In researching the A-8/S-8 engine for purposes of improving output, I have learned the following:

Stock A-8(6/94-99) have the following valve dimensions:

Intake: 32mm X 7 X 95.5

Exhaust: 27mm X 7 X 97.95

The first figure is the valve diameter, the second the valve stem diameter and the third, the valve stem length.

The A-8 4.2 which I own was manufactured in Oct 1997 and has 6mm valve stems. The earlier cars have 7mm valve stems and I dont know when this change was made.

The S-8 4.2 four valve has the following valve dimensions:

Intake: 32.75mm X 6X 105.65

Exhaust: 27 mm X 6 X 97.05

The intake valve stem is longer because this engine uses mechanical lifters on the intake valve.

There is also a 3.7 S-8 four valve that uses the following intake valve with hydraulic lifters:

Intake: 32.75mm X 6 X 94.This valve can be used effectively for improving gas flow with the stock A-8 head and aftermarket hydraulic sport cams.

The trick is that the slightly larger valve does not shroud the already crowded combustion chamber but allows the engine tuner the ability to radius the valve seat under the valve, move the contact area between and valve seat further out for much improved flow.

The valves as used by Audi are very high quality and require no additional machining for optimum performance and durability.

The physical dimensions of the combustion chamber do no permit fitting of larger valves w/o shrouding, seriously affecting gas flow. Audi is able to use the same head and valve seat on the S-8 engine but gets increased increased flow from recontouring the valve seat on the S-8 , with its slightly larger valve.

I have read and been told by the experienced engine builders that it is easy to screw up an Audi four or Five valve head by going overboard on porting. For a street engine, porting should be limited to smoothing and blending the intake port along with flattining the floor around the radius. The exhaust port shoud also be smoothened and soothened the roof f the exhaust widened. Be conservative because you will loose some low speed port velocity with any porting. Much effort should be spent on improving the valve seat contour particularly the intake.

CAMS.

I looked for a suitable hydraulic sport camshaft since I didn't want to be bothered with adjusting valves.

I followed the guidelines of Shrick cams i.e., "a four valve engine intended for street use with stable engine idle and ability to pass emissions should not have valve lift of greater than 1.1mm at TDC with hydraulic lifters."

Cat cams of Belgium ( www.catcams.be), a highly respected cam grinder in Europe and England has just the ticket:

The cam has the following specs:

Duration: 266-266

Duration at 0.050" : 212-212

Max Lift: 11.20mm/0.440" int & ex

lift at TDC: 1.10 -1.10

112 lobe centers

The cam uses stock valve springs and hydraulic lifters but you must use new lifters and check for coil bind and stem seal clearance when installing.

The came increases lift, duration and has more rapid acceleration of ramps producing more power and torque throughout the rpm range.

I will eventually take pictures and pass them along. I have finally read my digital camera manual and now know how to use the Macro photo close up feature. It is sitll a pain to down load the pictures on the net and I am working on it.

I'll pass along more information when I receive the Mahle piston catalog.

Everone interested in rebuilding one of these engines should review "Reconditioning of Aluminum Engines" on the www.msi-motor-service.com, web site

 

Jeff-

You are sure getting to know a lot about these engines! I'm really surprised about all the changes made.

Take a bunch of pictures, you can host them at audiworld.com under your picture poster, or when you get them taken, I would be happy to host them at audipages....this would be such a cool article to have there.

Good luck!

pw

 

Do you happen to have valve and cam data on the 5V A8 & S8 motors? The Bentley is rather incomplete.

What do you mean by "lift at TDC: 1.10 -1.10"? Shouldn't lift at TDC be zero with both valves closed?

Other than seating valves out and unshrouding them as much possible, I can certainly believe that porting can do more damage than good. Someties I believe porting is more art than science, but in any case, a flow bench can only confirm what really works.

And thanks for the interesting information. Please keep us posted.

 

The information on the 5-valve engine is still proprietary since it is still in production and thus is usually limited to those shops in Germany that have a direct relationship with the factory. Unfortunately, I do not presently have this information but will try to obtain it.

The valve sizes and cam data are unknown to me other than saying that I have seen a five valve cylinder head apart.The after market parts books that contain the data pertining to the four valve Audi V-8 engines do not yet offer information or parts for the five valve. I imagine that when Audi goes to direct injection and reverts to the four valve design Federal Mogul will offer after market replacement parts and will print all of the answeres to your questions with the exception of the cam data.

The shops that have modified the four cylinder heads which is very similar to your engine say that there little room for bigger valves and that in racing applications, the five valve head has some shrouding problems at high gas flow on the intake side that the four valve head does not experience. For street use the design allows a degree of flexability and output, particularly as set up on your, car with its three phase intake mnifold and variable timing on the intake cams, that the earlier four valve engins cannot completelymatch.However, that is not to say that the four valve is not capable of being significantly inhanced so that it will give the five valve S-8 a run for it's money.

B&B in Germany is claiming 400 HP with S-8 five valve engines that have gas flowed heads (Ported and racing valve seats.) They also claim to modify the iintake manifold, the exhaust manifold, the exhaust system and the ECU. In order to prep this engine, one would have to know the phasing of the intake manifold along with the variations of advance and retard on the intake camshaft, which is a feature of this engine .

The four valve engine is limited to a two stage intake manifold that helps significantly with low end torque and increases gas velocity at low speeds. The five valve S-8 apparently has a three stage intake manifold along with variable intake camshaft timing. This allows the engine designer to opimize the camshaft timing for low and high speed. Certainly , a great deal of benefit can be gained from cleaning up the ports and smoothing out the passages without enlarging the ports.This, along with a good three or five angle valve job will improve output. One may also be able to "phase port" this engine to take advantage of the intake manifold design. That is, enlarge only one or two ports in phases to stage increased flow. I don't kow how the intake manifold varies its gas flow but it probably restricts flow to one or two intake valves at low speed to increase port velocity and enhance combustion, and probably uncovers all intake ports at hige engine speed for maximum flow.




As of this date i don't know of any aftermarket cams available for the S-8 five valve. Ludlo Stessens at: www.catcams.be, could prepare a new billet but he would have to have the assembled heads with roller rockers to know the proper geometry.

He would also have to know the degrees of advance and retard on the intake cam along with the manner in which the intake manifold operates on this engine. The engine is a lot more complex because of the additional parameters but it also has a lot of possabilities if all of the information is made available. Unfortunately, only a few engine tuners such as B&B in Germany have all of the information necessary to optimise the output of the engine.

The S-8 five valve also lists a different part number for the throttle body over the standard A-8 five valve. Its probably a larger throttle body but I can't verify this.This along with slightly higher output injectors is most likely the case.

I response to your question on lift at TDC, camshafts in today's modern engine usually begin the intake cycle before the piston is at TDC. Sometime in the early 1900s engine builders discovered that if they began the intake cycle before the piston was at TDC, the engine would produce more power. What you have is a column of fluid (air) passing through the intake passages , through the manifold and into the engine. As the engine revs increase, begining the intake phase before the piston reaches TDC alows time for the column of fuel and air to begin movement and to catch up with with the time the intake valve is open for the incoming charge. The intake valve is opened; ...progressively more before TDC as the camshaft grind becomes more radical.
The S-8 five valve has an adjustable intake camshaft that varies the positioning of the intake timing depending upon the rpm of the engine plus other data processed by the ECU.

For more low end torque, the cams are advanced resulting in less camshaft lift at TDC.At higer engine spped, tha intake cam is retarded giving more lift at TDC.

At present there is limited data available on your engine. You may try e-mailing B&B in Germany with yur questions. You can also purchase an intake and an exhaust valve and finally, you can attempt to obtain the factory Bullitins available to the dealers service dept that no doubt contain most of the information you are seeking.

As I come accross information on the five valve, I will pass it along. I will try contacting a couple or engine builders in Gremany to see if they have the information you seek.

 

The correct valve stem length for the S-8 3.7 four valve hydraulic intake valve is: 94.75mm