bigger exhaust on turbo diesel

Submitted: Monday, Mar 08, 2004 at 13:31
ThreadID: 11094 Views:17907 Replies:9 FollowUps:2
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Hello Everyone,

Im thinking about putting a new exhaust on my SD33T patrol - but I dont know how a turbo diesel goes with a bigger exhaust.

I know from experience with petrol turbos that a 3" exhaust generally sees about 2-3 psi increase in peak boost (give or take a bit) - of course the Air Flow Meter picks this up on a EFI car and corrects for it in fuel and (where applic) ignition maps.

So what happens with a diesel - do i need to get it re-timed for the extra 3psi of boost? or do I just run it a bit leaner and not worry? does it somehow compensate?? (i cant see how it would but then im no diesel expert). I just dont want to shorten the life of my engine by leaning it out (400k kms and still going strong).

Oh and are diesels any quieter/louder than a petrol? I am still debating whether to go 2.5 or 3" (thats if i go a bigger exhaust at all in the end). Anyone notice a big difference with a bigger exhaust?

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Reply By: Member - Bruce and Anne - Monday, Mar 08, 2004 at 13:54

Monday, Mar 08, 2004 at 13:54
I have a 2.5 inch system on the MU and I don't think it has made a notable difference, a little louder and it has not increased the boast at all. I had it checked after winding out my waste gate. It is not a mendrel bent system though, but it is still pretty straight. MY system cost $180 I've seen the mandrel's costing $600 and more. Hope this is of some help.
AnswerID: 49520

Reply By: Wayne (NSW) - Monday, Mar 08, 2004 at 14:36

Monday, Mar 08, 2004 at 14:36

As I said in a earlier post, I changed the exhaust on the Cruiser to a 2.5" system. The pipe was as straight as possible and all the mufflers removed. I don't know how much extra toqure it produced but I could feel it. The extra toqure was at low revs too, which is what I wanted. The turbo kept it quite but it does have a note of a little more toqure. The turbo boost and fuel were not changed.

AnswerID: 49527

Reply By: Rosco - Bris. - Monday, Mar 08, 2004 at 14:46

Monday, Mar 08, 2004 at 14:46

From the little I've gleaned on the subject, it would appear there is no benefit in going any larger diameter than the outlet from the turbo, but a mandrel bent system is apparently a must. Adds significantly to the cost but dramatically improves flow by lowering resistance (friction) at changes in direction.

AnswerID: 49529

Follow Up By: Member - Ken - Monday, Mar 08, 2004 at 15:32

Monday, Mar 08, 2004 at 15:32
Hello all

You are spot on Rosco.

Before having my DTS turbo fitted I spoke to the Tech Adviser at DTS and in essence he said the exact same thing:

Exhaust no bigger than turbo outlet, mandrel bend & no mufflers.

I had the exhaust sytem fitted 10,000km after the turbo and found no change to either fuel usage or boost, however it virtually eliminated the 'turbo lag' or slow turbo spool-up. It is also supposed to significantly assist temp control, but as I have never had a problem with this, I can't really tell whether or not it is assisting in this area.

The exhaust system has a straight through muffler/resonator thingo which reduced the noise to less than pre turbo days.

The Troopy weighs 3.5tonne when fully loaded and gets boofed along at 110kph where allowable and drones along through the Simpson. Still no noise or temp problems.

Hope that assists those contemplating going to a turbo.

Ken Robinson
FollowupID: 311317

Reply By: Member - Jeff M (WA) - Monday, Mar 08, 2004 at 15:17

Monday, Mar 08, 2004 at 15:17
I'm no expert either but I would imagine you would be able to get rid of a little more heat with a bigger pipe, less pressure/friction?
Tell me if I'm wrong.
AnswerID: 49531

Reply By: Bob Y. - Qld - Monday, Mar 08, 2004 at 16:24

Monday, Mar 08, 2004 at 16:24

Had a 3" mandrel bend exhaust made up, and fitted myself. Bit noisier, more like a drone, but didn't change any settings. "Feels" stronger especially at lower revs, but it might only be imagination. Economy did improve by up to 5%. Boost pressure didn't change, but spools up faster, as mentioned above.


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AnswerID: 49537

Reply By: biscuits - Monday, Mar 08, 2004 at 16:37

Monday, Mar 08, 2004 at 16:37
Thanks for the replies everyone.

I could have sworn that the boost jumped from 6 to 9 psi on my CA18DET engine when I got a 3" exhaust - but then I am running a $10 boost guage so I dont know how accurate that all is! Plus the turbo was an aftermarket install job.

I suppose when I think about it, at the end of the day the wastegate is opened by the actuator which runs off the pressure in the compressor housing - so an exhaust shouldn't have any bearing on boost pressure...

Oh well, thats good I sorted that out at least. Sounds like a 3" with no muffler is the way to go - at least I can put a muffler or hotdog (resonator) on that if I think its too noisy.

Thanks again,
AnswerID: 49541

Follow Up By: Dave from Fraser Coast 4WD Club - Monday, Mar 08, 2004 at 18:18

Monday, Mar 08, 2004 at 18:18
yeah biscuits

the boost level is determined by the wastegate settings, you could run without any exhaust at all and there would be no change in maximum boost.
FollowupID: 311351

Reply By: Roachie - Monday, Mar 08, 2004 at 16:44

Monday, Mar 08, 2004 at 16:44
Same thing here too. I had a 3" mandrel bent system put on using a dump pipe made by Glen Binskin. This gave the turbo the quickest discharge possible. The system only has a small Lukey Hotdog muffler and sounds better than a standard 4.2T/D GU. I also seemed to notice it is better down low, but hasn't really done anything at cruising speed (eg: overtaking times etc).
I would imagine it would be necessary to re-calibrate the pump and timing etc to gain best effect.
AnswerID: 49543

Reply By: CMB - Monday, Mar 08, 2004 at 18:48

Monday, Mar 08, 2004 at 18:48
Hi all.

My first post.

The 4.2 TD GU has it's waste gate set at 8 psi so I guess your SD33T would be about the same (the blowoff valve seems to operate at about 13 psi). As stated by others, the boost will not be altered by a bigger exhaust. The back pressure will and fitting a bigger exhaust (I have 3" mandrel) will reduce this and allow the turbo to spool up quicker at lower revs. Mufflers etc make different noises. Just don't get caught by the Dept of T'port with a louder than standard truck.

Talk to Mannell Motors, Superior Turbo's etc about aneroid valve changes, timing, turbo refits (they machine the insides to take a different rotor) etc to get more power. It makes a HUGE difference to power. Being electronic, you could simply fit one of those power up chip things.

My biggest problem with having the above done is the heat under the hood that kills my dual battery yearly.

Good luck.

AnswerID: 49569

Reply By: Member - DOZER- Tuesday, Mar 09, 2004 at 22:31

Tuesday, Mar 09, 2004 at 22:31
Turbos hate backpressure, as they work on pressure differential, the less restriction after the turbo, the faster it can spool up and the faster heat gets out, and the cooler it all runs, but the waistgate will still control max boost, and SOMETIMES, a little more can be squeezed in without overheating things if it runs cooler with std boost. So better response from the pedal and welly, and possibly more from a bit more boost. Dont forget for any engine speed with the larger exhaust, you will be getting more boost than b4 and therefore more power can be made. U can try removing the muffler and fitting a hotdog resinator in its place to get cheaper benefit than whole replacement.
AnswerID: 49713

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