maxiiimus21

Yes i've read many threads and many have no solution to how to gain low end torque with a custom cat back. just to let everyone know, my setup is custom catback 2.25" with a megan racing n1. piping had to be custom cuz the smallest was a 2.50" inlet but outlet i donno, pretty big. many of my friends have suggested the k&n typhoon for my 05 civic lx auto. they say i will gain some low end torque that is some what noticable. is there anything else i can do to gain low end torque back? i am a newb. but i was thinkin of gettin megan headers with the intake? and the lx engines aren't vtec.

MyVtecGoesBWAH

nothing you can really do with simple bolt ons that will make you gain low end. slap a supercharger on that bitch? modified ecu,k-swap? or build ur D series interrnals. as long as that d is stock much stock you wont feel a difference even with headers,intake,exhaust etc. mostly every honda motor make more top end power... best way for low end torque B20 or J swap

D.T.P

2 problems:

1. Its auto
2. You are trying to get noticeable gains out of exhaust...

MPR

Ok, like the others already said, you're not going to gain much, if anything noticeable. Especially with an auto trans. Gains from exhaust can be had, but typically reqire equal modifications to the intake. These modification also typically lead to increases in mid to high rpm performance and a LOSS of lower-end performance and torque.

If you want low-end torque, put the stock exhaust back on and keep the stock exhaust manifold or swap for a header with a smaller collector (2.25" or smaller). Either that or build a custom exhaust with a smaller diameter or stock diameter piping. But this will result in no power gain or even a decrease in higher rpm performance, but should increase low-mid rpm torque.

This may seem backwards, but it's not. It's physics and fluid dynamics.

I'll try to explain as simple as possible. Basically, when you have air traveling through a tube (exhaust pipe or intake runner) at a certain volume flow rate (so many cubic feet per minute (cfm)...etc), when you change the diameter of that tube, it changes the velocity of the air travelling through the tube. The smaller the tube the higher the velocity. The higher the velocity, the more inertia the air (exhaust) has and the better the scavenging from the header. But this is only effective to a certain point or rpm, when the cfm of the engine becomes much greater and the smaller diameter tubing becomes a restriction because it can't allow enough volume to flow out at a fast enough rate. Thus you gain low end torque but loose high-end hp. It's a trade off.

Take my car for example. It's a b16a2 (2000 SiR). Stock exhaust is about 2-1/8" to 2-1/4" front to back. The stock cat and stock header collector were actually down around only 2" inside diameter. The car had ok low end torque, strong mid-range and top end, but started tun out of steam around 7600+ (peak hp rpm). The smaller stock diameter piping becomes a restriction in the higher rpm range.

I installed an ITR 4-1 header with 2.5" collector and built a fully custom 2.5" exhaust header-back with universal 2.5" cat and resonator. That was complimented with a custom short-ram intake and air box with a BPi velocity stack and K&N filter.

After completing these mods, there is a noticable drop in low-end torque, specifically below 3000rpm. You can litterally feel a bump in torque as it climbs above 3000. This loss of low-end torque is due to very low exhaust gas velocity in the exhaust system at lower rpm's which is due to the larger diameter piping. This causes poor scavenging from the header, and a loss in torque. However, once you climb through the mid-range past 4-4500rpm, I actually noticed a decent gain over stock, especially in the top-end after it changes over to the larger cam lobes (vtec) from 5600rpm on up to the limiter at 8200rpm. Throughout the mid range and higher rpm's the exhaust velocity remains ideal for scavenging and can support the higher volume flow rate. The engine runs very efficient above 3000rpm and just hauls-*** up top.

There is actually an "ideal" exhaust velocity which depends on your engine's characteristics. The displacement, port size, valve size, cam lift/duration and timing all play a role in determining the ideal exhaust and intake system for what ever type of power range you want.

Sorry for the novel...lol. I'm a geek and love discussing this type of stuff...

Basically, keep the stock diameter piping and you should be able to maintain low-end torque. Going with larger piping WILL decrease low-end torque. There are other effective things you can do, but they would involve modifying the intake/manifold and are probably not worth the cost for the gain.

Good luck.

iTaLiAn_vIrUs

^^

wouldnt a 4-2-1 header net more low end gains as apposed to the 4-1 ?

MPR

^A properly designed 4-2-1 header will net good mid-range gains as well as top-end gains. But overall, a header with a larger diameter collector will lose power in the low-end because of the slower exhaust gas velocity, whether it be 4-1 or 4-2-1. There are advantages in the design that can be implemented in 4-2-1 headers which makes them generally better performers throughout the rpm range than a typical 4-1.

You have to remember the flow of air through an engine changes drastically throughout the rpm range. When you optimize certain components for the best performance in one specific rpm range, it will have implications on the performance of the rest of the rpm range. For example, by installing venturi's in the intake runners and a header with a smaller diameter collector to increase air flow velocity for lower-rpm torque, those characteristics become restrictions in the higher rpm range by not allowing enough air to flow when it's needed.