which exhuast spool vs. apex

[ARunto]

bump for votes

 

[BinaryRotary]

what were the results?

I have a dyno chart somwhere but I picked up 26 hp and 17 ft lbs just from switching from the 3" to the 2.5".

 

[BinaryRotary]

true. not unless you're making some mad hp. but it's not gonna hurt anything except your wallet. might as well leave the door open for possibilities

Well, I'm running an HKS T4 single and made more HP at 26 psi with the 2.5 than with the 3.

 

[Striker187]

^^^^
why'd that happen?

 

[prospeed]

i have seen tests in mags with better gains with 2.5 over 3". plus with this small ass turbo there is no need for 3". plus with an exhaust to big you can lose low end power.

 

[Striker187]

true, you should lose some low end torque. but as a trade off, you should gain high end power right?

 

[t3ase]

Let me just chime in here. OrangeMSP had a turbo-back with a N1 can on his car and when I rode in the back seat, I could barely hear what they were saying in the front seat. This was at 40, maybe 50mph. It wasn't *LOUD*, it was just kinda "droney". Not ricey, but droney. I'd go with the spool setup just cause I know what I know about that N1.

 

[prospeed]

from the tests i have seen the 2.5 overall made more power. but every car is different. my last car which was a volvo 940 turbo i was only running 2.5 with 17psi and still making more power than people with 3".

 

[Striker187]

weird...why is that? an explanation would be nice so i can learn

 

[Installshield 2]

Engine displacement and amount of cylinders has more to do with piping diameter needs than engine output...

2.0Ls of displacement (and also 4 cylinders) will never make enough exhuast pulses to keep everything moving quickly out of 3" diameter exhuast systems...a 4900cc V-8 could....

I wouldn't say that 2.5" is the biggest an FS could use effectively, but I doubt 3" would ever be needed realistically...Unless you somehow were running major amounts of boost with a majorly increased redline (above 400hp and above 8,000rpm...maybe...)

 

[Installshield 2]

weird...why is that? an explanation would be nice so i can learn

I can give a brief explanation before I fall asleep...

I am assuming you know what an exhuast system does obviously, so I skip all of that...

Exhuast gasses move out of the system in the form of "pulses"...It is not exactly a steady stream of exhuast gas, but more of a series of blips caused by each exhuast stroke and valve open/close cycle...The best thing for power is to keep those pulses moving as fast as possible...

So first is back pressure...Everyone has heard of it, and a lot of people claim you need some of it...You don't...Somehow someone started the notion that back pressure creates torque, which it doesn't...Torque is helped at low rpm by smaller exhuast diameters that allow the exhuast gasses to get out of the system quickly...The smaller the exhuast diameter the faster pulses move at any given rpm...That is great for low rpm becuase the pulse rate (or sometimes called frequency, the amount of exhuast pulses that pass a certain point in a certain amount of time...it is directly related to engine rpm) is fairly long...Overall there is not enough individual pulses to get in the way of each other...

So you can see what happens as engine rpm increases...The frequency becomes bigger (in which case each individual pulse is much closer together) and as engine load increases the pulses themselves get physically bigger...With a small diameter exhuast at high rpm the pulses begin to bind up and join together, overall slowing down...This begins to create back pressure, in which there is force pushing back on the exhuast gasses not allowing them to escape the system...and overall making the engine have to work harder with each exhuast stroke to push the gasses out...That kills hp AND torque...In most cases back pressure is only really effecting hp, and it is most significant above 5250rpm...But some really restrictive exhuasts can cause back pressure at very low revs becuase of poorly designed mass produced bends in the system...

So with too big of an exhuast diameter, lowend torque is gutted becuase of the pulses getting hung up and hanging around in the system...With the big diameter they can spread out and move slower...which makes the whole line of pulses bump and push off of each other....basically doing a similar thing as what a small diameter high rpm combination does...But don't listen when people say "I installed a big ass exhuast and noticably lost lowend torque because I decreased back pressure"...They didn't loose lowend torque becuase they lestened back pressure...They lost lowend torque becuase the pulses at low rpm move slower out of the system (becuase of the bigger diameter)...If they gained any power, that is becuase they lost back pressure...

This is all directly related to engine displacement...Big displacement engines create much larger individual pulses...thus needing much more piping diameter to keep things moving efficiently...And this is the main reason on a lot of V-8's you find two individual 2.~ exhuasts dealing with one cylinder bank each...rather than having collecting the exhuasts after the manifolds into one giant 4.5" exhuast pipe...

The trick is to find a happy medium...You need a small enough diameter to keep the pulses happy and moving quickly at low rpm, and a big enough diameter to keep the pulses happy and moving quickly at high rpm...too big and you loose lowend torque...Too small of a diameter and high rpm operation is restricted and back pressure becomes a problem...

Also back pressure is a leading cuase of turbo lag, in which case it restricts efficient turbo spool up...Turbo'd cars usually can go with a much bigger diameter exhuast than the same engine naturally aspirated...

3" is too big for 2.0L...an FS will never reach the amount of power it would take to make it worth while, and an FS will never rev high enough to make it worth while...2.75" would probably be the ceiling, and reserved for a maniac that is making upwards of 350whp at over 7000 rpm with a big ass turbo...even then lowend torque probably wouldn't be as high as it would with a 2.5" system...but power would be made up for around redline...

 

[Striker187]

interesting writeup thanks. my car doesn't FEEL any slower at least and the turbo seems to spool quicker. then again, that may be my mind.

so Arunto, which size you think you're gonna go with?

 

[Installshield 2]

yeah sorry, I didn't mean to get too harsh about a 3" exhuast on an FS...You are Turbo'd so there wouldn't be as much as a problem than with a NA FS...Yours probably breathes great at high rpm, and possibly with a little smaller exhuast you would have kept a little more low rpm torque...and kept the peak hp...But if you don't have any lag problems, than your lowend torque is still decent and if it is good for you that is what matters...

 

[Striker187]

nah, no prob not harsh at all . i do agree with you that 3" is way to big for the FS if you're NA. and i don't need anymore torque down low....i got enough problems hooking up as is