trailblazr81

OK so Im looking at the LCE header and exhaust and the Downey header and exhaust. Looks like the LCE is 2 1/4 inch and the Downey is 2 1/2. I would think thats a little big for a 2.4L 4 banger. Perhaps the Downey kit could hurt low RPM torque. Tho after looking and reading it seems the Downey kit is header back with cat while the LCE is a cat-back for the same price.

ozziesironmanoffroad

biggest id go is 2.25". 2.5" would hurt your power

Wardamneagle

x2 2 1/4" is the only way to go.

norcalsvx

i run 2.5 (its crimp bent so it's smaller than a true 2.5" ) and has some bends so i still have some backpressure. I would go with the LCE 2 1/4 or just have a muffler shop weld up something for you for cheaper.

91Toyota

2.25...or die...lol

FredTJ

Wouldn't go larger diameter than stock at all.


It's not a "backpressure" thing, but rather velocity, loss of velocity.

It's sooooo funny when I see post about "opening up" the intake and or exhaust with absolutely no engineering to back it up.
A lot of people seem to think that bigger is always better, and it isn't, especially on these engines that move less gas than a gnat fart ;0




Fred

jped1981

Fred, I am not a mechanical engineer, nor am I a mechanic, I am not exactly disputing your argument, nor do i know if its better to flow faster or slower through the pipe, however this is my interpretaion of the exhaust piping arguments.

flow is defined as Q=VA where;
Q= flow rate
V = velocity
A= cross sectional area of the pipe

If nothing is changed in the fuel consumption or then the Q will remain constant. Hence you are correct, an enlarged cross section wil slow the velocity of the flow through the pipe. However the Q=VA equation does not account for headlosses due to friction and turbulence. I believe that the flow through a narrow pipe will increase the friction between molecules (think of sipping a milkshake through a narrow straw). However if the pipe is too large, there will be alot more area for the exhaust gasses and the Q=VA equation will be closer to holding true. and the velocity within the pipe will be lessened. So as you can see, you cannot be too narrow or too large. My 2 cents.

FredTJ

You're actually pretty close to being "right on"

In exhaust and intake gas flow (air, exhaust gases, etc.) velocity is your friend and turbulence is not.
Increasing the diameter of the pipe (intake or exhaust) will create less velocity and that leads to more surface reversion which is a bad thing. Lesser velocity can also cause turbulence, which is a bad thing.

Vehicle engineers spend a zillion dollars designing exhaust and intake systems to work just right for a particular engine/vehicle combo.

It's always funny to me and some of the other engineers that I work with that have an interest in vehicles when I see someone posting up wanting to "open up the exhaust or intake restrictions" when really the silly's simply have no clue as to what they're doing with no design going into their thought process at all.
Mmmmm, "bigger must be better"....
Not when we're talking about an engine that most less gas than a gnat fart



Fred

jped1981

Hey Fred,

I agree with you, that opening up the exhaust too wide is definitely not a good idea, nor is having a little pipe, but I wonder what the design criteria was when designing the exhaust pipe and manifold. i guess the design crteria would size the pipe. What do you think about the horsepower dyno gains? I do see them as typically in the upper rpm range, and area where its not a good idea to keep the motor at for sustained periods of time, but I think the goals of the dsigners should be considered before modifying the exhaust. I think that the automotive engineers would likely err on the safe side giving you a little play put a slightly larger pipe, however, I dont think it would benefit much on this 4 banger motor. I do believe that the gains achieved through exhaust modificationis fairly minimal and typically in non driverfriendly rpm ranges. My 2 cents

swaysidelife

i had the local muffler shop put a 2in cat back on mine with a flow master and it does fine sounds good too. the best part it only cost me $100 lol

20twinz

So whats a good header to go with then on a mild rig that does some off roading?

eBay headers?
LCE?
Downey?
Other?