Project: RS2 ABY

[Sonic]

Vas I have no idea why but my milltek cat-back apparently measures 2.5" diameter whereas I was always led to believe it was 2.75" - I'm still not too happy about this.

I'm pretty sure Ross has a 3" setup on his RS2 and we always meant to do a comparison between them both. His has a k26/27 hybrid similar but different to my kx500. Can't remember if his hot side is a 6cm2 or 8 but think it must be 8 like mine.

Everyone involved was always surprised mine dyno'd consistently at 431bhp / 600Nm, it should have made more top-end and you could see it choking. I just put part of it down to the cam setup and possibly inlet. I didn't consider for a second that my supposed 2.75" exhaust would be a restriction before 450bhp but upon revelation of its actual size me, Dmitri, Will and JP all collectively went "DOH!!!".

I'd have loved to just change the exhaust and see what the original setup would have made. I suspect more like 450-460bhp which is what I was originally expecting.

Given the anticipated power increases specifically targeted at the top with these latest upgrades it seemed clear even a 2.75" exhaust would begin to limit the flow and output as it's a fundamentally restriction to the flow rate and thus power output and efficiency of the engine.

If I'd have kept the 2.5" setup I fully expect all this work may have just resulted in a small 20bhp gain - that's an aweful lot of time, money and effort put in for a tiny reward all down to a flow restriction in the exhaust so it has to go.

Unfortunately Ross has never had his car properly running on the dyno to get any realistic figures, but from what we do know we expect his car to also have done around 420-430bhp. That's about spot on for his setup but clearly highlights mine was down on power.

[prj]

Could not have said it better myself. People have a trouble grasping the concept of cross-sectional area.

2.5" or 63.5mm - 3167 mm2
2.75" or 69.85mm - 3832 mm2 (21% bigger than 2.5")
3" or 76.2mm - 4560 mm2 (44% bigger than 2.5")
3.5" or 88.9mm - 6207 mm2 (96% bigger than 2.5")

It's a square increase. 2.5" will top out at around 430bhp on these, 2.75" around 520bhp, 3" around 650bhp, 3.5" around 850 bhp.
Of course this is just a rough mathematical multiplication. Timing also affects the power, so if you can run MBT you will get more out of the same amount of fuel-air mixture etc.

[Error404]

a 3" exhaust will flow 50% more than a 2.5 at any given pressure, which is not insignificant. Not to say that your 2.75" is restrictive, but it flows around 22% more than a 2.5" so....

[prj]

i have heard so many times this thing about if you dont have a 3inch exhaust you cannot make more than 450hp or etc so i just dont believe it to be that serious anymore. It could be easier to make more power but as always i hate absolutes and i have proven them in the past a few times.

What exhaust did Ross's rs2 have? he uses the same turbo/setup roughly doesnt he? If i recall he made similar numbers when his car was working properly. If milltek, then that is definitely a 3inch full exhaust as i used to have one.

I just posted you the same car with 2.5" vs 3" exhaust and 430hp vs 510hp with the only thing changed being the exhaust.
What is your problem? Which part of 2 bar backpressure do you not understand? We had a backpressure gauge on the dyno on both of these runs.

Ross' turbo and setup is nothing like Andy's car. Pointless comparison.

[A80Avant]

i have heard so many times this thing about if you dont have a 3inch exhaust you cannot make more than 450hp or etc so i just dont believe it to be that serious anymore. It could be easier to make more power but as always i hate absolutes and i have proven them in the past a few times.

What exhaust did Ross's rs2 have? he uses the same turbo/setup roughly doesnt he? If i recall he made similar numbers when his car was working properly. If milltek, then that is definitely a 3inch full exhaust as i used to have one.

[prj]

So you made 463 hp with a 2.75" exhaust, which sounds about right.
Andy's car had a 2.5" exhaust and this is why it was not making power before, as simple as that.

[A80Avant]

1. Your car has never been on rollers.
2. We have dynoed 2.5" exhaust on water meth and HX35 Super - 430hp and 610nm is all it would make, 2 bar backpressure in the exhaust. We made a 3" exhaust for the same car and it made 510hp and 693nm. No other changes apart from the exhaust.

So yeah, 2.5" exhaust is a huge restriction.

so on the above car you didnt change the boost profile or timing???

you may have forgotten Dmitri

my car has been on the rollers and made 463hp crank with 1.8bar and a sticky wastegate. While on the rollers, i also took the time to see the parameters i use for the virtual dyno and my perfomance box dyno sheet and it was pretty accurate. Based on that on 2bar over boost and 1.8 at 6300rpm i calculated on a fairly level road 480-490hp.

Dyno was dyno-dynamics which i hear is not an optimistic one. From the above, if i just then go and fit a 3inch cat back, would you pay for it if i dont get similar improvement in power??? (just joking)

[prj]

A bigger exhaust is always nice to have but I wouldn't put it down as a huge restriction. My numbers are close to 500 certainly above 450 and I run a 2.75 cat back with 2 cats and 2 silencers for ages now.

1. Your car has never been on rollers.
2. We have dynoed 2.5" exhaust on water meth and HX35 Super - 430hp and 610nm is all it would make, 2 bar backpressure in the exhaust. We made a 3" exhaust for the same car and it made 510hp and 693nm. No other changes apart from the exhaust.

So yeah, 2.5" exhaust is a huge restriction.

[A80Avant]

A bigger exhaust is always nice to have but I wouldn't put it down as a huge restriction. My numbers are close to 500 certainly above 450 and I run a 2.75 cat back with 2 cats and 2 silencers for ages now.

Great job. As always if you have a nice log of 3D and 4th gear along with the usual data of weight and tyre size let me know and I'll plot them on the virtual dyno

[s2_bo]

Cant wait for the vids

[pixsaudisex2]

Looking good...

[Sonic]

Will be happy to oblige just as soon as i can

[Fat Audi 80]

Three words to define my requirements!

Drive By Video!

[Error404]

Three words to define my requirements!

Drive By Video!

[AndyP]

Great work from everybody involved, glad you finally got it fixed

I can't wait to get mine back on the road now!!