Nissan 370Z Forum - View Single Post - Does synchro-rev really matter??

semtex · 04-06-2009, 04:16 PM

Actually, a heavier flywheel isn't so good for upshifts either, because with a heavier flywheel, the rpms don't drop as quickly in between gears. The higher inertia keeps the rpms spinning for a longer time when you disengage the clutch, but when you shift into a higher gear and re-engage the clutch, the new gear is going to start off at a lower rpm, so upshifts can be jerky as well, especially if you're shifting very quickly. The benefits of a heavier flywheel are less noise, and easier launches. My understanding is that Nissan chose a heavy flywheel specifically due to noise concerns. (They did this with the 350 and G35 as well, btw -- this isn't a new deal specific to the 370 and G37.) And I believe it, because in my 350 I had a JWT lightweight flywheel and that thing sounded dreadful. To the untrained ear, it sounded like the car had a mechanical problem, so installing a lightweight flywheel from the factory would probably have hurt sales as the average consumer would be turned off by the sound. Regarding launches off the line, a lightweight flywheel is much easier to stall, precisely because it has lower rotational inertia. Think of it this way. The flywheel is attached to the crank and is always spinning. When you engage the clutch from a standstill, you are essentially connecting the spinning flywheel to the stationary crankshaft. Now, at that moment, you're mating two opposing forces together. By virtue of inertia, the flywheel wants to continue spinning at whatever rpm level, and the crankshaft wants stay still. Which component will win? Well, the crankshaft will win if you don't give it enough gas. i.e., your engine will stall. With a heavier flywheel, the flywheel has more inertia, which means you have to give it less gas in order to offset the inertia of the stationary crankshaft, which in turn means it's easier to launch without stalling. Indeed, this same physics explains why your engine revs quicker all throughout the rpm band with a lighter flywheel. Remember that the flywheel is connected to the engine crank. When you give your engine gas, the crank spins the flywheel faster. The less mass the flywheel has, the more it's willing to spin faster (i.e., less mass counteracting against the crank). Make sense? (Sorry, I know that was a little long-winded, but I wanted to be as clear as possible.)

The 4 cyl. cars you mentioned probably didn't have a heavy flywheel because they didn't need one for optimal sound level. They were probably a little easier to stall at launch too. I had an S2000 that came with a light flywheel. It was really smooth to shift up and down, but it'd stall easily too from a launch. If I recall correctly, I had to rev it to about 3000 rpm to launch off the line smoothly. I don't have to rev anywhere near that high with the 370Z.

04-06-2009, 04:16 PM	#12 (permalink)
semtex A True Z Fanatic Join Date: Jan 2009 Location: Behind enemy lines Age: 54 Posts: 5,995 Drives: People to drink Rep Power: 32	Actually, a heavier flywheel isn't so good for upshifts either, because with a heavier flywheel, the rpms don't drop as quickly in between gears. The higher inertia keeps the rpms spinning for a longer time when you disengage the clutch, but when you shift into a higher gear and re-engage the clutch, the new gear is going to start off at a lower rpm, so upshifts can be jerky as well, especially if you're shifting very quickly. The benefits of a heavier flywheel are less noise, and easier launches. My understanding is that Nissan chose a heavy flywheel specifically due to noise concerns. (They did this with the 350 and G35 as well, btw -- this isn't a new deal specific to the 370 and G37.) And I believe it, because in my 350 I had a JWT lightweight flywheel and that thing sounded dreadful. To the untrained ear, it sounded like the car had a mechanical problem, so installing a lightweight flywheel from the factory would probably have hurt sales as the average consumer would be turned off by the sound. Regarding launches off the line, a lightweight flywheel is much easier to stall, precisely because it has lower rotational inertia. Think of it this way. The flywheel is attached to the crank and is always spinning. When you engage the clutch from a standstill, you are essentially connecting the spinning flywheel to the stationary crankshaft. Now, at that moment, you're mating two opposing forces together. By virtue of inertia, the flywheel wants to continue spinning at whatever rpm level, and the crankshaft wants stay still. Which component will win? Well, the crankshaft will win if you don't give it enough gas. i.e., your engine will stall. With a heavier flywheel, the flywheel has more inertia, which means you have to give it less gas in order to offset the inertia of the stationary crankshaft, which in turn means it's easier to launch without stalling. Indeed, this same physics explains why your engine revs quicker all throughout the rpm band with a lighter flywheel. Remember that the flywheel is connected to the engine crank. When you give your engine gas, the crank spins the flywheel faster. The less mass the flywheel has, the more it's willing to spin faster (i.e., less mass counteracting against the crank). Make sense? (Sorry, I know that was a little long-winded, but I wanted to be as clear as possible.) The 4 cyl. cars you mentioned probably didn't have a heavy flywheel because they didn't need one for optimal sound level. They were probably a little easier to stall at launch too. I had an S2000 that came with a light flywheel. It was really smooth to shift up and down, but it'd stall easily too from a launch. If I recall correctly, I had to rev it to about 3000 rpm to launch off the line smoothly. I don't have to rev anywhere near that high with the 370Z. __________________ "There are no small accidents on this circuit." -- Ayrton Senna 316.8whp & 248 ft/lbs (Dyno Dynamics) \| 319whp & 256 ft/lbs (DynoJet) (04/23/10) Stillen G3 CAI, CBE, Pulley / F.I. LTH / GTSpec Ladder Brace / Setrab Oil Cooler / UpRev-tuned by Forged Perf. Last edited by semtex; 04-06-2009 at 04:19 PM.