[1day]

I am in the decision process between the Invidia catted DP and the HM catted DP. Both are 3".
The Invidia has a divorced wastegate and the cat is near the turbo.
The Harmin Motive is an open bellmouth and the cat is at the end of the DP (looks just like the Cobb, but it does not have a 2.5" outlet).

Any experience with cat placement? How would the distance from the turbo affect back pressure and spool up?
Anything else I may need to know about catted DP's?

My car is stock. I plan on using an AP along with the DP. CBE will have to wait.

Thanks fellas!

[thekiffer]

I don't know about the back pressure and spool up but the closer to engine the hotter the air that travels through the CAT. Which if you care helps reduce emissions but it also makes the CAT wear out faster.

[Texas DanT]

That's a good question. I'm thinking about using my Legacy GT Invidia DP and adding a cat. I was going to put the cat towards the rear of the dp so the turbo will be able to spool faster and I'll still have a divorced wastegate. Now I need to figure out if I should use a 300 or 200 cell cat.

[Digitalfiend]

Originally Posted by Texas DanT

I was going to put the cat towards the rear of the dp so the turbo will be able to spool faster and I'll still have a divorced wastegate.

Actually, you won't because the Invidia is not a divorced wastegate downpipe, regardless of their "false" advertising.

This is a bellmouth with a wastegate divider:


Where as these are true divorced wastegate downpipe:






Note the difference.

With that said, there was a thread on this topic a while back, if I remember correctly. I'll see if I can find it.

[gormansscooby]

So i know that the divorced wastegate pushes it further down the exhaust stream. But what does that do for the car? And how much of a difference would the true one make versus just the bellmouth with a flange?

[cbarbu]

... and another thing: if the cat is close to turbo it might hit the gear box hsgn! I have an APS catted and it is very very tight!

[Texas DanT]

Originally Posted by Digitalfiend

Actually, you won't because the Invidia is not a divorced wastegate downpipe, regardless of their "false" advertising.

This is a bellmouth with a wastegate divider:


Where as these are true divorced wastegate downpipe:


Note the difference.

With that said, there was a thread on this topic a while back, if I remember correctly. I'll see if I can find it.

I know the Invidia does not have a seperate tube for the wastegate gases, but with the wastegate divider the turbo and wastegate exhaust gases are kept seperate or "divorced" long enough for them to flow in the same direction. That lets the gases blend together a heck of a lot smoother than with an open bell mouth.

And now that you mention it, I think I also remember seeing a post discussing downpipes and the different configurations. If you find it, please let us know.

[Texas DanT]

Originally Posted by gormansscooby

So i know that the divorced wastegate pushes it further down the exhaust stream. But what does that do for the car? And how much of a difference would the true one make versus just the bellmouth with a flange?

In no way am I an expert on this subject, but from what I've read...when the wastegate opens, the gases leaving the wastegate flow perpendicular or directly into the gases trying to leave the turbo. This causes some turbulence which causes some back pressure which affects the turbo's spool up time. So, if you can re-route the wastegate gases to flow parallel to the gasses leaving the turbo or if you can introduce the wastegate gases further down the downpipe, this allows the gases leaving the turbo to flow more smoothly.

Or at least that's how I understand it.

[cbarbu]

You might be right... for the stock one. On the stock one the wastegate side is almost obstructed and this force the gases to go at a sharp angle against the gases leaving the turbo.
On any aftermarket DPs that obstruction is not there leaving the wastegate gases to go almost inline with the turbo gases.

The way I see it, the desired DP is the divorced one, follow by the bellmouth w divider and finally pure bellmouth.

But of course each setup has his one weakness: for the divorced the wastegate tube is getting less hotter than the main DP tube => welds/tube prone to crack + high price; for the bellmouth you have the gases from both side => turbulences and small spool problems.

I have the bellmouth divider

That's the way I see it. Pls feel free to correct if you know otherwise.

Originally Posted by Texas DanT

In no way am I an expert on this subject, but from what I've read...when the wastegate opens, the gases leaving the wastegate flow perpendicular or directly into the gases trying to leave the turbo. This causes some turbulence which causes some back pressure which affects the turbo's spool up time. So, if you can re-route the wastegate gases to flow parallel to the gasses leaving the turbo or if you can introduce the wastegate gases further down the downpipe, this allows the gases leaving the turbo to flow more smoothly.

Or at least that's how I understand it.

[1day]

No offense fellas,
But what does all this divorced wastegate talk got to do with cat placement?

[cbarbu]

No offense at all,
The way I see it is if you go with a divorce you can't put a big cat (APS) at the beginning and you are force to put it at the end. If you go with the bellmouth you can put where you want!
Also the divorce allows more spool if you have a small cat by bypassing the wastegate gases after the cat! - see 2nd picture on this page
Pro and cons on everything !

...or go catless

Originally Posted by 1day

No offense fellas,
But what does all this divorced wastegate talk got to do with cat placement?

[1day]

Thanks cbarbu,

I thought of catless, but I really didn't want the headache of porting the wastegate and tuning issues.
Those were my 2 choices and the cat was at opposite ends of each. I was just wondering if it really impacted performance if the cat was close or far away.

[cbarbu]

for the 08 I don't know, for my 07 not yet... hope so!!
good luck in your choice... I'm sure if you'll do some digging you'll find a better answer!

Originally Posted by 1day

Thanks cbarbu,

I thought of catless, but I really didn't want the headache of porting the wastegate and tuning issues.
Those were my 2 choices and the cat was at opposite ends of each. I was just wondering if it really impacted performance if the cat was close or far away.

[Digitalfiend]

Originally Posted by 1day

Thanks cbarbu,

I thought of catless, but I really didn't want the headache of porting the wastegate and tuning issues.
Those were my 2 choices and the cat was at opposite ends of each. I was just wondering if it really impacted performance if the cat was close or far away.

I really don't think it makes that much of a difference considering most aftermarket exhausts use high-flow cats.

[1day]

You are probably right given the stock turbo an IC.