there is no need. its easy to see that the larger dia. piping, the better the flow. u do not want that back pressure building up on the exhaust tubin side of the turbo.

 

I love the people who think that some backpressure helps a turbo spool. I'd love to know how that works. Take any fan in this world and put a restriction on where the air is being pushed and explain to me how that helps it turn quicker or faster. Makes no sense. backpressure will never help any kind of fan turn quicker or faster. Not a turbo, ceiling fan, window fan, slimline fan, nothing.

The best exhaust on a turbo car is and will always be no exhaust. Now as far as power level goes, a true free flowing 3" exhaust is good for huge numbers, I'd say 600whp-700whp is a good maximum where I'd want to run a larger exhaust and I wouldnt go above 3.5" because of aesthetic issues. I have a 3.5" turboback now and its tight, I had to cut off a bracket from the hugger bearing that holds the driveshaft, and its still touching my front subframe a tad when i decel and the motor shakes. once I get mounts it should stop doing that.

 

Well, we all know that. But the question is.... how much better? Throw money into the equation, and you have hp gain/dollar. Toss in fitment and clearance issues too. Oh yeah, noise.

If we gained 100whp just from going to 4" from a 3", I think many would jump at it even with the cost of a whole new exhaust system, lots of extra noise, and possiblly banging the crap out of it on speed bumps.

But if you only gained 20whp, would you do it?

 

You arent going to gain even 10whp from going to a 4" so I wouldnt worry too much about it lol

 

Here is a comparo (sr20de-t ) between a 2,5 catback vs a 3,5 , from the turbo to the test pipe 3,5 inchs .
gt3071r at 12,5 lbs aprox very low timing etc.

 

That's mildly helpful.
Gains from exhaust aren't linear. Just because you gain some hp from a 2.5 to a 3.5 doesn't mean you're going to gain anything at all from a 3.5 to a 4.

If performance is all someone is looking for, they should run no exhaust at all.

Just dump the turbine exit onto the ground as well as the wastegate dump.

 

i freaking love how members on this forum feel as if they can tell the mods to delete a thread when they don't approve of the content.

 

I agree ... that its why Im goin to do a test between a 3,5 race `DP and a 4( I do the work and its cheap for my that its why Im doing it ), in aprox a month I will post the results.
cheers.

 

im not sure how much more flow u would get from a 3" to a 4". also, he said money wasnt an issue in one of the first posts. fitment would be an issue but i have seen a 4" turbo-back done by Pruven on a 42R they built. its huge, i must say but it fit.

and 20hp?????? that would be a huge gain, but its unlikely it would be that much.

think of it this way, take a small fan and somehow attach different size tubes to the front of the fan. gragually make the diameters of the tubes bigger and see what happens. they fan will perform best with no tube or the biggest tube u can fit on it.

 

The one thing the EVO really has going for it on making a lot of power on smaller diameter exhausts is that you can build an entire EVO exhaust with 2 90 degree bends and a 45 degree bend. The bends are where most of the pressure drop is and the fact that the EVO has so few bends really keeps the pressure drop down.

 

Very true.
You're one of the few people who have ever mentioned this and I think it really deservers more consideration.


An example in a different contect is AC ducting in your home. A 90 degree bend in ductwork is the equivalent resistance of putting in a 20-30 foot straight run of duct.

Those angled exhausts actually are designed to be that way for a reason.