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TG4

· Wlcm 2 costco, I Love You
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Discussion starter · #1 ·
The traditional exhaust for a N/A 2.3 is 3" reduced to 2.5 inches. The reduction is to increase the speed (and hence lower relative pressure) of the gas which due to the Venturi effect. My question is how would this work with gasses which are compressible and the effect is strictly about non-compressible volumes.

Wouldn't a reduction in pipe size cause an increase in back pressure (due to compressibility) to make up for the lower pressure downstream? (Newtons 3rd law)

So wouldn't a reduction drop horsepower?

Or does the siphon effect from the lower pressure make up for the back pressure; as pistons are only in the exhaust phase for a short time?

I'm just attempting to wrap my head around advanced physics.
 
#4 ·
Dude it sure as hell ain't easy. I've just about dedicated my life to 'the cause' and I still have a ton to learn, though I've learned a ton. I look forward to OHC230 making his return to the board, I learn so much from the guy. He's the one who taught me a bunch of **** which led to me landing my sponsor...
 
#9 ·
I'm in no position to teach anyone much of anything...barely qualified to give advice here :lol

I still have alot to learn, there's a whole lot I don't know. But hey, I'm doing pretty damn well considering I'm only 19!

I'm actually under the impression that I'm going to be able to learn a few things from TG4 here as well, seems pretty sharp!
 
#11 ·
I was attempting to figure this out on the side (stock Ranger goes from 2.5" to 2.25") while in school, but didn't have enough free time. I'll see if I can dig up my ATF, Fluid Mechanics and Thermo books and get it figured out while sitting in the hospital with grandpa. From what I remember, I put off doing the exhaust on my truck for fear of losing torque...
 
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#12 ·
Ok, I will do my best to explain what i understand of this. First the mustang exhaust starts out as a 2" outlet at the manifold (maybe 2 1/4"). If you are talking a 2.3 with a header, Which i assume you are, the point of the reduction in size it to keep the velocity up to help the scavenging effect. You obviously have a decent grasp on the situation, and fluid dynamics. The pressure isnt as important as the speed. a 2.5" exhaust is much better flowing than stock, and will still promote good scavenging. You also have to think about where in the system you downsize, you should always do it at the collector as close to the header tubes come together. The farther down the system you do this the less it helps. Thats why when you buy a new header for a street 2.3 the first thing you should do is chop the end off. Lol. Its painful to do that but its worth it. If you use the diffuser design, you actually take it down to a 2.25 or 2" at the small section, this also further promotes the scavenging effect and will actually increase torque and power.
 
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Discussion starter · #13 ·
arummel93 said:
I'm in no position to teach anyone much of anything...barely qualified to give advice here :lol

I still have alot to learn, there's a whole lot I don't know. But hey, I'm doing pretty damn well considering I'm only 19!

I'm actually under the impression that I'm going to be able to learn a few things from TG4 here as well, seems pretty sharp!
Click to expand...
Lol, im still in highschool!

---------- Post added at 12:05 PM ---------- Previous post was at 12:01 PM ----------

I am trying to learn as much as possible, but I wish I had more nuts and bolts knowledge.

---------- Post added at 12:47 PM ---------- Previous post was at 12:05 PM ----------

Judging from all that, those hedman headers look real nice, and with a 2.5" collector I wouldn't need a reducer, so the higher price justifies it. The only problem is that the bend for the collector to go under the car looks a little shallow, and what on earth are those holes, O2 and EGR. I thought that the O2 should be in the collector not the runner. Well that sucks, they look sweet, but the speedways still seem to be the way to go.
 
#14 ·
I was in Physics and A.P. Physics in high school, raced 2.3 mustangs on dirt for 5 years. then went to UNOH for High Performance and Automotive (I have my associates degrees in said courses) and ~30% of what i know about 2.3 mustangs i learned here.
 
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#15 ·
I got expelled from high school before I could get to the AP classes (for being too smart, go figure) and went straight into college making deans list with ease until I had to drop out for financial reasons. I spent so much money on college it literally cost me my first car, my beloved '93.

I'm just clever and have dedicated way too much of my life to knowing **** :lol
 
#16 ·
I'll jump in as the naysayer here...

I have a hard time believing that necking down the exhaust to 2" is optimal.
I think that doing this is just a way of 'optimizing' the parts already on the car. If other parts were used (exhaust primaries, valves, cam, etc)... I would be willing to bet that the impact of the neck down would change significantly.

I'm not arguing that necking down the exhaust is 'bad', as it obviously can be used to 'tune' a set of parts that have been thrown onto a car...
I'm arguing that if the exhaust needs to be necked down in order to optimize the parts that have been thrown on a car, then the wrong parts have been thrown onto the car.

The collector is the only exhaust location where I would expect an exhaust size reduction to be beneficial to any setup.


What data is available for discussion?
 
Discussion starter · #17 ·
You know siphoning a gas tank, the gas coming out sucks out the rest.

Its the same principal. Necking down the tube increases the speed of the gas which causes a decrease in pressure. As stated by Mike89us the 2.5" is still bigger than stock so it doesn't really cause back pressure. Also, remember that our engines are 4 stroke, so essentially your pistons are only blowing exhaust 1/4th of the time, by the time that it gets necked down, the valve has all ready closed, so back pressure doesn't matter. The increased speed in the pipe causes a pressure drop in the large tube, which in turn helps suck air out of the next piston. Then the process repeats.

---------- Post added at 03:49 PM ---------- Previous post was at 03:34 PM ----------

vristang said:
The collector is the only exhaust location where I would expect an exhaust size reduction to be beneficial to any setup.
Click to expand...
That's where the reduction were talking about would be, either at the collector or right under the firewall after the bend.

---------- Post added at 04:27 PM ---------- Previous post was at 03:49 PM ----------

Great, now I want to take Physics as one of my final classes, :crazy
 
#18 ·
I loved A.P. Physics, it was the only class my seinor year i wanted to be in other than Auto Tech

---------- Post added at 05:16 PM ---------- Previous post was at 05:15 PM ----------

vristang said:
What data is available for discussion?
Click to expand...
OHC has done dyno tests with the diffusers at the collector and stated that there was a performance gain.
 
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#19 ·
FWIW, I've got a B.S. in mechanical engineering. We never did anything like this though. Would take me a little to refresh, and then figure out the more complex stuff. Then all that could go to hell by the time I get to what the cats do to the flow...

vristang said:
What data is available for discussion?
Click to expand...
This is some need-to-know info. I could run calculations on what a stock Mustang/Ranger/whatever would do. But if you've got any head, intake or other work done then that would really screw with things.
 
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#20 · (Edited)
Hi folks, I have precious little time to post here, so you will have to excuse the hit and run nature of my answer. In reference to the original question, regarding changes in exhaust tubing diameter vs flow vs velocity; I believe the missing pieces you’re looking for; are the effects of temperature on the working fluid and the influence of velocity (not necessarily back pressure) on gas momentum.


It seems that the automotive world has become obsessed over the years, with the idea that more flow always equals more HP. Of course we all know this just isn’t true, but rather a careful balance of velocity and volume are required to produce a given scavenging effect at a given engine speed.


We could all agree that temperatures at the exhaust port quickly diminish as they travel through the rest of the exhaust system. If we also agree that heat is in fact a form of energy and energy/heat is rapidly absorbed as it travels through the exhaust system, then we should also agree that this lost energy/ heat could cause exhaust gases to slow down as well. Y/N?


On the momentum side of things, we could relate to this as well; if you light a match and wish to blow it out, do you hold it at arm’s length or closer to yourself to extinguish it? At what distance does the velocity from this action diminish to a negligible amount and would it be greater if you were blowing through a straw?


As Mike pointed out, exhaust systems are probably best evaluated as models of both “dynamic” flow and steady state flow. Many exhaust system parts are evaluated using a flow bench, which can be helpful in collecting some data, but that only provides a partial picture of what is occurring in the system. It would be wise to examine all aspects of a system, but the interpretation of the data can get very complex, even for a seasoned engineer.


Good discussion, you guys were definitely moving in the right direction on this. Gotta run.

OHC.
 
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Mikel89us
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