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Lightened flywheel options

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DerekJ

DerekJ

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I have been running my car in sprints and hillclimbs and this winter Im considering various upgrades I might make. One is to lighten the flywheel as quicker acceleration will suit the hills I'm running on.

I think my car still has the original flywheel. It has a BJ8 engine, with forged pistons, a DWR8 fast road cam and B&M Super Damper.

What is the weight of the original flywheel and what suggestions would people have regarding how much lighter I should go, - bearing in mind that I drive the car to the evnts so I need it to be driveable.
 
My understanding--caveat: I don't have a lightened flywheel--is that the lighter flywheel won't do much if anything for the acceleration of the car (when you think about it, the flywheel is a very small part of the overall mass of the drivetrain and vehicle). The main advantage is a quicker-revving engine, good for quick blips of the throttle; e.g. to facilitate downshifts. At least, that's how an experienced racer explained it to me.
 
I am an experienced racer. A lightened flywheel will make a huge difference! A BJ8 flywheel is about 28 pounds. Taking 6 pounds of weight off the flywheel will allow the engine to spin up much faster and there will be a noticeable improvement. It will feel as if you have more horsepower. The car WILL accelerate at at a much faster rate.
The most important place to remove weight is at the outer diameter of the flywheel. This is more important than simply removing weight. You can achieve even more weight reduction by scalloping out material between the mounting holes for the clutch.
 
I defer to Richard with his actual experience. Still, it doesn't make sense to me since the weight removed is negligible compared to the overall mass of the car. I just can't see the physics involved. Removing weight from the car, aerodynamic mods, less unsprung weight, etc. make sense, but removing a small bit of the rotating mass wouldn't seem to make much difference, except for faster (unloaded) revving.

Anyone have before/after dyno numbers with just flywheel lightening? Track times? Hill climb numbers?
 
Bob, this was how it made sense to me...

Tie a nut on a piece of string; hold the string about a foot away from the end and spin the nut around, noting the "speed" at which the nut makes a revolution.

With your other hand, pull the string so that now there's only about 6" of string length and note how much faster the nut is spinning.

When cutting a flywheel, the object is to remove the most (excess) mass from the outer circumference, freeing it up to spin faster. The flywheel in my car was cut decades ago, and yes, it really makes a difference.

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Oh, and another little bonus benefit, is that the car will also decelerate faster too; lift your foot off the gas and the speed drops noticably faster__great for autocross type events.

The really uncanny part is when you switch the engine off__immediate silence.
 
Thanks guys, great information, just what I was looking for.

Randy, good point re the cut off - I guess that will also help eliminate the run-on that I have from time to time

...and Randy... you have a photograph of absolutely everything! Brilliant.

Bob,

The reason for the improvement is that a fly wheel is really a device to store energy, and it also smooths out an internal combustion engine. In a performance engine you don't care about the smoothing and so with a lighter flywheel energy is freed to directly power the car rather than be stored in the flywheel.
 
It's more of the same with road wheels too; racers want the lightest combination possible, to reduce the amount of rotational mass they need to accelerate.

Guys that put those humongous wheels on a car might as well be tying an anchor to the rear bumper!
 
Between Randy's and Richard's comments I'm convinced a lightened flywheel improves performance, but I'm still confused about the physics. The mass of the car is about 2,400 lbs. Assume that the engine makes, oh, 100lb-ft of torque off-idle. All the gear reduction--roughly 4:1 in the gearbox and 4:1 in the rear-end gives about 16 times 'amplification' of torque. A little slipping of the clutch to get the car moving and the combined engine torque and gear reduction is adequate to start moving the car forward, and as momentum is increased less torque--hence higher gearing--is desired.

The analogy I thought of is this: imagine a Healey engine on a stand turning a large, heavy wheel; i.e. a huge flywheel. With the weight and resistance of the flywheel, clutch and gearbox factored in a large wheel weighing--I'm just guessing here--about 1,600 or so pounds might be equivalent to the mass of a Big Healey. I just can't understand how removing 6lb or so would make a huge difference (but two of the most knowledgeable people on the forum say it does). What I do understand is if the 1,600 lb. wheel is, say, 10 feet in diameter removing 6lb from the periphery would make a difference, but if the wheel is thick but small diameter--say 3ft.--removing 6lb from the periphery wouldn't be much difference (the 'string theory').

I'm still looking for hard data; i.e. 'my track/quarter-mile times improved 3% after I had my flywheel lightened.' I don't think you'll see any difference on a dyno, unless there is an actual increase in rate of acceleration and the dyno can compute that.

The large, heavy road wheels I understand, as you're looking and maybe 10-20lbs. times 4 at the outside of almost 30in. of diameter.

While we're talking physics, can anyone explain why 'unsprung' weight is worse than, well, sprung weight?
 
DerekJ said:
Thanks guys, great information, just what I was looking for.

Randy, good point re the cut off - I guess that will also help eliminate the run-on that I have from time to time

...and Randy... you have a photograph of absolutely everything! Brilliant.

Bob,

The reason for the improvement is that a fly wheel is really a device to store energy, and it also smooths out an internal combustion engine. In a performance engine you don't care about the smoothing and so with a lighter flywheel energy is freed to directly power the car rather than be stored in the flywheel.
Click to expand...
I submitted and magazine article from the early years showing how much to lighten the flywheel, https://www.healey6.com/Technical/Competition.pdf ,(John Simm's website). The article is very informative on getting more horses from the "6".
 
Thanks. I'm convinced. That's the kind of 'proof' I was looking for. In short, lightening the flywheels is effectively equivalent to lightening the car, and I can understand how that increases performance (esp. acceleration). For me, lightening the flywheel might offset slower acceleration due to the taller (3.54:1) rearend in my BJ8.

Can anyone explain why 'unsprung' weight is bad (I suspect the logic/math is similar to the flywheel explanation)?
 
DerekJ said:
Here is another good article that explains the pros and cons. Overall there is no real increase in 'power'. The difference is in how it is delivered. Lightened flywheels are recommended for motorsport use.

https://chicaneculture.com/2010/05/basics-flywheel/
Click to expand...

Careful, my Avast anti virus just blocked a trojan horse from that webpage
 
Another way to look at it (and I'm sorry I didn't take the time to read the referenced articles, maybe this week-end), is that all engines develop their rated power at their rated "higher" rpms. If you have a 90 HP motor it ain't going to put that out at 1000 rpm unless it has humongous pistons and displacement. So it makes sense to me that if it spins up faster it can deliver power faster, regardless of the fact that the overall mass of the car doesn't really change much. You will wear out your clutch faster, it will not be as smooth as previously noted.
 
Horse power is Torque (ft/lbs) x Revolutions / 5252.

When an engine is quoted as producing 200 hp that is at or close to its maximum revs. That is the only time it produces 200hp. Less revs = less hp. That is why mid range torque is so important.
 
Derekj, food for thought, I used & ran a Dynojet dyno for years at a turbo shop. Many engines(most) quit making hp lomg before max rpms, some turbo cars will keep climbing w/enough fuel or boost. IMO & experience max revs doesn't equate to max hp, that's why cars were run on dyno to determine when to shift & what rpm,. Just commenting
 
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