Gentlemen and Ladies;
I've spent the winter rebuilding my 1.8 up to 2142 (or there about). The
question I would pose is about the weight of the flywheel I need to purchase.
My new Eagle Rods and KB pistons are much lighter than the stock equipment
from 1974, does the sum of the mass in the case need to correspond with the
mass of the flywheel somehow? I've seen two choices: 17lbs and 12 lbs. Your
thoughts? Thanks. ScottD

No

Copied from the Nissan site: se-r.net. Experts?? Check the math. Is it good math or fuzzy math?
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As far as perceived acceleration goes, your engine sees the mass of your car as a point stuck way out on some lever arm that it has to twist. If your engine is direct drive (i.e. no gear reduction), and you have an M3, you'd need quite a bit of torque to get that 3175 lbs moving faster. So somebody invented gears, which has the effect of changing the length of the lever, as far as the engine is concerned. In an M3, for example, first gear is 4.20:1 and final drive is 3.23:1 so what looked like 3175 lbs to the engine out at the end of that lever without gear reduction now looks more like 234 lbs (3175/13.57), assuming your rear tire has a radius of one foot give or take a few inches.

Suppose you had a magic flywheel with all the mass concentrated at the outer edge. Now the flywheel is stuck directly to the engine, so you can't reduce its effective moment via gearing. The only way you can reduce the moment is by lightening it and/or changing its mass distribution. If you could somehow remove 10 lbs from the rim of the flywheel, and the flywheel's radius was also one foot, then that would have the same effect on acceleration in first gear as reducing the mass of the car by 10x13.57 or 135.7 pounds. Now I am guessing the flywheel's radius is more like six to eight inches or so, so 10 pounds off it's outer edge would have the same effect as reducing the car's mass by more like 70-100 pounds (in first gear). Only you can't take that much weight off the edge, and moments of disks look more like 1/2mr^2, etc. etc. Point is that in first gear, the mass of your car appears to be only 20-30 times that of 10 pounds out at the edge of your flywheel, as far as the engine can tell. So the reduction of weight of the flywheel begins to be pretty significant. Expect bigtime effective acceleration improvements in first gear for proper flywheel lightening, similar to what you'd expect from reducing the weight of the car by anywhere from 70 to 100 lbs or more. The benefits decrease in higher gears in proportion to whatever the ratio is.

Obviously, the lighter your car is to begin with, the bigger an acceleration improvement you'll see since the flywheel mass represents a larger portion of the perceived mass of the car.

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bill's info is correct. a lightened fly is easier on the engine to spool up it's all in the start though...


but starting out is so much more direct with a 10 lb fly than with a 17lb fly... 7 lbs may not be a big reduction to a 400hp engine, but it's huge to a 140hp engine. it really increases throttle response.

Let's not forget the inertia provided by a spinning flywheel. The heavy wheel provides much more energy for launching the car which was not mentioned by the ricer tome.
Back in the day, drag racing, the standard rule was: big motor, light wheel, small motor, heavy wheel. The small motor needed the extra stored energy that the heavy flywheel provided. This was back in the days of "heads up" class racing where the first guy across the finish line won. We used to launch at 7k rpms with a 301 chebbie & a 40 lb flywheel.

I found a comparo in Hot Rod where a light flywheel showed an improvement of .7 mph over the quarter mile on a given car. I don't recall the car set up.

The light clutch/flywheel combined with a tall 1st gear in the shitbox has made it a stone bitch to launch around the typical 90 deg start box. It does provide an overall weight loss for the car, however, and that is a plus.

What about idle characteristics or other side effects? Does a lighter flywheel, combined with high CR make for a rough idle?

Not that I could notice on a 10.5:1 motor

not that I've seen either... my idle is smooth

With a lighter flywheel, your engine will spin up faster. The lighter the flywheel, the faster it will gain revs. However, it will also spin down faster when you let off the throttle. As long as you keep your foot on the throttle, , you are going to be golden with a light flywheel.

Getting started in 1st gear can get tricky the lighter a flywheel goes. The less mass in the wheel, the more precise you need to be with your clutch and gas transition. With a full weight flywheel you almost do not need to add gas to get moving, it is very forgiving as the wheel contains a lot of kinetic energy, keeping the motor from stalling. The less mass to the wheel, the less energy it stores, and the less forgiving the transition is. This is the reason you will hear folks say that a light flywheel is "not streetable."

The other thing that a heavy flywheel will do is mask an engine that is a little out of balance - smoothing it out a little. A flywheel is like a gyroscope, and it will smooth out minor balance issues with an engine.

So, thats the theory. Match the weight of your wheel to the type of driving that you want to do. The more stop and start driving, the heavier a wheel you want for comfortable driving.

My 2056 used a 10 or 12 pound wheel for a while, but it needed to be resurfaced and did not have enough meat to be ground again. I went with stock weight for the replacement (17 pounds) as I happened to have one on hand. It is noticeable, but the loss of performance is more then offset by the ease of driving around town and when I get stuck in traffic. When I am done with school and have the time to start AXing the car, I might find myself wishing for that lighter flywheel again.

with these guys.

I'd do it even though a four-cylinder will be a little more susceptible to some vibration because there are 33% fewer fires per revolution of the crank. The lighter weight will produce less shear in the crank, but in any case don't romp on and off of the throttle a lot with the engine unloaded.

My 6.75lb flywheel has done just fine since 1999, and that's combined with an early push-style 911 clutch and pressure plate (like yours but 225mm... unless you've converted from the 215mm) in a 3.6L car.

I was an avid Porschelist/Rennlist guy back then and when I put out a post like yours the response was almost uniformly "Don't do it!". Turns out few had and it was just popular opinion that the idle would suffer and the 901 would wear out or break quickly. Neither happened.

Their reasoning, and my initial concern, was that since the original pressure plate and clutch-pak in a 993 is ~55lbs then approx. 33% of that would be foolish! The entire setup I have in the car now is less than 20 lbs... I think I remember 18, but I can't remember the exact number. I'll weigh 'em again during a transmission swap in a week or two.

I'll never go back to stock parts in my machine especially with FI. That said, with huge cams and big carbs, low speed streetablility could suffer.

Good luck!

I can't comment on Zach's experience... when I had a full size flywheel, the engine was a dog... a flywheel of any weight wouldn't help. that said... I have not had an issue with "streetability" of my engine... my only issue in traffic is my left leg getting tired.. it takes very little gas to take off in first now that my idle is set correctly(about 1000-1100)

In my Corvair days, I was always told "Don't Do it", you won't like it. Especially by "Experts"
As mentioned in another reply, I don't know if they were working from "real' experience ?
I got to drive a turbo Corvair with an aluminum flywheel once and it was very interesting. Took off great, it seemed to counter the dreaded "turbo lag" and when you let off the gas, it was like stepping on the brakes. I really liked it.. But, that being said, I don't know how long I'd like it on a Daily Driver. Then again, that aluminum flywheel was VERY light.. If I came across one for the right price, Sure, I'd use it.

Look at a flywheel as a device that "stores energy"

For a daily driver a heavy flywheel allows for easier stop and go driving since you have mass spinning that is more difficult to slow down. Let the clutch out a little too quick and the stored energy prevents stalling.

A lightened flywheel will rev quicker and have better throttle response since it doesn't have to accelerate or decelerate as much mass. A nice touch for racing. It also helps to brake the car when you lift off the gas pedal since there is less mass to slow down. However, if it's too light the engine does not have enough spinning mass and becomes overly sensitive to letting the clutch out...it will want to die.

Many engines need a little mass to idle smoothly, others don't. Light cars and low gearing reduce the negatives of a light flywheel

A common complaint with Porsche engines is the rattling that comes from shaking the syncro assemblies at idle. Lighter can aggravate that.

Bottom line, it's personal preference and the decision should be based on the primary use of the vehicle. Like most things in life, you give up something to gain something......

I think it works better in a racing experience...
I have 1....it took getting used to(strictly street)
but now that I'm used to it.....I like it!!!

My experience with lightened flywheels has been with air-cooled VWs. In general:

a lightened flywheel. . .

- increases throttle response
- revs and decelerates quicker
- decreases fuel mileage slightly (less stored inertia)
- results in a jerkier ride going through the gears (can be a PITA on a street car)
- makes the car "feel" like it has more HP during acceleration

a heavier flywheel. . .

- allows for smoother, more relaxed gear changes (you don't have to shift quick to be smooth) This is nice for street cars.
- increased fuel mileage
-revs and decelerates slower

I like the feel of a stock flywheel on a street car. I don't care for the jerkier shifting and less fuel mileage that always accompanies a light flywheel.

The only time I want a heavy flywheel is when I am pulling a trailer.

Thanks Gentlemen for all the great input. I'll go with the
lightened FW for now and see how it works. One nice thing is that it would
not be a major job to switch to a std. weight if it doesn't work out.

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Thanks too to Jake and all that helped with other info to walk me through
the rebuild process. ScottD

hijack:

what carbs are those? a 2l+ should have at least 40s, those look like solexes...

This is the Flywheel I like to use on my 6 cylinder cars - a 906 Unit that weighs in @ 7 lbs.

Like others, I was advised not to do it - Hogwash. Does it spin up & down Faster - Yes, Does it take some getting used to - Yes. But I like it.

T.C.

Click to view attachment

Yes, I suspected the carbs are under sized. They are Weber 34 ICT's I
bought before any research. Never ran them. Now I'm looking for a pair
of 40's.

how do you intend to use the car?
this should be known to make good recomendations although most of teh comments above are good

No plans to race the thing, but not a daily driver either. Living in the
NorthEast, I'll be picky about the days I will take her out on the road.
Weekend touring mostly, we have a lot of fun windy roads around here.

I should post a WTB on the Classified Forum for a pair of used 40's. Saw a
set on ebay recently, but the price was up to $350 and reserve still not
met. Maybe they are worth it.

Also look for Dellortos. Great carbs