Disclaimer upfront: This is a very unscientific test. Your results may vary (and of course so will your opinion)...

When it came time to replace the clutch at the end of the season last year, my Dad wanted to go back to the dual mass flywheel (he didn't really appreciate the gear rattle). So I reinstalled the DMF with a new stock GM clutch.

We raced at the Corvette challenge this past weekend and got some data with the new/old dual mass set-up. The car is much easier to launch now (if the car has traction). This past Saturday, I was launching at 3500 and getting better and more consistent short times then with launching at 6000 with the single mass (the single mass would bog at launch RPM's less than 5000).

In fact, I had to change my driving technique slightly. With the single mass, it was hit or miss, it would bog or it would spin, so I adapted and would "slip" the clutch to get the best short time. With the dual mass, I can now launch at a lower RPM of 3500-4000 and "dump" the clutch without fear of "bogging". This makes my reaction times and short times MUCH more consistent. So it seems as though the dual mass is much better for drag racing assuming you can put all of the power to the ground (no real surprise there).

So I decided to compare runs in similar weather conditions. The only difference is the SMF to DMF, and the launch technique (I keep all this info in a spreadsheet). On both runs, I dumped the clutch, 5000 RPM for the SMF run and 3500 for the DMF run.

Here are the two runs (incremental data to 1000'):

SMF 10/27/01 1.846 5.243 8.036 88.53 10.412 Launch at 5000 RPM
DMF 04/06/02 1.852 5.247 8.045 88.54 10.452 Launch at 3500 RPM

So where is the 0.2 and 2 MPH that everyone talks about when going from DMF to SMF??

The other "nice" feature is that full-on power shifts are MUCH softer with the DMF. The DMF absorbs the shock of the shift. The SMF does not and consequently the shifts are very harsh with the SMF.

So for anyone considering going to a SMF, I wouldn't recommend it (at least not for drag racing). The DMF allowed me to launch more efficiently and at lower RPM's. Remember, the ZF6 has a numerically low first gear ratio, so the stored energy in the heavier flywheel really seems to help the car get off the line better. Plus it's much easier to live with around town (no more gear rattling, easier 1st gear starts from part throttle, etc).

That's my 2 cents...

 

I think most would agree with you. But they may convince themselves other wise due to the $$$$$$ of the DMF :D
I myself would never go for a SMF in a street driven car especially daily driven. Now on a track car, of course.

 

Great info, Thanks

 

I've always been told that for drag racing you want a heavier flywheel to help launch the car.

If you are running roundy roundy, use a lighter flywheel to help get the rpms up.

 

We will see. I just put in an aluminum SMF and better clutch not long ago and getting ready to head to the strip. I always thought that GM used the DMF mainly to quieten the tranny- they didn't use them in the F-bodies with the T-56, did they? In fact, are there many other drag cars using the DMF set-up? I really don't know- that's why I'm asking. If nothing else, it did cut about 30 pounds from the car. Now if only I could do the same that easily. :lol:


[Modified by Dr. Doom, 2:41 PM 4/9/2002]

 

Yes, that's true. The ZF tranny has straight cut gears and is very noisy without the dual mass to dampen the vibrations. F-bodies used a SMF.

I posted this info because the common belief related to C4's is that going to a lighter flywheel yields a 2 tenth reduction in ET and 2 MPH increase in speed in the 1/4 mile. My data indicates that this isn't the case.

The way I see it (and others too) is that you should only be putting in a SMF for road racing or because it's significantly cheaper. For drag racing, if you can get your car to hook up, I'd stick with the DMF.

In your case, you are making a ton of power so all bets may be off with you!


[Modified by Eric Fischer, 9:53 PM 4/8/2002]

 

Click my pic.





[Modified by Mr Mojo, 10:53 PM 4/8/2002]

 

I'm one of those that did notice a reduction when I switched over. The car does rev much better and I picked up a little more than 2 mph but about 2 tenths is right... However, I do agree that the car is MUCH harder to launch now!! It is very critical and there is a very small margin of error with the SMF as opposed to the DMF. But, then again, my car doesn't hook worth a shemp anyway... And I certainly am not big on the noisy ZF, but I don't have a whole lot of choices here. A lot of us that aren't running one-piece seal cranks HAVE to run a SMF because there isn't a two-piece dual mass!!
-Jeb Burnett


[Modified by jburnett, 4:37 PM 4/9/2002]

 

Have been real pleased with the SMF in my LT-1. It is actually quieter in my 396 than the 350. MTI set it up with the 396 which probably helped. We need to take a survey on six speed LT-1's than have run in the 11's. I think you'll find most of them have the SMF set up.

 

Please educate me. Can someone explain the physical differences between the DMF and the SMF. :)

 

The whole point of the flywheel is to store energy and provide constant angular momentum. A lighter flywheel will let the engine vibrate a bit more. Each cylinder firing provides a peak in the power and torque delivery, and each space between firings is a trough. Flywheels catch and store the peaks and average them in with the troughs for smoother power delivery.

I can't tell you the difference between a single and a dual, but a more-massive flywheel will always provide smoother power delivery at the cost of responsiveness and efficiency. The engine has to spin up the flywheel as it revs up.

A less-massive flywheel provides an engine that can change rev speed much faster, while being harder on the components and providing more vibration.

 

I disagree with a few claims made. A heavier flywheel does not decrease efficiency. It is a stored energy device and some of the most efficient cars in the world are driven by giant flywheels. The "sensation" that an engine with a lighter flywheel "rev's quicker" is based upon "useless" no load situations with the clutch in or in neutral. In this case, the angular acceleration of the flywheel is fictitiously large. This does not mean the engine will rev significantly quicker under the full chassis load that includes driveshaft, halfshafts, ring gears, four large 40 lb wheel/tire combos, rotors and a 3K lb chassis. The additional loading due to a dual mass flywheel does not exceed a couple HP in the extreme situation of maximum acceleration thru 1/4 mile. As was illustrated in the original post, this factor is negligible. Single mass flywheels are for those obsessed with lightness and a few road racers that want better downshifting performance and want to minimize tire breakaway due to poor speed matching. I doubt if a poll of 11 second rides will provide much insight. A similar poll will show red cars are faster and get more speeding tickets than other colors. The color or flywheel type is not a factor.

 

Maybe the aluminum might be to light for some, but the 40lb dual mass is a bit heavy for a Vette. IMHO that is. The Vette can certainly stand a lighter flywheel. When you are in gear and rolling, the throttle response will be better with a lighter flywheel. Ever wonder why sprint cars do not use a flywheel?
I'm goin aluminum when I replace my clutch. :D

 

Maybe we should all agree that the change to single mass flywheel should be based on cost:)that is pretty black and white...:D

I saved some 400-500 bucks by going with a Camaro flywheel...:yesnod:

 

Rick,

EXCELLENT POST! It makes sense that the loss of 20 #'s at the flywheel is insignificant compared to the heavy forces required to accelerate a car at full throttle.

The only advantage I can see for drag racing is IF your car has more power than traction, a lighter flywheel could help you get off the line with more traction. Maybe this is the case with the high torque of those 11 second stroker motors.

 

I think you are right Eric.. when we used to race the D/A we used a very light flywheel, but that car was totally different than a street car... it would leave the line at over 7000 rpm & be in 2nd gear before the 60' clocks.

As for traction we'd adjust how hard the clutch would hit...depending on track conditions..

 

Rick, you got me. I did not consider the relative mass of the flywheel in comparison to all of the driveline components downstream of the flywheel.

My old IC Engines professor just got out of bed to mark up my post with a big 7/10.

"An internal combustion engine is a machine with insufficient power." Dr. Alfred E. Gardiner, Fall 1992.


[Modified by Matt Black, 9:59 PM 4/10/2002]

 

IMHO, I would assume that 99% of the people who change flywheels are also changing the clutch at the same time. I don't really see how you can compare the two when you have a worn and slipping clutch vs. a new clutch regardless of the flywheel. When I replaced my GM stock clutch and installed a SPEC Stage II, the car feels much more responsive and now I've got traction problems with the ASR comming on all the time and that's with 315s on the rear...