Turbo Exhaust Options
- Thread starter diabloam
- Start date
TTM
Well-known member
Hmmm... As much as I can understand that somewhat "restricting" the exhaust out from combustion chambers can have an influence on mid range torque, I'm not sure I understand how it could also apply past the turbo on a turbocharged engine?
Many parameters may enter the equation such as engine, turbo size?
Suffolk944
Moderator
I can vouch for that ! [
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Diver944
Active member
Just to confirm -
Any car previous to August 1993 (K reg) has a relaxed emmisions test to pass and our cars will not need a Cat to pass it.
The downpipe (Turbo Discharge or TD pipe) has an internal diameter of just over 2 inches - Thom has a picture on his engine build thread. These pipes are only about 6 inches long and cost less than £200 but you have to remove the turbo to fit it so the labour cost is high unless DIY.
The rest of the Turbo exhaust is 2.5 inches
There are very few places in the Uk that can make a proper mandrel bend on 3" pipe without it distorting, but you can buy pre bent lengths in different angles
ORIGINAL: Diver944
I like that explanation Graham, it makes sense [
However I wonder how much extra gas is introduced to the piping from the wastegate discharge. This pipe joins the main section just before the back section and must therefore add volume into the mix [&:]
I wondered that, but then thought that the gasses coming out of the dump pipe have been diverted from the turbo anyway - total gas flow must still be the same. Plus there is no silencer on the H&S front section so the dump pipe ties in to the start of the straight 3" run. Apologies if this doesn't read correctl, I'm using my iPhone on beach!
diabloam
New member
Hmm Now I am even more unsure what to go for. Its seems I might be best to wait a bit till can justify splashing out on a whole system including discharge pipe, or alternatively get middle and rear system and then when I change my wastegate tackle the front section. Why do you need to remove the turbo to fit the discharge pipe, surely it just bolts to it and follows the same route. For the guys with the fabspeed exhaust, who much did it cost including shipping for the full 3" exhaust? Cheers
Just to clear things in my mind the turbo sytem goes like this yeah?
Manifold is two piece and goes to wastegate (via X-over pipe) and turbo, then you have turbo discharge pipe going to centre section, then centre section is two into one (one from wastegate and other from turbo discharge pipe and this then goes to back box. Is this correct, cant recall and aint done anything to mine since got car so unsure, thanks
Just to clear things in my mind the turbo sytem goes like this yeah?
Manifold is two piece and goes to wastegate (via X-over pipe) and turbo, then you have turbo discharge pipe going to centre section, then centre section is two into one (one from wastegate and other from turbo discharge pipe and this then goes to back box. Is this correct, cant recall and aint done anything to mine since got car so unsure, thanks
Yes, that's pretty much it. However, changing the wastegate doesn't really impact on the front(centre as you call it) section and vica versa. Also, the downpipe needs the turbo removing, as I found, because although you can disconnect it from the turbo ( took me 2 hours!) there is still not room to physically get it out of the car unless you can also get the studs out of the turbo.
Suffolk944
Moderator
No you need to source a 3" downpipe if required, I don't have one at present though if I do end up with a new turbo fitted then I will be getting one.
sawood12
New member
ORIGINAL: Diver944
I like that explanation Graham, it makes sense [
However I wonder how much extra gas is introduced to the piping from the wastegate discharge. This pipe joins the main section just before the back section and must therefore add volume into the mix [&:]
I would imagine that the exhaust system after the turbo would be quite important. Turbo's are driven from the pressure drop across them so if you have a restrictive exhaust post turbo you are increasing the pressure at the turbine outlet which will reduce turbo efficiency, so a larger exhaust will mean greater pressure drop.
The wastegate discharge will be so far down the system for a reason - I'm guessing that because the pressure of the exhaust gas in front of the turbo is high, higher than the pressure of the exhaust gas at the turbine outlet, if you were to just dump this right after the turbo then you will increase the pressure behind the turbo and hence reduce pressure drop across the turbine and may even cause it to stall. By dumping the wastegate gases further down the system you are allowing it time to cool and its pressure to drop to a level that is similar to that of the gas at the point of the system where it is introduced, thereby not affecting upstream pressures.
You could of course always dump the wastegate bleed to atmosphere. At least you'd be able to hear when your wastegate is working!
u63af
New member
I managed to remove my down pipe without taking the turbo off when I did the clutch. I was thinking of having a 3" turbo back system made with one silencer at the back and the wastegate venting to atmosphere by a screamer pipe. I was speaking to Rick Cannell and he said his car used to sound amazing with a screamer pipe.
Question for those of you with experience of tuning turbos: Does reducing the on-boost back pressure, by fitting a less restrictive exhaust, or de-catting, for example, in principle affect the AFRs? Given that once you are on boost at full throttle you are working on a looking table rather than from the airflow metering, it seems to me that it you might end up with more boost than the fuelling table is expecting. Or am I worrying unnecessarily?
Diver944
Active member
Yes you will start flowing more air from a lower RPM as the restriction is reduced and you will probably make a little more peak boost depending on your setup. If you are still running the factory AFM it will not be able to compensate for this extra air.
Whether it is enough to make you dangerously lean can only be determined if you measure it properly. It would be advisable after a change like this to get the Air Fuel Ratio measured on a Dyno so you can double check you are still safe.
Whether it is enough to make you dangerously lean can only be determined if you measure it properly. It would be advisable after a change like this to get the Air Fuel Ratio measured on a Dyno so you can double check you are still safe.
Turbine outlet is only 2" on the K26/6, pretty much matched to the downpipe size. Wonder if there is much benefit to replacing the downpipe with that size outlet. Most of the energy in the gas is going to be flow when it comes out of the blades isn't it? Will take it a while to turn into pressure again. Perhaps slapping a really big downpipe on would just cause a load of wierd turbulence at the turbine outlet?
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