Finished RWD Conversion

I finished the RWD conversion that Capt. Obvious wrote up as a DIY a while back. First impressions are that I'm too damn tired to go for a long drive - but a turn around the block did confirm that everything's working as expected. The steering feels loads lighter, but I can't tell if that's due to the conversion or due to me being accustomed to driving my 'Vette for the past few days. It is remarkable how much weight is lost - about 80 lbs, if I remember Capt. Obvious's post. Unexpected benefit: If you're an international drug smuggler, you will be able to fit quite a bit of product in the center tunnel once the drive shaft and differential are gone. Looking forward to some long drives to get used to the feel...

To avoid if possible: Dropping the differential on your face. Ow.

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Important safety tip: check. Thanks for that heads-up! Thanks for the international drug-smuggling tip, too. It's always good to learn a new trade.

Regardless of the weight reduction, the car should feel quicker because the HP loss through the AWD system is removed. Well done.

Agreed. The Getrag unit is likely fairly efficient, but not as efficient as NOTHING.

I dont get the point of this. Why not just buy a C2? The AWD is a great benefit of the C4 / C4S so why change it?

Why replace the IMS bearing with a different design? Why use different exhaust tips than stock? I bought my 996 because it was ALMOST exactly what I wanted - manual transmission, aero kit, full suspension upgrades, etc. The only thing I wasn't crazy about was the AWD - so I "fixed" it.

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maybe he wanted a wide body rear wheel drive car

Nice to hear about another C4 to C2 convert. After your nap tell us how your Porsche handles after losing some weight and adding some 'feel'.

The lighter steering feeling isn't just you, I noticed the same thing as well. Glad my thread about the conversion was helpful. Also, another benefit of the AWD->RWD conversion is the C4 chassis is quite a bit stiffer than the C2 chassis, which is why they use a C4 chassis for the GT models.

^^^ It's not because it's wider?

Well he just has a standard C4 not a C4S, which is not any wider than a C2 for the 996, just like a GT3 is the standard width body-wise for the 996 era

Correct. My C4 is not a wide-body.

Aha, so theres no PSM to mess up?

Whether or not the front wheels are driven does not affect PSM operation. The '99 C4 has PSM, but PSM is in no way freaked out that the front drive has been removed.

Porsche would not approve of your blasphemy. I would think that as the rears lose more grip the PSM would have to notice the fronts are not pulling. Can you test this and post for those wondering? I bet if you tried it on jackstands it would play havoc with PSM.

Is it PSM or just Traction Control?

I have RWD on my car now as well. The only time PSM has ever engaged for me is on the track when the car is being hustled through corners. I turn mine off as soon as my helmet goes on. PSM is useless on the track as it does spooky things when the car is loosing grip, like checking the brakes in a 100 mph off camber sweeper. I should install a limited slip diff to take full advantage of this mod. But an LSD from Guard Transmission is around $2500 plus install... ouch

I believe all the AWD 996 cars are equipped with PSM. Also, PSM works by applying the brakes to various wheels to "right" the car when it's sliding off if the intended line of travel. I believe it will also cut engine power if necessary. If you ever do a track day in a 996 and forget to deactivate PSM, you'll burn through your rear brakes pretty quickly.

I agree with Hardback - PSM has caused me far too many skipped heartbeats - having one of the front brakes lock when you're driving near the limit isn't fun at all. The combination of AWD and PSM has always made the car a little neurotic. AWD with PSM disabled still resulted in the car fighting with itself - the rear end WANTS to swing out, and you want to countersteer to catch it, but it never really happens right. It's like trying to get a date's bra off for the first time - not nearly as smooth as you'd like it to be. Driving it today felt much more natural - the one time I was able to kick the rear end out, it was very mellow and predictable - much more like my Boxster used to be, much more like my Corvette is. I know the 911 rear end can get away from you when you start adding more speed, but I'll pay $1 to anyone who does the conversion and then decides it was better the old way.

PASM may cut power; I do not believe PSM does anything except modulate the ABS channels.

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The AWD system is 100% mechanical, there are no electronics or sensors of any kind associated with it. All PSM cares about is whether the wheels are turning or not, not if there is power going to a specific wheel.

The steering feel alone is worth doing the mod. The steering is now as light as my Boxster's was.

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PASM only has to do with suspension, so it doesn't cut power. I thought there may be traction control associated with PSM that would cut power.

Heh. I have no PASM, so it's just some other acronym to me. I have not noticed PSM reducing engine power. If it's doing it, it's much more subtle than the ASR in my Corvette... which is why "press the ASR button" always follows "turn the key."

Some really good reading from Renntech (link below): "Included in PSM is the ASR anti slip regulator, and ABD. If one tire slips the automatic brake differential applies the brake to the slipping tire, hence reacting and behaving just like a limited slip differential. If BOTH tires spin, then the ASR slip regulator kicks in and retards the ignition and cuts power so that traction is regained. Turning PSM OFF will disable the ASR, four corner braking montiors, and pitch/yaw sensors...... BUT the Automatic braking differential stays on at ALL times up to 62 mph... That is why you will notice that even with the PSM OFF light showing you will still see a separate flashing warning light letting you know that ABD is still kicking in during certain rear traction sliding situations to try and behave like a limited slip differential...." http://www.renntech.org/forums/topic...-insider-guys/

Mine will indeed reduce power in a forced oversteer. I have tried it with PSM on vs. PSM off. This is not possible w/o sensors.

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That has nothing to do with the AWD being there or not, though. It has to do with the speed differential between the four tires (wheel speed sensors), along with input from the yaw sensors.

The only sensor involved with the front wheels is the ABS sensor (which is why you have to reinstall the stubs from the half-shafts, since the ABS encoder teeth are part of those castings). The output to the front differential is via viscous coupling - so there is no computer magic involved with the F/R torque split. I didn't bother looking, but the plate on the front of the transmission is still spinning away, even though there's nothing connected to it. It's awesome that people who haven't done the mod are the ones arguing about how it can't possibly work.

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I can attest to this... One weekend at the track I didn't deactivate PSM and my rears wore out faster than my fronts!

I may go totally crazy and pull the PSM fuse next AX.

Interesting reading in this thread. A couple of comments based on personal experience and some on what I've read. 1) I have the AWD system intact in my car and have never tried it without. I'd love the weight savings, but in my past investigations into the subject I've seen most people that have tried this at the track end up reversing it. Do a few searches to see what I mean (mainly turbo folks). 2) While I still have a lot to learn when it comes to driving, I've done my share of DE's over the past five years. I don't disable the PSM. The only time it kicks in is when I've made a mistake. It does allow a small amount of tail rotation before kicking in, but it needs to be small and controlled. I've never chewed through brake pads due to the PSM interfering. If your PSM is working that hard you're either really driving right at the limit of your tires or are not a very smooth driver. I won't attest to be driving at the limit so I can't say for sure what happens at that point. This is born out by the track tests in which Porsche has had professional drivers complete laps with the PSM engaged and disabled. They result in the same times because PSM doesn't kick in...

Porsche Stability Management System A racer’s perspective By Jack Miller Thank you to Jeff Southall of Porsche Cars of North America for the technical information in this article. April 29, 2001 marked the official return to Formula One of electronic driver aids, including traction control. In racing as elsewhere, technology that enhances (or interferes with, depending on your perspective) human performance is controversial. Potential Porsche buyers face a similar controversy in deciding whether or not to purchase Porsche Stability Management System (PSM) in the new Carrera 2, Boxster, or Boxster S. PSM is standard in the Carrera 4 and Turbo and unavailable in the new GT2. If you never intend to race your new Porsche, the decision to purchase PSM is simple. If you can afford it, buy it. It provides a level of safety impossible to achieve by driver skill alone. Here’s why. PSM monitors the ABS sensors (which measure the speed of each wheel), engine speed (RPM), throttle position (via E-Gas), gear selection, lateral acceleration (side to side), yaw (the car spinning in a circle), and steering wheel position. This enables the PSM to detect oversteer and understeer. It basically determines the slip angle of the front and rear tires, or more simply, when the car is not going where the steering wheel is pointed. Oversteer is minimized by automatically applying the brake on the outer front wheel in a bend, slowing the rotation of the car; understeer is minimized by applying the brake on the inner rear wheel, speeding the car’s rotation. No driver will be able to do that until Porsche develops a car with four brake pedals. However, PSM is not only a braking system. If you lift off the throttle in a low traction situation (wet, snow, etc.) and the back of the car gets loose, PSM will increase the engine speed (blip the throttle) to keep the car in line. Also, if traction is low, PSM can use engine braking (EDC – engine drag torque control) to slow the car. PSM can calculate the amount of available traction by comparing wheel speeds at all four corners of the car. Recognizing that even street drivers expect excitement from their Porsches, PSM allows approximately seven percent slip angle before intervening. Five to seven percent is generally agreed to be the limit for modern, high performance tires. The biggest difference between PSM and the other systems on the market today (Mercedes Benz, BMW, Jaguar, etc.) is that PSM is programmed to allow a good deal of slip, as you can see. All of these other systems clamp down the moment any slip (i.e., fun driving) is detected. However, if you require more fun, you can turn the PSM off. When you "turn it off," you are taking only the outputs offline. The PSM system is still collecting data from the ABS system, the yaw sensor, the lateral acceleration sensors and the steering wheel position sensor. If you have PSM off, and the levels of slip are exceeded, and you do not touch the brakes, the car will continue to slide. If you have not exceeded the levels of slip allowed, and apply the brakes (no matter how hard), PSM will not active its outputs. However, if you have exceeded the levels, AND apply the brakes (no matter how hard), PSM will activate until the car has regained control or you get off the brakes, at which point PSM stops outputting. PSM assumes that since you hit the brakes that you are not comfortable with the level of sliding and that you want it to help. This answers the question, posed by Mike Furnish on the PCASD forum, that inspired this article, "what happens in a spin when you put both feet in?" Presuming that you put in the correct two pedals, PSM will activate. So what about PSM and racing? At this point in my career, PSM is an asset to my racing. It has allowed me to more confidently explore the limits of traction on the first few laps at a new track, particularly in scarier corners, e.g., Turn 8 at Willow Springs. I was very happy to have it at Phoenix International Raceway, a track with concrete barriers everywhere. When PSM activates you can feel it, much like you can feel ABS. It will show you where you are losing traction while keeping you on the track if the loss was unintentional. When it engages, it may slow you down where you might not want it to later, i.e., where you really do want more oversteer, but on those first few practice laps, who cares? You can actually throttle steer the car quite well with PSM on as long as you are smooth, the yaw is not excessive, and the corner is fast enough to allow smooth inputs. This in itself is a good training tool. So PSM is good for practice, but what about when it matters, during timed laps? In a time trial situation, it would depend on the course whether it would matter if PSM were on or off. On a tight road course, you would most likely want it off. On an autocross track, you want it off for sure. If you had sufficient presence of mind on a road course you could turn it on and off depending on the corner. You could make sure it’s off for Turn 2 and 4 at Willow Springs, turns where throttle steering comes into play. You could turn it on for Turn 8, the last place on earth you want to see your tail catching up with you. I've never done this, but it illustrates the point. So far, so good. Since you can turn PSM off, why wouldn’t you want to buy it, even for a car you intend to race? It seems like the best of both worlds. However, remember above where I said that when PSM is off, it is still collecting data and if you hit the brakes when the levels of slip are exceeded, it will intervene. That could be a negative in one racing technique, trail braking, where you are obviously on the brakes and turning. There are two reasons to trail brake, one in which PSM is neutral or even a positive, and one in which it can interfere with the driver’s intention. The first is when you are trail braking to lengthen the straight or to maintain a higher speed through the first part of a turn. In this case, you want the car to stay on its directed path. If things are going as intended, PSM is very unlikely to engage even though you are on the brakes. If it does, it is probably because you lost rear traction in a pretty big way. By engaging it didn’t cost you time since your intention was to slow down anyway and it may have saved you from spinning. The second use of trail braking serves a different purpose. If you are trail braking to induce some oversteer intentionally to tighten the corner, PSM could interfere in the same way as when it is on and you lift to oversteer. While I have a lot of experience throttle steering the car, with PSM on and off, I don’t brake to loosen the rear of my 996 C2. Lifting is normally sufficient. However, I have seen this technique, in the form of left-foot braking, used in a friend’s 993 C4 in Turn 4 at Willow and Turn 5b at Spring Mountain and presume it would be useful in the newer 996 C4. Since the 993 does not have PSM, I cannot tell you to what extent it would have interfered. If you are smooth, probably very little, if at all. But, this is one possible negative to weigh against the aforementioned positives. I think it’s worth it, but let me give the last word to Porsche. "We wanted the car to perform like a Porsche not a family saloon, so the system has been designed for minimal intrusion," explained Thomas Herold, the Carrera 4 Project Manager. "Its limits are really high and you can reach the same lateral g-force number with the system in or out on a steady state cornering circle. Thus, if you are a good driver, you can keep the power on in a drift and even adjust the car’s attitude on power in a corner without interference. But if you lift off suddenly or brake, and the car is in danger of destabilizing, the system will reach out and save you." "The difference is small around the Nurburgring for a skilled test driver," he explained. "Within one second a lap in fact. This is the way the car is made. If you are smooth, there is no interference from the system. But if you are ragged, the system will be cutting in all the time to stabilize the car, so an aggressive driver will be slower with the system on."

I've read the same article - it's a good read. It's also nice that Porsche engineered the system so it does not cut in when slip angle > 0, which my X5 and my Nissan Murano both do. What I can attest to is that PSM on my vehicle, with my combination of suspension, wheels, and very short sidewall tires, activates far more often than I'd like, in situations where its activation is unnecessary and sometimes potentially dangerous. Example: a mountain road I've been driving since I was 16 - I probably have at least 500 round trips logged on this 40-mile stretch - PSM kicks in in several corners that I can take 5-10 MPH faster with PSM disabled. Since this typically results in the outside front tire locking momentarily, the result is that the car drifts closer to the centerline and nearer oncoming traffic. I absolutely understand that most drivers leave PSM enabled, and most drivers prefer AWD. I just don't happen to be one of them.