This has always irritated me (exhaust back pressure)

[txplayboy001]

Any gasoline burning motor DOES NOT NEED BACKPRESSURE to operate correctly. No back pressure is the best. You just need to up your fuel to compensate for the increased air flow through the motor if you have less backpressure.

Sorry if this is off topic, but this backpressure myth gets on my nerves. :OT:

 

[CManT1914]

Any gasoline burning motor DOES NOT NEED BACKPRESSURE to operate correctly. No back pressure is the best. You just need to up your fuel to compensate for the increased air flow through the motor if you have less backpressure.

Sorry if this is off topic, but this backpressure myth gets on my nerves. :OT:

Did that no muffler thread get on your nerves?

 

[txplayboy001]

Well, not really that, it just reminded me about a phone call I got last night. Spent an an hour arguing the physics of how an engine operates.

 

[Mavrick]

Then why would a stock mustang feel noticeably less powerful when running an open H pipe and no mufflers/tails?

Maybe back pressure isn't a necessity for the basic operation of a gasoline engine, but it sure is beneficial to a 5.0L Mustang, and that is what this board is all about.

 

[87'GTstang]

A little is needed for maximum low-end torque but once the exhaust starts flowing, backpressure starts limiting potential high-end horsepower. An engine will run fine for the most part, just go ahead and strip all the exhaust down to the manifold/header. However though, no back pressure will a lot of times cause a backfire sound when going from WOT to complete let off and return to idle simply because the engine is basically "sucking" air back in due to no backpressure present. To a degree backpressure is harmful but helps low-end torque to an extent for those that care greatly about it. All I can say is that too much can sometimes be just that - too much. No sense in putting a dual 3" exhaust system on a stock engine - which is why manufacturers of lesser displacement engines don't.

 

[txplayboy001]

Well, like I said, less back pressure = need more fuel
You take away back pressure, it causes a lean state.
Just like any other mods you do to your car, it will effect the way it runs. Every change you make needs to be compisated somewhere else, if the computer can not do it.

 

[CManT1914]

Well, not really that, it just reminded me about a phone call I got last night. Spent an an hour arguing the physics of how an engine operates.

oh, I hate getting into arguements like that, but I gotta go with Mavrick on this one. The engine always needs a certain amount of backpressure depending on how much power they're making. That's why these top-fuel dragsters run open headers, cus they need to since they make like 5000 hp! lol

 

[txplayboy001]

I agree to a certain point. Forced induction cars have big fuel upgrades (for instance), no back pressure is needed there. Stock, I agree, especially on these 5.0 motors, won't run right.

Here is an interesting link:
http://www.uucmotorwerks.com/html_product/sue462/backpressuretorquemyth.htm

Not trying to argue, just stating a fact, and trying to bury a myth.

 

[10secgoal]

Well, like I said, less back pressure = need more fuel
You take away back pressure, it causes a lean state.
Just like any other mods you do to your car, it will effect the way it runs. Every change you make needs to be compisated somewhere else, if the computer can not do it.

I was running no pipes or mufflers for a while (nasty driveshaft problem took the both out ), which I would guess is as close to no back pressure as you can get. I went and put on exhuast and picked up et and 2 mph. I don't think and motor NEEDS back pressure, but because it doesn't need it to run right, that doesn't mean that adding some will not benefit the car.
I think the whole idea of backpressure thru exhaust is misunderstood. It's not the actually restriction on the way out of the exhaust that causes the low end torque. It's the scavenging of the cylinder gases caused by the vacuum behind the exhaust pulse infront of it. A gas pulse exits the header and enters the pipe. On the way out it creates a suction behind it. When the next valve opens, the gas is actually pulled out of the cylinder, rather than being pushed out ALL by the piston. If your exhaust is too big for the amount of gas exiting, it will not form a "seal" if you will. And if it doesn't, you now have no vacuum behind the pulse and no scavenging effect.

 

[5spd GT]

Sorry txplayboy...but an engine does need some backpressure to run better overall...be more specific with your generalization next time...

I'd reply with some more but I'm going to go throw on a 4in exhaust on my car because I don't need back pressure at all...

 

[crunchie12268]

Start talking to those ricer ppl and they will argue you to death about back pressure and there turbos..... I would think you need some back pressure to be beneficial but I don't know.

 

[87'GTstang]

I think the whole idea of backpressure thru exhaust is misunderstood. It's not the actually restriction on the way out of the exhaust that causes the low end torque. It's the scavenging of the cylinder gases caused by the vacuum behind the exhaust pulse infront of it. A gas pulse exits the header and enters the pipe. On the way out it creates a suction behind it. When the next valve opens, the gas is actually pulled out of the cylinder, rather than being pushed out ALL by the piston. If your exhaust is too big for the amount of gas exiting, it will not form a "seal" if you will. And if it doesn't, you now have no vacuum behind the pulse and no scavenging effect.

Exactly! It is the same principle behind induction components. If you go too big, you get "lazy" flow and it creates turbulance and a "pushing" action needed to get it out of the way. When things are sized accordingly and matched, then it is like syphoning, once it starts flowing, displacement equals replenishment and you get an automatic flow due to a vacuum created. This vacuum created by positive displacement instead of pressure created by laziness is what helps pull airflow/gases through the combustion system and ultimately through the exhaust.
Myth - I think not. Some just don't know the exact physics and principles of the concept and distort the "theory".

 

[mackey]

Start talking to those ricer ppl and they will argue you to death about back pressure and there turbos..... I would think you need some back pressure to be beneficial but I don't know.

Someone needs to read up on turbos

 

[crunchie12268]

"Someone needs to read up on turbos "

Yeah I have read up on turbos and know all about the back pressure. But I was talking about the N/A ricers, they think the same rules apply. My friend kept bugging me about a test that these people did with different fart cans... like they picked up two hp from a folgers coffee can, somehow this all related to back pressure but now I forgot... any ways I don't know much about turbos but I know all about the back pressure.

 

[Boss_Mustang_69]

I was running no pipes or mufflers for a while (nasty driveshaft problem took the both out ), which I would guess is as close to no back pressure as you can get. I went and put on exhuast and picked up et and 2 mph. I don't think and motor NEEDS back pressure, but because it doesn't need it to run right, that doesn't mean that adding some will not benefit the car.
I think the whole idea of backpressure thru exhaust is misunderstood. It's not the actually restriction on the way out of the exhaust that causes the low end torque. It's the scavenging of the cylinder gases caused by the vacuum behind the exhaust pulse infront of it. A gas pulse exits the header and enters the pipe. On the way out it creates a suction behind it. When the next valve opens, the gas is actually pulled out of the cylinder, rather than being pushed out ALL by the piston. If your exhaust is too big for the amount of gas exiting, it will not form a "seal" if you will. And if it doesn't, you now have no vacuum behind the pulse and no scavenging effect.

This may be a stupid question, but if there is a vacumn behind the first pulse, which pulls the second pulse, wouldn't the first pulse be pulled backwards, thus being redundant?

 

[HISSIN50]

there would be a slight slowing of the frontal pulse, but because the system is tuned, the frontal pulse continues and then is accelerates again. the analogy to syphoning was a good one. if the pipes were too large, then the 'seal', as it was phrased, would be lost.

also, about ricers. hmmm. putting a sewer pipe after a stock 1.5 inch pipe makes no difference - the stock pipe is still the bottleneck. i dont understand the fart can thing though myself.

 

[giddyup306]

You want to build back pressure after the collectors to make torque. Read my post in the cat thread.

 

[giddyup306]

also, about ricers. hmmm. putting a sewer pipe after a stock 1.5 inch pipe makes no difference - the stock pipe is still the bottleneck. i dont understand the fart can thing though myself.

They're after the reverse venturi effect.

 

[_jb_]

But if you have too much backpressure the exhaust gases will be pushed back into the chamber. I don't really think too much of backpressure. I know what type of exhaust to run for the best results.

 

[wesl56]

10secgoal is dead on.. it works the same way with an intake... think about it... thats why long intake runners build torque, cause of velocity and the "siphoning" thing....