3" downpipe to 2.5" exhaust???

[9!'clipseDOHC]

The background: I have a full (header back) 2.5" exhaust on my non-turbo 4g63. I am doing a full 6-bolt turbo swap this summer. I will be keeping the 2.5" pipes, except for the headers/non-turbo downpipe, for a while to keep the cost down.

My question: Would a 3" downpipe to a 2.5" hi-flow cat and 2.5" cat-back system cause any problems? Or would 2.5" all the way be bettter? I don't really want to get a 2.5" downpipe as I would eventually like to upgrade everything to 3". Please let me know your opinions. Thanks

 

[Calan]

A 2.5" all the way would be better.

Normally, when the exhaust goes from the smaller DP into a larger exhaust, it cools, expands, and looses velocity. With a larger DP and smaller exhaust, you would actually be causing a bottle neck where the expanding exhaust gas meets the smaller pipe.

As a (very) loose analogy, think about water flowing into a funnel.

 

[streetdsmpwr]

Well the guy who originally owned my car removed the stock downpipe, cut the flanges off with a good couple of inches of the pipe still on them and welded them onto a 3" pipe. He did the same thing from the cat to the muffler. My exhaust goes 2" at O2 housing to 3" pipe to 2" cat to 3" pipe to 2" muffler. My car runs fine but there is some noticeable unwanted back-pressure. I think you would be ok for a while until you got the 3" cat-back as long as you don't run too much boost, but don't quote me on that. I run 15 psi fine but thats on a tiny T-25. Don't worry I'm replacing the whole thing with a full 3" exhaust in a couple weeks.

 

[misterskeetz]

Whatever you do...always start your exhaust upgrades from the back of the car. Catback then downpipe then 02 housing/exhaust manifold/headers.

 

[streetdsmpwr]

I don't think there is any danger in running 2.5" to 3" but you just won't get the performance gains you want out of a 3" downpipe. Take it from the guy who has the worst exhaust system ever.

 

[idt512]

Normally, when the exhaust goes from the smaller DP into a larger exhaust, it cools, expands, and looses velocity.

This is the part I dont agree with, I know ive read it this way many times before but from what I remember in science class, it is hot air that expands, and cold air contracts (remember putting a plastic bottle in the refrigerator and it shrinks after about an hour causing it to have vacuum due to the air cooling?) with that in mind, it should be 3" downpipe to 2.5" exhaust because the gas has cooled and contracted, therefore needing a smaller area or volume. But even after knowing this I still went and got a 2.5" downpipe to 3" exhaust, I figured there must be something im missing if everyone else does it this way

 

[dsm-onster]

That's right.

The sooner the restriction to the turbine the more it affects the turbine. The turbine works off of differentials. A 3" down pipe to 2.5" exhaust shows a greater difference to the exhaust manifold back pressure, than a 2.5" down pipe and a 3" exhaust.

OF couse you want a smooth transition from 3" to 2.5" down steps are always bad.

 

[gorf]

This is the part I dont agree with, I know ive read it this way many times before but from what I remember in science class, it is hot air that expands, and cold air contracts (remember putting a plastic bottle in the refrigerator and it shrinks after about an hour causing it to have vacuum due to the air cooling?) with that in mind, it should be 3" downpipe to 2.5" exhaust because the gas has cooled and contracted, therefore needing a smaller area or volume. But even after knowing this I still went and got a 2.5" downpipe to 3" exhaust, I figured there must be something im missing if everyone else does it this way

Combined Gas Law, Calculate volume, pressure, temperature.

If you have 2.5" dp the pressure will be higher in that pipe than in a 3" tailpipe. But the volume stays the same. Play with the numbers in the above calculator.

Another way to look at it is to think Air conditioning.... Freon is compressed into a hot liquid, it goes through a nozzle which allows it to expand and cool off the evaporator. Same principal, So no your thinking is backwards....

 

[dsm-onster]

Right! OR wrong for saying he's backwards. But RIGHT! you hit the nail on the head for why larger exhaust earlier is better than larger exhaust later. Since the pressure goes up with more heat added to the gases if volume stays the same (after all you linked the ideal gas law), you need a larger volume for the same number of moles of gas to lower the backpressure since the heat energy output and molar flow from the turbine wheel is a constant WRT the exhaust system You lower the back pressure by lowering the volume right after the turbine housing to lower the temp out. Thus where the gas temps are higher larger volume is more beneficial. Turbine Work = Specific Heat X Mass Flow X (Temp in - Temp out).

Temp makes the individual particles move faster but remember they are all moving in random directions. Increasing temp doesn't make the particles more faster in a certain direction any more than another. Hense why you increase temp and keep volume the same pressure goes up. Pressure is the effect of the momentum from the individual particles all over the surface of the container.

 

[snakeyes711]

well, Its hard to find an affordable 2.5 inch down pipe any ways. I've looked, and there are stainless 3" pipes all day for around 70 bucks shipped. But I can't find a one 2.5" downpipe anywhere for that kind of price.

 

[9!'clipseDOHC]

well, Its hard to find an affordable 2.5 inch down pipe any ways. I've looked, and there are stainless 3" pipes all day for around 70 bucks shipped. But I can't find a one 2.5" downpipe anywhere for that kind of price.

I've noticed the same thing, 3" downpipes are much more prevalent than 2.5”. I guess a 3" downpipe with a reducer into my 2.5" converter sounds like the way to go, both price and apparently, although disputably, performance. Thanks for all the input!

 

[snakeyes711]

G/L on finishing it.