mroberts

So while we're on the topic of exhaust ... and just to fan the flames, what about the argument that too big an exhaust can hinder performance ?

I've heard that going to (say) a 4" exhaust at 250 hp will hurt performance because the flow exiting through the exhaust expands and cools, actually increasing back pressure. The ideal situation would be the turbo exiting directly to atmosphere, but do you pay a price for a large diameter, long exhaust ?

ehall

"Why close or delete the thread? Some of it is actually interesting to read.

Marc"

I'm just laughing at the legs on this thread. You never know which threads can take on a long life. Hell no don't close it.

As for a 4" pipe, it all depends on the ability to maintain flow all the way to the end of the pipe. If the turbo exhaust is forcing out heavy enhaust, with say a higher volume/efficiency turbo, then theoreically it could be a benefit, but it could possibly be too much for the stock 26/6. That's speculation on the 26/6. I haven't seen data. Some of the cup cars had a 4" exhaust, but they ran larger turbos. (26/8? maybe 27/8?) Anyway, it's a good point. The best exhaust is none.

951_RS

Yeah, I've heard speculation on smaller turbos if too big an exhaust will hinder performance, but I really doubt this honestly. People get confused with "on a small turbo, a 3 inch compared to 4 inch is a very very small difference in power" and then someone takes that and says "a 4 inch is overkill on a stock turbo" further, we are then led into "a 4 inch is bad for stock turbo."
So we can see how through the grapevine things get misconstrued. While it's true a stock turbo will not see much power gain from 3 inch to 4 inch, it doesn't mean the 4 inch hurts, just doesn't help as much as stock to 3 inch did. Once you get a bigger turbo though, we start to see how much of a difference a larger exhaust makes.

This is how I see it anyways

SD Porsche Fan

Since I've deleted the cat I've installed a Lindsey DP WG, Vitesse chip and mbc. The car pulls very strong upto about 12-13 psi. I've suspected a minor boost leak somewhere but spool up is quick and I can't hear anything. So is my 2.5" a limiting factor? How much boost have people been running on a stock exhaust?

Evan70

I've run up to 17psi on the stock exhaust with just a cat delete. However, I'm now fighting a boost loss problem, that I believe is the stock exhaust collapsing where I had the cat removed and a straigt pipe welded in. Loosing about 2-3psi consistantly no matter what boost leverl I set it to (i.e. 12psi when set to 15, 14 when set to 17).

special tool

Evan - no matter where you set it to?

This indicates a boost spike. The end boost level is the real boost level and the first may be a spike.
What are you using for boost control? What type of air lines?

Evan70

Yes, no matter what I set it too. Also, it is slower to make boost as well (used to make max boost around 3200, now it takes until almost 4000). I have a Blitz SBC-iD EBC, Lindsey DP wastegate, lindsey vacuum lines, and one of the 5 port vacuum manifolds that someone was selling on here a while back (Laust maybe?). I've checked all the vacuum lines, and there are no leaks that I can find, nor has my idle vacuum changed any since the problem started. Checked the wastegate pipe, and it remains cold at idle so I don't think the wastegate is leaking. Also checked the bypass valve, and it's fine as well. Also checked all the intake pipes/hoses and can't find any leaks.

special tool

Well, what does the vacuum manifold have to do with your boost circuit?

You don't have them connected in some way, do you?

Evan70

Boost gauge, and wastegate pressure lines run off of it, but otherwise, no it really doesn't have anything to do with it.

mclarenno9

I think your wastegate pressure line should be tapped somewhere between the turbo and the IC since there is a ~ 2 psi drop through the intercooler. That could be affecting things maybe?

Evan70

Routing of the lines hasn't changed any, and worked fine as routed for over six months. The EBC uses two pressure lines:
One runs from the manifold to the EBC brain and provides the boost level that is used for the gauge display and triggering the solenoid that opens the wastegate.
Second line is run from the stock banjo bolt to the solenoid to provide pressure to open the wastegate.
I am getting a "whoosh" when on full boost and when letting off the throttle from full boost, but I can't find any leaks anywhere. This has been going on for a couple of months, but I just started loosing boost on Tuesday.
I did just find a post where someone said there idle air control motor was leaking and causing similar problems, so I'll check that this weekend.

ehall

"Yeah, I've heard speculation on smaller turbos if too big an exhaust will hinder performance, but I really doubt this honestly." 951 rs

I doubt it too, but wanted to get back on topic. For that specualtion to be correct, it would implie that somehow a pressure build up of cooler low pressure air could pool near the end of the exhasut. I find that highly unlikely. That thinking also refutes the concept that no exhaust at all is preferable. The only place I don't want bigger tubing is on the exhaust manifold. I'm not after racing characteristics, so I'm a torque guy.