Factory Dump Pipe & Cat Replacement

[xlnt6]

Macka,

Regarding your boost graph query on Boost1a. This flatspot in the mid-RPM range was done purposely to limit the mid RPM torque. When the original tuning was done the boost was limited below 5000 RPM or so to 10 psi, above this RPM boost was allowed to go to 12 psi. This was to protect the OEM T5 and clutch, and stop it from destroying the gearbox !! The T5 is rated at 450 NM, and the car was making 600+ NM. So any more would be asking for trouble.

Now that a T56 and twin plate have been fitted, the boost has been allowed to peak at 15psi (refer Boost2a) and no mid RPM limit is required. Torque now goes up to 875NM in the mid-RPM range.

Robin.

[Lawsy]

The reason you all of a sudden got an increase in boost with the new dump pipe is actuall attributed to flow characteristics of the new pipe work...

Ie, this is a slightly different situation, but you can see the similarities.

One of the workshops (I think it was the nizpro guys) stuck one of there own pressure sensors on the outlet side of the turbine, and measured the back pressure and intake pressure as well. They did this with a standard car and then did the same thing with there complete custom inlet pipework (im pretty sure it was nizpro). They noticed that the boost suddenly jumped to 8psi and peaked close to 9psi. What they realised is that the pressure in the excess pipework on the standard car was actually forcing the turbo to work harder to produce the standard 6psi (this is why they measured the backpressure) and that the backpressure was nearly the same when producing the 9psi.... So to make 6psi in standard form, the turbo is working as hard as if its trying to make 9.... I hope that made sense.

Now, this is from an intake mod, but in reality the same basic principle works on the exhaust side as well. If suddenly the turbine has less pressure on the exhaust outlet, it will spin more freely with the same amount of pressure.....

So the sudden increase in boost is clearly attributed to the better flow of this dump pipe and means that the turbo is working under a similar stress level to produce the extra boost. This is good, its like free boost (though, you payed for the dump pipe so I guess not ).

Correct me if I'm wrong, but I'm fairly sure this is what is happening here, but it has been a little while since I did basic dynamics, and I've put off the advanced stuff for a bit so..... I hope this helps.

[xlnt6]

Lawsy,

You are correct !!! I was aware that the freer flowing dump/cat, meant less restriction, less backpressure and as a consequence more boost. But what I was trying to measure was the difference in power/torque based on the gain of the dump pipe/cat as opposed to just increasing the boost.

Case 1 - OEM dump-pipe / cat with 10psi/12psi boost profile (boost1a)

Case 2 - SMB dump-pipe / cat now with 13psi

Case 3 - OEM dump-pipe with 13 psi boost

ie. Power Gained due to SMB = Pwr Case 2 - Pwr Case 3

Now the power made in Case 2 is partially due to the SMB and the increase in boost itself. So how could I determine what the SMB pipe would make if the boost conditions before and after were maintained the same ?

What I didn't measure was Case 3, so instead I tried to gauge the power made with Case 4.

Case 4 - SMB dump-pipe with 10psi/12psi boost profile

ie. Power Gained due to SMB = Pwr Case 4 - Pwr Case 1

Sure I could have just left the boost higher and claimed more power gain, but how much better would Case 2 have been versus Case 3. I'll never know. My before dyno run was as per Case 1, so to make the playing field level I wanted to compare it to the power achieved for Case 4.

Hope that make sense. If I've missed something, let me know ....

Cheers

Robin.

[ktford]

an extremely informative thread... thanks guys   This is what its all about

Ain't it the truth

Scotty

[xlnt6]

Just thought I'd update this post to show the latest tinkerings. I've always liked the Herrod quad outlet system, so I thought I have a go making up my own.

The tips are SMB RT253 which are 2.5 inch inner with a 3 inch outer skin. I've replaced the factory rear muffler section with mandrel bent 2.5 inch mild steel pipes, then flanged to the two rear sections. These are 2.5 inch stainless steel using SMB 4 inch resonators (2.5 inner straight thru), then onto my own twin outlets on each side.

Note it excellent, a nice burble, without being too groany (if that's a word). No droning, and I'm still using the OEM centre muffler section. This was mainly done for looks, not performance. Most if not all exhaust gains are to be had upfront.

Cheers

Robin.