Tom- Impreza wagon project

Post by **Stuart** » Sat Jul 09, 2011 11:55 am

The Skyline got down to around 6mpg when used hard. Bigger engine, two turbos and more weight.
Flew through the MOT today with Mark, Russ and I all pleased that the underside is actually very good condition. Driving very carefully (only moderate use and amount of boost) I've done 146 miles with just over half of the

Rob S · Post by **Rob S** » Sat Jul 09, 2011 1:11 pm

Congratulations mate, chuffed for you.

ScoobieWRX · Post by **ScoobieWRX** » Sat Jul 09, 2011 2:22 pm

Sounds like you're really enjoying the experience. Well done for getting the wagon. I love mine, specially on the many trips to scotland with a boot full of dogs and all our holiday crap on the back seats. Briliant choice sir!!

Jon · Post by **Jon** » Sat Jul 09, 2011 8:47 pm

ScoobieWRX wrote:
As for exhaust....I would get rid of the NURspec system regardless of noise, which i presume is a 3" system and therefore robbing you of torque, and go back to a 2.5" OEM spec tube diameter exhaust whether it's a decatted OEM system or aftermarket stainless steel jobbie. That should get you back under trackday noise limits, give you a beautiful burble and max torque which is just what you want for track work.

Just out of interest, how will a (after turbo) exhaust affect torque???
I understand the workings of the up-pipe, headers and porting giving gains and early spool, but a bit confused about your earlier statement? Enlighten me please

PS
Pinky is looking bang on the money Stu

Post by **Stuart** » Sat Jul 09, 2011 8:50 pm

Thanks Jon!

Interested to hear about the torque comment but I have heard it before somewhere else.....like everything else on the net I guess.

turbomart · Post by **turbomart** » Sat Jul 09, 2011 9:26 pm

Jon wrote:Just out of interest, how will a (after turbo) exhaust affect torque???

Must admit I was baffled by that by that statement... maybe he thought old pinky was an NA, in which case it might of had some merit.

Nik · Post by **Nik** » Sat Jul 09, 2011 10:36 pm

Congrats on the MOT mate. A mate of mine has some brand new drilled and grooved solid not vented rear discs for sale at

ScoobieWRX · Post by **ScoobieWRX** » Sat Jul 09, 2011 11:47 pm

Keeping a narrow diameter exhaust system, like OEM tube size, creates more back pressure. On a lower power car with a standard turbo that's desirable. The pistons have to work a bit harder to push exhaust gasses out and that's where the torque comes in. If the engine works a bit harder you are seeing higher engine loads from lower rpms which translates into more torque.

3" exhaust systems let the engine breathe more easily and aid exhaust gas escape so pistons don't have to work so hard to push exhaust gasses out. Great for bhp higher up the scale but not so good for torque. Because the pistons aren't working so hard you see much lower engine loads and in effect make less torque.

Example: GrpN Subaru's run OEM diameter exhausts (2.5/8"), not 3". They make only c. 300bhp however they still manage to make between 580 and 600ftlb torque. Admittedly they run 25psi of boost from 2000rpm but that does make the engine work very hard from very low down. Trying to squeeze so much exhaust gas out the OEM diameter exhaust really works the pistons. Subsequently you see huge engine loads and in turn an abundance of torque.

That's why i recommended to Stuart that he runs an OEM diameter exhaust system over the 3" jobbie. Because the car isn't mapped, going back to the OEM diameter system will get the car running better, more smoothly, increase torque, be more fuel efficient, and pull more strongly from low down.

If you don't agree with me no worries, it was just my opinion.

Post by **Stuart** » Sun Jul 10, 2011 7:50 am

I also seem to remember that an oversized rear can allows/encourages overboosting which can result in the seals on the turbo 'blowing' i.e the tell tale signs of oil swirls coming out of the exhaust under hard acceleration.......my old GTR used to do it whilst the Impreza does not. (Thankfully)
For reasons of noise and trackday compliance alone I'm interested in switching to an OE bore system. If the torque thing stacks up then all the better.

25psi of boost sounds fun

ScoobieWRX · Post by **ScoobieWRX** » Sun Jul 10, 2011 10:11 am

The overboost created from running the 3" system on your average subaru shouldn't be an issue so far as turbo reliability is concerned unless it's trying to create too much boost and you start getting compressor surge or stalling. I doubt you're even getting fuel/boost cut from what bit of overboost youre getting.

The issue is with the extra air being crammed into the cylinders. You've got to feed that air with fuel otherwise you're going to be running lean. If your ECU isn't mapped to feed enough extra fuel on boost you risk causing Det (knock). If you run an induction kit as well that can contribute to increased lean running. Induction kits often come with larger than OEM induction tubing and although that increases the volume of airflow it decreases airflow speed and your MAF sensor will read that as same as or less airflow, and you get even more lean running.

It would be worth running a wideband up your exhaust and hooking a set of detcans to the engine, and taking it out for a blat just to see how it's fuelling generally and on boost. It may well be fuelling fine in which case excellent, however for peace of mind when hoofing it i would want to know that all is well. Best done on the road in a real world situation. At this stage I wouldn't bother doing it on a rolling road because you don't tend to see as much airflow, engine load or boost compared to running outdoors.