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I have just finished some work on the 928S4 throttle valve housing to see what can be gained. A larger 80mm butterfly valve (stock size 75mm) was installed in two welded and ported housings and in one stock housing.
Welding on the outside of the throttle housing was made in order to obtain more material, more meat for a proper porting job with large radiuses to the up chutes. However the welding resulted in many difficulties through shrinkage and warpage. In spite of the steel girdle I made and bolted to the housing, the C-C between bolt holes across the housing shrank about 2,5mm and the distance between the inner walls of the up chutes shrank by about 8mm. The latter had to be compensated by adding more material through welding of the short side radius inside the up chutes. The bolt hole locations had to be adjusted. The housing also shrank toward the bottom of the valley resulting in about 2mm of flycutting to the surfaces connecting to the intake manifold.
Shrinkage also made the needle bearing bores to be out of a straight line, making the shaft to turn sluggishly. As I did not want to install the new needle bearings, I made some bushings for testing. After much testing and figuring I made a guiding shaft and a special cutting tool for recutting of the bearing bores. A thin wall bushing had to be made and placed on the outside of each needle bearings for a proper interference fit.
Boring out the housing for the larger 80mm (stock 75mm) butterfly valve went well but the old bore was way out of round from warping.
The shaft and throttle lever mechanism had to be shimmed properly for the butterfly valve to turn freely. For some unknown reason the throttle valve in the welded housings did not open fully which had to be adjusted.
Flow testing gave the following results. Note that my flow bench is too small for this amount of air flow but as a comparison between the different throttle housings it should be pretty accurate. The figures have been converted for 28" of system pressure.
- Stock throttle housing with 75mm valve 896,1 CFM.
- Ported throttle housing with 75mm valve 922,3 CFM.
- Stock throttle housing with 80mm valve 933,9 CFM.
- Ported throttle housing with 80mm valve 1044,4 CFM.
The small flow increase of 4,2% by just boring out the housing and installing a larger 80mm valve make sense as I remember a Rennlister posting of an almost non existing power increase after installing a larger 79mm throttle valve. The 16,5% flow increase for the full ported housing with 80mm valve is the best that can be done, I cannot see any further improvements possible with the stock throttle housing.
The engineering firm of Porsche consider themselves one of the top automotive manufacturers but they cannot cast an intake manifold that will match the intake ports. For the S4 there is a huge mismatch of almost 6mm. On later 5R heads they have chamfered the floor of the port in order to try and overcome the problem.
Åke
Take a very close look at the intake gasket. It is very thick and offset at an angle and works well. This is why porting the intake and port matching is difficult to get right and then also create a gasket that will not impede the flow of air.
The engineering firm of Porsche consider themselves one of the top automotive manufacturers but they cannot cast an intake manifold that will match the intake ports. For the S4 there is a huge mismatch of almost 6mm. On later 5R heads they have chamfered the floor of the port in order to try and overcome the problem.
Åke
This is interesting.
I suspect that the factory was mainly trying to solve the following problem. The S4 intake manifold is thermally insulated from the heads and the block. Not perfectly, but to some extent. This causes the block to spread apart relative to the intake when the engine heats up. To prevent cracking of the intake manifold, the factory specified a flexible coupling between the intake and head, both in terms of anchors and the very thick, angled, reinforced, and flexible gasket. The thick gasket is a part of the flow path.
The S3 and earlier intakes also had compliance built in the rubber couplers, so this was really an S4 problem.
If porting the intake, I would be tempted to take some risk and roll the die a bit. I’d consider adding dowel pins to the intake-head interface, using o-ring or sealant, and bolt the intake rigidly to the head. Then port match the interface. This would take more risk with cracking the intake, but I’d probably take that risk.
maybe it was incredible engineering foresight - they know you'd have to pull the heads to rebuild and cut the surface at some point which would move the ports closer together...
Some hp estimates from a highly approximate formula:
- Stock throttle housing with 75mm valve 896,1 CFM.
Loss is 6hp with a 300hp engine, 15hp with 400hp engine and 28hp with 500hp engine.
- Ported throttle housing with 75mm valve 922,3 CFM.
The corresponding hp numbers are 6, 14, 27. So just porting the throttle body gives you 0.5-1.5hp depending on the engine.
- Stock throttle housing with 80mm valve 933,9 CFM.
6hp, 13hp, 26hp.
- Ported throttle housing with 80mm valve 1044,4 CFM.
5hp, 11hp, 21hp.
Conclusions: It only makes sense to overbore the throttle body if one also adds material to the outside and ports the throttle body for higher flow. This will be expensive, and should only be done for high hp engines that need to retain the stock appearing look. The gain is expected to be about 7-8hp. I guess it would make sense to do on a stroker engine if the budget is not an issue and the car goes thru some sort of a visual inspection.
Back to the intake manifold mismatch. The engine block will get hotter than the manifold on a running engine. How much will the difference in temperature be? If we assume the block is 50C hotter than the manifold, the block will expand 0,27mm or 11 thou more than the manifold. That is quite a small amount.
Åke
Back to the intake manifold mismatch. The engine block will get hotter than the manifold on a running engine. How much will the difference in temperature be? If we assume the block is 50C hotter than the manifold, the block will expand 0,27mm or 11 thou more than the manifold. That is quite a small amount.Åke
Would it make sense to make thick aluminum plates pinned to the manifold and head for precise location and use them to replace the stock gaskets? Those would allow one to transition very smoothly from the intake manifold to the head without increasing the cross-sectional area at that point.
Would it make sense to make thick aluminum plates pinned to the manifold and head for precise location and use them to replace the stock gaskets? Those would allow one to transition very smoothly from the intake manifold to the head without increasing the cross-sectional area at that point.
I have considered exactly that, I have some 20mm material which can be used. Will take a closer look.
The engineering firm of Porsche consider themselves one of the top automotive manufacturers but they cannot cast an intake manifold that will match the intake ports. For the S4 there is a huge mismatch of almost 6mm. On later 5R heads they have chamfered the floor of the port in order to try and overcome the problem.
Åke
Ake,
I think Porsche was well aware of the problem - there are several different castings of the intake manifold. My spare 1989MY GT engine - a lot of material had been taken out from both the heads and the manifold ports in an attempt to match them better. I also bought a cheap early S4 manifold years ago to stick in my race car - similar to the one in your picture - even the shapes of the port openings were vastly different from the ones found on my GT. I threw it in the bin after discovering a mismatch to the tune of 3mm+ vs. GT/GTS heads on the port and also a botched powder coating effort by an incompetent shop. Last, but not least - my 1993 MY GTS also had properly port-matched intake and head, with a lot of material removed on both sides. Yet a third casting number and yet another slight variation in port shape v.s. the 1989 MY GT intake. Also, the inside was coated in the smooth plastic finish. Btw, the exhaust is not much better on the early cars...
I made a template from the S4 intake manifold. The pictures show the amount of mismatch I have. For the newer 5R heads, the factory has chamfered the floor of the intake ports in order to try to overcome the mismatch problem. Nothing has been done to the roof of the ports.
I've got a pair of Euro S heads (2V) in to a local, highly-regarded, speed shop. The expert looked at them and thought they looked fine except for taking down a ridge on the inside of the port.
Reconditioned valves for 928S 2V Euro. From left to right 40mm exhaust, 45mm intake and 48mm high performance intake valves having 8mm stem for weight reduction. The 48mm valves are not entirely finished, keeper grooves need to be machined.
Åke
When performing cylinder head porting jobs a valve grinding machine is a vital piece of equipment. I bought this old Van Dorn valve grinder forty years ago from the Swedish air force. Its is a very sturdy and heavy machine, all made in cast iron. It is still working good but sometimes new spares are needed. I managed to source rubber drive belts from a guy in England. Grinder wheels can be had from Goodson but expensive. I found some low cost wheels in India which seems to be OK but they were way out of balance making the heavy machine to walk around on the bench. Had to make a shaft and put up the grinders wheels in the crank balancer for a proper balancing job. Now the machine is running very smoothly, no vibrations at all can be felt. Maybe I should give the Van Dorn machine a paint job to make it look like new.
Åke
12-06-2017, 10:36 AM
