Turbo Spool Up Test


April 6, 2006

Having had my curiosity piqued by the theorists who asserted that aftermarket downpipes are bad for the car’s turbochargers I decided to review the data I had on hand for any evidence of this.

The theory goes something like this. Aftermarket downpipes are less restrictive to the exhaust gases leaving the turbochargers and flowing through the exhaust system. This reduction in restriction means there is less back pressure within the exhaust path. With this reduced back pressure comes an increased pressure differential across the turbochargers turbine section allowing for quicker spool up of the turbocharger.

Many who have switched to aftermarket downpipes have commented on how they noticed significantly quicker spool up of the turbochargers with the freer flowing downpipes. This improvement is all the more impressive on a car such as the S4 which has barely any turbo lag to begin with.

Unfortunately, some claim that this increased rate of spool up puts added strain on the turbochargers, thereby shortening their useful life span. The turbine now accelerating more rapidly than it was originally configured must surely be experiencing added stresses and is on the road to failure.

Thus we reach the conclusion stated by some, that aftermarket downpipes, designed to improve exhaust flow, are in fact a significant contributor to turbocharger failure.

My personal experience after having installed aftermarket downpipes on my car was that there was no noticeable difference in the time it took for the turbochargers to spool up. Not wanting to completely discount those who could ‘feel’ the difference I dug through the various data logs I have collected to see if I could find an answer to this question.

I went and looked at the two most extreme cases I could find. The worst-case scenario involved my stock downpipes mated up to my measly 70mm diameter Supersprint exhaust. Surely I must be nearly choking the life from my cars engine with this restrictive exhaust. I then looked for the best-case scenario, the massive 3″ diameter Autospeed downpipes attached to a Milltek SuperDual 2.25″ dual exhaust. The turbocharger would nearly be flying from it’s housing it would be spooling up so fast.

I plotted out the curve of boost pressure versus time for several cases of hard acceleration from both set ups. With the very short spool time and long interval between data samples it was not possible to develop a high-resolution plot. However, what emerged was interesting even in its crudeness. There does not seem to be any significant difference in the spool up rate for either configuration.

K03 Turbocharger spool up
K03 turbocharger spool up recorded via VAG-COM

My theory as to why this is the case is as follows. The difference in restriction between the stock and aftermarket downpipes is probably small. Especially at lower flow volumes. The turbochargers spool up very quickly, fast enough that the turbochargers are already providing maximum boost before any back pressure affects can be realized. Once at boost with a greater volume of exhaust flowing, the free flowing aftermarket downpipes begin to show their benefits, namely more power.

So, in conclusion, what do I think about the theory that aftermarket downpipes are bad for turbochargers because they allow them to spool up too quickly? I think it is wrong, and until somebody can show me some proof to the contrary, I stick by the results I have provided.

Lastly, while the results I obtained are interesting, to me at least, I did not perform a rigorous scientific test. This is data collected on my car on two separate occasions approximately two weeks apart. There are too many variables that I cannot account for. Still, I think the results say more than those who claim they can ‘feel’ a difference.

Audi B5 S4 Information and Testing