40v 4.2 V8 engine differences, A8, A6, S6 and S8

no experte here, but dont the s8 lumps have a higher compression?

in for more detailed info

Looking at the S6 and S8 40v motors, I think that they both have 11:1 comp ratios.

I half expect this to be just heads and cams along with a different ECU tune.

I'm wondering if it is cams, are the pistons different? I would look this up by part numbers... but there aren't any! I can't find any 4.2 40V pistons listed in any VW/Audi parts catalogue.

MG

Cams are different. S6 has more aggresive intake cams, S8 has more aggresive intake and exhaust cams.
S6 and S8 have a dual stage plenum (suppose to be better) vs a A6/A8 three stage plenum.

I believe the rest of the motors are the same, well other than the tuning obviously.

Both my 40V S8 engines have the three stage inlet manifolds.

MG

The shortblocks are the same except for the rods. The S rods are forged and the others are powdered metal. The S6/S8 have different cams, different valvetrain (higher rocker ratio), and a different tune. Also the S cars have a different main pulley and power steering pulley for slower accessory speeds.

The S engines have a different intake manifold than the A6 and A8.

Thanks that's great news re blocks.

Any idea about the pistons?

MG

Mickey said:

Looking at the S6 and S8 40v motors, I think that they both have 11:1 comp ratios.

I half expect this to be just heads and cams along with a different ECU tune.

I'm wondering if it is cams, are the pistons different? I would look this up by part numbers... but there aren't any! I can't find any 4.2 40V pistons listed in any VW/Audi parts catalogue.

MG

Click to expand...

You wont find a part number in etka for the pistons as they wont sell you the pistons. Have to buy a complete short block

You wont find a part number in etka for the pistons as they wont sell you the pistons. Have to buy a complete short block

Click to expand...

Actually Audi finally released internal V8 engine parts a few years back. Pistons are available and are the same from S6/A8/S8/A6, they are cast.

The RS6 pistons are a lower compression and are cast as well.

I think I'll start looking for a stock A8 40V 4.2 to transplant the heads onto.

If I can find one more piston, I'll have a full set of S8 pistons and rods from it too but I'm not sure if it's worth changing them.

I half expect that the S8 pistons will have more clearance for the valve heads if the cams have more lift and duration.

Nobody seems to know much about the stock pistons in these engines.

MG

I've been looking into this again and I think (please someone correct me if I'm wrong) that the pistons are the same between the A6/A8 4.2 40v 300bhp engines and the S6/S8 4.2 40v 340/360bhp engines.

I'm still looking for that one piston but looking at the 300bhp spec pistons, they look identical to the piston I have here to replace.

Has anyone put them side by side?

I know that the heads, cams and inlet manifold are different and I hope that the difference in compression ratio is in the heads.

MG

Pistons are the same, combustion chambers in the heads are different volumes.

Does anyone know what the difference in the S6/8 intake and exhaust cams are compared to A6 or RS6 4.2?

I'm doing a big HP RS6 build and want some better cams.

Cheers

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RS_66601 said:

Does anyone know what the difference in the S6/8 intake and exhaust cams are compared to A6 or RS6 4.2?

I'm doing a big HP RS6 build and want some better cams.

Cheers

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Click to expand...

Watching with interest. I happen to have an RS6 engine waiting to go into a project and an S8 40v engine in pieces.

MG

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MG

Any chance you'd be interested in getting the RS6 and S8 cams measured?

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RS_66601 said:

MG

Any chance you'd be interested in getting the RS6 and S8 cams measured?

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Click to expand...

Sorry I'm not pulling the RS6 engine apart just yet, any upgrades until it's running in the project. The S8 lump is 180 miles away and I don't know when I'll be there next.

MG

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Oh sorry thought they were apart

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