Tried Mitsu technical services etc, and they either don't know or cannot tell over the phone due to legal reasons. Myyyy goodness!!!! :confused:

It's 2.5 inch isn't it?

It's 2.5 inch isn't it?

I thought it was 2.25, mandrel bent.

Some people who said they measured it said it's closer to 2.4" from extractors/exhaust manifold to rear suspention where it decreases to about 2.25" as it curves up and over the suspention components :D

My theory is that it is 2.25" all the way, I think people are getting confused with its diameter(sp?) becuase of the exhuast sheilding that covers the pipes all the way upto the rear suspention making it look bigger

Well, I was sure that the external dia at one point was about 2.15 - 2.2 inch. Mitsu told me once that it was 60.5mm (almost 2.5) but that was for a 3.5

2.25 at smallest point isnt it on the 96? ur exhaust is only as good as ur smallest point :cool: wow i sound msart

'] wow i sound msart
:bowrofl: (sorry dude couldn't help it)

3.0l - 2.25 Inch

3.5l - 2.5 inch

Trav

3.0l - 2.25 Inch

3.5l - 2.5 inch

Trav

So Trav your saying if we were to bolt up a 3.5 zorst to our 3.0 we should see an improvment, with a new high flow muffler ofcourse??? :P

u mean 2.5?

no. you will lose power. you will lose all back pressure, even just by getting a high flow muffler. i had my system replaced with 2.25 inch and a ultra flow 2.25 inch lukey muffler, and i lost 3 HP. why? no back pressure. i am now going back and putting a tubro muffler on, which flows in a Z shape through the muffler, therefore givign something for the back pressure to push against, and inprove low down lorque etc.

just think of a rocket ship. why do they take off froma platform?

Trav

just think of a rocket ship. why do they take off froma platform?
Heh heh... nice analogy :P

I'm thinking of fitting an exhaust from a 3.5 to my car but only after headers and if I did other crazy stiff like cams etc. People have gone from 2.25 to 2.5 on a 3.0 and have said they saw good gains but it's mainly due to the muffler. A mate went from the 2.25 to press bent 2.5 and saw gains but that would be due to the muffler.

u mean 2.5?


just think of a rocket ship. why do they take off froma platform?

Trav

No I mean 3.5 as in 6G74 zorst on 6G72 :D

And Rockets take off from a platform to keep them upright, and to allow astronughts(sp?) to enter the capsuals. Due to the force of the rocket engines they need to be steadied in the first few seconds because the vibrations, back pressure has nothing to do with a rocket engine :D

Well I have a 2.5" catback on my old 3.0 TF with a K&N Panel filter and Unichip and recorded an increase in power through out the whole rev range and a 12kW @ the wheels increase top end lol

did u dyno b4 and after the exhaust thou? might have had little to do with your 12kw increase.

Trav

did u dyno b4 and after the exhaust thou? might have had little to do with your 12kw increase.

Trav

Ahhhh you got me there :D , the zorst on it's own didn't do much for power, dynoed 98kW at the wheels with the zorst and K&N (had better response though), I put this down to incorrect air fuel ratios created by the breathing mods, the Unichip fixed this up along with advanced ignition timing :)

Mind you, my muffler is a dog-leg design as opposed to a straight through design like the VRX so backpressure isn't really an issue lol

thats good then.

hmmmmmm.......backpressure hey, i reckon its a topic that will just never die. I myself am not a believer of backpressure.

http://forum.hot4s.com.au/messageview.cfm?catid=17&threadid=17889&highlight_key=y&keyword1=back

i've found a person who shares the same thoughts too!

I think back pressure isn't really a correct name for it. It's more about the piping being too wide to flow efficiently. Too wide an exhaust means gas speed slows down and less scavaging occurs.

Firstly, thanks to everybody replying to my question.

Seems to be quite common issue, this back-pressure (bp) matter. You beat me to it again, Redav. I thought you were meant to be working in the bush, not on the Forum ;)
Firstly, 4-stroke engines do not require bp, only 2-strokes do. Bp is actually bad for 4-strokes.
Why would a free-flow exhaust then decrease the engine power? Could be as a result of leaner mixture due to improved breathing, yes. But good ECU should fix that. Older carburettor engines would need to be re-tuned.
So, when ppl claim that after they built large exhaust pipes, they experienced power loss, not gain, why does it happen? I would say the reason is in the incorrectly designed pipe. Best results are achieved by cutting the exhaust off soon after headers’ collector. Bit noisy, though, hence not recommendable on street use. Heh. But seriously.
Too large pipe will result in to the gas flow being slowed down and then restarted again after next cycle. Think of water coming from a garden house in to a large container - the flow will be disrupted and becomes almost a static puddle, depending on the size of the secondary container. Exhaust gases work in a similar way. The idea of having certain type/size exhaust system is to create and maintain the flow as fast and steady as possible.
Too small pipe will give lot of bp, resistance (power loss), especially on higher revs.
Too large pipe will create “standing” gases on low revs, again, resulting to some amount of power loss, due to lack of nice flow.
So, any std exhaust pipe is usually a compromise between these too extremities. Sigh, not easy case that one, cos engine revs vary from 750 to 8000 – big difference in the gas flow/speed. Hmmm, that’s an idea: create variable size exhaust systems. Similar things already exist on intake manifolds.
Hope this helps to clarify the issue about back-pressure. Unless you have a bad driver’s seat….
I have spoken.







I think back pressure isn't really a correct name for it. It's more about the piping being too wide to flow efficiently. Too wide an exhaust means gas speed slows down and less scavaging occurs.

I agree killer, thats what I said in my previouse post, I noticed a big improvment (relativlly speaking ofcourse :D ) with my Unichip, low down flat spots were greatlly reduced and above 3-3.5 grand is absolutlly screams (more so sound, but does pick up noticablly faster) :D

i think the analogy of the waterflow into the pool applies to the muffler more than the piping, seconadry collectors flow into piping which is smaller, unless u did something huge like 3.5" piping. its only when u flow to the muffler or resinators do u get to an open space that can be compared to the pool.

i measured the factory pipng (not including the heat shielding) on my 3.0 with calipers and its was all 61mm or 2.4in before the stepdown over rear suspension. so very slightly closer to 2.5 than 2.25, it then stepped down to 2.25 over the rear suspension and into the muffler.
I doubt they would bother to make different piping (and flanges, cat converters, resinators) for the 3.5 :cool: 2.4" is already a big size for a factory system on a 6 cyl.

No I mean 3.5 as in 6G74 zorst on 6G72 :D

And Rockets take off from a platform to keep them upright, and to allow astronughts(sp?) to enter the capsuals. Due to the force of the rocket engines they need to be steadied in the first few seconds because the vibrations, back pressure has nothing to do with a rocket engine :D

Well I have a 2.5" catback on my old 3.0 TF with a K&N Panel filter and Unichip and recorded an increase in power through out the whole rev range and a 12kW @ the wheels increase top end lol

I think we need dyno figures on the rocket, before and after the platform. :D

I just did a heap of reading on the tpoic of back pressure. Basically the way it stands, or at least the way I see it, is as follows.

The best possible exhaust setup, (excluding top fuellers becuase they're in a class of their own, very unique design) extractors with no exhaust piping at all is best. (Obviously not legal or practical on the street) This is because firstly:

-With the way extractors work with the high pressure/low pressure system is more effective then anything else at removing (scavenging) exhaust gases from the cylinder, hence the piston doesn't have to work (expel energy (ie: expel horse power) to push the gas out). This can be compared to no headers at all, or factory headers.

-Secondly, by having extractors or some form of headers this prevents burnt valves, as compared to no headers at all. Open exhaust valve and exhaust gas doesnt come into contact with oxygen as soon as the valve opens.

-Finally, there is no restriction at all from exhaust piping or mufflers etc, maximum flow.

Trying to shine some more light on this topic heres a few facts (supposed facts) I've picked up:

-The reason your exhaust piping can be too large and you lose power is because the exhaust gas spreads out in the larger diameter piping. The gas cools, becomes denser hence it moves slower, and has less energy (in the form of heat energy) there fore it moves slower, and less scavenging occurs.

-A better flowing exhaust port means the intake works better, giving a leaner mixture. The leaner the mixture, nearing optimal fuel to air ratio gives more horse power when compared to a rich mixture, although this also ties in with igntion timing etc, gets very detailed.

-Cylinder scavenging produces more power as exhaust gas is sucked out not pushed out, and a cleaner cylinder is left clean for the next combustion. A poor flowing, restrictive exhaust, too big or too small, means less scavenging as the engine is still having to work to force exhaust gas out of the system, as the flow isn't great enough to keep up with the scavenging effects of extractors etc.

In summary, its not actual back pressure we need, you just dont want to big exhaust piping that the air cools down to the extent that it flows slower, slower flow equals more restriction which means lost power.

Anyway, just a few things I've put together from reading a few articles and having a few discussions on the topic, not trying to say that this is all right, as there may be some inconsistencies, feel free to make some constructive criticism as we all may learn something. Sorry for the long post.

Cheers.

Excellent info , dude.

Makes heaps of sense.

Mate in Burwood used to run his Group A VK around on slicks, AVgas and
supercar exhaust (exit under drivers door) :nuts:

Phenominally(?) free revving for a 308 but helps to have a dad (Ian Tate) who
can build a 400+hp engine for you as well. :D

yeah top info josh, who told you that? :cool:

yeah top info josh, who told you that? :cool:

You just told me things I already knew chump, and I know how to use google.