Cab v. Coupe engine support bracket

[Colby]

Ok, this is building off an earlier thread I started. I am prepping an '84 cab for weekend racing (and some street use). I was considering putting solid mounts in just the rear to gain some chasis rigidity. Then it was mentioned that a cab should first switch to a coupe/targe style engine mount to gain any benefit. I looked at pics of the two different setups. I see the cabs have the huge round rubber thing in the center, where as on coupe/targa's the engine mount mounts directly to the engine bracket.

My questions are:

1) Why did Porsche use the two different setups?
2) What all is involved in swapping a cab to a coupe setup (the crossmember and the bracket on the engine it bolts to)?
3) Do I have to drop the engine to get this bracket off (not the crossmember)?
4) What gain would I see if I were to do this?
5) Would the engine vibration shock be unnecessary?
6) Is this conversion necessary to get any benefit from solid engine mounts in the rear?

Colby

[Colby]

btt

[APKhaos]

Only bone i can throw you is that the later cab motor mounts are firmer that the coupe parts. They make a good compromise between the softer coupe mounts and the solid race mounts.

You need someone who knows the cab crossmember arrangement for the rest of your questions.

[Tyson Schmidt]

1. Because the Cabriolet chassis "wiggles" over sharp one-wheel bumps. This oscillation of the chassis is undamped. The mass of the engine/trans. is used as a ballast, and the oscillations are dampened by the shock on the engine mount. The big rubber center allows the twist, and the shock dampens it, absorbing the oscillations. This gives the car the illusion of being more solid than it really is, and increases driver comfort/feel. Problem is, it doesn't help at all in the handling department. Coupes don't need it, so they don't have it.
2. You need the piece that bolts to the engine, and the long crossmember piece that bolts to the engine mounts.
3. You can remove the muffler and rear engine sheetmetal. Then support the rear of the engine whith a floor jack and drop it down enough to gain access to remove it all.
4. The chassis will flex less in twisting motion, making the swaybars more effective and steering inputs more responsive. Shifting will be more predictable and precise when under hard cornering/braking/accelerating.
5. No
6. Yes

[89911]

I would seriously reconsider and lengthy chassis and suspension upgrades with a cab. You have a chassis the is missing its entire top half which is going to negatively counter any upgrade and pehaps make it worse. If you are seriously going to race this then you will need a full cage for safety and stiffness. Might look kind of crazy, cruising around with the top down with a metal scaffold around you .

[Colby]

Ok Tyson, I'm convinced; you are the Porsche tech GOD! That being said, help me devise a plan of attack to maximize the potential of my cab. I know, driver skill comes first. I'll work on that (I have taken several courses already and been racing for several years). Tires second. I will get good race rubber. That being said, what would you recommend to get my '84 cab in good weekend race warrior status? I have done the following:

Torsion bars (21/27)
Weltmeister sways (22/22)
Poly bushings (f/r)
Lowered to just under euro spec
street tires are 17x8 (225/45/17) and 17x9.5 (255/40/17) 3 piece forged
Race tires are fuchs 16x6/7 (looking for 8's for the rear)
Weight reduction: AC pulled, fiberglass rear bumper
fiberglass 930s style valance, whale tail
race seats soon
Roll bar soon
SSI/Supertrapps

I know the cab is a lil more flexy and ultimately its potential is less than the coupe, but how can I make it as stiff as possible w/o hurting more than helping?

As stated in this thread, I considered replacing the engine support crossmember and bracket to a coupe style and then using solid mounts for the engine, sport mounts for the tranny. Obviously Porsche gave the cabs a different mount for a reason, but it sounds like for comfort not functionality. Where is the source of the "wiggle"; in the rear corners?

Will this benefit me by stiffening up the cab chasis or is it not worth the effort? What will the side effects be? Any insight would be greatly appreciated.

Colby

[Tyson Schmidt]

The source of the wiggle is simply the twisting effect caused by one corner of the car absorbing the shock from a road irregularity. This causes one corner of the car to rise or fall in relation to the rest of the chassis. Without the rear engine mount damper, it is undampened.

However, with the damper arrangement, there is more movement. By putting in the coupe crossmember, and using solid mounts in the rear, you stiffen the chassis by tying the center tunnel to the rear of the car. This may lose the dampening, but you have less flex.

To further reduce flex, I'd recommend welding in rocker stiffening plates. These run the length of the inner rockers and tie in the front and the rear of the car more solidly. These are available from Eurotek. They custom make them, and they are very effective. Around $260 for the pair. John Walker has their number. I do too, but I'm typing from Jack Olsen's computer right now, so the number is 1,100 miles away.

And to address those who think an open car is too flexible to get good handling out of, try your luck at the track against a well prepared Honda S2000. It will be an eye opener!

[Graham Archer]

As an alternative point of view, I would suggest that the general design of the motor and transmission mounts is simply to isolate the engine/transmission from the frame (for both driver comfort and perhaps to reduce fatigue stresses on the eng/trans case). I would very much doubt that in 1982 the engineers used the eng/trans as a tuned mass damper system to damp out chassis flex. But then, what do I know about vehicle dynamics.

I've always thought that the more flexible mounting setup in the cab was to provide additional comfort by further isolating engine vibration at idle from the frame (necessitated by the increased flexibility of the cab chassis). I would think the little damper is there only to damp vibrations at idle.

[Tyson Schmidt]

One of the main reasons that the Targa was developed and used instead of the full Cabriolet, was that the engineers back in the sixties couldn't get rid of enough cowl shake/wiggle. They stiffened the car as much as they could, but the wiggle was still there. It's the type of thing that gives the car a "cheap slapped together" feel.
Even though the car was acceptably stiff enough, the cowl shake was still there. It wasn't until the eighties that they devised a setup that would get rid of these oscillations, and make the car feel like a quality, well built car. Which it is.
If you can't somehow dampen the shake, stiffening the car further will simply raise the shake's frequency. Instead of big, slow wiggles, you get really small fast ones. This is still annoying, and can make for squeaks and rattles.
If it is dampened by a ballast, you only get the flex during the actual bump in the road, so they coincide. If the flex coincides with the bump impact and stops right after from the dampening, you attribute it to the bump and the car feels solid.
For a track car that is set up like Colby's is, ultimate stiffness takes precedent over "percieved comfort".

[North Coast Cab]

Digging up an old thread.

Does anyone have the contact info for Eurotek? Or has anyone used the rocker stiffeners?

John

[aigel]

Colby:

My friends 86 cab has soldid motor and transmission mounts, an autopower rollbar, a weltmeister camber brace and a fully hopped up suspension (along the lines what you have listed, a bit stiffer torsion bars). No change to the cab engine mount setup.

I can only tell you one thing: This 86 cab is one of the most well planted cars I have driven in AX. It may not be as stiff as a coupe, but it is stiff enough for competition driving in AX and the big track. My friend does very well at DE and timed runs. It being a cab is not a limitation.

Just a datapoint.

George

[widebody911]

There's always "Plan B"

[North Coast Cab]

Thom,
Can you summarize your suspension and stiffening upgrades? I know Tyson is quite the experta as well. I've done a fair amount of research and would love to compile all the data into a comprehensive list. We're not talking about making a cab a race car, we're talking about improving the characteristics od a track/street car.

John

[MichiganMat]

I converted my cab brace in my targa to a coupe style brace. There was a definite improvment in the shift action from 1st to 2nd, and from 2nd out to 3rd. The conversion was worth it just for that reason if nothing else. Its also cool to see the entire car shake when you rev the motor.

[widebody911]

Originally posted by North Coast Cab
Can you summarize your suspension and stiffening upgrades? I know Tyson is quite the experta as well.

Well, I'm blushing to be mentioned in the same paragraph as Tyson!

I have 22/29 torsion bars, Weltmeister "race" plastic bushings in the front, Neatrix rear spring plate bushings, worn out stock inner trailing arm bushings. TRG sway bars, 930 brakes.

The side bars made a huge difference in the torsional rigidity. I also did a smaller bar across the back seat, right about where the seat belt mech would bolt on. My doors would pop loose on a good jolt; not any more!

[North Coast Cab]

Thom,
Do you have any detailed pictures of teh door bars and that rear mount bar?

John

[KobaltBlau]

Originally posted by widebody911
[B]There's always "Plan B"

MWUHAHAHAHA

Originally posted by MichiganMat
I converted my cab brace in my targa to a coupe style brace. There was a definite improvment in the shift action from 1st to 2nd, and from 2nd out to 3rd. The conversion was worth it just for that reason if nothing else. Its also cool to see the entire car shake when you rev the motor.

er, what? your targa shouldn't have had a cab brace, targas and coupes are the same, just cabs have the "cab brace"

[widebody911]

Originally posted by North Coast Cab
Do you have any detailed pictures of teh door bars and that rear mount bar?

Check out http://66.34.72.138/howto/cab-brace/index.html

[North Coast Cab]

Thom,
Now I can see what you've don along the door sills and behind the seats. I have to wonder if all of this could have been simplified by adding to the roll bar and creating more of a cage. I've been thinking of doing the same along the door sills with some 1 1/2" square tubing. I need to change the door panels to remove the pockets in order to gain full access, but if done properly I don't think you'll even notice them. I run a DAS Sport bolt-in bar so I need to keep the rear fairly stock. I'll let you know if I dive in.
Also, have you done the engine mount upgrades described? I bought a coupe mount, but didn't install it becasue it didn't have the A/C bracket. A year later and having not used the lame A/C I'm thinking about putting it in with two solid mounts.
The car is definitely a dual purpose ride so I'm trying my best to improve handling without completely sacraficing the streetability.

John