[Skiddins]

I have been reading some interesting info on the effects of reducing the weight of the flywheel etc.
http://www.pumaracing.co.uk/flywheel.htm

It seems that cutting back on small amounts of weight can create large differences in the lower gears.(I'm sure most of you knew about this sort of thing as well)

I have also read recently on this board about people disabling the balancer shafts on their 9000's.

Can anyone tell me what the weight of any of the parts involved in the balancer shaft assemblies is, and what their RPM's are with respect to the engine RPM's?

Thanks
Skiddins

 

[California Saab Tuner]

Cutting back on rotating weight does drastically increase power in low gears. But on FWD cars this usually just translates to wheel spin.

For a Saab the cheapest way to go faster for anyone with a turbo Saab is just to give it more boost in the lower gears. Because unless you've got slicks, full boost in 1st, and probably even in 2nd will just make your tires smoke. Although, I have heard good things about aluminum flywheels and gas mileage around town. Could be worth it if that's what you're looking for. Beware however, removing TOO much weight can on some cars cause idle problems. If the car missfires at all at idle, or has lopey cams sometimes it can stall due to the reduced idle momentum. Shouldn't be a problem on a good running Saab though. Cheers and Ciao!

 

[ms08]

Whilst removing weight will certainly aid the speed with which the engine increases revs, I think it is more about increasing the rev range of the engine at the top end(with the potential to widen the power band) which is probably more important to race cars where they will be working more frequently than road cars. I would think that may benefit N/A cars more than turbos and idling issues do come in when they become 'lumpy'. Removing 'dead' weight is always of benefit provided it doesn't compromise strenght.

 

[Mark B]

I'm having to dig into the little used part of the back of the brain here, so I may say something that's not 100% correct, but here goes:-

It's not the mass of the components that's the issue, but their "polar moment of inertia". For linear acceleration, simple mass is sufficient for determining the force/acceleration relationship. Think of "polar moment of inertia" as "rotary mass". Items with a high PMOI are difficult to accelerate in rotary motion, items with a low PMOI are easy to accelerate in rotary motion.

Imagine a 1m bar with 1kg masses at either end. When lifted in the centre in one hand, it's quite difficult to start & stop rotation. Move the weights to the centre and it's a lot easier. If my memory is correct, PMOI is proportional to the square of the radius at which the mass rotates.

Things like balance shafts have a low PMOI as they are long and thin (low radius). Balance shafts rotate at twice crank speed. The flywheel has by far the biggest effect, as its radius if large and it is thick at the edge, so most of its mass is at a large radius. If you want to reduce the PMOI, reduce the mass of the flywheel by removing metal around the edge. This does not increase static max engine power output, but when engine speed is increasing, during acceleration of the car, less power is used to accelerate the flywheel, so more is available to accelerate the car. The effect is greatest in the lower gears as engine acceleration is greatest then.

All crystal clear?

 

[Mark B]

Whoops. Sorry, second post by mistake.

 

[mark e]

Mark, I agree with your statement about PMOI.

The flywheel is quite important for a steady idle and I wouldn't take very much if anything off it.

However I do think their are gains to be had from taking the balance shafts out. Although their PMOI isn't very high by comparison, they are still fairly weighty beasts and the drive mechanism to them will rob the engine of power.

I'm looking in to blueprinting my engine at the moment and the way to go seems to be to leave the pure rotating mass (ie the flywheel) untouched but lighten the non-rotational mass such as the piston stems. As the balancer shafts are by nature eccentric I can only see it being of benefit to remove them for performance gains.

 

[California Saab Tuner]

Lightening con rods, and the pistons themselves (if possible without compromising strength) gives out right horsepower as they must be lineraly accellerated with each engine stroke. Less weight, less inertia, more hp. But lightening the flywheel won't necessarily cause idle problems. It "can" but if you've ever changed your own clutch you'll realize that the weight of the clutch assembly NOT including flywheel is more than enough to keep a good running engine idling smoothly, unless your engine runs quite poorly. Still though, it's expensive to lighten given that you're not likely to ever use the power you gain from it. The gear you're going to notice the biggest difference is 1st, and you can't run full power in 1st anyway.

 

[johnw]

Here's a picture of my alloy flywheel, you also get the benefit of the bigger 9000 clutch when fitted to a 900... http://www.saabcentral.com/forums/viewtopi...er=asc&start=20

 

[Eric van Spelde]

Got two kgs skimmed off my c900 flywheel years back (it now weighs 7 kgs btw). Idles as before and getting the car up and running smoothly from a standstill is still very easy (right, Bill J?) - but throttle response and rev-ability have vastly improved, which IMO is the greatest benefit of a lightened flywheel by far.