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bunkyT18
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Pilbara
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November 2004
Turbo/exhaust question please help. Mon, 15 August 2005 00:23 Go to next message
If I had a very restrictive stock exhaust. Like off a carby 3TC that was welded to a 2" dump pipe attached to a stock CT20 (3TGTEU motor, stock apart from pod filter). Would the backpressure or restriction be so severe that it would actually affect the engines ability to boost past a certain level?

The reason I ask is that I installed a Speco ("Danger to the manifold!") Boost gauge and a bleed valve on to my T18 with the above setup and from running the stock boost level, I can tell the gauge is a bit out, by about 1psi less. Anyway, I raised a the boost little by little but found it stopped just short of 10 PSI (calculated real boost not dodgy speco reading) , no matter if I adjusted the bleed more or not. I found that it actually ran heaps better after backing it off a little but the boost gauge reading did not change.

I'm thinking that if a free flowing exhaust allows a turbo to boost better, then the reverse must surely apply for restrictive exhausts.

What do youse think?

[Updated on: Mon, 15 August 2005 00:25]

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TurboRA28
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Re: Turbo/exhaust question please help. Mon, 15 August 2005 00:45 Go to previous messageGo to next message
That sounds valid enough to me..
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October 2003
Re: Turbo/exhaust question please help. Mon, 15 August 2005 01:44 Go to previous messageGo to next message
If the wastegate opening pressure is stock i.e. no bleeding of signal then a free flowing exhaust will raise boost as the turbine is now flowing more exhaust but the size of the wastegate port is still the same size and spring pressure.

If you are bleeding air off you should just keep making more boost as the turbo is pushing more air into a sort of closed system or brick wall and the wastegate is closed.
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bunkyT18
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Re: Turbo/exhaust question please help. Mon, 15 August 2005 06:23 Go to previous messageGo to next message
Thanks for the responses guys.

Quote:

If you are bleeding air off you should just keep making more boost as the turbo is pushing more air into a sort of closed system or brick wall and the wastegate is closed.


Nah, I don't buy that mate, although I appreciate your logic. Bear in mind that this bugger is so restrictive that you can actually feel it choking once you get to around 10 psi. The boost is not read from the turbo but from the manifold.

If the exhaust is so restrictive that it actually flows at low boost levels but increased boost levels severely affect flow. A turbo will simply not 'spool' or boost when what you have said above occurs.
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Re: Turbo/exhaust question please help. Mon, 15 August 2005 08:48 Go to previous messageGo to next message
Does the engine rev as quickly and hard as when the engine was NA?

If not I wouldnt go blamming your restrictive exhaust, it could be that the restriction is now on the intake side i.e. carby or the engine is not nearly getting enough fuel, I assume it is jetted stock still?

Quote:

The boost is not read from the turbo but from the manifold.


You will get the same boost reading from the turbo as the manifold. The only time this changes is if you have a restriction between the turbo and the manifold such as an intercooler.

Why dont you put another slight restriction in the exhaust, say block off the tailpipe and test again. If 10psi is not meet either the restriction is most likely on the inlet side or your turbo is fuxored, timing is out, not enough fuel etc.
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bunkyT18
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Re: Turbo/exhaust question please help. Wed, 17 August 2005 10:33 Go to previous message
The motor is a 3TGTEU so there is no carby, it's efi.
Fuel is pumped in using a lift and inline pump, surgetank combination plenty of fuel.
timing is fine as it's running on the factory computer with no drama's. I might point the finger at the knock controller next...

And no I will not block the exhaust more and see what happens. Just got a gut feeling it would not tell me anything I don't already know.

[Updated on: Wed, 17 August 2005 10:36]

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