A Marriage of Two Engines...4.2L RS4 Components & S4 Foundation, Wishful Thinking???

[Manystylze]

I'll start with a few disclaimers:

1. I have No Idea which thread this belongs in as this is a forth coming DIY pending any information you all may offer
2. I have these parts already BEFORE you ask why don't I do X or Y
3. It's more a genuine interest in the potential challenge

That said...I have a blown RS4 engine that grenaded two cylinders and the block has significant damage in those cylinders. As such, I have a S4 complete engine but needs some cylinder head work. Naturally, considering that the blocks are similar, I am pretty much considering transferring the good components that are left from the RS4 engine (crankshaft, rods, pistons, etc) onto the S4 block. I have already torn down the RS4 engine and know that 2 pistons were damaged so I am sourcing those now (and also considering a new set I found overseas). I've noticed a few differences in some of the fasteners (cylinder head bolts) and was just wondering whether or not this fateful marriage may/may not work.

From initial research, many components are similar between the two block with much of the difference in the RS4 being forged internals, differences in the cams and cylinder heads, and many flow control components.

I have experience tearing engines down but not totally familiar with Audis. I'm hoping that some of you with more Audi experience with the 4.2L v8 can chime in and offer some ideas or considerations when comparing the 2 blocks along with potential haphazard.

[VinnysS4]

I hate to be the bearer of bad news, but I don't think it will work out. The only thing the two engines share in common are the looks and the displacement. Just about everything else is different between the two. They take different head gaskets, different timing components, different fuel system, different rods, different camshafts, etc... I'm no expert, and I would absolutely love to see this work out. I just don't know if it is possible without a bunch of custom work. If that's what you're into, then absolutely give it a shot.
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[Manystylze]

I hate to be the bearer of bad news, but I don't think it will work out. The only thing the two engines share in common are the looks and the displacement. Just about everything else is different between the two. They take different head gaskets, different timing components, different fuel system, different rods, different camshafts, etc... I'm no expert, and I would absolutely love to see this work out. I just don't know if it is possible without a bunch of custom work. If that's what you're into, then absolutely give it a shot.
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I understand the things mentioned, but in consideration what are your opinions of the following:

1. S4 block mated to RS4 heads, crank, rods, and pistons - Unless the cylinder chambers are different in length, this should address some of whats mentioned. Would have to see how the timing components match up of course. Cooling may be the main thing with different flow ports and channels.

2. Fuel delivery can be addressed later, have the parts to swap. Just have to get the low and high pressure RS4 pumps.

3. Between myself and my father, most of the custom work (if necessary) can be done. Plus have an additional engineer/gearhead offering assistance. Going to take pics along the way in the event everything works out. The only issue may be with the proper amount of tuning. Just realized I may need to consider a new MAF and throttle body to handle the airflow.

[VinnysS4]

when I was rebuilding my S4 I was sent RS4 head gaskets. I compared the two and the oil and coolant ports did not line up. I don't remember if the head bolt placement was the same either.
I would suggest getting your hands on both car's head gaskets and seeing if you can make it work

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[Manystylze]

when I was rebuilding my S4 I was sent RS4 head gaskets. I compared the two and the oil and coolant ports did not line up. I don't remember if the head bolt placement was the same either.
I would suggest getting your hands on both car's head gaskets and seeing if you can make it work

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Totally concur. Being that I have RS4 heads, the only issue is aligning the gasket to the block itself and checking ports to see if the gasket covers anything. If there are any blockage issues that can't be addressed, then I may just move the hardware (valves, springs, etc) from the RS4 head to the S4 head. It would essentially give me the same thing in theory.

Concerning head bolts, alignment can be checked as well using the head gasket. The 1 thing boggling my mind is the head bolts themselves. I have noticed that OEM RS4 head bolts have been significantly lower in price than S4 head bolts. Apples to Apples was an avg of $36 for the RS4 for a set of 10 vs $94 for S4....with $43 being an aftermarket set of bolts for the S4. Any ideas there?

[Nollywood]

I thought the RS4 valves were different to those of the S4. I might be wrong though.

[Manystylze]

I thought the RS4 valves were different to those of the S4. I might be wrong though.

You are correct, they are a bit different but nothing that can't be worked around.

[ven0m]

As said above, apart from sharing the manufacturer name and similar displacement these are two different engines. After a quick google:

The 4.2L 40V V8 has many variants. They are the same basic engine but that doesn't mean they're completely interchangeable. Below is data from ETKA.


BBK: Audi B6 S4, Audi B7 S4
BHF: Audi B6 S4, Audi B7 S4
BAT: Audi A6 (04/04-05/06)
BFM: Audi A8 (11/02-05/06)
BGJ: Volkswagen Phaeton (05/03-07/03)
BGH: Volkswagen Phaeton (08/03-05/10)
AHK: Audi C4 S6 Plus (06/96-10/97)
AQJ: Audi C5 S6 (09/99-05/01)
AHC: Audi D2 S8 (09/96-12/98)
AKH: Audi D2 S8 (08/97-12/98)
AQH: Audi D2 S8 (05/99-02/01)
AVP: Audi D2 S8 (09/00-09/02)
BCS: Audi D2 S8 (09/00-02/01, Japan)
AYS: Audi D2 S8 (02/01-09/02, Japan)


The 4.2L 32V V8 FSI, which is the RS4 motor, was used on many Audis/VWs including the A8, Q7, Touareg, R8, etc. That motor is completely different from ours.

There was also the older 4.2L 32V V8 non-FSI from the D2 A8, C5 A6, C4 S6, and C4 S4.

In regards to your valve exchange endeavor, what do you wish to achieve by getting bns valves to fit the bhf? Don't get me wrong, I like machining custom parts and making things work. But the desired end result is questionable. If it is for the sake of making Frankensteins engine, by all means, go for it and post pics! If you only wish to have a working performance engine you will be better off with upgrading the BHF with stronger internals, fixing the timing components (as you already have a BNS, the timing guides are interchangeable) and fitting a blower. The BNS is shot if the block is damaged. AluSil is not forgiving for repairs.

You are correct, they are a bit different but nothing that can't be worked around.

Apart from 8 valves missing and requiring custom valve guides for the different clearance? The S4 BHF engine is 40-valve EFI, the RS4 BNS engine is 32-valve DI

[roboto_1337]

Something else to bear in mind is that the timing sprockets throughout the engine may be different.... more/less teeth. Should look into that.


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[ottocycle]

Something else to bear in mind is that the timing sprockets throughout the engine may be different.... more/less teeth. Should look into that.


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On the BNS the idler gears have 38 and 19 teeth. The camshaft sprockets have 25 teeth. The BHF has 30 teeth on the camshaft sprocket.

[Manystylze]

On the BNS the idler gears have 38 and 19 teeth. The camshaft sprockets have 25 teeth. The BHF has 30 teeth on the camshaft sprocket.

Good information to note. Thanks

[Manystylze]

As said above, apart from sharing the manufacturer name and similar displacement these are two different engines. After a quick google:



In regards to your valve exchange endeavor, what do you wish to achieve by getting bns valves to fit the bhf? Don't get me wrong, I like machining custom parts and making things work. But the desired end result is questionable. If it is for the sake of making Frankensteins engine, by all means, go for it and post pics! If you only wish to have a working performance engine you will be better off with upgrading the BHF with stronger internals, fixing the timing components (as you already have a BNS, the timing guides are interchangeable) and fitting a blower. The BNS is shot if the block is damaged. AluSil is not forgiving for repairs.



Apart from 8 valves missing and requiring custom valve guides for the different clearance? The S4 BHF engine is 40-valve EFI, the RS4 BNS engine is 32-valve DI

Indeed a Frankenstein of sorts. The difference between the heads should only be an issue pending cooling channels between the blocks. My comment that "nothing I can't work around" was only pending there is too much an issue utilizing the RS4 heads and possibly interchanging hardware. I will definitely post pics as I go along and record/describe the journey. The RS4 block was pretty much shot after the damage to the 2 cylinders (one maybe salvageable by sleeving but the other too far gone). May just have to source another or Q7 block...was just hoping to see if the challenge of utilizing a block I already have would be beneficial, plus I like the challenge.

[roboto_1337]

A Q7 short block might be the best case. I’m not sure of the differences there. But more likely to have abstained abuse


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[S4Bennett]

RS4 pistons are DI-specific and are relieved for a 4 valve combustion chamber. There could be valve clearance and misfires if you do not use the RS4 pistons to match the cylinder head.

[Manystylze]

A Q7 short block might be the best case. I’m not sure of the differences there. But more likely to have abstained abuse


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You're correct. The Q7 and RS4 are supposed to be the same exact engine and block with differences in the heads, intake, etc. Keeping my eye out but at the same time, curious to see if I can make magic with parts on hand. It's the Engineer Mechanic in me

[Manystylze]

RS4 pistons are DI-specific and are relieved for a 4 valve combustion chamber. There could be valve clearance and misfires if you do not use the RS4 pistons to match the cylinder head.

I have 6 of 8, two of mine grenaded when the flap pieces fell into the cylinders. I am sourcing 2, but hoping to find a full set to ensure they are matching, balanced, etc

SO the RS4 heads and parts I have are from the same engine/vehicle.

[Manystylze]

A Q7 short block might be the best case. I’m not sure of the differences there. But more likely to have abstained abuse


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Actually revisiting this idea. Found a Q7 block at a price I can't refuse. May see about grabbing it. Looking through some more articles online I see that my biggest milestones will be cooling paths, timing gear, and software.