What are his acceleration times? 3" makes more top end power but would it not take him less time to travel (let's say 100 feet) using a 2.5" because of the faster spool up?

 

Sounds like a 2.5" setup is more geared towards the autoX crowd, who want that burst of torque
coming off the apex of the turn, whereas the 3" is more for the straight line crowd...

 

Might go over your settings on your Xede to verify that it is a exhaust issue?

 

Of course The Xede is most certainly NOT the issue. The only differerance is the 2.5--->3" exhaust

 

I recently had an AVO turboback (3 in.) installed but left the stock cat in place. I don't have any dyno figures but the butt dyno says better lo-end and a slight, albeit noticeable, gain up top (compared to stock).

 

Take the cat off. Yeah ah when you go to the track. Take the cat off. Ah yeah off road use only

 

I've been stating this for quite a while now. The torque loss at part throttle is really noticeable when going from 2.5" to 3".

I'm surprised that you haven't been flamed yet, with 'every dyno proves the superiority of the 3" vs. 2.5" exhaust"...........blah, blah, blah.

 

Ive got a series of testings lined up over the next few months to get the facts on this "AS IT APPLIES TO THE EVO" thats important. I also dont think that this is a "POSITIVE ATTRIBUTE" of a 2.5" exhaust system I think its actually negative flow efficiencies that create a high pressure spot right at the wastgate(or wheel if WG is closed) causing a peculair side affect which in this rare instance just happens to be a "POSITIVE SIDE AFFECT" of overspinning the turbo past 100% (completely closed WG). But I also feel tuners specifically smooth the power and torque curve for some unknown purpose as if there is a contest for smoothest dynochart.

 

MP5,
I think you touch on some very key issues. I can agree with alot of your assumptions. I say assumption but i use the term loosley. Reason being is that it is more about misplaced fact then anything else. Your opinion i think is right on target with what many tuners have found.
Now NOTE: This is my honest opinion based on my own tests and learned thru reading and actual events.
Any one who is able is allowed to rebut these comments.

Now i have a small theory to what you are experiencing. Much of it you have already mentioned but i will reiterate.
A larger pressure differential is ideal for a turbo charged setup. For many reasons. They say that it will lower cylinder and manifold EGT's. This is definately the case. What i think happens is the expanding endgases in the exhaust runner does not have the same "energy" expanding and spooling the turbine. In comparison to a 2.5 inch discharge.
I believe that in the lowend of your band you are relying on the stock volumetric efficiency of your engine, induction system and exhaust scavenging to induce and promote enough exhaust energy to reach boost threshhold. At that point it has built up enough "pressure" (gas energy), that it will spike sharply and FEEL torquier under that specific rpm range.
I do think though that you will have a little less torque at that exact level on a dyno graph but will proportionately have more thru out the range much earlier than with the 2.5
Hope some of this makes sense. If i can clarify for you please ask and i'll try to go thru my theory again.
My 2 cents

Let the flaming begin.... LOL

 

With the wastegate open you get the same pulsing effect that custom header fabricator/designers have to consider when calculating header tube lengtha and diameter. Like tuning a tuba or trombone, proper sound/pulse/exhuast gas flow creates a suction where the proceeding pulse drags pulses behind it. Even with the full exhuast running through the turbo, there is this effect, but to a considerably lesser degree.

 

Hmm understood with the WG open this would be an issue

Perfworks thnx I appreciate your input. I also realize that the TQ might actually be less at the "hit" than at the corresponding point with the 3" and the "rampup" might make the "feeling" less- thats why Ive got a series of tests lined up with a 2.5"/ 70MM/ and 3" on the track and dyno as well as any street logging I can accululate into useful truthful data

 

I have to throw in my 2 cents here. I just put on a 3 inch dump back w/cat and resonator plus cams (264/272e) and cam gears (-5 degrees). This combo SHOULD have worse low end than stock right? Wrong it's way, way better. It has been documented by respected tuners that exhaust energy builds torque (everywhere) on turbo cars. Even off boost the car is better with that annoying off idle dead spot gone. A win win if you ask me (I know, I know you didn't ) Go for the full 3" and run a cat too for the best of both worlds

 

Crono I do have a 3" with cat. You just did your stage 1+ and the TQ and HP are huge I understand you newfound enthusiasm. Ive had the stage 1+ for a while now and have had 2 different exhaust setups I am mearly comparing butt dyno and the crude data results that I have seen. Stock to stage 1+ is overwhelming difference I see why you are satisfied. I am talking about the same system differences with just changing exhausts

 

Cool, make sure you dyno combos. How about an average hp and torque from say 3-7k (wouldn't that be a cool standard). I do like the having the big torque hit (and of course you can modulate it with the throttle). Maybe you are talking about timing/ mixture/ boost issues? There is more than one way to tune any given combo

 

I hope your tests include timing the "sections" of your rpm band in question (say WOT runs from 1000rpm of either side of your "window" your looking at)... in that, you will find your answer... and a fist full of other questions...