Porsche 928 technical highlights
Discussion
Following on from Dogsharks "shot in the arm" post I thought I would post some of my thoughts on this under appreciated beast:
Wanted a 928 for a while after Uni and finally bought a manual S4 in 1998.
Owned it up until last year.
There are some of my technical comments as an engineer on the engineering:
Hypereutectic block- uses the Reynolds 390 process- 18 % silicon alloy (conceptually pioneered by AMC ). Very stiff cylinder block, however potentially brittle. Because of this the block uses a bed plate instead of the usual contemporary (for the 1970s) main bearing caps. The aluminium is acid etched from the bores where the silicon is left proud and then honed. Piston skirts are either iron or chromium plated(at 700 grams each for the 100 mm bore variant they weigh enough!). Engines are capable of incredible mileages with minimal bore wear. The engines are NOT nikasil as stated wrongly in two UK Porsche magazines!
The non catalyst cars run leaner at part load and are the ones to go for. They don't run closed loop control fueling. My 928 could regularly get around the 30 mpg mark at the legal limit-driven with considerable restraint (which betters the period 75 mph consumption quoted by Porsche –26 mpg).
The manual 928 S4 runs a 28 mph/1000 rpm 5 gear, where as the US 928 S4s run closer to 40 mph/1000 rpm (due to a taller 2.2 diff), the later GT runs a shorter 26mph/1000 rpm.
The S4 and later cars use a twin plenum resonance system intake manifold. This system has the volume of each plenum sized so that it's natural frequency provides a boost in torque at about 3000 rpm and then later at 4100 rpm. This is otherwise known as resonance. The engine runs as a twin plenum up to about 3500 rpm then switches to single plenum mode (there is a big butter fly in the centre of the intake system- you can visually see a rubber black plug which sits over the blade) and then runs back to twin plenum mode above about 5200 rpm. Each plenum groups up even firing cylinders. The manifold is made out of magnesium. Unfortunately the intake set up while reasonably good at providing a broad torque curve presents a tortuous path for the air flow into cylinders 1,4, 5 and 8. This hits top end performance and probably contributes to the poor 67 bhp/litre figure.
The pent roof combustion chambers have twin 37 mm inlet valves-which is ample size. By comparison the BMW M3 S50 engine has 35mm valves (53 bhp/ 534 cc cylinder), as does the old S62 M5 V8 engine (50 bhp/ 625 cc cylinder) so there's AMPLE head room for performance upgrades. The ports aren't as straight and don't look as good flowing as the Bimmer items and the cam timing has too early an Intake Valve Closing Event and too late an Exhaust valve opening even for great performance so there's some more room for optimisation. The 944S had an almost identical cylinder head to one 928 S4 bank and managed 190 Bhp, although this figure CAN NOT be doubled due to the un-even firing of a V8- it shows there is still flow potential in the cylinder head and flow performance is what gives power more than anything else.
Other factors of note to those wishing to either modify their engines or use them for performance track days is that if you habitually use high revs and High G cornering- lack of oil in the sump pan can cause 2/6 rod bearing failure. It is caused by the fact that oil is pumped into the head in large quantities by high rpm and the head acts as a reservoir. The crank also spins up a batch of oil and keeps it in suspension under these conditions. Worst case scenarios would be over 1 G corners using an excess of 6500 rpm. Devek market a longer stroke crank that can take the S4/GT capacity out to 6 litres. This crank has oil way mods to rectify this potential issue. (with the 928s 122mm bore centres someone ambitious and with vision could easily take the engine out to beyond 7 litre capacity!)
The only other thing of technical interest I can think of is that the battery was "hung" at the back of the drivetrain (check out the above pictures posted) to act as a "damper" to stop nasty "modes" from occurring in the drivetrain: the drivetrain can be thought of as a system of springs and masses and at certain rpms a nasty resonance can potentially arise- where a "wave" form forms and its vibrations could be felt potentially. The engineers at Weissach used the battery to get rid of this nasty resonant mode or "tune it out".
The 928 was a "designed" car where as the 911 is an evolved car. Out of the two the 928 has a more timeless style and is much better value for money. The car I owned had normal issues, such as starter motor failure, clutch master cylinder, water pump failure and I had a coil pack up. Nothing major, but a lot depends on your level of competence. In terms of charisma any 911 excites me. I was always left with an incredible respect for the 928, and I felt annoyed that they were so under appreciated. Now the car has been sold (current owner is REALLY pleased with it BTW) I don't miss it the way I would a 911 or indeed one of my 1970s BMWs…….
[I don't have access to technical pictures at the moment, but if anyone feels they would like to- please feel free to post some pictures...]
Wanted a 928 for a while after Uni and finally bought a manual S4 in 1998.
Owned it up until last year.
There are some of my technical comments as an engineer on the engineering:
Hypereutectic block- uses the Reynolds 390 process- 18 % silicon alloy (conceptually pioneered by AMC ). Very stiff cylinder block, however potentially brittle. Because of this the block uses a bed plate instead of the usual contemporary (for the 1970s) main bearing caps. The aluminium is acid etched from the bores where the silicon is left proud and then honed. Piston skirts are either iron or chromium plated(at 700 grams each for the 100 mm bore variant they weigh enough!). Engines are capable of incredible mileages with minimal bore wear. The engines are NOT nikasil as stated wrongly in two UK Porsche magazines!
The non catalyst cars run leaner at part load and are the ones to go for. They don't run closed loop control fueling. My 928 could regularly get around the 30 mpg mark at the legal limit-driven with considerable restraint (which betters the period 75 mph consumption quoted by Porsche –26 mpg).
The manual 928 S4 runs a 28 mph/1000 rpm 5 gear, where as the US 928 S4s run closer to 40 mph/1000 rpm (due to a taller 2.2 diff), the later GT runs a shorter 26mph/1000 rpm.
The S4 and later cars use a twin plenum resonance system intake manifold. This system has the volume of each plenum sized so that it's natural frequency provides a boost in torque at about 3000 rpm and then later at 4100 rpm. This is otherwise known as resonance. The engine runs as a twin plenum up to about 3500 rpm then switches to single plenum mode (there is a big butter fly in the centre of the intake system- you can visually see a rubber black plug which sits over the blade) and then runs back to twin plenum mode above about 5200 rpm. Each plenum groups up even firing cylinders. The manifold is made out of magnesium. Unfortunately the intake set up while reasonably good at providing a broad torque curve presents a tortuous path for the air flow into cylinders 1,4, 5 and 8. This hits top end performance and probably contributes to the poor 67 bhp/litre figure.
The pent roof combustion chambers have twin 37 mm inlet valves-which is ample size. By comparison the BMW M3 S50 engine has 35mm valves (53 bhp/ 534 cc cylinder), as does the old S62 M5 V8 engine (50 bhp/ 625 cc cylinder) so there's AMPLE head room for performance upgrades. The ports aren't as straight and don't look as good flowing as the Bimmer items and the cam timing has too early an Intake Valve Closing Event and too late an Exhaust valve opening even for great performance so there's some more room for optimisation. The 944S had an almost identical cylinder head to one 928 S4 bank and managed 190 Bhp, although this figure CAN NOT be doubled due to the un-even firing of a V8- it shows there is still flow potential in the cylinder head and flow performance is what gives power more than anything else.
Other factors of note to those wishing to either modify their engines or use them for performance track days is that if you habitually use high revs and High G cornering- lack of oil in the sump pan can cause 2/6 rod bearing failure. It is caused by the fact that oil is pumped into the head in large quantities by high rpm and the head acts as a reservoir. The crank also spins up a batch of oil and keeps it in suspension under these conditions. Worst case scenarios would be over 1 G corners using an excess of 6500 rpm. Devek market a longer stroke crank that can take the S4/GT capacity out to 6 litres. This crank has oil way mods to rectify this potential issue. (with the 928s 122mm bore centres someone ambitious and with vision could easily take the engine out to beyond 7 litre capacity!)
The only other thing of technical interest I can think of is that the battery was "hung" at the back of the drivetrain (check out the above pictures posted) to act as a "damper" to stop nasty "modes" from occurring in the drivetrain: the drivetrain can be thought of as a system of springs and masses and at certain rpms a nasty resonance can potentially arise- where a "wave" form forms and its vibrations could be felt potentially. The engineers at Weissach used the battery to get rid of this nasty resonant mode or "tune it out".
The 928 was a "designed" car where as the 911 is an evolved car. Out of the two the 928 has a more timeless style and is much better value for money. The car I owned had normal issues, such as starter motor failure, clutch master cylinder, water pump failure and I had a coil pack up. Nothing major, but a lot depends on your level of competence. In terms of charisma any 911 excites me. I was always left with an incredible respect for the 928, and I felt annoyed that they were so under appreciated. Now the car has been sold (current owner is REALLY pleased with it BTW) I don't miss it the way I would a 911 or indeed one of my 1970s BMWs…….
[I don't have access to technical pictures at the moment, but if anyone feels they would like to- please feel free to post some pictures...]
Marquis_Rex said:
Other factors of note to those wishing to either modify their engines or use them for performance track days is that if you habitually use high revs and High G cornering- lack of oil in the sump pan can cause 2/6 rod bearing failure. It is caused by the fact that oil is pumped into the head in large quantities by high rpm and the head acts as a reservoir.
The reason this post jumps out at me is that I thought the 'head oil reservoir causing 2/6 failure' theory was fairly recent based on my perusal of Rennlist, but it seems you were there nearly ten years ago.Gassing Station | Porsche General | Top of Page | What's New | My Stuff