I have a 2270 motor that I will eventually be putting into my '71 with a sideshift tranny, once I get some dress-up and parts collected. It does not currently have a flywheel, clutch, or pressure plate.

1. Is there a benefit to finding a lightened flywheel if the motor has lighter internals?

2. I assume I should use a 6-spring clutch disk or an aftermarket performance material, but maybe this isn't necessary for a 150-HP/TQ motor? Ditto for the pressure plate.

I posted this on another thread just the other day:

"Lightening the rotating mass does not necessarily give more horsepower.
Reducing the rotating mass of the engine gives an almost negligible difference as compared to removing the mass from anywhere else on the vehicle.

So removing mass from the rotating assembly is the lowest ROI as far as lbs/$$ goes.
Now if you are building a race car from the ground up then, by all means, reduce weight wherever you can, including the engine.

The best reason for reducing rotating mass is to minimize the forces on the crank and allow higher RPMs.
For the greatest effect, the best place to reduce the mass is the conn-rods, wrist pins, & pistons.

Reducing the mass of the flywheel (or crank) cannot give higher HP readings on an engine dyno (or the dyno operator is doing it wrong)
(let the flames begin :-)"


I thought that my statement would have started a larger debate.
It started a shitstorm on my other car forum when I suggested that a lightweight driveshaft or crank pulley would make no noticeable difference in performance - OTHER THAN the effect of removing the weight from the car in general.

Thanks Stu! I did actually read that post, but I didn't want to hijack the thread. Plus I want to know about the clutch disk itself and the pressure plate. Any suggestions there? The car will not be meant for racing, but I don't want to risk slipping the clutch or frying it too easily either. I will probably just take the flywheel and pressure plate that is on my current 1.7, have the FW resurfaced, and put a new disk of some sort in.

Although not on the 914, I had to replace the flywheel and clutch package on the 911 when the engine was rebuilt as it was knackerd

We did lighten and balance the bottom end

I will bow down to Stus greater knowledge on performance and the best bang for you buck and not really why I did it (actually a lower cost option than stock) but I do know the engine wants to spin freely and and safely

nailed it ,little more to say really.

I've been building engines close to 30 years my personal T4 performance engine is 2600cc 180 HP (more details in my signature) summer daily driven.
My flywheel, pressure plate and disc (6 spring, both were new Sachs) is basically stock with a .010 undercut on the fly for a bit extra clamping pressure.

Really, at your power level, you don't need anything more and a lightened fly will just be a PITA on the highway.

I agree with Stu, and wanted to add something. In my travels, I've always looked at the flywheel as an energy storage device. Think of a single cylinder engine: (Especially the "Hit and Miss" vintage engines at the fair.)
You get a power stroke that turns the flywheel, then the flywheel keeps things turning until the next power stroke.

In an automobile, the energy stored in the flywheel helps with smooth starts and easier shifting. (The RPM's don't drop as much when you push in the clutch.)

Lightening reduces the flywheel's capacity to store energy, but it also reduces the work the engine needs to do to spin it up. This makes for a very "revvy" engine that may feel faster, but does not actually increase HP.

If I was road racing or autocrossing, I might consider a lighter flywheel. But for a street car, I wouldn't bother.

Balancing the rotating assembly is money better spent.

I agree with all that has been said ,youll hit on it ,,motor revs faster but you loose torque and thats what gets the car moving good luck

I have a 78mm X 90.5mm 2006cc type 1 engine, that was built by me from a full Gene Berg kit, that with the Berg carbs cost me $5K in 1991. Any that may remember Berg is that he was one of the best of the best in the T1 world.
It has a 12.5 Lb flywheel lighted flywheel, it does spin up quick and I don't really consider it a hassle around town and short commutes. If your speed is being governed by the flow of traffic no problem.

But on a really long trip my foot gets sore from constantly adjusting the gas pedal. You have to constantly watch your speed, you glance down and you're 5 mph under the speed limit, then you glance again and your 25 over , then under....over... under...
It's a real PITA keeping a constant speed.
Basicly you have to keep one eyeball on the road and the other on the speedo.

Hold on, everyone here has a 914 because they like to DRIVE, right? If you want a well behaved street car, you get a Camry. Snappy engine response is one of the things that makes shifting a joy. Maybe the stock flywheel gives you what you want, but it is at least something to think about.

Another key concept (apparently completely lost on the younger crowd on the BRZ forums):

Horsepower measured by a Dyno is really measured at a lot of discrete points of constant RPM in the "curve" as the RPMs are raised.
When you look at "dyno pull results" (like this one from Vans 914 build page: http://www.ephotomotion.com/914engine/page50.html)


It looks like a constantly changing RPM and most people believe that you want to try to do the "pull" as fast as possible and that a faster pull means more HP.
That is not the case, in fact, the SAE requirements for dynamometer measurements are that the dyno is supposed to pause at each discrete measurement point for a specified duration before measuring torque & RPM.
In practice, dyno pulls are done in a dynamic sweep that seems to be one continuous pull, when they are really performing hundreds of discrete measurements.
The most accurate dyno measurements are done as slowly as possible.

What I am getting at is that each discrete measurement of torque is supposed to be done at constant RPM.

So how does the rotational inertia of the flywheel and crank affect the torque measurement at constant RPM (NOT accelerating)?
Answer: It doesn't.

I was essentially unable to convince the naysayers that you could not "feel" the difference of replacing the OEM driveshaft with a carbon fiber driveshaft on a BRZ.
And they REALLY love talking about how they can feel the difference of replacing an 8 lbs crank pulley with a 4 lb one in terms of vehicle acceleration.
I calculated the difference between a 0-100 MPH run with the OEM driveshaft and a run with a "magical" driveshaft that weighs ZERO.
There was a theoretical difference of 10 milliseconds in a 0-100 MPH run, but they were still convinced that you could FEEL the difference with their "Butt Dyno".



This IS, in fact, the only noticeable difference a driver will "feel" with a lightened rotating assembly is throttle response when the car is out of gear (again - as compared to removing the weight from anywhere else on the car)

I used to have a very light flywheel on my 914 with the first revision of my 2056. It spun up really quick, but also spun down really quick. It was fun for short drives but it got old pretty quick on longer ones or if I got caught in traffic anywhere. When the engine came apart for Rev2 (its currently on Rev3) the light flywheel was replaced with a stock weight one (maybe a TAD lighter - I don't honestly remember). It made the car a lot more fun to drive IMO.

Zach

The only thing I find snappier is sitting at the traffic light, clutch pedal in, blipping the gas. Once you have it in gear I've never felt anything that suggests that it has more zip except maybe a hair in 1st/2nd.
Seeing the 914 is a road/track car that doesn't make a hill of beans difference.

I put a lightened flywheel in Betty's car.

She loves it. She says it makes the throttle response much better.

Give the customer (my wife) what she likes and wants.

What if she wants no head gaskets?

Ha ha ha, you can be pretty funny, Mark... sometimes.

I too have always said lightweight flywheels are for racing, not the street, but something's come up. I have two WRX/914's here. They're pretty much the same but have a few differences, so it's interesting to compare. One car has a standard OEM WRX transmission/clutch/flywheel, the other the same transmission but a performance clutch and a lightweight flywheel. The car with the heavy flywheel is easier to drive in traffic, the lighter flywheel is more fun to drive everywhere. It probably doesn't accelerate any faster but it performs faster, if that makes any sense. It may only be psychological, I don't know, but it seems to spool quicker. I like it and next time the car with the heavy flywheel needs a clutch it will also be getting a lighter flywheel.

There's a big difference with these cars, though. Both drivetrains come from 3100 lb cars, so they're relatively unstressed in the lighter cars, and the lighter flywheel might even be proportionally correct for the 914's lighter weight. The situation might be different with a T4 engine and a 901, that I can't say. But I do like it in my car.

Yep, lighter flywheels will make for a faster revving motor. But only when it's decoupled from the transaxle. Once you add all of the rotating mass of the rest of the driveline, that weight reduction doesn't amount to too terribly much. You'll get more benefit from shedding weight on the wheels...

-Josh

edit: It will make for zippier double-clutching though...

I agree wholeheartedly
I love the snappy response and have never had a problem driving my car.
All of my 2.0 motors will always have lightened flywheels!

Thank you all for the very well-stated opinions! Lots of good arguments for both sides!

Since I have a standard flywheel, I'm just going to use that. I don't think it will be worth the extra expense, although if I find a deal I might reconsider

Well, exactly, which is what happens every time you shift. As you move from one gear to the other, in some situations you use the throttle to get a certain RPM before engaging the next gear. Faster response gets you back in gear faster.

I like lightened flywheels:

When I built my 2.4 (Circa 1990), it was recommended to lighten the flywheel.

The reasons given where better throttle response, and less rotating mass for the clutch to absorb in a hard shift.

No one talked about lightening the overall weight of the car.

Rumor has it the Type 4 was designed as a stationary engine for pumps and generators, and hence was designed to run at very steady low RPMs. Lower RPMs need heavier flywheels to get the same stabilizing effect.

Of course we were idiots in those stone ages!

stock flywheel, balanced and solid clutch disk with no springs.

I don't think you can rev it quicker! Cam has a big influence.

Once you are in 3rd,4th,5th gear a light flywheel doesn't make a hill of beans, this is where gearing changes will make the huge differences.

A lot of the mindset for a lightened flywheel is born from racing, drag racing in particular, but also road racing as well.
It also comes from the VW type 1 world where lightening a flywheel you are going from 18lbs to 12.5 lbs, on a T4 to get to 12.5 you are only losing about 2lbs.

Now on the machining end of lightening a flywheel, on a street car even performance you still want to see a good service life. I strongly recommend that you do not remove material from the backside of the friction surface. Doing so will risk warping of the flywheel, especially if you overdo it.
Really in lightening reciprocating mass (RM) it only makes sense to remove material from the outside of the fly. lt's also interesting that most street performance apps you want a counterweighted crank, thus increasing RM.

In the type 1 world it's highly recommended that you do not lighten a fly without counterweighting even the stock crank, but then you are removing more weight from the fly. It has to do with introducing harmonics, as the weight is also there to even out the firing pulses.
My Gene Berg 78mm counterweight crank set has a lightened 12.5lb fly, but then it also has a 6.5 lb pulley. So all told the complete assembly is close to stock weight, but it has a tighter center circle of the RM.
Original Berg stroker cranks are the holy grail of T1 cranks, I likely could part out my engine and get $3-4K just for the crank assembly on evilbay.