Large Down pipe from turbo

[karlblues2]

Hi guys. I am wondering what the impact is of fitting a larger (3") diameter down pipe from the standard turbo Should it give more power, will it induce more lag, will I lose some 'drivability' / responsiveness Please advise

[Pisobiker]

Larger diameter (read "freer flow") exhaust sistem, will reduce performances in the rpm range below boost treshold, but will increase turbocharger responsivness and power output in the mid to high rpm.

The reason is because turbine's efficiency is proportional to pressure drop between inducer and exducer.
Therefore, to increse turbocharger's efficiency you have either to increase inducer pressure, or decrese exducer pressure (better known as exhaust "backpressure") or both (which is even better ).
This way responsivness of the turbine is improved while power transmitted to the compressor side (therefore boost and flow capabilityes) is increased as well.

Some carefull matching of exhaust system with turbocharger and engine layout must be taken into account, though. Here, most important thing, as sayed, is FLOW. It is not always true that the largest the exhaust the better the flow:

to minimize backpressure, flow speed inside the exhaust must be kept high enaugh, while increasing section will reduce speed. Of course at high rpm flowing requirements increase, therefore needing bigger pipe sections, which are about proportional to power output.
Moreover, in a turbocharged application, flow comes out of the turbine in a very turbulent manner, which slows down flow therefore increasing backpressure. The optimum is obtained when increasing flowing section enaugh to produce flow gains in the high rpm while keeping flow as straight as possible, maximizing flow speed (for which some high flow metallic cat-converters seem to work pretty good).

The 3" turbo back system is known to be the best choice in the 350-430 hp configurations. For a std. K24 turbocharger I personally wouldn't go larger than a 65mm ID exhaust system.


Marco

[karlblues2]

Marco. Thanks. You're a gent Just the info I need to take to my exhaust guy next week as he is trying to manufacture out some of the resonance and off-boost lag I have been experiencing since the 3" turbo down-pipe was fitted. In a nut shell, should I stick to standard diameter down pipe

[Albert]

Marco: What wolud You think on changing 3" do 3,5" downpipe ? Is there a reason to do it ? I have a preaty big turbo and exept more boost and more advanced ignision i'm runing out of ideas for not so expensive tuning.

[Pisobiker]

Marco. Thanks. You're a gent Just the info I need to take to my exhaust guy next week as he is trying to manufacture out some of the resonance and off-boost lag I have been experiencing since the 3" turbo down-pipe was fitted. In a nut shell, should I stick to standard diameter down pipe

Pleased to help, when possible

Anyway, if you are running a stock K24 with up to 280-290 Hp output, you will probably have a good enaugh increase in high rpm power using the stock downpipe with a good 2.5" system from there back, using a single high flow 150-200 cells per square inch metal cat converter. This should provide free enaugh flow to squeeze all of the power the K24 can handle, while still having a good low rpm responce.
Going to a better manufactured 2.5" downpipe might give some small high rpm gain, but don't know if it will be worth the hassle and cost of having one made.


@ Albert,

if my calculations are correct the 3.5" (which should mean ~ Ø85 mm ID) turbo back system should be good in the 500-600 Hp range.
In the 430-480 hp range I would go with a Ø76 mm ID system.

Marco

[Pisobiker]

@ Karl,

by the way, forgot to say but, apart from exhaust, a good "homefrie" remap, such as the ones our ECU experts can provide (Paul N. and Mihnea), can really make wonders on low rpm torque and throttle responce.

Read through the "Paul's ABY Home Fries" topic in the "Engine and Turbo" section for more informations about them.




Marco

[karlblues2]

Thanks Marco I'll let u know how I go