the horsepower calculation on engine dynos. Conversly, because mass and time are part of chassis dyno calculations, things (weight and MOI) that have nothing to do with power production (combustion) are part of the horsepower calculation.

 

So, the correct answer is:

 

1. Your car will not lose any horsepower (as measured by SAE J1995).

2. Your car will accelerate slower.

[ecp48]

John,

 

Not disagreeing with your analysis, but weight reduction in the moving components is a necessity in producing maximum HP. This is because weight has to be moved and the more weight, the more Horse power it takes to move the mass. While mass is not measured on an engine dyno, heavier moving components, flywheel, crank, rods and pistons cost horsepower and will to some extent lower ultimate RPMs, because it takes power to generate those RPMs.

 

However, I don't think the HP in this case will be significant, just driveablity.

 

 

Ed

[Tony D]

"but weight reduction in the moving components is a necessity in producing maximum HP"

 

This is a technical difference ECP. Reducing the weight of rotating components on the chassis does not produce any horsepower.

 

It eliminates a parasitic loss of the chassis.

 

Like John C stated, when measured on an engine dyno, you will see no increase in HP. The engine makes what it makes. It does not "make more" due to the elimination of a chassis component. Only more is read / sensed due to the methodology of the testing.

 

Weight reduction inside the engine may assist in production of HP through friction reduction, etc, but external items do not make horsepower, they only reduce a parasitic loss so what the engine is producing gets to where it needs to be.

 

This is why many teams test on the engine dyno, then retune on the chassis dyno.

 

The two methodologies quantify very different things.

[blueovalz]

I've got to side with John here. The key here is acceleration (or change in velocity). I believe horsepower is a measure of work performed over a given time. At a steady 5000 rpm, the work/time accomplishment is measured as horsepower. At a steady 5000 rpm a 100lb flywheel and a 15lb flywheel, on the same engine, will show the same horsepower, and torque. The issue is how fast can that TORQUE accelerate the mass. If you claim a heavier flywheel reduces horsepower, then it's like saying my LS1 placed in a Excursion will produce less hp and torque than if it were placed in a Z car. But in reality, the hp is still the same, and the torque is still the same. It just takes longer to accelerate the Excursion than the lightweight Z car because of weight tacked onto the engine.

 

And FWIW, I've found the most desirable characteristic to a lightened flywheel is the decreased shift time. With a more rapid decrease in off throttle rpm, I can shift much quicker. When you add the added inertia of 4 rotating wheels, plus the chassis, then the added or reduced mass of the flywheel becomes much less significant. But disengage all that from the motor (pushing in the clutch pedal) and letting the engine slow down faster then becomes (to me) and over-riding advantage.

[(goldfish)]

It takes power to change an items velocity, thank you mister newton. Moving masses that would change hp readings would be any that has to change direction and, or speed of movement, ie. pistons. At a constant rpm the flywheel would have no change in velocity, therefore taking no hp. The pistons change velocity( up and down), so lightening them would free up hp.

[Guest bastaad525]

Okay so basically, my engine is not making any less power (I figured ) but it WILL show less power to the wheels on the dyno due to higher parasitic losses from the heavier flywheel. Makes sense, kinda sux... if I had thought of that before hand I'd have paid the $75 to have it lightened again... thought I read somewhere that you actually DIDN"T really want a light flywheel in a turbo application but maybe I remember wrong.. anyways that was the main reason I left it stock.

[Savage42]

Finally, we get to the point. Yes, RWHP might be slightly lower. Of course, turbo cars make better power when under load, so it would probably be fairly insignificant with your car than a NA car. Gearing is also a big deal with a turbo car. I see that you run 3.54s, which is the way to go. I had 4.11s in my 510 w/ VG30ET and went to 3.70s and it was much better!! A little more flywheel weight is a good thing with a low compression motor, as it keeps the low end from bogging down. It's all a compromise, but you should be fine.