ok I'm not the most technically well endowed car guy. but can any of you Audi guys help me answer a question that has been plaguing me for over a year:

How does Audi get 420hp for the RS4 but only 340hp for the S4? Why does Audi do this to their engines?

IMO they should keep the 4.2 around 380hp for the S4 and S5 and bump more hp. I think sales would go up and the car would be more bank. does anyone else feel this way?

 

The RS4 uses direct injection FSI technology and the S4 motor does not. Even still, there are a few other internal differences as well, including the 8200 rpm redline on the RS4.

 

What ^ says. The S4 & RS4 engines are 'only' different at the injection. Which doesn't mean that you can easily fix a S4 to RS4 specs.

 

actually. I am pretty sure that the only thing that they have in common is the same displacement and they are both made by audi. I am pretty sure they are different blocks as well.

 

no
wrong
the 4.2 engine in the rs4 is COMPLETELY different than the 4.2 s4 engine

the 4.2 s4 engine has 5 valves per cylinder , whiloe the rs4 has 4 valves per cylinder... completely different heads, different bottom ends... crank, pistons, and all that..

they are two ompletely different engines, that happen to be 4.2 liters

 

^^
+1 completely diff engines

 

I was going to say that, but doubted myself so I checked a car site for the displacement, which is exactly the same. But that is indeed one of the few similarities.

 

thats the ONLY similarity (4.2 liters)

and don't think that the s5 4.2 fsi is a "de-tuned" rs4 fsi.. its not.. different components aswell, pistons,cams, etc

 

Thanks for answering my question! but what if the started using detuned FSIs in the S4 and S5? do you guys agree there would be something special?

maybe use a turbo charged V8 for the RS version?!?

 

very informative!

 

I'ma big time motor guy, built tons of engines thru the years so this really interests me. I'm in the process of buying an S5. If I understand what some of you guys have said is the S5 FSI 4.2 closer to a RS4 engine then the S4 engine is? or is it all different also? To me it doesn't make any sense that Audi has a few totally different engine families for something that is pretty close in specs. It must be terrible cost wise.
getting a 340-354 Horse motor up to 420 or so can be done with camshafts,heads, exhausts and programming.
Does anyone know if there is a website or something that has more info about the 4.2 motors?

Tonny

 

The S5 motor is an S4 motor with Direct Injection. The RS motor is completely different from the block out. The most I have seen pulled from the S4 motor with out forced induction is ~390 HP.

If I remember correctly the RS4 pistons are travelling at a similar rate to a modern day F1 car at redline. The RS motor is amazing and was one of the first large displacement motors to have a redline this high.

 

FYI, the 2.4L V8s being used in F1 are revving up to 19,000RPM.

 

that whole traveling at the rate of an f1 car was false advertising which audi even admitted.

and the s5 motor is not an s4 motor with fsi.

 

There was a discussion like this on Audizine, where a member posted a very informative post. Remember, the whole process of making RS4's is not terribly effective cost wise, its why the damn thing is so expensive.

------- Start Cut & Paste -------

There is alot of confusion over the fact that Audi makes 3 different 4.2L V8's. There's the port-injected V8 (S4, S5, A8, ...), the FSI V8 in the Q7, and the FSI V8 in the RS4.

The two FSI engines are related, but still different. You see alot of quotations describing the "high-revving version for the RS4". Most of these facts are from the Audi self-study program document floating around, and refer to the differences between the RS4 engine and the Q7 engine.

The S4 engine and the RS4 engine are not related. They only happen to have the same displacement. The RS4 engine is not a "tuned-up" version of the S4 engine. You might say it's a "tuned-up" version of the Q7 engine, but the modifications go way beyond tuning.

A few characteristics of the RS4 engine:

Only one belt, driving the alternator. All other accessories (A/C, water pump, oil pump, hydraulic pump) are chain drive or shaft drive from a special accessory drive gearbox.

Stress-honed cylinders, using a torque-plate.

Tubular headers.

FSI system and heads.

12.5 :1 CR

Dual-mass damper system with unbalance (for high RPM smoothness).

Q7 rods are 36MnVS4, RS4 rods are 34CrNiMo8. Not sure about S4 V8.

Q7/RS4 share pistons (forged), 290 grams.

New cyclonic oil separator system.

4 drive chains with different sprocket sizes RS4 vs Q7.

Quoting ("basic engine" refers to the Q7 engine, not S4):

To match the higher engine power output and RPM, the
following cylinder head components were modified:
Intake ports are charge optimized (based on larger
cross-sections)

Intake valves are chrome-plated hollow stem valves
(for weight reduction)

Valve springs are made of a material with higher
tensile strength and also have higher spring force

To meet the higher fuel requirements, the injectors are
designed for higher flow rates.

Roller rocker arms are more robustly designed, with
peened rollers for higher strength

Camshafts have different timings and larger opening
lengths

Intake valve opening angle 230 crank angle degrees

Exhaust valve opening angle 220 crank angle degrees

The lifters were adapted from the 3.2L V6 engine found
in the TT and A3. They have a larger ball stroke which,
in the course of testing, proved advantageous for the
high-revving engine (with regard to the inflation of the
hydraulic valve clearance compensation element).

The cylinder head has a modified water jacket which
circulates coolant to the area between the intake port
and the injector, thereby reducing the temperatures in
the cylinder head combustion chamber plate.

Due to a modified camshaft drive reduction ratio, the
camshaft adjuster has 25 teeth for the chain drive, as
opposed to 30 teeth in the basic engine.

A reliable supply of oil in all driving situations is critical,
especially in a sports car such as the RS4. The oil supply
system in the high-revving engine was designed for
racing applications in which it is subjected to lateral
acceleration of up to 1.4 g. To ensure this, the sump in
the RS4 has an additional system of flaps.

Design
Four flaps, whose axis of rotation is parallel to the
longitudinal axis of vehicle, are arranged inside a
housing. Each of the flaps opens toward the inside of the
intake end of the oil pump.

Function
When the vehicle is cornering, the oil flows inside the
sump toward the outside of the corner. The two flaps
facing the outside of the corner close and hold the oil in
the sump intake.
At the same time, the two flaps facing the inside of the
corner open to allow additional oil to flow into the intake.
This ensures a sufficient supply of oil to the oil pump.

The 4.2L V8 FSI engine in the RS4 does not use the mapcontrolled
thermostat as found in the Audi Q7 engine.

To achieve more effective cooling, two additional coolers
are used. Coolant flows continuously through one of
the additional coolers. The second additional cooler is
opened via a coolant thermostat.

To avoid excessive heating-up after shutting off the hot
engine, the coolant run-on pump is activated a preset
period of time after the engine is shut off. The pump
run-on time and the need for additional activation
of both radiator fans are determined on the basis of
characteristic maps. Various measured quantities
are included in the calculation (engine temperature,
ambient temperature, engine oil temperature and fuel
consumption).

Q7 has variable-length intake system, RS4 does not.

To enhance the sporty character of the RS4 engine, a fantype
manifold is used in the RS4.

Good separation of the exhaust pulses is ensured by
keeping the individual exhaust pipes apart until they
merge into one. These fan-type manifolds are also
attached by means of a clamping flange system.

The primary and main catalytic converters are metal-type
catalytic converters. Their advantage is that they have a
lower flow resistance than ceramic catalytic converters,
which is good for engine performance.

The front silencer for each of the exhaust pipes is housed
in a common casing. However, the exhaust pipes are kept
separate. The front and rear silencers are absorption-type
silencers. They are notable for their low flow resistance.

Engine management in the new 4.2L V8 FSI is by two
versions of the Bosch MED 9.1.1.

A single control module is used in the Audi Q7 engine.
There are two control modules for the RS4 engine. A
master-slave concept is required here due to the requisite
processing power at engine speeds up to 8250 RPM.