variable exhaust?

[nemesis]

What kind of results can I expect from putting a variable exhaust on my vehicle?

[Old Tele man]

...variable what?

• pipe = length, diameter, temperature?
• valve = lift, diameter, timing?

[pounsfos]

hes talking about variable exhaust valve that sits in the exhaust pipe
(most are butterfly valves like your throttle body butterfly valve)

it closes the butterfly vavle at low cruizing loads to improve low end response (as you now have more back pressure so gasses escape easier compared to a bigger exhaust)

then it opens as load increases (like w.o.t)

all this is controlled either by your ecu or by engine vacuum

as for an increase in fuel economy well i wouldnt know but if you ask me you could invest your money in something abit more worthwhile, like a "economy grind camshaft" or a some intake mods and stripping emissions devices to increase fuel economy.

do a google search and some reading on it and report back as you have me interested on if this might do something or not (if it does then i'm moving upto a bigger exhaust hahahaha

[nemesis]

Originally Posted by pounsfos

hes talking about variable exhaust valve that sits in the exhaust pipe
(most are butterfly valves like your throttle body butterfly valve)

it closes the butterfly vavle at low cruizing loads to improve low end response (as you now have more back pressure so gasses escape easier compared to a bigger exhaust)

then it opens as load increases (like w.o.t)

all this is controlled either by your ecu or by engine vacuum

as for an increase in fuel economy well i wouldnt know but if you ask me you could invest your money in something abit more worthwhile, like a "economy grind camshaft" or a some intake mods and stripping emissions devices to increase fuel economy.

do a google search and some reading on it and report back as you have me interested on if this might do something or not (if it does then i'm moving upto a bigger exhaust hahahaha

Well that's what I have now, but I am going to redo the whole thing with one high flow muffler and 2" piping ( good for 150ish hp flow wise) and butterfly valve that's going to open at 5k rpm controlled by ecu, and the other one with a stock muffler and 1" piping that should be good for up to 40ish hp ( perfect for light throttle cruising). Basically the exhaust is going to have to go through stocking muffler and 1" pipe 0-5k rpm ( velocity should be at it's max around 4.5k) and after that 5k-13k the exhaust is going to be routed to go through both pipes and both mufflers all together should be able to support 200hp or so which is about what I need. I could have done 3 stage set up when it starts with 1" pipe, then goes through 2" pipe and then both pipes, but I'll see how this works out and if it does good 10% or so I might step up to a 3 stage set up and go little further in the head, plus intake variable length that is little more complicated, but is doable. The name of this game is velocity, try to keep it as high as possible without backpressure, because we all know, backpressure is not good.

[pounsfos]

it closes the butterfly vavle at low cruizing loads to improve low end response (as you now have more back pressure so gasses escape easier compared to a bigger exhaust)

sorry ment higher velocity

[Ryland]

So you are going to have more then one exhaust pipe?
Smaller exhaust pipe is not used for more back pressure, it's used for greater scavenging, back pressure is caused by blockage, you don't want blockage, you want your exhaust to use the pulses to your advantage because exhaust is not exiting as a steady flow.

[Frank Lee]

I cringe whenever I hear talk about exhaust backpressure.

[Old Tele man]

...there are TWO determinants happening in exhaust "tuning":

• SONIC (acoustive) velocity (1100 to 1700 fps) which is temperature dependent.
• MASS (gas slug) velocity (up to 300 fps) which is pipe cross-sectional area dependent.

...one is 'why' headers are sometimes wrapped to hold-in heat, and the other is 'why' headers use different diameters and lengths.

[Allch Chcar]

Sonic Velocity is why headers are wrapped and Mass Velocity is why tubing is sized, right?

From my weak knowledge, the diameter of the tubing determines the torque range based on the engine configuration.

I can't help with the variable exhaust but I do know of a variable intake design.
Ford uses a second set of throttle bodies in their engines to improve the low end air velocity for better low end torque. I've personally seen them in the 3.8 liter V6 in the Ford Windstar where they were located one in each intake just before the fuel injectors. But I know they also used them in the Ford Ranger at some point. The switch to full open was in the 2k-3k RPM range.

Sorry that I can't be of any help.

[Duffman]

Originally Posted by Frank Lee

I cringe whenever I hear talk about exhaust backpressure.

Ditto,
If this really worked, every car would already have it.