924/944/968 Frequently Asked Questions

14.0 Performance Modifications

14.1 Chips/Chipping

If your car is new, Porsche's official response is that using an aftermarket chip will void your warranty. The 968 (and 993) Bosch management system will lower the rev limit if it detects a non-factory chip installed.

The factory tends to underspecify their car's power slightly. Plus, chip vendors are usually highly optimistic on their claims of power increase. Take their fancy chart-o-graphic specification sheets with a grain of salt.

It's possible the chip is not smog legal in your area. It may also cause problems with the new IM-240 "rolling" smog test, in which a fixed program of RPM ranges is used to give a more accurate picture of your vehicle's emissions.

And remember: there is no free lunch. The factory did what it did for a reason, if there were an extra 20 hp that could be gained without any loss in flexibility, reliability, or safety, you can be sure they would have used it.

Things a chip can do:

Increase throttle response by modifying the throttle profile (Turbo, 944S, S2, 968)

This similar to adding a round throttle cam to a regular 944. The normal throttle cam and throttle profile are flatter when you first begin to accelerate. This helps prevent lunging and makes your driving smoother. A modified throttle profile makes the response quicker - less pedal movement equals more gas. No actual HP results from this, but it feels racier.

Increase Boost for Turbocharged Cars

Yes, increasing boost gives you more power. Possible damage could result, see below under "Performance Gains".

Raise the Rev Limit

This probably won't give you more power. It can come in handy when autocrossing. It may allow you to take a turn without up shifting where you normally would due to the rev limiter. On cars where peak-torque is very high, it may also help your straight acceleration times if your power doesn't roll off too steeply after a certain RPM. Normally the rev limit increase by these chips is fairly small (for example, 500 RPM), so damage to your engine is unlikely if you use the extra revs rarely.

Increase spark advance and/or lean out the mixture to use 92 octane gas

This sounds like the holy grail. Free HP, and all you have to do is use better gas! Well, yes. This is where the chip-maker's claims usually stem from (as you can see, in non-turbo cars, the other items aren't going to give you more actual power). However, most of the high compression cars already require 92 octane gas. In older, lower compression cars, you're more likely to see some improvement.

Performance gains:

Normally Aspirated Cars

Zero to minimal. Some chips will trade low-end torque for some loss at high end (minimal, in the 5 hp and below range). Damage is unlikely to result to your car, but the engine could be damaged if the programmers were clueless like running the engine too lean which would lead to detonation (knocking, pinging) and the subsequent engine destruction. You may also notice a slightly different engine note because of the advanced timing.

Turbo

Yes, more boost = more HP.

With that being said, all the chips increase boost by controlling the waste gate. The factory tends to roll off boost above ~4500 RPM to lessen the strain on the engine. The after market chips usually increase both the over-all boost and not roll it off as much at high RPM.

However, the head gasket is not a strong point of the 951 engine. The factory has revised the part several times, changed the torque values for the bolts, etc. Almost all the chips claim to have special mappings for the engine control module that allegedly minimize the chance of gasket failure. Some may be better at this than others. But don't fool yourself: running more boost puts more load on your head gasket, the engine, and the rest of the car.

There are several after market suppliers that carry copper, stainless steel and other "exotic" head gaskets. Replacing your head gasket and upgrading the bolts to something like Raceware fasteners may allow you to run more boost without losing a head gasket - for a while. How long your rods and bearings will last is questionable.

Kevin Gross has this wise advice (paraphrased):

"If you overboost the engine, or run the timing wrong or any of a number of sins, something's going to break. Head gaskets are cheap and easy to replace. Holed pistons, burnt valves, etc., etc., are expensive and difficult to replace. Think twice before compromising your 'safety valve' head gasket."

15 PSI or 1.1 bar of boost appears to be the maximum amount of boost you can run with a reasonable margin of safety. Going beyond this (such as the injectors maximum of 17 psi) would require extensive modification such as replacing the fuel pump, seals, transmission to handle the extra power, etc., etc.

If you must have a chip:

See if you can find a used one. Damage for the normally aspirated cars is unlikely to result, so you might as well pick up a chip cheap. If you buy one new, make sure you buy from a vendor that lets you return it "no questions asked". If the chip doesn't live up to your expectations, you can return it.

14.2 Engine & Transmission

14.2.1 Using a 944S or S2 ring & pinion in a 944 Turbo transmission.

I recommend against use of the 944S ring & pinion. There have been many reports of 944 Turbo transmissions self-destructing because of this modification. Basically, the 944S ring & pinion just can’t handle the power of a turbo, especially when you throw in a chip to raise the boost. So just say no to this modification. It might be ok for street use, but a transmission rebuild is mighty expensive.

On the other hand, the 944S2 ring & pinion set does seem up to the task. It is a popular option for 944 Turbo owners and gives about the same, shorter ratio as does the 944S r&p.

Note that any time you install a different r&p, you must reestablish correct pinion depth, that is the shimming under the front pinion shaft bearing. This requires use of the VW/385 measuring tool set, which a specialist shop will have.

14.3 Lowering/Suspension

See section 10.2 for cautions regarding extreme lowering.

14.4 Wheels

14.4.1 What wheels and offsets fit where?

The following is included with the permission of Jim Pasha. This information was originally printed in the June 1995 issue of Excellence magazine in the article "Wheels for the front-engine Porsche" on page 107. The article contains much more useful information besides this chart, highly recommended. Thanks to Jim for giving permission to include the chart in this FAQ. The chart is pretty much unedited, besides removing the 928 information.

924/944/968/928 Wheel and Tire Size and Offset Information


Year/Model
Original Tire Size
Front Rear

Wheel Size
Backspacing
mm / inch
Offset
mm / inch
924, 1976 to 1984 All 4 bolt on
108mm diameter (4.25")
165 HR 14
185/70 HR 14
205/60 HR 15
165 HR 14
185/70 HR 14
205/60 HR 15
5 1/2 J x 14
6 J x 14 (alloy)
6 J x 15 (alloy)
110mm/4.33"
110mm/4.33"
110mm/4.33"
33.8mm/1.33"
33.8mm/1.33"
33.8mm/1.33"
924, 1979 to 1984 All 4-wheel
disc brake, 5 on 130mm
185/70 VR 15
205/55 VR 16
185/70 VR 15
205/55 VR 16
6 J x 15
6 J x 16
143mm/5.63"
143mm/5.63"
53mm/2.09"
53mm/2.09"
944, 1982 to 1986 1982/83:


Sport 1983 to 1986 front:
rear:
185/70 VR 15
215/60 VR 15
205/55 VR 16
215/60 VR 15
185/70 VR 15
215/60 VR 15
205/55 VR 16

215/60 VR 15
7 J x 15
7 J x 15
7 J x 16
7 J x 15
8 J x 15
125mm/4.92"
125mm/4.92"
125mm/4.92"
125mm/4.92"
112.2mm/4.42"
23.3mm/0.92"
23.3mm/0.92"
23.3mm/0.92"
23.3mm/0.92"
10.6mm/0.42"
944, 944S 1987-1988
sport: front:
rear:
M030 Option: front:
rear:
215/60 VR 15
205/55 VR 16

225/50 VR 16
215/60 VR 15

225/50 VR 16

245/45 VR 16
7 J x 15
7 J x 16
8 J x 16
7 J x 16
8 J x 16
153.9mm/6.06"
153.9mm/6.06"
153.9mm/6.06"
153.9mm/6.06"
153.9mm/6.06"
52.3mm/2.09"
52.3mm/2.09"
52.3mm/2.09"
52.3mm/2.09"
52.3mm/2.09"
944S2 1989-1991 front:
rear:
M030 Option: front:
rear:
225/55 ZR 16

225/55 ZR 16

245/50 ZR 16

245/45 ZR 16
7 J x 16
8 J x 16
7.5 J x 16
9 J x 16
143.9mm/5.67"
153.9mm/6.06"
160.25mm/6.31"
174.3mm/6.86"
55mm/2.17"
52.3mm/2.09"
65mm/2.55"
60mm/2.36"
968 1992-1995 front:
rear:
Sport Suspension, front:
Club Sport rear:
205/55 ZR 16

225/45 ZR 17
245/40 ZR 17

225/50 ZR 16
7 J x 16
8 J x 16
7.5 J x 17
9 J x 17
143.9mm/5.67"
153.6mm/6.05"
150.3mm/5.91"
166.3mm/6.54"
55mm/2.17"
52mm/2.05"
55mm/2.17"
52mm/2.05"
924S 1986-1988
Sport/Club Sport
Special M030 Rears
205/65 VR 15
205/55 VR 16
205/65 VR 15
205/55 VR 16
205/65 VR 15
6 J x 15
6 J x 16
7 J x 15
153.9mm/6.06"
153.9mm/6.06"
153.9mm/6.06"
52.3mm/2.09"
52.3mm/2.09"
52.3mm/2.09"
944 Turbo 1986 front:
rear:
205/55 VR 16

225/50 VR 16
7 J x 16
8 J x 16
125mm/4.92"
112.2mm/4.42"
10.6mm/0.42"
23.3mm/0.92"
944 Turbo 1987-1991 front:
rear:
Sport/Club Sport: front:
rear:
M030 Option: front:
rear:
205/55 VR 16

225/50 ZR 16

225/50 VR 16

225/50 VR 16

245/45 VR 16

245/45 ZR 16
7 J x 16
8 J x 16
8 J x 16
9 J x 16
7.5 J x 16
9 J x 16
153.9mm/6.06"
153.9mm/6.06"
161.6mm/6.36"
174.3mm/6.86"
160.25mm/6.31"
174.3mm/6.86"
52.3mm/2.09"
52.3mm/2.09"
60mm/2.36"
60mm/2.36"
65mm/2.56"
60mm/2.36"

Note: While research has been done to verify all the dimensions in the table, there may be discrepancies between this published data and factory documentation. Listed information is based on actual measurements and some calculated data where an example wheel could not be obtained. Should you have any doubts, measure the wheel yourself for actual verification.

14.5 Brakes

14.5.1 My front brakes seem to be doing most of the work and wearing out twice as fast as my rears, can I adjust the front/rear proportions?

The answer is, yes, you can change the front-to-rear braking bias. Unless your name is Michael Andretti, you have probably heard that many race cars have adjustable brake bias systems, which their drivers use to tune the car's front-to-rear braking.

The 924, 924S, and the eight-valve, normally aspirated 944 was not equiped with a brake proportioning valve. Because of this, the front-to-rear proportion was fixed, determined by two things:

The caliper piston diameters can usually only be known by taking off the caliper and measuring. The front and rear piston bores within the brake master cylinder are usually stamped or cast into the master cylinder. With the 944S, Porsche introduced a 33/5 proportioning valve. For the 944 Turbo and 944S2, Porsche originally used an 18/5 proportioning valve, then later superseded it to the same 33/5 valve used on the 944S and some 928-series models.

Why is braking proportion important? You may remember from your high school physics courses that the frictive force generated by an object moving across a surface is directly proportional to the weight of the object, that is, the force with which it pushes down on the surface. So if you double the weight on a car's tire, it can grip the road twice a strongly.

For tires, the contact patch of rubber has a total limit to the force it can generate while still rolling. This force depends on the weight on the tire: the more weight pressing a tire down onto the road surface, the more force that tire can generate along the road surface. When this force is exceeded, the rubber loses grip with the road surface and the contact patch slides along the road surface instead of interlocking with it. The car's wheel stops turning and is said to have "locked."

Another important bit of physics takes place when you use your car's brakes: weight transfer. The harder you use your brakes, the more weight is transferred to the front wheels and tires, and the less weight is borne by the rears. This affects the braking force which can be generated: the less weight on the rear tires, the less ultimate braking force they can generate before locking up.

 

So, it turns out that as you brake harder, it's best that the rear wheels do proportionally less of the braking. Since it's hard to change caliper piston size on the fly, a proportioning valve does the trick by instead reducing line pressure in the rear circuits. The chart on the right shows how this would look ideally, rear pressure increase gradually rolling off as system pressure (pedal effort) increases.


 

A proportioning valve gives you an approximation to this ideal curve, using two straight-line segments. Up to the point of the change-over pressure, both the front and rear brake circuits receive full system pressure, i.e., the pressure within the brake master cylinder. Above the change-over pressure, only a fraction of additional system pressure is shared with the rear brake circuit.

There are two ways to affect the proportioning in your Porsche. By far the simplest, and perhaps safest, is to add or change the factory-style proportioning valve, which threads into the brake master cylinder's port for the rear circuit. The 33/5 designation on the valve currently used in 944-series cars is decoded as follows. The "33" refers to a 33 bar change-over pressure. One bar is 14.5 PSI, and so this valve starts reducing incremental pressure to the rear circuit when system pressure exceeds 478.5 PSI. The "5" indicates a reduction factor of 0.46, so that each additional bar of system pressure will add only 0.46 bar to the rear circuit's line pressure.

 

A second approach to modifying braking behavior is the installation of an adjustable proportioning valve. These are commonly used in professional race cars, whose drivers will fine tune the change-over point to cope with changes in outdoor temperature, tire compound, track surface, and wet/dry condition.

Before you rush off to implement any of these features, keep a few things in mind. The first is that it is perfectly normal for you car's front brakes to be doing most of the work. For this reason, you will always go through front brake pads and rotors faster than rears. The only way to change this is to reduce front-to-rear weight transfer under braking. And the only way to change weight transfer is to alter the car's center of gravity, its weight, or its wheelbase.

Another thing to keep in mind is the effect of locking up under braking. For a pure racecar, the ideal set-up is one such that the rears and fronts lock at the same pedal pressure. For a street car, and for most track enthusiasts, biasing the set-up in a way which causes the fronts to lock a little sooner that the rears is ideal. If your car locks its rears, it becomes extremely unstable in a hurry. Think of a pendulum being held with its weight above its axis. If you lock the fronts but keep the rear turning, you may flat-spot your tires and dampen your linens, but your car will remain controllable.

If you have the 18/5 proportioning valve on your car and switch to the 33/5 valve or an even more radical valve used on late-model 911-series cars, you should be aware it may cause rear brake lock up. If your car has ABS, some folks feel that the change is worthwhile and safe, especially for track usage. However, others have expressed the fear that locking up all four tires at once would cause the ABS system to think the vehicle is stopped. The p/n for the 5/33 valve is 928.355.305.02 and lists at around $69.05.


home previous next