checkmate1996

From previous threads there is a 78-79 flywheel and 80-83 flywheel. What are the differences and does impact performance?

MarkRobinson

I'd say with 95% confidence, no performance differences as the weights are probably within a pound of eachother. That said, all '78-79's were cis, & all US-80-83 were AFM & I believe have cranks sensor posts in the flywheel. Not sure if that impacts the 78-79 clutch or not, might want to check on that.

Mark

mark kibort

remember, when you are in gear, the total inertia is the flywheel AND the entire car's mass. so, in effect 7lbs on the flywheel or clutch will have the effect of near 200lbs in 1st gear if in the car, 70lbs in 2nd, 30lbs in 3rd, 16lbs in 4th and almost nothing as you get in the other gears.

think of the effect of one lb.

MK

GlenL

Mark,

I'd like to see how you're getting those numbers. (Of course I would.)

To Brad's question, I don't think it makes any difference. The small lip is inconsequential for changing the moment of inertia. Are the friction disks the same size? That'd be what's important.

Jim bailey - 928 International

The clutch packs are very similar no real effect. And crank sensors only started with LH injection 84 Euro 85 USA. Light weight flywheels only show when revving out of gear blipping the throttle between gears contrary to popular belief have much less effect on overall acceleration of the car. Especially when bolted to a 50 lb crankshaft.

JEC_31

FWIW:

One of my coworkers (the crazy maintenance guy) used to run a '55 Nomad at the local dragstrips, and just this morning he and I were discussing flywheels. He said that for launching at the light, the best thing is the heaviest dang thing you can bolt on so that you don't bog and lose RPMs when you drop the clutch. He went on to explain that the horsepower spent accelerating that round chunk o' metal is a drop in the bucket compared to the benefits of a good launch. It also makes the car smoother and more streetable.

Now the opposite comes from my RX-7 buddies, who have featherweight cars with (non-turbo) Wankel rotaries that don't have much useable low-end torque. Also these are not dragsters - they are 99% autocross, road course, and twisty backroads cars that need to be able to acclerate out of corners. One of the best mods for them is a flywheel that's much lighter than stock, as it has huge benefits for freeing up the motor to acclerate quicker and while doing so spend it's power on the wheels not the flywheel. This of course makes the car less streetable, as it's now quicker to drop RPMs between shifts and you have to fine-tune your "blip" to compensate.

Choose your weapon for which battle you want to fight, I say...

mspiegle

I don't know about a 928.... but my friend just went from a 25# flywheel on his M3 to an 8# and while it revs quickly (in neutral), we couldn't detect any faster performance. He was pulling 5.3sec 0-60 times with ONLY a short shifter (stock car). He went to r-comps and added mo' power and the light flywheel and he's pulling 5.0sec 0-60 times. Between the r-comps and mo' power, I doubt the flywheel helped any of that .3 second decrease in time.

I have another friend with a 350Z who went the whole 9-yards with a tilton 4-plate clutch and light flywheel. Other than better shifting on the track and loosing some weight, he gained nothing in the 1/4.

JEC_31

Exactly what my friend said, that drag racing needs heavier flywheels. I'm starting to get the idea that only low-powered featherweight cars will see measureable (and not large not large) straight-line performance increases with lightweight flywheels. It's the quicker shifting and helping-out with any lack of low-end power that are the main benefits.
However, both the sweet DOHC inline 6 of the M3 and the ballsy 24V OHC 3.5 V6 of the 350Z have no lack of low-end power!

Did the either the M3 or the Z pilot have any trouble adapting their shifting to the lighter flywheels?

mspiegle

Strangely, both of them reported that it was EASIER to shift while at speed. Since they're both "performance-oriented" drivers, they already shift fast (faster than their revs drop with a stock flywheel), so having the lighter flywheel helped their shifting become smoother. The only downside was starting out because of the inertia issue. I think almost anyone will agree that the flywheel is good for performance-oriented driving (track/auto-x/twisties/etc), but it doesn't help in a straight line like everyone (read: vendors that sell these things) says they will.

mark kibort

think of how much hp it would take to spin up a disc of 7lbs from 3000 to 6000rpm in 2, 4, 7 seconds. depending on the gear you are in. (ie 1st is a much faster rate of acceleration than 3rd or 4th), so you can see its effects are dependant on rates of acceleration. a 1hp milling machine can spin up the disc in a few seconds to 6000rpm.(and thats from 0rpm)
so, again, it depnends on the rate of acceleration. thats why its effects are much more noticable in neutral!
in gear, that 7lbs or, as we were talking about , 1lb, is just a part of the entire mass, even though its carrying more effects do to its spinning. but its not a big number!
.xxHP to 20hp depending on a bunch of factors when in gear and at the drag strip. on the road race track, more like 1-5hp.

again, thats for 7lbs. if we are talking about 1lb, its going to be more like 30lbs in the car (sub 3hp) in 1st gear and way under 1hp for every gear after that.

as a rule of thumb, if we are talking about tires, that weight on a rim is 1.4x as if it was in the car and 2x if it is on the tire. still no where near the effects that most folks hear about from the magazine ads or tv shows.

Acceleration = power/(mass x velocity) but with spinning mass, we have to find a mass equivilant to make the equation work, but you get the idea. as you find that Hp to weight ratio, you can see the effects of weight with HP /weight by changing the weight values.

Mk

GlenL

Mark,
First off, F=ma works perfectly for this analysis. The power applied in immaterial in how the flywheel design affects acceleration. No technical need to repeat your favorite equation.
Secondly, it's not the weight that matters, per se, but the moment of intertial.

bd0nalds0n

If I have a lighter flywheel and a 3.10 rear end, how much faster will my car accelerate?

checkmate1996

...if two trains were leaving a train station at 1:00pm with a 7lb flywheel ....

FlyingDog

Is one train Bugatti powered?

mark kibort

its not only the moment of intertia,
its the rate of accelerating it as it applies to hp cost or gain.

yes, both a=F/m and a=power/(mass x velocity) work here. no argument there.

when wheels and tires are being used, its:
M=I/R^2

when looking at flywheels, its:
M=I x (gear x final ratio)/r)^2

with a 16" rim, 1 lb acts like 1.4x in the car

with a 9" diameter flywheel , all the increased weight of 7lbs at the flywheel edge equates to :
200lbs in 1st, 71lbs in 2nd, 31lbs in 3rd, 16lbs in 4th and 10lbs in 5th.
(done for a honda civic) you can see if the weight was evenly distributed, the values would be much less.

Mk