CTS

I had a couple people mention to me that they like their 2wd-converted 996 Turbo a lot better without the 4wd, but on occasion they felt the car was somewhat dangerous under hard acceleration. I thought about this for a while and I think this can be mitigated in 4 different ways:


1. The 315/30 18 rear tire that most people use is completely inadequate. This tire has a load rating of 98, which translates into 1653 lbs per tire at the maximum inflation pressure. Typically a modded 996 Turbo is going to weigh in at 3600lbs with driver and fuel, and has 30% of its weight on each rear tire. So with that tire you are using 1080/1653 or 65% of the capacity.

In contrast the 2023 GT3RS weighs in at around 3400 lbs with fuel and driver and use rear tires with a 109 load rating which translates to 2271 lbs. If the GT3RS also has 30% of its weight on each rear tire, then it is using 1020/2271 or 45% of the capacity.

I guess we can infer from this that Porsche feels better about running the rear tires at around 45% instead of 65%. The commonly available 305/30 19 has a load rating of 102 which is 1874 lbs and easily fits on the rear of a 996 Turbo. On our example 996 Turbo, this would use 57% of the tire's maximum capacity.

All the 200TW tires that are used in endurance racing offer great performance but do not last very long on the street. I think that the expectation of modern 911 rear tires lasting more than about 8000 miles is probably unrealistic. These 200TW tires also don't offer good cold weather performance so traction expectations in winter even on dry roads need to be realistic.


2. Most of the time when you buy Xona/Tial turbos, they are going to be delivered with wastegate springs that open around 1 bar of boost pressure. This means that the ECU cannot control boost at under 1 bar, which likely means that the engine will deliver a large percentage of the available torque at small throttle openings, especially at lower rpm. This makes the throttle feel highly non-linear, with the torque coming on very explosively at lower rpm. This makes traction modulation much more difficult. The solution is to ask for .5 bar wastegate springs which would be similar to the K16/K24 wastegate springs that the cars came with. This will result in a lower maximum possible boost pressure, so this has to be considered.

Also consider getting a tune that does not max out the available torque in the lower midrange. The car will be more fun to drive and more reliable with a flat torque curve instead of the typical falling torque curve.


3. The stock subframe mounts and control arms use rubber bushings so the ride is comfortable and quiet. This compliance allows the toe to move around under high load which means that the rear of the car will be steering. This will get worse as the tires get sportier, because the racing or near-racing tires develop a lot of cornering force at very small angles. Note that the GT2 used the stiffer GT3 subframe mounting. I would also monoballs and hard mounts for track use and 996 GT3 front control arms with rubber mounts in the rear for a car I wanted to be quieter inside.


4. 996 Turbos all came with an open differential. This just isn't appropriate for 500+ ft/lbs and rear drive. I think for a road car I would use an LSD with low preload because that will have less power oversteer in rain or snow. I have not tested a Torsen differential on a sporty street car. They handle somewhat weird on the racetrack between coasting and throttle and this seems like it could be even more obvious on the road. A Torsen diff would probably be better in rain or snow. I hate the idea of not having any lock up in coast on the road or the track though.


The last thing is to just leave the PSM on unless you know the tires are sticking and the road conditions are good. That offers a second chance if things go sideways.

Chris

racer959

2fcknfst

I can definitely get the traction control to come on more often, and it is certainly interesting in low traction conditions, such as rain.

I am also a little more leary about pushing the limits of the chassis as it obviously does not pull out of a corner the same as it does with AWD, the sensation of the back wanting to come around is definitely in my mind.

Your synopsis of rubber choice and loading is very interesting. I would be curious to do a back to back comparison of the 18/315 660 against a 19/305 PS4, the PS4 being my desired street tyre. The 660 is good, but noisy and has very stiff sidewalls, so pretty sure that contributes to poor rain performance, something we get a lot of here on the west coast.

Straight line acceleration most certainly suffered with the removal of the AWD system. Even with an LSD, breaking the back free in first, second and even third (when tyres are cold), is a more regular occurrence.

I have said in another thread that I am of the opinion that at 580hp and rwd only street car, it is getting very close to what can be managed in an enjoyable manner.

Some very interesting things to consider though.

s65e90

This guy must be an engineer...

2fcknfst

It's Chris Cerveli, engine builder extrodinaire.

Definitely food for thought.

theprf

The 305/35-18 NT01's have a load rating of 101, this is the tire I have been using. I have not had any issues with spinning these tires with a clutch-type LSD and stock power levels.

The wastegate spring is something I would never had thought of, as the boost control relies on being able to feed the wastegate an artificially low pressure to trick the wastegate into closing.

Bernard IV

Aside from frugality, this is why I keep the AWD, it just feels safe to me. I also have 245 / 315 wide tires with soft rubber and they've never slid one inch on the road for me.

powdrhound

Yeah, the OP is Chris Cervelli. We had been through a lot of what he talks about in the early stages of setting up my car years ago when the car was driven on DOT R-comp tires. As he mentions and as I've advocated for many years, the 315/30/18 is a horrible tire on these cars. Back in the days, when we switched from the NT01 315/30/18 to the much taller 305/35/18 with is substantially higher load rating, it was a night and day difference in performance. This is likely not going to be very evident on street driven car driven at 50-60% but If the car is driven aggressively or on track with intent, the short 315/30 absolutely gets destroyed in short order . Back in 2002 when the GT2 was designed, there simply were not many choices but obviously that has changed. You can sort of begin to understand why Porsche had increased the rear tire height / load rating on the 997 gen cars.

When Chris and I went about setting up the Xona turbos we went to great pains to set up the wastegates to give us a nice even progressive power band without any torque spikes. The stock springs that Tial used to include with these turbos are all wrong and the tuners generally pay very little attention to it. You always wanna check what springs Tial delivered the turbos with because you never know but they used to come with the blue spring. You wanna use the softest Tial WG spring which is the Green (6.27psi spring). This spring will allow you to run upwards of 1.5 bar or so on the top end if needed. Chris also machined a longer WG rod coupler (pictured below) to allow for a lower preload and a cracking pressure of 4psi and full open pressure of 14psi. With the stock couplers the cracking pressure is around 7psi. It is paramount that the WGs are set up correctly on the Xona turbos prior to tuning.

2fcknfst

Couldn't you modulate boost by having a BOV on the intake, and using throttle position to control load?

Seems to work pretty good on an automatic diesel, keeps the boost on, but will bleed pressure by lifting a bit.

powdrhound

My guess is the 305/30/19 PS4 will be far superior to the 315/30/18 660. Compare the load rating for one. If looking for max performance in the summer, the Porsche N-spec 305/30/19 Michelin Sport Cup2 would be my choice.

2fcknfst

I am starting to see that as a viable solution; about .8" taller than my current setup - probably will help the front lip too.

powdrhound

The wastegate by default is closed. The N75 frequency valve uses boost pressure to open the WG and start bleeding of pressure. The softer the spring, the earlier this can happen (the earlier you have mechanical boost control).

CTS

You don't have much control over the throttle position with a stock or tuned ECU. At some point I will be putting together a post which describes this in more detail. The throttle position does not faithfully follow the gas pedal position during the onset of boost.

You don't want to let boost escape thru a BOV except when the throttle is fully closed because the turbos can overspeed.

Chris

2fcknfst

I wasn't thinking just lifting completely, partial lift, enough to open the BOV so it bleeds a bit of boost, but also keeps some boost on tap.