Exhaust back pressure - some things to consider

[Steve]

Hi all,

this may be a bit of a such eggs to some of you, but nonetheless, I have been quite surprised by the effect of upgrading my exhaust.

I fitted an Nismo exhaust, 80m with two straight through mufflers, to quieten my car down to legal, from the N1 that was fitted, 80mm, small final muffler, no middle resonator.

At the time I was making around 200rwkw, and noticed an increase in lag and drop in power, but nothing overly dramatic.

Later I fitted a HKS 303756T with a 0.61 turbine AR, and as expected, a huge increase in power, around 256rwkw at 1 bar with an interim tune, as I was doing cams and a couple of other things.

Boost came on too hard at 3000rpm and even in 4th gear, with light throttle, the car would go sideways at 100kph, when boost hit.

To combat this, and to reduce back pressure/free up top end, I went to a 0.87 turbine housing. HUGE lag, over 5000rpm until 1 bar was achieved. I was very disappointed, as other cars running similar combos were no where as laggy as this.

The first thing I blamed was my custom manifold, as it was the only 'unknown' quantity on my car. I took the car to be dynoed, it now made 285rwkw at 1.2bar, and 270rwkw at 1.1 bar, but the lag was still up around 4800rpm for 1 bar. Better but by no means what it 'should' be. At 1.5 bar I only made 300rwkw, and at 1.7 bar 309rwkw and power dropped off between 7000 and 7500rpm.

People I know who have run these turbos, told me that their experience was very different, with a massive increase in power for every 0.1 bar of boost being wound in above 1.2bar. In once case on an SR20, making 30rwkw more going from 1.3 to 1.4bar.

I now suspected the car was being choked, so went off to get a new catback. I found a HKS Super Dragger 90mm, small centre resi, to 'try' and see if it resulted in much improvement.

It was a dramatic improvement.

things I found

- boost comes in 700rpm earlier

- off boost is alot crisper

- off boost, it is now possible to drive at 50kmh in 4th gear, and the car can now be driven under 2700rpm all day - very smooth (and nice and quiet if plods are around)

- economy has improved, from 190-250kms for 50L fuel I am now up to 170km and only just made 1/2 tank!

- The car is so much snappier to drive, and really brings a grin to my face every time I get in the drivers seat.

- It would appear that an exhaust system will flow reasonably until a flow is reached where back pressure starts killing power.

- It also seems that the bigger the turbine housing, the more imperative that the exhaust be less restrictive.

- my wastegate held 1.7 bar no probs with the nismo exhaust, now it wont allow more than 1.3-1.4bar. The (lack of) back pressure seems to be making the solenoid, and wastegate spring work harder (soft spring)???? but the car pulls alot harder at that boost pressure than 1.7bar before:)

The car isnt getting a retune until I replace the intercooler to plenum pipe (2 1/2 inch, being upgraded to 3 inch) as I am sure this is something of a restriction too, and hope for a further improvement. Also, it seems I need to upgrade my wastegate spring to around a 1bar unit, or I wont be able to get much more boost.

Bottom line, DONT underestimate the importance of a well built, free flowing exhaust especially when you start making a bit more power than stock:)

Hope you find this interesting, as I cannot believe the massive difference just changing my cat back has made:D and remember, a noisy exhaust is your friend:p

[EnricoPalazzo]

Hahah well done mate. NOW u better b comin on sunday. OK!

[2rismo]

Also, it seems I need to upgrade my wastegate spring to around a 1bar unit, or I wont be able to get much more boost.

What is the std spring set to? Would make your boost controller work a lot less hard if you upped it as well. Nic report mate, thanks.

Adrian

[Bl4cK32]

Well done Steve

[Busky2k]

Pretty interesting. What would have been nice was to see if you could have got an actual backpressure reading before/after!

[Steve]

2rismo, I have to change the spring, as its external gate. It has been as low as 10psi without spiking, so perhaps less than 10psi spring.

I would have liked to do a before/after, but I would sooner spend the money it would cost on mods:) Plus, I had access to a couple of different cat back systems to try and see how things went.

Dean, to reiterate, meow:p

[sweetr33]

yeah very impressive figures there. What car do u have steve?? Good of u too fill others in on the importance of a free flowing exhaust.

[EnricoPalazzo]

in his sig its says r33 gtst. Haha u should use ur eyes more

[sweetr33]

oh very true it does..... lol my bad. Damn eyes. Or brain? lol

[GTS-t VSPEC]

Steve, Good to hear that your drama's have finally been resolved. I've always gone for very hi-flow exhaust, i.e. no cat, no center muffler, and 3" from the turbo back, the response is great even on a GT30:D

[Steve]

Thanks Paul, not quite out of the woods yet, but nearly there:) some people just have to learn the hard way - makes it more fun I spose, just gets a bit expensive.

I think your small turbine housing is helping alot too, I may end up stepping down to 0.73 - have to see how it all goes - if I can get power to 8000rpm without dropping, I will stay with the housing I have. Currently (before this exhaust anyways) it starts dropping off at around 7200rpm.

[whatsisname]

Steve,

I still remember the Tilbrooks dyno day a yr or so back when Spoolin's (SAU nick) car ran a very low 219rwkw, very low considering his GTS25t had serious mods ex-Japan. The exhaust had a shitty comp cat & backyard cat back section to keep it quiet. The lads replaced the cat with a pipe & fitted the original Jap brand name system (came with the car ex Japan) The car went back on the dyno with tune & set boost level untouched. It ran 282rwkw.

Perfect example of how much difference an exhaust can make!

[Steve]

Jeez Matt, thats crazy. I dont think mine was quite that bad, then again, it would be nice to pick up 60rwkw with an exhaust change!

[rob77]

The smallest part of my exhaust is the shitty 2.5" cat used for compliance. I have always wondered how much difference a 3" straight through pipe would make, the only thing stopping me (apart from the fine) is the noise level. With the N1 system my car is very noisy already and I dont want to draw more attention than I have already to the cops.

[Guest INASNT]

Very interesting read steve!

My cat will also have to be changed because everytime its on the dyno, after the run it spells like rotten eggs, and this is a high flow 3 inch cat.

[Steve]

rob77, I know someone who has a pipe only hanging off the back of an SR20, and its acutally not alot louder than the N1 system, if at all. Only difference is it sounds a bit more aressive if you know what I mean.

He also has an apexi flap in, and with it shut, makes less noise than a N1 system - but still has its pretty aggressive note.

[whatsisname]

A restrictive &/or blocked cat, especially one that's smaller than the rest of the system, is a huge power killer!

[predator]

HKS make some damn good exhausts, in fact most of the japanese systems are pretty good - and the aussie ones that can be fabbed up lack a lot of the refinement. I think one of the reasons my car runs so smoothly is simply because of a nice exhaust setup.

yeah.. definitely whatsisname! Ok...this is a whole different type of car, but same concepts apply. My brothers car is an old XF which i was driving around for a few weeks while my skyline was in the shop. It was basically making hardly *any* power, and would struggle up hills, or basically under any attempt at accelerating fast. Basically we thought the rings or something had gone and the engine was on its death bed. Somebody recommend we should change the cat, we did and TOTALLY different car. Power came right back again, and actually went pretty hard for an old 250c.u. 4.1L. So basically the totally clogged cat was killing the backpressure and starving the whole engine of any power.

Lifespan of a cat is 10 years or so, so probably about now the old factory cat is starting to get restrictive on a skyline, if it hasn't been changed.

[Steve]

I have been told the lifespan of a cat, to be actually doing its job (not when it gets blocked up) is only a couple of years, and no more than 80,000kms.

this may be why the emissions standards of ADR37 only apply for 5years/80,000

[SKYLINED]

QUESTION: Besides monitoring the gases coming out of the exhaust, how do u know a cat is stuffed?