Engine / Trans Split

There seems to be some anecdotal evidence around that under severe acceleration, there is a tendancy for the engine/trans combination to try to open up at the bottom of the bellhousing. It seems logical to me when I think about the torque transferred to the engine/trans unit, and the fact that all the bolts between the g/box adaptor plate & the 302 block are at or above the crankshaft level.

Does anyone have any experience of this problem, or a bracing set-up to address it ?

Kind Regards,

Peter D.

Peter,

This doesn't make much "dynamic" sense. Under accelleration, the engine will try to rotate up (counter to the torque at the wheels). This will put the bottom of the trans under compression. Of course, with a sloppy downshift, the opposite forces will occur...

Hi Peter, you have an e-mail on the way outlining a "fix" for your concern that DRB has come up with.

From what I can gather, there used to be problems with the very early adaptor plates - I think that they were just cut from a flat sheet of aly. Mine is one of the later ones (probably like Bill's), made from 5/8 " billet aly & has about 1/8" machined out where the bellhousing goes. This effectively forms a slight stiffening flange around the whole of the outside.

I am trying to get details of the DRB "control arm" mod & will post the info when I get it.

Kind Regards,

Peter D.

Peter/Dave, Is this peculiar to the adapter that DRB uses? I have a Porsche G50 with a Kennedy adapter, should this concern me? Dave, I'd love to take a look at the work around DRB came up with if you don't mind emailing to me as well.

Thanks!

[ March 12, 2003: Message edited by: Lynn Larsen ]

Lynn,

I haven't had a bit of trouble with mine.

Perhaps somebody forgot to Loctite a bolt or

two?

My engine puts out well over 400 ft/lbs

of torque. There is no flex between the

engine and gearbox, with the DRB adapter.

My adapter may be of a different style than

those on earlier models.


Bill

I spoke to Peter Ransom (DRB/GT40 Autralia) about the "control arms" & he tells me that they run from the rear engine mount bolts on the sides of the block to the lower 2 studs which mount the g'box to the adaptor plate. He is selling them as a kit which includes the rods, longer engine mount bolts & longer g'box studs. The only constraint is that the engine needs to have a relatively standard sump (in terms of width at the back) to provide clearance for the rods.

Bob, with regard to the dynamics of the "split", I too thought that it didn't make sense when I first heard about it. Then I thought about the diff setup - if the pinion gear is at the back of the crown wheel, it is trying to lift the back of the crown wheel, therefore the pinion gear would be pushed down, thus trying to split the assembly. I am assuming that the pinion is at the back of the diff (simply because that makes it nearer the g'box & avoids another shaft having to go past/through the diff). As you can tell, I have no idea of the innards of the G50 g'box, but this seems to make some sense. Perhaps there is someone out there who can explain it in more expert terms ?

Kind Regards,

Peter D.

Peter,
I agree with Bob. Unless the trans is coming appart you need to look at it as a fixed unit. Then you look at the net forces on the system. Under accel the wheels are applying a torque trying to lift the front of the motor (and car) up in the air. Since the mounts are pulling the motor down, the top is in tension and the bottom is in compression.

My 0.02.

This sure is a tricky one. I agree that the front of the car has to lift under acceleration - if you think of the engine/trans assembly a rigid beam with a centre of rotation around the drive shafts, we are applying an upwards force just behind this point with the pinion driving up the back of the crown wheel & thus pushing the pinion down. This will tilt g'box end of the beam down & therefore, the engine end of the beam upwards.

But, the "beam" is not totally rigid - it is supported fore & aft of the centre of rotation (& the pinion download), so it must tend to "bend" down at the pinion force point.

Ergo, a conundrum ! How do we apply Einstein's "Unified Theory" to this & bring the two apparently opposing propositions together ??? !!!

Maybe, the downward force (bending moment) on the chassis at the back of the gearbox is the thing which lifts the front of the car & the engine is just going along for the ride ?

Kind Regards,

Peter D.

In thinking about this a bit more (always dangerous...), the reaction torque depends on where the engine/trans unit is mounted. Given a mount at the engine/trans split, there will be NO reaction at the interface. Moving the mount further back on the trans increases the static bottom separation force but increases the compression forces under accelleration.

Peter,

> Maybe, the downward force (bending moment) on the chassis at the back of the gearbox is
> the thing which lifts the front of the car & the engine is just going along for the ride ?

I think both are correct. They don't have to be
mutually exclusive. The downward force at the
back of the gearbox occurs, while the engine tries
to yank itself out of the mounts. Granted, it's
been a while since I dabbled in physics/mechanics

Since it is a mid-engined car, the chassis flexing
upwards does not lessen the resistance at the
engine mounts as much as a front engined car
since the engine is closer to the fulcrum
instead of the end point. Angular momentum
is greatest at the radius.


Ian

[ March 18, 2003: Message edited by: Ian K ]

Regarding the issue of the motor to transmission "stays".

Newton's first law of motion states "For each action there is an equal and opposite reaction".

For a GT40, with a transaxle attached to the motor, to make the car to move the motor torque is transmitted through the transmission to the drive axles, through the wheels and tires to the road surface. Due to the friction between the tires and the road surface the vehicle moves. However, there is an equal and opposite reaction of torque, which must be resisted by the transmission/motor unit and chassis. This torque reaction causes the nose of the car to lift when under acceleration.

Therefore when you accelerate the front of the motor tends to lift and this force is transmitted through the motor mounts to the chassis and this lifts the nose of the car.

To join a transmission Bell housing to a Ford Windsor motor, the adaptor plate bolts to the full circumference of the Bell housing, but to only the top half of the motor as the engine block only extends down to the centre of the crankshaft. Therefore the bottom half of the adaptor plate is not supported.

GT40 Australia have undertaken some R & D work on the adaptor plate issue and have found that Billet aluminium (both GT40 Australia and Kennedy Engineering manufactured adaptor plates) and also mild steel plates can flex when large torque motors are run on Chassis Dynomometers.

The flexing issue does not appear to cause any issues as several DRB GT40's have now travelled in excess of 50,000 miles fitted with aluminium adaptor plates and Porsche G50 transmissions without any problems being experienced.

However it was considered that it was good engineering practice to more rigidly connect the motor to the transmission. So in conjunction with developing new Urethane motor mounts (that do not rely on the bonding of the rubber like standard mounts) we have produced a "motor to transmission stay kit". The stay brackets bolt to the motor mount at the front and to the adaptor plate, on each side at the bottom with bosses bolted to extended studs through the Porsche G50 Bell housing. These stays have eliminated the adaptor plate flexing issue.

I trust that this clarifies the motor to transmission stay bracket issue.

Best regards

Peter Ransom
GT40 Australia

Thanks Peter. It makes sense that the unconnected bottom would be the weak link no matter which direction the forces are under during accel, etc. This would provide added stiffness.