MG-Cars.net

Welcome to our resource for MG Car Information.

Recommendations

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG MGB Technical - Lightened fly wheels

Has anyone ever tried to lighten a Mg fly wheel and if so how did you do it? and are there any significant different in aceleration etc..
Ross

We machine around 4 lbs of metal from the engine side of the flywheel, scooping it out from the central boss to within 1 1/2 inches of the outer edge around 1/4 inch deep.

The result is an engine that revs both up and down more quickly - good for quick gear changes.

The downside is a lumpier idle.
Chris Betson

So will it stall out with a really lumpy cam? My car already is really lumpy right now but I dont really mind it.
Thanks
Ross

Check this out from the archives---

I wouldn't lighten a cast iron flywheel at all by machining it. It reduces it's inherent strengh and you don't want it coming apart at revs. Sure a lot of people have done it before, but most machinists won't do it these days. Mine wouldn't.

The best advice saftey wise is to buy a lightweight steel version (or maybe aluminium if you're racing). A lot of tuning shops offer these, Hans Pedersen sold me mine http://www.hi-flow.com/ But I don't think he sells them anymore, the guy who was making them retired or something. I'm sure Moss ST in the UK, or MG Motorsports or Cambridge Motorsports could supply.

Cheers,
Neil.



Barry Petaluma Ca parki44@hotmail.com Posted 02 September 1999 at 04:16:10 UK time
Shaving a flywheel is unlikely to cause it to fly apart. The area shaved is geometrically by definition the outer radius. The lightened outer radius reduces the centripetal force required to hold the wheel together. The lighter the flywheel the less centrifugal force. Besides being more area and more weight at the perimeter, the perimeter is further away from the center and consequently has more effect. an ounce removed one inch from the center of rotation has 1 ounce inch of rotational inertia. An ounce removed 12 inches from the center has 12 x the inertia. Shave the weight from the outer perimeter -- not the center. Obviously if it gets too thin the clutch pressure plate could bend the wheel. The thin flywheel has less mass and will heat faster with a slipping clutch. As a practical mater a flywheel so weak would have so little mass as to be a dificult engine to drive. Idle would be erratic. Hi revs and slipping the clutch would be needed to start the car moving wi
th out stalling the engine. Some engines have balancing weight as part of the crankshaft system. Shaving those can be a disaster as it can change engine balance. MG wheels are symmetrical in balance and can be shaved without much dificulty.
BP



John in Austin Posted 02 September 1999 at 19:05:53 UK time
The flywheel has 3 purposes: 1) clutch mechanism, 2) starter gear, and 3) smooth the idle (momentum/inertia).

by taking off a conservative 3 - 5 lbs, you reduce the amount of rotating mass that the engine needs to overcome to rev. stock, later model engines will see no benefits, but highly tuned race engines do because they have the power to rev that quickly anyway. On the stock car, the flywheel is not a limiting factor.

Think of it like a cyclists wheel from the Tour de France. If racer X had heavy duty touring tires, he would never win a sprint becuase his effort is not only going to propel him forward, but also to simply spin the wheel.

Just as it tales less energy to get the flywheel to a certain rev, it will also slow down faster and there will not be as much rotating mass to 'even' out the sequential power strokes, so the engine may get a it lopy if taken to the extreme.

Typically, the clutch side is decked even and with a finish to allow bedding of the clutch and wheel, then the engine side of the wheel is shaved even with the ring gear towards the middle until the desired weight is removed. If more needs to be taken off, chamfer down from the ring gear so the gear is never overhanging the wheel.

All of the work should be chamfered to eliminate stress concentrations.

The whole assembly would then also benefit from a balance.



Daniel1312@aol.com Posted 02 September 1999 at 19:27:35 UK time
A cast iron flywheel can burst if excessively lightened and the clutch dumped at a high rpm on a car shod with slicks - or so I'm told. I have never seen a disintegrated flywheel.


Fred Horstmeyer

I had my flywheel lightened by 5.5 lbs and rebalanced. Idle is only slightly lumpier, not a problem. The car revs noticeably quicker, so I get into the power band of my Piper 270 cam more quickly. The result is quicker acceleration though not a lot- .5 sec 0-60 improvment. As Chris says, the revs go up and down more quickly and I can shift faster. I like the shifting feel as much as the slightly improved acceleration. You could expect a greater accel. improvement with a hotter cam like the Piper 285. Glad I did it.
Terry

Terry, 5 0r 3 main flywheel?
Fred Horstmeyer

My 5-main flywheel is lightened by about 3 lbs (as far as I can remember). So many other things changed at the same time, that I couldn't tell what if any diffference it made. Car goes great, shifts quickly (it feels even better with my shift shortened 1 1/2") and pulls quickly from a dead start. To what extent is the flywheel responsible...I'll probably never know. The difference certainly wouldn't warrant the outlay of cash required for the light flywheel as depicted in the catalogs.
Barry Kindig

I'm generally for lightened flywheels but one note of warning if this is on a daily driver. A lightened flywheel will make it more difficult to idle slowly along in traffic or through town. As people have mentioned, the lighter the flywheel the more quickly it will want to stop. For this same reason, a lot of 4x4 guys go to heavier flywheels, so the inertia from the flywheel will keep the motor and wheels spinning at low rpms over big obstacles. Does that make any sense? From driving a car with a lightened flywheel, lots of stop and go driving can be a pain in the ass.
just a thought
Bill
Bill Mertz

Problem is, some guys like Terry say idle is no problem, and other guys say it will be a problem. I can easily get stuck in 90* heat and stop and go traffic. I'd like to hear some real life testimonials from others with lightened flywheels.
Fred Horstmeyer

Fred;

I run a 16 pound flywheel on my MGB, with a 300 degree cam. The engine idles all day at 900 rpm, regardless of temperature. It is a little bit lumpy, but nothing significant and only really noticeable in the exhaust note.

The lightest I have heard that you can take the stock flywheel to is 13 pounds, but that is not recommended for street use. That process involves both shaving the flywheel and drilling holes. This requires careful work, especially chamfering of all the holes to eliminate stress cracking.

The only reason for the 22 pound flywheel is that it makes it much easier for the engine to keep idling when the engine tune is slightly off, which is typical of most cars of the MGB era. If you keep your engine tuned up all the time, you should be able to run an 18 to 16 pound flywheel without any noticeable difference.
Ron Kluwe

I'm wondering what the merits are to the different amounts of lightening that people have used. Barry removed #3, Chris removes #4, Terry has removed #5.5, Steve S. recommends no more than #6, which Ron reports doing. Reading between the lines, I would guess that Barry didn't get much difference, but Terry did. Also, if you have the older, heavier con rods, does this matter in the vibration department? Should you remove less to accomodate the vibration, or more for the extra weight already in the system? Certainly, dynamic balancing of the engine should be done with a lighter flywheel. It seems that removing metal from the outermost portion will have a greater effect than if removed from the center out. Comments? Tom
Thomas Lennon

Ross,

Try Cambridge Motorsport but I do not consider this a DIY job.

Paul
Paul

OK, I'll climg in the ring,too.

As Chris pointed out, the advantage of lightening the flywheel is that faster shifting becomes possible due to the engine changing RPM faster.

Ron is correct concerning the necessity of keeping the engine well-tuned if it has a lightened flywheel.

Ross-
Yes, the less flywheel inertia, the greater the tendency to stall. The "lumpier" the idle of the cam, the worse the problem will be as the flywheel is lightened.

Fred-
I wouldn't lighten a cast iron flywheel unless it had been X-rayed to be sure that there were no cracks in it. A disintegrating flywheel can remove a leg just like a piece of steel from an exploding artillery shell can.


Terry-
The lower inertia of the flywheel would have the same theoretical advantage in terms of acceleration regardless of the camshaft used. In other words, a lighter flywheel would not be of no greater advantage with a Piper 285 than it would be with a Piper 270.

Thomas-
You guessed right. The heavier the connecting rod, the greater the interia of the flywheel will need to be to decrease the vibration that will result from the acceleration/deceleration of the reciprocating masses. Dynamic balancing will reduce the amount of power that is lost to the production of vibration that results from both primary and secondary imbalance. Dynamic balancing will not, however, effect the acceleration/deceleration forces that will result from a too-light flywheel.
Steve S.

I have a 5 main engine- 18V, and again, I have no idle problems. I do have to slip the clutch a bit more to take off gently from full stop, but I don't consider this a problem either. I gladly traded slightly more cluch wear for the revability increase.
Steve:in general, the hotter the cam, the higher in the rev range the power band is. The true advantage of a lightened flywheel is faster rev rise to the cam's power band. You will see a bigger difference between a stock 285 and a lightened flywheel 285 than corresponding 270's because of this.
Joe Huffaker, the MGB engine guru and multi SCCA national champion in the '60's and '70's recommends removing a maximum of 6 lbs from the flywheel. I have heard of a 13 lb flywheel also but like Ron, I wouldn't consider it for street use, or track use either, come to think of it.
Terry

Terry-
The reason that a Piper 285 will rev quicker than a Piper 270 between, say, 4,500 RPM to 6,500 RPM is because it makes more power in that range of the power band, not because the flywheel is lighter. It's a simple matter of power vs mass.

On another note, I'm very interested in what you've done to your engine to get it to idle so well with a Piper 285 and a lightened flywheel. You say that your engine has an "only slightly lumpier" idle. My experience with the Piper 285 is that even with a standard flywheel the engine will have a decidedly loping idle. Perhaps you're the one that's finally hit upon the combination that's the long-sought-after solution. Just specifically what have you done with the head, intake, exhaust, and ignition? Do you use the 2" SU? Do you have the mediocre low-end torque problem that afflicts this camshaft? C'mon, share your approach. This is something that we performance addicts all need to know.
Steve S.

Steve: I'm running a 270, not a 285, Weber DGEV, 45D dizzie with pertronix ignitor, cast iron exhaust manifold with low restriction glass pack, the lightened flywheel and no emission controls on a '77 18V. There is no noticeably rougher idle than with the stock setup on my '73 GT. There is a noticeable difference in revability- revs rise and fall more quickly. The tach needle returns to idle when you blip the throttle in neutral quicker than any MGB I've ever been in. I'm happy with it. (cylinder head soon to be dispatched to Burgess). I have no experience with a Piper 285 so can't comment on it. I intended to use one sometime in the future but your note about mediocre low end torque gives me pause.
As a fellow "performance addict" I am awaiting word of your experience with the Burgess modded cross-flow. SAFETY-FAST
Terry

Terry-
Yes, the Piper 285 is "cammy". Lots of high-RPM output, mediocre low-RPM characteristics (doesn't like full-throttle below 2,600 RPM at all), a loping idle, occasionally difficult cold-weather starting (gotta watch that ignition timing and the carburetors!), etc. Unless you have the head reworked to Fast Road specifications you'll lose more at low-RPM than you'll gain at high-RPM. It will function with 1 3/4" SU HIF4s, but does its best with 2" SU HS6s, a bigger-bore intake manifold and a 2" Big-Bore exhaust system. Your downdraft Weber will have to sit on a bench in the garage. Properly tuned, power will go to 7,000 RPM with its peak at about 6,600 RPM, so you'll need to lighten the valvetrain with chrome-moly pushrods and install stiffer valve springs if you don't want the valves to kiss the pistons. It's obviously a very serious camshaft for the seriously committed speed demon for whom the compromises involved are irrelevant. I doubt that anything hotter would be practical on a street machine.
Steve S.

Steve,

It's a nit (and I know you're already aware of this so it's for the benefit of others) but HS4/HIF4s have a 1 1/2" throat, while HS6/HIF6's have 1 3/4" openings. You need to go with something like an HD8 (for example) to get to a 2" throat. FWIW, I understood what you meant.

Barry
Barry Kindig

Barry-
Thanks for the correction. We don't need mistaken information in the archives. SU HS4/HIF4 both do have 1 1/2" bores, while the SU HS6/HIF6s do have 1 3/4" bores. HS8 is indeed 2". Readers of my previous posting are advised that the nomenclature of the carburetors therein is correct, but the bore size given for them is not. I need to stop staying up past midnight sanding on Rick's B.
Steve S.

Steve: will you replace your Piper 285 cam when you go to the x-flow head or keep it to see if it will work better with the x-flow than with a cast iron head? If you are planning to use a different cam, which one? Thanks
Terry

Terry-
Like you, I chose the ever-popular Piper 270 camshaft, coupling it with 3 1/4" K&N aircleaners, SU HIF4s, a Maniflow intake manifold, a Peter Burgess Fast Road head, an electropolished exhaust manifold, and a 1 3/4" Peco exhaust system to promote a nice, healthy top end. I love driving on the nearby winding mountain roads of West Virginia, so low-RPM and midrange torque is more important than blazing straight-line top speed. That's one of the main reasons I'm going to build an engine with a Derrington crossflow head: it's prodigous mid-range output. For the reasons given in my previous posting I find the vices of the Piper 285 to be a bit too much for me to live with. As to just which camshaft that I'm going to use with it, that's still very much under investigation. Although I'm not normally an advocate of 1950cc Big-Bore conversions due to the tendency of the cylinder walls on some blocks to flex, I may be on to a solution that will make such a conversion practical: Oversize Sleeves. Imagine a reliable 1950cc with a Burgess-modified Derrington head and a pair of Weber DCOE 45s (makes your skin tingle and your head swim, eh?). If it seems reliable, (and it should be, at least in theory) that will have a big influence on my choice of camshaft.
Steve S.

Steve,
If you are going that far, why not go with one the direct replacement fuel injection kits for the 45s.
Leland Bradley

Leland-
That's a perfectly legitimate question: Why not go for a direct replacement fuel injection kit? Four reasons:

1) It wasn't available when the MGB was in production. Derrington heads and Weber DCOEs were, so the "Period-Correct"/Nostalgia Factor is there. (Why do you think I own an MGB? It's not just because I love to tinker, you know.)
2) Computer mapping is an alien science to me.
3) I love showing Miata owners just what really could be done with the technology of "The Good Old Days."
4) If I wanted something more modern, I'd just get a Miata.
Steve S.

I have an EFI datson 510 with tripple DCOE bodys and I must say it was worth the fooling around to get it to work.
Ross

This thread was discussed between 24/04/2002 and 03/05/2002

MG MGB Technical index

This thread is from the archive. The Live MG MGB Technical BBS is active now.