40 replies to this topic

[B4TT]

True.. would it not be just as easy to cover all bases and keep it 3" the whole way?

[chrisc]

i have a full exhaust setup from fast fit with a magnaflow twin tip muffler on my liberty, got it all for $280 aswell. its quiet for the first 10 minutes of driving then it sounds loud but not annoying like some other exhausts

[aekOne]

In “theory” reducing the diameter after the first 90cm down river from the source aka turbo is not only ok but can help keep the cooling gas flowing at the same velocity as when it started at its max peak temp.

For me the reduction in size was simply to try and keep the exhaust volume down. I had a 3” X-Force cannon on it originally but it was loud as buggery (3” bellmouth dump into a 3” center pipe and 3” cannon – this system had no cat or resonators in it). What we actually found once we changed to the Prodrive muffler was that it made no difference to the final power output (I initially though was going to use the Prodrive for city driving and swap over to the 3” cannon for track days etc). This was only at 180kw at the wheels so in a higher power application I’m sure it would have a more detrimental effect on peak performance but at that relatively modest power the reduction of ½ an inch from the diff back made little to no difference at all

You can pick up a prodrive muffler from anywhere between $500-$650 plus shipping (I did see them on sale at Vivid Racing for $450 recently). Search around online for a good deal…FPV are the Australian Prodrive distributors so you could contact them and see what they could do one for?

[Matt]

Its proven that a drop from 2.5 from 3 inch from the diff back actually gives a boost in mid range aswell

An "S" bend in a specific section of the diff back can help move the mid range boost around

[Matt]

Quoted from Michael South Racing website

"The diff-back pipe leading into the rear muffler in the exhaust system has a huge bearing on the torque band. Using an "S" bend leading into the rear muffler via 2x 90° mandrel bent pipes back to back produces the most torque in the midrange. Reducing the pipe angles from 90° to 45° moves peak torque to a higher RPM with a slight loss in midrange torque. An almost straight pipe with an angled muffler produces a very linear powerband with a smooth delivery of power due no "kick" as a result of the noticable loss of torque in the midrange. As a result, the "S" bend has a huge bearing on midrange torque. After consulting with Prodrive, they had also confirmed this to be the case.
Changing the diameter of the diff-back pipe that leads into the muffler from 3" to 2.5" also helps improve midrange torque. Pressure at the turbo is not affected by this decrease in diameter. As a matter of fact, it actually helps to keep exhaust gases flowing for better power.

As the pipe is straightened out, I noticed that the torque band shifted up in the RPM range. A straight pipe may offer more top-end power, but the loss of low-end power outweigh the power gains."

[Shaz]

it not Hi-Tech (Your wallet would know if it was a hi-tech!!)

Pretty sure i can see a "Supercat" stamp on the bottom of it

No idea. I'll have a look. I remember the guy telling me I could pay $X for this one, or $XXX for this one, and I chose the latter.

[aekOne]

yeah i've heard plenty of people say this as well BUT I've also heard plenty of “experts’ discount or have varying ideas the theory too. There’s a good piece written somewhere by one of the engineers at Garrett who disagrees with theory and provides a bunch of mathematical equations to illustrate his point. This is one big KW applications mind you

[Matt]

Yeah its a theory, though Michaels spoken about it with me for ages and he seems to have a lot to back it up with after chopping and changing bits and pieces and playing on the dyno and in particular on the road where "real torque" is generally felt

It really depends who you talk to, Michael knows his shit though

[aekOne]

yeah he does for sure, but for me the proof was in the pudding, i don't need anyones here say anymore, i have two dyno sheets to illustrate the similarities between the two

[Matt]

By all means, im in no way pushing it upon anyone...just a bit of theory about what was asked

Also just a question, but when you changed pipes was the tune altered or fiddled with? Or just bolt up and get a dyno reading?

Ive always thought dyno torque wasnt exactly correct? My torque figure read 798nM and i know that aint correct? lol

[alexGT]

2.0 this is the case, 2.5 not so much.

in saying that i have a 2.5" diffback on my 2.5 and don't think it gets choked up at all.

[B4TT]

Interesting reading, thanks guys.

Just a quick one, what engine where you fiddling with Aek and what kind of power figures etc

Also Matt, same for the cars you are talking about if you can, just wondering if said theory only applies to certain set-ups making big power or the other way around...... Or not at all

[Matt]

I have a 3in turbo back to 2.5 diff back on my std v8 ej207 with the vf37 twin scroll...its a tcpauto system with an s bend on it (needed one to get around the diff bits anyways) and was pushing 198kwatw

And dont qoute me on this...but Michael would have done his testing on the same motor in his rally car but built bottom end with a vf36 (with restrictor pushes about 180ish...more so now with higher comp and e85)..and also on his daily 2 door sti 2.5 build which used to run 307kwatw or thereabouts and is being prepped to run much, much higher 370ish or so

Torque figures...i dunno, mine were different with nearly every dyno run....mines had like 26+ runs whilst tuning in one day and they were never matched figures...but it was being fiddled with a lot

[B4TT]

Discard dyno torque figures, the power figures can vary enough lol.
Reading where power is developed tells the tail of torque anyways given power is a function of torque x rpm.
A torquey engine just makes power low in the rev range

[aekOne]

Quoted from Michael South Racing website

"The diff-back pipe leading into the rear muffler in the exhaust system has a huge bearing on the torque band. Using an "S" bend leading into the rear muffler via 2x 90° mandrel bent pipes back to back produces the most torque in the midrange. Reducing the pipe angles from 90° to 45° moves peak torque to a higher RPM with a slight loss in midrange torque. An almost straight pipe with an angled muffler produces a very linear powerband with a smooth delivery of power due no "kick" as a result of the noticable loss of torque in the midrange. As a result, the "S" bend has a huge bearing on midrange torque. After consulting with Prodrive, they had also confirmed this to be the case.
Changing the diameter of the diff-back pipe that leads into the muffler from 3" to 2.5" also helps improve midrange torque. Pressure at the turbo is not affected by this decrease in diameter. As a matter of fact, it actually helps to keep exhaust gases flowing for better power.

As the pipe is straightened out, I noticed that the torque band shifted up in the RPM range. A straight pipe may offer more top-end power, but the loss of low-end power outweigh the power gains."

^ That is the reason I understood the size reduction to be beneficial

Low power mate. 180kw at the wheels.

A lot of group N cars run the reduced diameter system so there’s something about it!


Kind of off topic but one of the places I found the most differences in the exhaust system was going from a 3” divorced waste gate dump to a 4” bellmouth to a 3” dump. The bellmouth dump felt better right across the range. Why? I have no idea as I’ve heard time and time again that the split pipes are better?

[Scott]

^ That is the reason I understood the size reduction to be beneficial

Low power mate. 180kw at the wheels.

A lot of group N cars run the reduced diameter system so there’s something about it!


Kind of off topic but one of the places I found the most differences in the exhaust system was going from a 3” divorced waste gate dump to a 4” bellmouth to a 3” dump. The bellmouth dump felt better right across the range. Why? I have no idea as I’ve heard time and time again that the split pipes are better?

you should see the torque curve on Michael's rally car tho lol

im running the 3" to 2.5" also

my car made 15kw and alot more torque going from a more expensive twin dump pipe 3" to a bellmouth 3" to 2.5 diffback


i had a twin dump on my 2L and it loved it, but the 2.5L didnt perform with it

[Matt]

Hahaha his rally car is nuts!! E85 just made it better

@Aek-Ive always kinda heard different abouth the bellmouth vs twin dump

A massive bellmouth with a proper splitter seems the way to go

[aekOne]

haha yeah i bet! like i said, the reduced diameter is a system frequently used on group n rally cars so on a finely tuned system im sure there are real are benefits from it

A massive bellmouth with a proper splitter seems the way to go

definitely the way to go!

[B4TT]

My dumps would be bell mouth, they are still sitting I the shed, I wanted to make them get black before fitting.

[Matt]

Get black via ceramic coating?

Do it!!