If I were to measure the time for a full throttle RPM increase of a V12 E-type engine fitted with a standard V12 manual flywheel (with pressure plate and clutch, gearbox in neutral), say from idle to 3000RPM and then fit a BW 3-sp auto with torque convertor fully loaded with ATF (in neutral or park),
which would rev up quicker?
Would the difference be substantial - enough to notice just by listening to it rev up?
When I fitted the light weight Fidanza flywheel to my EFI’ed, E-type, I was stunned when I felt how heavy the manual flywheel was. Such a heavy flywheel must have tremendous rotational inertia (surely a primary reason it was so easy to spin the wheels). So I’m wondering how much inertia difference there would be between manual and auto.
Keep in mind, though, that with an automatic, the torque converter IS the flywheel, and the flywheel is replaced with a very light flex-plate. What matters is the relative rotational moments of inertia of the two systems. Without measuring/calculating that, I don’t see how anyone can give you any answer beyond a wild-ass guess…
[quote=“Ray_Livingston, post:2, topic:387837”] the torque converter IS the flywheel, and the flywheel is replaced with a very light flex-plate. What matters is the relative rotational moments of inertia of the two systems. Without measuring/calculating that, I don’t see how anyone can give you any answer beyond a wild-ass guess…
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Thanks Ray, Understood.
This community has a fantastic collective knowledge. I concede that my question is perhaps way “out there”, but we know such weird things about our cars, why not this?
Also, perhaps someone has both the complete flywheel, pressure plate, clutch plate combo as well as a torque converter lying around, and there just happens to be a scale handy and he just happen to have nothing better to do than to weigh this lot…
On the torque convertor, would one include the weight of the oil in the convertor? My gut tells me one should as it also has to be accelerated when revving up.
Absolutely!! And most likely assume it is all at or near the periphery, which is the worst place for it in terms of MOI.
Regards,
Ray L.
Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
5
The flywheel for the V12 is stupid heavy. The only reason for any mass at all is to absorb heat when feathering the clutch, and hopefully not much of that is going on.
The all-steel flywheel in my 289 Mustang was much lighter. That’s not because the Ford V8 needs a lighter flywheel than the Jaguar V12 – neither engine needs any inertia at that location at all. The difference is that the Ford flywheel was designed by somebody who knew what he was doing. The relative positions of the mounting flange, the clutch friction surface, and the ring gear all lined up properly so the flywheel could be flat and thin. The Jaguar POS, OTOH, has the ring gear a full inch away of the clutch friction surface so even if you did make the flywheel thin you’d still have this massive flange at the OD in order to step over to where the ring gear needs to be. And that probably wouldn’t be good for fatigue life, so you’d need to beef up the corners so the thing doesn’t crack.
I think there is a lot of reasons why a manufacturer would use a heavy flywheel. Easier clutch engagement, less gearbox chatter, are two that come to mind. I think for performance the heavy / light flywheel is a bit of hooey. At a stop light your engine will probably rev faster, but when you engage the clutch the engine has a lot more weight than the flywheel to deal with.
comment, as I am not 100% sure I understand exactly what you are asking, but when I replaced a BW12, with a 5speed manual & slightly light flywheel, (in a Saloon), all I can say is it was a no contest for on road acceleration, and the general feeling of acceleration. Maybe I was just fooling myself, but judging by the bulging eyes and white knuckles of my passengers when I really put the boot in, probably not. I understand the Torque convertor soaks up quite a lot of power in a BW12,maybe ~15% ?
I do not know the facts, but I theorize all torque converters are not equal. But if I am not mistaken, I believe most/all drag racers use torque converters. If so, I doubt they use them if they prevent quick acceleration.
Tom
I would have thought you could impute the difference in the numbers by knowing the 0-60 times for both types of car.
I’m not sure i buy into a lot of what Kirby is saying, as it is going off-topic (and for the wrong reasons). He is implying that the Jaguar engineers didn’t do a good design job by comparison to the Americans, whereas in reality the Jag flywheel is a legacy item from the 6 cylinder car, which originally dates from the 1949 xk120. He can check how much beef his argument has on his flange corners by simply looking at the Jag designed flexplate, which was designed by Jaguar. They are not known to crack and look pretty flimsy to me.
I do know plenty of shops who mill away plenty of material (Very slowly) from the standard item, usually for racing customers.
I think you are right, there is a big difference between a road going Jaguar torque convertor, and a high stall item fitted to drag racers, which according to that article Robin attached, are not as well suited to manual clutches, for the reasons stated in there, including heat dissipation, reliable takeoff etc
I never did timed comparisons, but didnt Jaguar publish them with 0-60mph always slightly quicker with manual transmissions ?
One of the more fascinating bits of bob, in dragsters: if you watch the launch, ofttimes you’ll see a puff of black, behind the cars: that’s the clutch material.
I never drove a proper nitro dragster, but always wanted to try an alcohol car.
Remember to always add the weight of the (heavy) pressure plate (and clutch plate) to the manual flywheel, so dropping 5kg off of the flywheel weight is not twice as good as dropping 2.5kg off - it’s obviously less than less than 2x, so to speak.
Top fuel drag racers use a multi plate clutch that operates like a centrifugal clutch. On launch the engine is given full throttle and revs to 8000 rpm which is more or less maintained down the strip. The clutch plates are sequentially engaged automatically, as set up by the crew chief (usually) to transmit maximum power to the rear wheels without causing them to break loose and spin (a potential disaster if it happens). Most of the plates slip against each other, with the final plates and the clutch locking up as the car nears the finish line. Setting up the clutch properly is the most important job - it’s completely rebuilt and set up after each race. Top fuel dragsters are primitive beasts but absolutely fascinating. So much fuel is pumped into them the engines operate at near hydraulic lock up. A visit to the Wikipedia page on them is worthwhile.
Torque converters are effectively one fan connected to the engine blowing hydraulic fluid through another fan attached to the transmission and are not as efficient as a mechanical connection. That led to lock up torque converters which restored the efficiency.
Modern automatics, like the ZF Jaguar and many others use are so good, with virtually instantaneous shifts and multiple speeds (8 speeds now, though some transmissions are 10 speeds) that they are able to hold the engine within a relatively narrow range where maximum power or economy exists. I’d be very surprised if a standard transmission equipped F Type could beat a ZF automatic to speed.
I was involved with the latest Targa Bambina rally here in NZ and was really surprised at the performance of some of the support vehicles 3.6 diesel 8sp autos going through the stages at quite high cornering speeds whilst staying on the correct side of the road(when it was wide enough )
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Kirbert
(Author of the Book, former owner of an '83 XJ-S H.E.)
16
Restored some of the efficiency. A/T’s still have a hydraulic pump which runs all the time, a parasitic load on the engine.
i dont see anyone mention overall diameter!
small diameter will be lighter , but also rev quicker!
my torque converter i much lighter and smaller dia. also lightened the flex plate!
there i NO comparison on free rev. or car accelleration, it is cut in half, especially in the lower gears, where weight is more effective and diameter!
Informative thread,
just for clarification, my initial comments refered to the old iron case BW12 & its Torque convertor, vs a manual clutch (which I was informed was one of the original Borg & Beck clutches for a manual XJS, very high quality and it sure felt tight and put the power on instantly)
Some of these 6-8 speed autos I have driven, they go like hell when you drive them really hard, and imo, you would have to be immensely skilled to keep up with a manual trans, (if it was even possible)
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