964 C2/C4 chassis differences?

[j-rho]

Hello,
First post here, apologies if this has been hashed out previously.

I'm trying to find out if it would be possible to implement the 964 Carrera 4 drivetrain components in a Carrera 2. Or, put differently, if there are any chassis-level differences between the two models, such as an additional drivetrain tunnel or a different shaping to the front trunk floor.

I'm planning to build an AWD 964 for use in the SCCA Street Modified 2 Solo2 autocross class, and I'm trying to figure out if I can start with the lighter RS America platform.

Many thanks!
Jason

[Bill Verburg]

If you want AWd start w/ an AWD chassis, then strip out the parts you don't need.

[j-rho]

I do plan to start with an AWD chassis. It is sort of a bizarre dance through this particular ruleset - I cannot put RS America door panels on a C4, but I can add AWD to an RSA. If the resulting combination is the same either way, it doesn't matter which chassis I start with, as long as the base chassis stamping/moldings are identical.

[Bill Verburg]

The 964 AWD setup is not something that can easily be added or subtracted. The tubs are different for one thing.

If you want AWD I would perhaps think more along the lines of the simpler 993 setup. But even there you can take it off easily enough but not add it after the fact.

[Garrett]

From a wiring standpoint you're WAY better off starting with the AWD chassis given the brake system, ABS and differential locking system involve a different set of hydraulics, electronics, and control units. The 964 all-wheel drive system is electronically managed, which is performed by the same control unit that manages the ABS. The 993 system is simpler in that it is mechanically managed within the components that make up the system: viscous coupling (993) versus electronically managed (4 speed sensors, , brake pedal sensor, two accelerometers) hydraulically controlled (two separate hydaulic slave cylinders) clutch plates (964) - even the words are complex!

Additionally, with the 4x4 components hovering over the front end, you will appreciate the excellent power steering that an RSA does not have to begin with (it's only major mechanical difference from a C2).

An RSA, in most trim examples, is not any lighter than a C2.

C4's are fun race cars. Good luck!

[Garrett]

The "tubs" are the same on a C2/C4, if by "tub" you mean the sheet metal that makes up the body. The PET lists the body part number the same for these models.

[Bill Verburg]

Originally posted by Garrett
The "tubs" are the same on a C2/C4, if by "tub" you mean the sheet metal that makes up the body. The PET lists the body part number the same for these models.

Pet is not showing the multiple differences in the tubs, one example
C2 has vacuum boosted brakes, C4 and RS have electro-hydraulically boosted brakes, very different installs.

[j-rho]

I appreciate the replies thus far gentlemen. Let's approach this dilemma from a different angle.

I'd like to construct an AWD 911. Ideally it'll weigh approx. 2900 lbs., my class's my minimum weight. Modifications to include:
A/C, stereo, and back seats removed,
race seats,
carbon front fenders and flared/carbon rear 1/4s.,
lightweight battery,
cam'd, TPC supercharged, no-cat but otherwise stock 3.6 (target 400whp with excellent low-end torque)
18x10.5" front wheels w/285/30's
18x11.5" rear wheels w/315/30's
and a good enough suspension to make it all work.

I'd like to wrap this in as compact a package as possible, with an emphasis on minimizing the total width at the rear axle for the given wheel/tire size. It's my understanding that the 993 received a much-improved rear suspension design, but from the specs I've seen, the improvement came at the cost of 3" greater width.

Strange as it may sound, I believe I would prefer to be 3" narrower than to have better rear suspension geometry, meaning I would favor the 964. However, I'm not sure that the extra width of the 993 isn't due to larger factory wheel fitment or whatnot, meaning it may be the same width as the 964 once my chosen wheels/tires are put on, I'm not sure...

Thanks again for the input everyone!

[Tom W]

I think the 993 suspension is better and would start with a 993 C4 if you plan to pursue this. However, given the project description above, it will not really matter as clearly money is not a constraint. If you simply want to get to 2900 lbs a lightweight battery will not be necessary.

What class do you expect to run in with that configuration? Do you really expect the supercharged engine to withstand track use? Why are you limiting yourself to 10.5 and 11.5 wheels? If you are paying to redo the bodywork, go to an RSR body and run 11" and 13" wheels. Will you run slicks?

[Bill Verburg]

Here are some idea's for you

964 AWD l/w

[j-rho]

Originally posted by Tom W
I think the 993 suspension is better and would start with a 993 C4 if you plan to pursue this. However, given the project description above, it will not really matter as clearly money is not a constraint. If you simply want to get to 2900 lbs a lightweight battery will not be necessary.

What class do you expect to run in with that configuration? Do you really expect the supercharged engine to withstand track use? Why are you limiting yourself to 10.5 and 11.5 wheels? If you are paying to redo the bodywork, go to an RSR body and run 11" and 13" wheels. Will you run slicks?

The class is the SCCA's Solo2 (autocross) class, Street Modified 2. In this class, we have essentially unlimited drivetrain allowance (as long as the engine uses a block from the maker of the chassis, it's OK), and suspension is unlimited, as long as you re-use the factory pickup points. This means it's okay to use adjustable rod-ended links, but you can't convert struts to SLA or something like that. We can change front fenders and flare the rear. We can change seats and steering wheels but interior has to stay stock otherwise.

In autocross, the runs are only about 40-60 seconds at a time with lots of time to cool between runs, so heat management isn't as much as an issue as at the track.

Also in autocross, many of the maneuvers are through very tight elements, like 30mph corners and tightly-spaced slaloms. Here, there is a definite tradeoff between the additional lateral grip available from wider wheels and tires vs. a narrower car that has to move less to negotiate these elements. By my estimation 285s and 315s should be "adequate", though I suppose in the future I may decide to go to a 12.5-13" rear wheel and a 335 or 345 section tire. The tires I'll be running are DOT-R's, like the Kumho V710 or Hoosier A6, but there is another class with similar allowances that also permits the use of full-slicks, which I may also try, though that is not the target.

The venue for the Solo2 national championships changed this year, from a very grippy concrete surface to a relatively slippery asphalt one. Since most of our courses are run between 25-65mph (second gear), this led to the AWD cars putting a hurt on the RWD cars, since the former were much better able to apply power to the slippery surface. The one benefit is that this surface is very very smooth, so in my mind, having an "ideal" suspension geometry isn't as important as it was before, since between the smoothness and lack of grip, it will be moving less in general.

I also have some questions about how to make the drivetrain durable in a drag-style launch (not what it was built for, I know, but it's what we do in ProSolo), but that can wait for another day.

Thanks for the replies, and for the pics Bill!

[mjshira]

Bill

Is that the lwt 964 C4?

[Bill Verburg]

Originally posted by mjshira
Bill

Is that the lwt 964 C4?

yes

[mjshira]

that car ROCKS!

[group911@aol.co]

Bill, I've built a few 964's and I can't picture any structural difference between the 2 and 4wd infrastructures. I know it sounds like a major pain but I think if he had the 2 cars sitting side by side, the changeover is manageable - the worst part being the wiring. Also, if I recall, the 89 4wd system is the closest to the 959. Of course, the system in the LWT 964 you pictured is the 959 adjustable. I understand Porsche made a few of the LWT kits for customers to convert their cars. The short geared trans in the LWT would probably be ideal for his application.

[j-rho]

group911, how sure are you that there are no structural differences between the C2 and C4? I recall reading somewhere that the C4 sacrifices some front trunk space to fit the front differential and other components.

With the diameter tire I plan on running, a 7250rpm redline would net a ~73.6 mph top speed in second gear (2.118 ratio, 3.444 final drive), which is just about perfect.

I've read a PDF out there on the operation of the AWD system in the 964; it would be neat if one could devise a way to adjust the amount of lockup the system provided, perhaps with some sort of brake bias type of valve or something, so it isn't so "on/off"...

In any case, I do plan to start with the 964 C4, it's just through a weird twist of rules that if the C4's chassis is the same as the C2, that I can use parts from other US-sold C2's, including the RS America.

Many thanks!

[group911@aol.co]

The fuel tanks are the same so I can't imagine there would be a difference for the front diff. I'd have to crawl back under both to be sure. Even the fron spindles are the same. The c-2's just used a blank axle end. Look at Bill's LWT picture for the ultimate bias adjustment. Same as the 959.
Or, you could just buy one of the LWT cars.

[Tyson Schmidt]

Why would you want all wheel drive for autocross?

Anyway, the tubs are the same, but there are many other differences. The C4 uses hydraulic brake assist, where the C2 uses a vacuum booster that fits where the front axle would be on a C4.

It would be possible to swap the C4 drivetrain over, but like group911 said, you'd need to have another car to swap all the parts you'd need over.

The 964 AWD is not suited to autocross. Unless it rains.

[j-rho]

Hi Tyson, thanks for the reply. Followed a lot of your work on Jack's car, I ran OTC in 2003 in U2 and T3 in S2000s.

The new surface we run our National Champioships on is a very slippery asphalt, not quite like a wet surface, but close. We run almost entirely in second gear, and I plan on running TPC's roots supercharger; being able to put power down is of utmost importance.

In my class, brakes and drivetrain are unlimited (must use OE block), suspension is limited only to OE mounting points. I think I can make the car handle pretty well within those bounds, and I think the AWD would be helpful in putting down the fat torque and roughly 400whp the car will need to keep up with its competition.

I have to run @2900 lbs. but through my rules I would like to be able to use some of the RSA parts up high in the car and add some ballast down low.

[Tyson Schmidt]

Makes sense.

See if you can find a non-sunroof. Maybe a salvage title RSA, and a crashed C4, and put together the best parts of both.