With this rational we might as well all run a 3.5" exhaust. We don't though because like the 3" there is a diminishing return on power. Once the pipe has hit a certain size the back pressure has dropped so low that it makes no sense to go bigger. This is why 2.5" catless systems are deemed "good enough". The cost to go to 3" isn't worth the miniscule gains from a 3" system. Also you still ran a cat on that JCW DP, so you still have back pressure. Could it be that the 3" helped simply because it makes up for the restriction you have in your system still?

A back to back test of a 3" and 2.5" would show a difference, but how much is the question? TBH if the gain was significant enough you could have your answer from looking at 2 different charts because it would be obvious. From what I've seen the gains or loss between the 2 could easily just be discrepancies in runs that occur even when no changes are made to the car. In other words it's so small it's not worth it,

Not to be rude, but all the Alta DP's I've installed or seen have not been able to run the lower shield, maybe you made changes to it since then, but that's been the issue I've dealt with so far. Not having to deal with melted fans doesn't mean it doesn't happen. Although I'll admit all I've seen have been track cars, it's still a risk for any one that decides to push the car. When we are talking about parts that are meant to increase power it's a safe bet that some of those owners will track or drive the car hard.

The problems I was referring to was for drive-ability. Building a large amount of torque lower in the RPM range can cause wheel spin and torque steer issues. Both these will get worse as the peak torque is achieved at lower RPM's and as the amount of torque is increased. The relevance of this is that your argument for the 3" was increased spool and the faster onset of peak torque. Like I said peak torque for a 2.5" catless exhaust is some were in the 2.3-2.5k range, there is no realistic need to spool or build torque faster then that since your just going to spin the tires any way.

 

I have never been interested in Alta's products (except the AP, which is not available in Europe) because they don't convince me.
My opinion over the 2,5 vs 3 inch exhaust is that I think you Alta guys should learn the basics of fluid mechanics and the laws of gas expansion. In there lies the truth.
It's the laws of Physics.

Also, how come you don't offer a 3' DP anymore?

One of the best exhaust manufacturers "Akrapovic" is offering a 2,5 inch "only" exhaust system and DP for the R56. Have they lost their mind???

Enough with the b@llsh$t, we are tired of nonsense.

Honestly,
Themis P.

 

Exactly

 

what about manifold sizes?

 

No-one mods the exhaust manifold, the removal and replacement is a huge job, and apparently the gains are minimal.

Porthos fitted a WMW exhaust manifold I think. I don't think he liked it.

 

I was gonna bring up the the the 2 JCW tuning kit manifolds. I know the first one didn't really do anything, but I wasn't sure about the second, aka WMW manifold.

I've been waiting to see what Porthos thought, but never read any thoughts on it after installation.

 

As far as I remember, he never installed it once he saw that its a much more involved job than it seems.

 

im pretty sure i got one with my JCW kit but can't tell what gains each part did. i only ask because my mate just got a big shiny new manifold on his evo and reckons its good.

 

Most of the ones for the Evo's work because they have the room to make them equal length and a little bit larger. Our cars are kinda stuck with one shape essentially, unless you want to relocate the Turbo and/or some other major parts.

 

I hope everyone understands that I'm not saying that there's a problem with 2.5" exhausts at all. It is a good improvement over stock for sure. I just want you guys to know that there's no denying that a 3" system will make more power. There's nothing to be lost with going with a 3" system, it will give quicker spool, and more overall power because of more flow.

Here are results from a JCW that we did with our old turbo back taking off one cat. This proves that a Mini can gain power with less restriction.

http://blog.perrinperformance.com/?p=860

 

The problem I have with this is it's based almost completely on theory derived from other cars. I get where your coming from having worked on many other turboed cars where 3" pipe is pretty optimal, but as I said before there is no universally perfect pipe sizing for all cars that are turbo'd.

Your graph does little to prove 3" is better then 2.5". It simply shows that you made power over a stock system with stock cats.

The biggest power gain you saw from removing the one cat was in a very small portion of the power band. The rest was either line for line or very minimal. This would lead me to believe your not seeing much of a gain with 3" since you gained so little from removing such a large restriction.

As you can see in the graph below we saw the same gains if not better from a 2.5" catless over the JCW exahaust on an MCS around stock JCW boost levels.

The solid line is 2.5" catless turbo back and AP
The top dashed line below it is the JCW exhaust and AP
The bottom dashed line was base line JCW tune and exhaust

 

Great post Mike. Puts things in order.

 

Car only made 200whp?

Other mods?

I have a catless 2.5" and am making ~220whp

 

The car I tested for those graphs was basically stock with the exception of the AP and turbo back exhaust. The tune it self was not all that aggressive.

I've done similar set ups with the addition of a FMIC and retune and have seen numbers in the 220 WHP/ 270WTQ range.

This is only really relevant to the dyno I use though.
Peak numbers can't really be compared between dynos. Even the same model dynos can give diffent numbers. Given your location and tuner I would assume you were tuned on the Church Auto Dynopak. This dyno is pretty well known to read high. This doesn't mean your tune is bad, I'me sure your car is quick, it's just unfair to compare the peak numbers from it to anything else.

What an engine gains and where it gains is the most important part to look at.

 

I was just wondering, I wasnt trying to discredit anyones numbers at all.

My tune was done with a leaking DV sadly.. so I am hoping with the new one my numbers are a little higher now especially wtq since I was dropping boost off :(

Also going back to a stock airbox haha.

Just was curious what the mods were that is all, not trying to start a tuner battle if you know what I mean haha.

 

Oh no worries, I didn't take it like that at all. I just wanted to answer your question with the proper perspective of the numbers.

 

Subscribe. This is better than the presidential debate

 

What isn't better than the presidential debate?

 

Sorry Mike, I should have explained it better. This is they dyno sheet:


Shows that this JCW with our 3" turbo back exhaust with 2 high flow cats system made power by simply removing one of the high flow cats! This shows that even with a 3" system with high flow cats, the JCW benefits from higher flow.

So far just about everything is theory, since no one that I know of has done a back to back test of a 2.5" system on a 3" system. But I'd bet my sandwich (that's a pretty big deal because that's pretty much my whole lunch) that the car with the 3" system will make more power.

 

Watching the presidential debate

 

With this dyno sheet you have proven that removing an obstacle is better for the flow. Peace of cake. I totally agree with you. That being said, why was the obstacle in the way obstructing the flow in the first place, bearing in mind that power is what we are after.
Technically, You should prove that the gain you claim to have with your 3" system over the 2,5" systems exists. That way you will end this debate and all will be fine and customers will use your 3" because it's the best power-making exhaust in the market. I kindly dear you.
But something tells me that you won't do the test.
And I will bet my PhD in Thermodynamics...

Themis.

 

Well I have an MCS with a 2.5" and am local to Mike@COBB.

More than happy to drive down for some dyno runs and if they have a 3" system im all for test it out.

 

i would love to test it out with my car too. im mins away from machv. and i have a stock dp and a 3in dp back exhaust

 

Most of the exhaust bottle necking is in the DP.

Stock DP and 3" cat back does pretty much NOTHING for performance.

 

Thank you for pointing that out. But I know that I'm just getting everything ready to get an AP cause if you didn't know the 2011 can not be flash tuned right now and I don't feel like throwing up CELs for switching out the dp to a bigger one right now. So yeah.... Thanks for rubbing it in though