What are the major differences between the two? Can one buy an A8 and invest some money in basic tunning to achieve similar/better results? Obviously, I would prefer the S over an A8, but the pre-owned S8s seem pretty salty.

I am considering the S6 Avant and the allroad for my next ride, but the the car wearing the four rings with the #8 stamped on her rear has always been in the back of my mind!

 

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the 50 horses can be offset by a chip and exhuast for about a 30 hp gain but the final drive ration thats another thing all togher. beside the 50 horses and the geared tranny its this ratio that makes the S8 move

 

1. The S8 has stiffer springs, shocks, and sway bars and is lowered about 1" -- The H&amp;R suspension on an A8 probably provides better performance, is ride height adjustable in the front, but has a bit stiffer ride.

2. The S8 has 4-pot 2-piece Brembo front calipers and larger rotors front and rear -- A comparable brake upgrade for the A8 is readly available from several sources.

3. The S8 has a lower final driver ratio -- As Chuck said, you can feel the difference on the street. On the track, I don't believe the A8 is at much of a disadvantage. You never use 1st gear, and 4th gear is a disaster (too tall) in both the A8 and S8. The A8 spends most of it's time in 2nd and 3rd; the S8 falls on it's face above 96 mph in 4th while the A8's 3rd gear is good for 109 mph.

4. The S8 has 50 more HP -- The A8 can make up most of the difference with a chip and exhaust if you feel it's necessary.

And escapA8 is correct -- Paul's A8 comes pretty darn close to the S8.

 

There are two versions of the 4,2 S-8 engine, the four Valve S-8 produced between 9-96 and early 99 and the five valve S-8.

The four valve S-8 was never imported into the US and was orginally designed for the higher avaliable octaine gasoline of Northern Europe.

It is possible to modify the intake and exhaust valves of the four valve A-8, use the higher output injectors of the 4 valve S-8, replace the stock A-8 cams with new high performance hydraulic sport cams , re-chip the engine computer; etc, etc.

The engine output will be similar to the S-8 five valve with slightly less torque.

If you select the correct camshaft you will have good increase in horsepower and torque and will still easily pass the California emissions which the original S-8 4 valve may not be able to do.

The engine modifications can all be carried out if the proper parts are sourced , but the best results are with engine modifications in conjunction with using the S-8 transmission/final drive.

My project is one of replacing my A-8 transmission with an S-8 tiptronic and rear differential as well as replacing my A-8 engine with an S-8 four valve. The lower part of the engine is rebuilt with new A-8 pistons From Mahle but the compression is approx 11:/1 insted of 11:6/1 of the original engine which is too high for the available gasoline in California. The valves are S-8 and the heads and valves are all hydraulic instead of using the mechanical lifters of the original S-8 intake cams. I am using an set of hydraulic lifter sport cams manufactured in Belgium which will work with the stock A-8 heads. Obviously, with proper porting, valves and valve seat reshaping even more power can be obtained from stock A-8 heads.


All of the S-8 four valve modifications that I am doing can be done to an A-8 four valve engine. The engines are basically the same with changes in pistons (Higher compression)better exhaust valves but with smaller stems, bigger intake valves with sport/performance valve seats.Higher output fuel injectors,etc.,etc. It just depends upon how much you have to spend.

 

Keep us posted on that project, would love to see pictures!

 

I ran some simulations on a reasonably sophisticated computer model of the S8 motor.

1. Increasing the compression ratio from 11 to 11.6 requires a 2 point increase in (R+M)/2 octane to maintain the same knock index, from 92 to 94 for example.

2. The S8 motor is most vulnerable to knock between 5500 and 6500 rpm. The knock index is about goes from 1.6 to 1.7 when increasing the CR from 11 to 11.6. A knock index of 1.0 indicates no detonation, 2.0 indicates detonation very likely, anything in between means possible.

3. Increasing the CR from 11 to 11.6 is worth about 4 peak HP.

One alternative worth trying -- switch to surface gap spark plugs.

A surface gap plug gets the electrodes out of the way of the fuel/air mixture trying to get into the cylinder, and this alone is probably worth 3-5 HP from the improved volumetric efficiency. Indexing spark plugs is an old racer's trick, but it's difficult to get an electrode out of the way with multiple intake valves. The other benefit in this case, though, is that a surface gap plug is equivalent to about a one deg ignition retard, solving the problem of a 11.6 CR with crummy California fuel.

BTW: F1 and CART motors all use surface gap plugs.