Stock Intercoolers

How bad are the B5 S4 stock intercoolers?

Stock Intercoolers
Audi B5 S4 Stock Side Mount Intercoolers


The assumption that the B5 S4 stock intercoolers are terrible has been around for about as long as the S4 has.  I myself long ago abandoned the stock intercoolers and started upgrading with aftermarket intercoolers; first with the AWE Tuning side mounts, followed by Evolution Racewerks (ER), Addict Motorsports (AMD), and presently I have a custom set of Silly Rabbit Motorsport (SRM) IC’s in the works.

The Plan:

During the period of time between selling my ER IC’s and having the AMD IC’s delivered I had a few weeks to spend either not driving my car, or sticking the Stock intercoolers back on to keep my car on the road.  I decided that I would explore how much of a drag the stock IC’s were on the car with K04 sized turbo’s.  At the time I was evaluating the FrankenTurbo F4H-BT which was close in performance to the Borg-Warner K04.  Having just removed the ER IC’s, arguably the best cooling SMIC option available for the B5 S4, and soon to be using the highest flowing IC I have tested, the AMD IC, I realized that I had the opportunity to benchmark the stock intercoolers against two of the best products available, on a turbo setup that is one of the most widespread upgrades on the B5 S4.

The Data:

The results from the Intercooler Faceoff were clear, the Stock intercoolers have a greater pressure drop than any intercooler that has been tested on my PTS flowbench.

SRM V2 CFM chart

But does that matter?  The function of the intercooler to transfer heat from the charge air to ambient air necessitates a pressure loss and possibly the pressure drop was an indication of a design favoring heat transfer over pressure losses.

Temperature Comparisons:

The first comparison I made with temperature data was of the Stock IC’s and F4H versus AWE IC’s and K04 turbos.

stock_vs_awe_iat_riseThe stock intercoolers are represented with yellow and the AWE intercoolers with blue.  It’s clear that by the end of a FATS pull, going wide open throttle in third gear at a low rpm and holding that through 6500 rpm, that by the end the Intake Air Temperature is higher when using the Stock intercoolers.

The interesting aspect of this comparison is that at 6500 rpm the difference is about 14 degrees Fahrenheit, but at 5500 rpm the difference is about 7 degF. From a practical application standpoint the temperature difference of the charge air is not that much greater with the stock intercoolers over most of the range evaluated. From 2500 rpm to 4000 rpm the difference is minimal and from 4000 rpm to 5500 rpm the difference only grows by a small amount.  It’s only at the point where the pull is about to end that a significant difference manifests.  75% of the acceleration event shows a small IAT difference between the two.

It’s worth noting that in third gear 5500 rpm equates to almost 75 mph.  For a vehicle being operated on public streets within the posted speed limits it is likely that the improvements that the AWE IC’s provide in holding IAT’s in check may not often be realized.

Next I compared the stock IC’s with ER’s IC’s.

Stock IC with F4H @ 21 psi
Stock IC with F4H @ 21 psi


ER IC with F4H at 21 psi
ER IC with F4H at 21 psi

No surprises here, at 5500 rpm the ER is keeping the charge air temperature about 66% cooler than what the Stock IC does and the advantage of the ER grows as the rpm’s continue to climb.

Temperature FATS Comparison:

While the IAT rise is useful to gauge how the intercoolers compare it’s ultimately how the car performs that is of greatest importance.  Of course it is impossible to only evaluate temperature affects of these two IC’s on the vehicle performance when Pressure Drop affects are unavoidable.

Bearing that point in mind, here is how the ER’s and Stock SMIC’s stack up on a few FATS pulls.


Avg = 3.66Avg = 3.93

Pressure Drop FATS Comparison:

With unambiguous data coming from the flow bench tests the question I had was how would those pressure drop differences play out on the road.  Shown below is a comparison of the cars acceleration while using the AMD and Stock IC’s.

AMD versus Stock IC Acceleration Comparison
AMD versus Stock IC Acceleration Comparison

The FATS number comparison between these two:

Avg = 3.50Avg = 3.93

While these results are not surprising, when looking at the big picture there is an interesting point to be made.  In a relative sense the ER cools the charge air much better than the AMD IC, but the AMD IC has much less pressure drop than the ER.  It would appear that ‘better’ flowing intercoolers produce more benefit as exhibited by the better FATS numbers with the AMD IC’s.

I suspect what is taking place is that the temperature difference is not so great as it seems to be.  When considering that the turbocharger compressor is outputting air that approaches 300 degF and the intercooler then draws it down to below 100 degF, a 10 to 15 degree difference between two IC’s at the very end of the pull is a relatively small difference.

Conclusion One:

The FATS numbers show the benefit of having aftermarket intercoolers, better charge temperature and pressure losses result in a better performing car.  How the magnitude of the improvements is characterized is up for debate.  My experience with the car and FATS would suggest that for every tenth of a second the FATS drops the peak wheel horsepower number increases by 10 whp.  Using stock intercoolers at 21+ psi is going to significantly degrade the vehicle performance in comparison to how it could perform with a top end intercooler.

Intended Operating Environment

The analysis above was done with the stock intercoolers subjected to boost levels 150% greater than what they would encounter on a stock vehicle.  Even a Stage 1 B5 S4 will reach boost levels approaching 100% greater than stock boost levels.

How do the stock IC’s fare with a K03 equipped S4 operating at chipped boost levels.

For that evaluation I looked at some data logs I have from a GIAC chipped S4 that was operating with stock intercoolers and then upgraded to the Silly Rabbit Motorsport SMIC’s.

The FATS numbers compared as follows:

Avg = 5.21Avg = 5.01

The results contradict those found with F4H/K04 turbos operating at 21 psi.  One contributing factor was that the SRM IC pulls started with an IAT approximately 10-15 degF greater than the stock IC’s.  The IAT chart is shown below:

SRM versus Stock ICs
SRM versus Stock IC’s with GIAC Stage 1 Tune on K03’s

It can be seen that the SRM IC does a better job at keeping the charge air temperature rise in check, but the difference is less than what is seen with a K04 size turbo at 21 psi.  What does not appear to be occurring here is an improvement in pressure drop resulting in a faster car.  This is likely a result of less dense charge air having less of a pressure drop – the flow improvement that can be achieved with the SRM IC’s isn’t realized at low boost pressure levels, or at least it does not translate to the car being faster as was the case with the AMD IC’s at 21 psi.

Again thinking about the practical use of the car, limiting the acceleration event to one where the car reaches approximately 75 mph, the stock intercoolers keep intake temperatures right near the starting level, even with Stage 1 range boost levels.

Stage 1 boost with Stock IC's
Stage 1 boost with Stock IC’s

Here’s another car with stock IC’s and an APR Stage 1 tune over a public street acceleration range:

APR Stage 1 boost IAT profile
APR Stage 1 boost level IAT profile

Stock IC, Stock Turbo, Stock Tune:

The final evaluation is of the stock IC’s in their intended operating environment, cooling the charge air from K03 turbo’s operating at stock boost levels.

Stock IC IAT to 6500 rpm
Charge Temperature Drop with Stock IC’s and Stock Tune -> 6500 rpm

In this chart the Orange line (COT) is the temperature of the charge air at the turbo compressor outlet.  It’s clear that the intercooler is doing a good job at bringing down the charge air temperature.  In fact, at the end of a pull up to 6500 rpm, approximately 95 mph, the ending intake air temperature is lower than the starting temperature!

Conclusion Two:

On K03 turbo’s there does not appear to be much benefit to using aftermarket intercoolers based upon the type of evaluation made here.  Of course there could be different situations that could highlight more significant differences, such as operating a Stage 1 B5 S4 on a road course at a high performance driving event.

For a stock tune and even a chipped B5 S4, when operated as a daily driver and within public street speed limits, there is probably little benefit to upgrading to aftermarket intercoolers.

Final Remarks:

I’m a proponent of using quality aftermarket intercoolers in all conditions even if there is no measurable improvement in the vehicle’s performance.

The improvement in pressure drop allows the turbocharger to develop the requested boost level at a lower pressure ratio, so that the turbocharger is not spinning as fast as it would otherwise.  Readings I have taken with the stock intercoolers, k03’s, at stock boost levels show that the K03’s are developing more pressure than what is recorded by the MAP sensor.  In the interest of turbocharger longevity, by reducing the load on the device the likelihood of failure should also decrease.

This chart shows how much of a pressure difference there is between what the ECU requests and what the K03 must output to meet the requested level when it must go through the Stock intercooler.

Stock FATS Boost Levels
Pressure Drop of Stock Tune -> 6500 rpm

If an aftermarket intercooler is able to appreciably close that gap and not hurt temperature performance I’d call it a good investment in the long term health of the turbochargers.

For some additional perspective on IC Efficiency read this post.

Some additional data from a boost profile that I don’t intend pushing the k03’s to routinely.  16 psi at the compressor around 6500 rpm put the turbo right at the max speed line of the compressor map.

The chart shows that towards the end of the pull the IAT starts to swing upward, netting around a 23 degF rise, which is not uncommon for a stock IC on a full pull to near redline on elevated boost.

K03's on high boost to 6500 rpm

Audi B5 S4 Information and Testing