PseudoKirby

yes I have seen this asked before, but they are quickly dismissed, so I have been doing some research:
as far as I can tell the RS5 and S5 engine differences come down to:
Pistons (a 1.3 compression ratio increase)
Cams
ECU and tune
and the RS5 has VVT on intake and exhaust, whereas the S5 only has it on intake

so there should be a decent bump in switching to the RS5 intake manifold, the castings are the exact same, meaning the S5 has the same sized ports
Honda guys get something like 20hp in swapping a K20 intake onto a K24 and they dont even have the same sized ports as each other
realistically +40hp isnt too far fetched
so while not expecting the whole 450hp number, but it may be possible to come close to the 400chp number with the manifold, a tune, and exhaust

you can bolt the manifold onto the block as they have the same patterns for the lower runners
the valve covers look the same but are just red, so breathers all match up
injectors are the same, as are the fuel rails and associated pressure sensors
throttle bodies are same part numbers
manifold position sensors are the same

the only issues I can see are:
dual throttle bodies
MAP instead of MAF
relocating the coolant tank and or oil filter and housing
and tuning

now I am assuming you can split off a single throttle body signal and Y it off to duplicate to have both operating in unison, cant find a wiring diagram for an RS5 to verify how the ECU controls the throttle bodies, but I cant think of any reason two throttle bodies should operate in anything but unison
but I doubt the S5 can run without a MAF signal, and wiring in the MAP sounds difficult along with a RS5 ecu swap, so you would have to feed the dual intakes from a Y'd off single intake monitored by a single MAF, which sounds messy, but doable?
and how tuneable is the 4.2 ecu? can a tuner activate or add a MAP which would be wired in? speed density?


FAQ:
"just buy an RS5"
you can get the manifold for under a grand
"the RS5 makes all its power up top from a raised redline"
audi overbuilds, the S5 rotating assembly can handle an extra 1000rpm if needed, but:
RS5 dyno graphs show only a 15hp climb past 7k, the S5s redline
"it wont change anything"
how much horsepower do you suppose a little over 1 point in compression ratio increase and cams can make? the intake manifold and tune has to be a good portion of it
"its not worth it"
I'm bored and I like thinking outside the box, so play along with me

hahnmgh63

Where are you located? I've got a spare RS5 intake manifold. Injectors are also a different part # so you might want to upgrade those, not sure of the fuel delivery differences. Bosch injectors charts are easy to find but I think the RS5 are Seimens injectors.

PseudoKirby

Im in california
according to audi parts website they share the same part number between S5 RS5 and R8
https://parts.audiusa.com/p/Audi_201...9906036AA.html

MFB01

Ask this guy. If anyone would know for sure, I think he would......

https://www.youtube.com/@VAGTechnic/featured

PseudoKirby

is this a friend of yours?

hahnmgh63

Vagtechnic is a U.K. based Audi repair shop that has some great Youtube videos. I watch them every once in a while. A lot of 3.0t and 4.2 cars that they work on.

PseudoKirby

update, didnt notice the B7 RS4 uses a single throttle body, but is only 420hp, wondering if the B8 4.2 has that extra 24hp from those dual throttles

also those intakes are $1000 used...

hahnmgh63

Compression ratio is listed at the same. B7 RS4 lists bhp 420@6800 & lb/ft 317@6000 or (430NM which equals 317.15lb/ft @5500 depending on the Audi brochure) whereas the RS5 lists 450@8250 & lb/ft 317@4000. That's a fairly big difference where Aud lists the maximum torque rpm between the two as well as the maximum Bhp difference. Very likely attributed to the intake and/or exhaust manifolds for the most part and possibly some tweaking of the ECU. RS5 does have some other minor gains to be had in the variable output oil pump and electrically controlled waterpump thermostat. Definitely a freer flowing intake manifold so I'm sure the exhaust designs is freer flowing.