has anybody here done this with a custom exhaust?

for example, if you're playing with a hose, to increase speed/velocity of water, you pinch the end of the hose.

couldn't the same "theory" be applied to exhaust pipes? i.e. have the collector on the header be 2" and then connect a pipe that would gradually increase to 2.5"

 

I don't understand your analogy. If you want to "pinch" the exhaust flow, you'd need a *smaller* pipe toward the rear. In any case it's not going to work. While you may "speed up" the flow, you are actually only speeding up a fraction of the total, meaning your *total* flow is less.

Not good.

 

You've got a good understanding of part of the theroy behind exhaust designs.

However, it is somewhat off.

Yes, you want an exhaust pipe to be small enough to create high exhaust velocities. But the key is "mass flow rate". If the exhaust is too small, the mass flow rate will start to go down even though the velocity is high.

This is why you don't want to use a turbo exhaust on an N/A Engine.... (besides the fact is loud as help) .... it's too big! Your velocity will actaully decrease to a point where the mass flow rate slows down even though it is very free flowing.

There is a fine line between what is too big and what is too small. And it all depends on the amout of power your engine is going to be putting out.

For example:
Someone wishing to run a 230whp N/A engine should use a 2.5" exhaust. Any bigger and the velocities would slow down too much, and hinder mass flow rate. Any smaller, and the mass flow rate would go down due to the small size, even though the velocity increases.

The key is Mass Flow Rate.

Also, you want to maintaince this consistant pipe size the whole way through. Once you've got the ideal pipe size figured. You want to run that size the whole way back. Any variations from that size and you will lose some Mass Flow Rate.

Here is a very good artical from Team-Integra.net that goes in more detail
Team Integra: Advanced Exhaust Design - Back Pressure & Area

 

hey guys, thanks for the replies, especially for that article.

but do any know the logic as to why just "axle-back" exhausts produce even a a nice gain?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by FOBsquad &raquo;</TD></TR><TR><TD CLASS="quote">but do any know the logic as to why just "axle-back" exhausts produce even a a nice gain?</TD></TR></TABLE>

I would guess that the logic behind axle-back gains (if there are in fact any gains) is that you have in effect freed up some of the restrictions, in effect making the flow slightly improved. If I understand your original post, you are thinking that increasing the speed of the exit of the exhaust will improve efficiency. "Pinching" the exhaust will actually have the opposite effect. As w/ your hose analogy, when you pinch the exhaust or the hose, all the gas or water building up behind the "pinch" is under a greater amount of pressure, thus reducing efficiency. The best solution is to free up the entire system, and the sliding scale of improvement decreases as the amount of improvements made to the system decreases. In other words, axle-back systems offer slight gains since they are a slight improvement. The more you free-up the better it gets, the more you "pinch" the worse it gets.

 

A stock exhausts are usually "chambered" muffler (or some variation of this). Because this design is the the best/cheapest for a quite exhaust.

Very rarely are they "straight through" mufflers.

In a stock exhaust system, the stock chambered muffler causes the biggest restiction in the system.

So, buy changing the chambered muffler to a straight through muffler by adding an axle back exhaust, you are fixing the stock mufflers weakest link.

Now the weekest ling is in the stock exhaust header, cat, or pipe size.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by FOBsquad &raquo;</TD></TR><TR><TD CLASS="quote">has anybody here done this with a custom exhaust?

for example, if you're playing with a hose, to increase speed/velocity of water, you pinch the end of the hose.

couldn't the same "theory" be applied to exhaust pipes? i.e. have the collector on the header be 2" and then connect a pipe that would gradually increase to 2.5" </TD></TR></TABLE>

Your example is not the greatest simply because it compares the characteristics of a liquid to that of a gas. As we all know water is not compressible; it forces it's way through and always occupies the same volume of space. Gases are compressible and behave differently when given less space to operate in.

A car's exhaust should be made to scavenge the hot gases away from the motor at a high rate. The best way to do that... leave it to the brilliant highly trained engineers. I'll gladly bolt it on and enjoy it. It is fun to think of new things though.

 

couldnt one gain from gradually increasing the size of the piping?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JeremyL &raquo;</TD></TR><TR><TD CLASS="quote">couldnt one gain from gradually increasing the size of the piping?</TD></TR></TABLE>No, per the analogy, you have it backwards.

I suppose there's some logic to making it very slightly tapered *smaller* on the way back since the gases are cooling off and are taking less space. But then again making or buying a tapered system is nuts.

 

i meant start at say 2" and gradually increase the size of the pipe as you work to the muffler up to 2.5"

 

If all if this is true, then why do people pick up so much horsepower when running open header?

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by CRX PAT &raquo;</TD></TR><TR><TD CLASS="quote">If all if this is true, then why do people pick up so much horsepower when running open header?</TD></TR></TABLE>

cuz there is no resistance when letting the gasses go out of the engine, meaning, when you have an exhaust, the gases leaving the engine need to travel about 6 feet of pipping and mufflers which makes resistance, and when an engine is NA it needs the least resistance to make the most power, so when you run open header there isn't any resistance which translates into more HP and TQ

 

Thats why turbo peeps run an open DP even tho its loud as heck......

Blaze

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by CRX PAT &raquo;</TD></TR><TR><TD CLASS="quote">If all if this is true, then why do people pick up so much horsepower when running open header?</TD></TR></TABLE>

Sometimes backpressure is necessary to make it run well at certain RPM ranges. That's why a good exhaust is tuned for the engine and car. Open header would not always be best.

 

<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by CRX PAT &raquo;</TD></TR><TR><TD CLASS="quote">If all if this is true, then why do people pick up so much horsepower when running open header?</TD></TR></TABLE>

The lack of exhaust means there is less resistance (backpressure). Backpressure is never a good thing (except maybe on a turbo car but for different reasons). Since the exhaust has to go through the header, velocity is still preserved.