But if JKav is correct, how do you explain this quote:

Let us say the turbine discharge of our stock turbos is what, less then 2.5" probably right? Let's say that opens up to 3" in the space of the turbo elbow. Is that too soon?

Also, there is talk of turbines acting on pressure ratios, well if you increase the pressure at the turbine outlet via smaller diameter piping, does that not mean that the exhaust gasses are leaving the turbine outlet at higher speeds?

And I can't help but remember how the low end of my car felt with the pipe off from the downpipe back. This tells me changes in pressure even as far away as the downpipe outlet must effect spool.

I'd be happy to test this... on my car, 2.4l / 35r... I'll do a boost log in 3rd gear. Then I'll take my 3" exhaust off just after the downpipe, then I'll do another log. I'd be willing to bet spool will be hurt.

The higher velocity is a result of greater pressure. That pressure is pushing in ALL directions, not just the path of least resistance.

In an NA engine, that velocity is used to create a Bernoulli effect in adjacent primaries via the collector, which is why it works (to a point anyway). There is no such thing happening in a turbo downpipe.

Not really, as he was making a general statement and the EVO happens to be an exception. Let me explain. Most cars have a considerably higher number of bends in the exhaust. This are rough numbers here, but a 90 degree 3"OD mandrel bend on a 3" radius has the pressure drop of roughly 90" of straight 3" OD tube. Most cars have 3 or more 90 degree bends, so the bends actually dominate the pressure losses. The EVO on the other hand has an exhaust that can be VERY straight. The buschur exhaust for example looks to use 6" radius bends on the cat back and has maybe 135 degrees of total bend. The 6" radius compared to the 3" radius makes a HUGE difference in pressure drop right off the bat. But also, because there is so little bend in the path, it acts very similar to a straight pipe. Something like the buschur 3" probably flows as well as most 4" exhausts on things like Hondas that have 4 or more bends in the path. This is the ONLY reason the 3" works so well on the EVO. It only works well if every effort is made to reduce the number of bends and make the bends as gentle as possible. Add an extra bend or 2 and that 3" exhaust is going to choke a 400HP car.

Do you really believe it doesn't have an effect? Because look at all the ebay O2 housings that everybody loves. They have a 2.5" tube that closely matches the turbine outlet diameter. I guarantee that if you turned it into a undivided bell mouth so that you had a significant cross sectional area change, you would undoubtedly see a performance loss. The closer to the turbine wheel and the more abrupt the change, the larger the impact will be.

To respond to your statement though, I used a tapered O2 housing on the custom exhaust that I built. It smoothly tapers from 2.25" (turbine discharge diameter) to the final exhaust diameter and the wastegate port smoothly blends into the main flow path using a tapered crescent shaped port to disperse the flow as evenly as possible to reduce flow tumble.

This is how my Apex'i N-1 is designed on my Eclipse. It goes from 2.75" to 2.94" and finally to 3.35" right before the muffler. I think its the way all Apex'i exhausts are designed. I used to contemplate changing it out for something 3" all the way through, but it gets the job done so I never did. It also has a removable neckdown joint for when you grow out of the stock cat. I wonder why in the world Evo exhausts aren't designed with that removable piece?

I totally understand the N/A logic. I'm not saying back pressure is good on a turbo motor. I'm saying velocity is good. If you increase the velocity at the turbine outlet, yes the pressure increases, but it probably also causes an increase of pressure at the turbine inlet, so the pressure ratio through the turbine stays the same, yet velocity is increased for the same amount of flow, and perhaps the increase of back pressure at the turbine housing inlet does not negatively effect the VE of the engine significantly. It's a balancing act obviously but I wonder if we are always thinking on the too large side of where the balance is.

Ted would you run a a turbo elbow with a 2.5" outlet into a 3" downpipe? Would that differential between the two pipes cause problems with spool? If so, why?

IMO this is one of two things.
1. Placebo effect: The car is so fecking loud with an open downpipe that it just feels slow because it's making enough noise that it seems like it should be going faster.

2. Your tune messed up the car once you pulled the downpipe off.

I can saw without a doubt, pulling off the exhaust has always made my car faster to get on boost.

I was thinking that, because I drove my Eclipse around uncorked and felt the same sensation.

Don't rule out the probability that a significant decrease in pressure will affect tuning upstream. This is a case whereby swapping parts may create differences in tuning requirements.



I don't know how much of the pulse energy is lost after being churned through the turbine wheel, but if there is a change that increases turbulence and/or pulse reflection in the system, that could create standing waves that could actually cause rhythmic increases in pressure against the turbine wheel.



And if it did, that would manifest itself as a decrease in VE. No pressure on the back side of the turbo is beneficial.


I would use a low angle transition. I don't want to speculate about what turbulence will or won't do, so it's just best to eliminate it from the picture altogether.

Interesting points 03'.. so the turbine discharge of a stock turbo is 2.25"... and most Evo owners expand that to 3" either by the turbo elbow outlet, or at the downpiple inlet (same place really). The length of the turbo elbow is probably about 10". So it is quite soon after the turbine outlet.

So three points/questions I'm making:

1. Is 3" possibly way, way too big for many Evo owners.

2. Is being way bigger than needed detrimental to spool.

3. As we enlarge the exhaust diameter, at what range from the turbine discharge must we be concerned about the effects on spool?

Not really. It depends on where you are talking.

Pre-turbo, it may or may not be a good thing. First of all, realize the exhaust flow during the blow-down period can be above mach 1 depending on port design. Reducing velocity (without increase pressure losses) can actually be a good thing. It's all relative to the engine operating conditions though so a blanket statement of velocity being a good or bad thing cannot really be made.

A higher velocity post-turbo means a higher Reynolds number which means higher frictional losses. Despite what you want to believe, the less pressure at the turbine outlet, the better. End of story.

The pressure ratio is merely a function of shaft power requirements. The compressor needs a given amount of power to make a certain amount of boost at a certain mass flow rate. The pressure ratio is a function of compressor usage and exhaust mass flow rate. It's an energy balance thing. For pressure ratio to remain unchanged means your power requirement is unchanged, which means mass flow rate and engine boost is unchanged. This will not be the case as the increase in exhaust backpressure will cause a decrease in engine VE (massflow rate) which means to make the same power, your pressure ratio will actually have to increase because of the decrease in engine VE.

You are negating the fact that as you increase pressure, your density increases. Thus, that given massflow rate now actually takes up less space, thus reducing exhaust velocity. Without running numbers, I'll call it a wash, but I wouldn't be a bit surprised to find velocity in the exhaust manifold decreases as exhaust backpressure increases.

Higher backpressure will ALWAYS hurt engine VE. Just like you said, it's an air pump. Reduce the pressure differential across the motor and you will reduce engine VE. That said, most motors will operate with more exhaust backpressure then intake boost pressure. You are just making the situation even worse and increasing the likelihood of exhaust reversion by increasing exhaust back pressure.

My large turbo sucks up small cats all the time. Good thing I have an air filter. Have to pull over and get them unstuck from it all the time.

Got this off a website.

"Our DPs have a 2 1/2" stainless steel first bend with a 2 1/2" stainless steel flex section. They then expand to 3" right after the flex section right under the oil pan area. Why start out at 2 1/2"? If you are running on the stock O2 sensor housing, even ported it is 2 1/2" inside diameter. When the exhaust gasses are hot, they flow better, 2 1/2" is plenty for the first 12". As the gasses cool , bigger helps."

1. No
2. There are no exhaust products for the evo which are way bigger than needed.
3. Its not about size as much as it is about the transition from the turbine outlet. Basically you don't wan't a 5" pipe immediately after a 2" turbine outlet, but if you merged gradually from a 2" pipe to a 5" pipe, you wouldn't be hurting anything.

In terms of Evo's, you should be shifting focus from the DP and exhaust, to just the turbo elbow.

A reference I have on engine modeling suggests that a turbine can be modeled as a 50% area reduction nozzle when dealing with the pulse reflection/propagation model. What this equates to is relatively equal portions of the wave are reflected or allowed to transmit through. With it being an open ended nozzle you get a 0 degree phase shift on the reflected wave... Or is it 180 degree phase shift??? I never can remember which it is...

I think that recommendation was based on a constant pressure type manifold though. Can't remember what it recommended for a pulse-flow style manifold.