Exhaust Conundrum

[Cosmo Dude]

) = bend
> = elbow

[LordWorm]

Originally Posted by Cosmo Dude

) = bend
> = elbow

Show me a picture

bet you its a mandrel bent elboe =)


The only reason i coudl possibly think they'd want backpressure in a dragster would be to keep gas in the chamber to stop burning through the valves (one of the excuses offered by backpressure fanatics....doesnt hold water in modern cars though).

However, for a car whos engine has to last a grand total of 900 revolutions before rebuild, including staging and burnout, i wouldnt think they would worry.

find picture.

[Cosmo Dude]

The elbow is mandrel but sharp (exhaust has mass)

For a street car that has more pistons than tail pipes this arguement is useless (no matter how right I am) ..|..

[LordWorm]

Originally Posted by Cosmo Dude

The elbow is mandrel but sharp (exhaust has mass)

For a street car that has more pistons than tail pipes this arguement is useless (no matter how right I am) ..|..

Its still a mandrel bend... the diameter of the pipe at the bend is still the same as the diameter of the pipe at the opening. Gas accellerates around corners.

And again, I ask you to explain why you are right. Give me an argument that defeats the no-backpressure argument. Explain it in scientific terms, and tell me then why the worlds most expensive headers (burnstainless custom headers) are designed to defeat backpressure?

Explain why highflow cats are great?
explain why if you want backpressure why you dont just put a potato in your exhaust pipe?

And then, once your done with that, explain to me how much backpressure is needed to do whatever it is that its spose to do, and how to tune the backpressure to a given RPM to do good things for power.....

Then I'll reconsider.

Not meaning to be an ass about this...but If you are right, you need to be able to explain why you are right - and then explain why F1 cars have no backpressure, drag cars have no backpressure, and the best street machines going have no backpressure...if you think about how the exhaust system and the engine is working, backpressure just doesnt stack up.

[twilightprotege]

backpressure = evil. it's a misused term in the exhaust industry.

what exhaust w@nks call backpressure - they are really referring to velocity, they are just to dumb to use the real term as it doesnt sell.

put a 2" mandrel system on my car and the bottom end will rock. the exhaust gas will be moving at a very high velocity and thus that is what helps with power. rev the car up to 5000rpm and then the pipe cant take any more - THEN AND ONLY THEN do you get back pressure - when the pipe cant take anymore and the gas starts staying in the cylinder.

in a perfect world you want a small exhaust system for low rpm and then expands as the rpm increases so you keep the exhaust gas velocity at optimum.

so it's essentially a trade off. there is no perfect exhaust system for any car. decide where you want power, then get your exhaust system.

oh and the reason why stock cars come with small systems - low end power. no other reason.

[chicaboo]

I'm going back to 2.25" mandrel when I can, but before then I need a FLYWHEEL and some other stuff.

[Cosmo Dude]

Originally Posted by LordWorm

And again, I ask you to explain why you are right. Give me an argument that defeats the no-backpressure argument. Explain it in scientific terms, and tell me then why the worlds most expensive headers (burnstainless custom headers) are designed to defeat backpressure?

Headers are not an exhust system!

Originally Posted by LordWorm

Explain why highflow cats are great?

Where did that come from? Now you are clutching at straws

Originally Posted by LordWorm

explain why if you want backpressure why you dont just put a potato in your exhaust pipe?

Back pressure in the right place will give you power where you want it. A potato in your tail pipe is as useful as a potato up your a$$

Originally Posted by LordWorm

And then, once your done with that, explain to me how much backpressure is needed to do whatever it is that its spose to do, and how to tune the backpressure to a given RPM to do good things for power.....

Visit your local motor-cross track and talk to the 2-stroke lads about tuning exhausts. Not as critical in a 4-stroke.

Originally Posted by LordWorm

Not meaning to be an ass about this...but If you are right, you need to be able to explain why you are right - and then explain why F1 cars have no backpressure, drag cars have no backpressure, and the best street machines going have no backpressure...if you think about how the exhaust system and the engine is working, backpressure just doesnt stack up.

They all have some back pressure, they all have tuned length exhausts and all have put more money into their exhausts than you have put thought into your arguements.

[LordWorm]

Cosmo Dude.. What are the headers if not hte very beginning of the exhaust system?

If backpressure is such a good thing, why restriction free exhaust system componants (hence my question regarding highflow cats)

F1s, drag cars etc, They all have no backpressure. None at all. Zero. Zip. Nada. Nothing, at their operating RPM. Well in excess of that RPM, where the pipe becomes a restriction again, then backpressure would develop... but if they were capable of operating at such an engine speed, they'd put wider pipes on, to push the point further back.

Power where you want it is dicated by the length of the header primaries. Beyond the header, nothing more int he system can have a positive effect on power - it can only add restriction (backpressure.).

I suggest you read the link i posted. I suggest also you get onto the burnstainless website..

if backpressure is so good/required/useful tell me how much I need, at say, 3000rpm, with a 2.5in system, to achieve "power" at that point. Then explain to me why it is so....To which I'll answer, I'll tune my header primaries to that length, so that i am getting maximum velocity at that point, with a system that has no backpressure. If you are right, show me that you are right. Provide a scientifically based argument. Tell me EXACTLY what this backpressure is doing in the exhaust system What effect is it having on the flow of exhaust gasses out of the engine? How is it aiding headers to work correctly?...

Backpressue is the biggest missunderstanding and missleading thing in the world of exhausts (and more than any other performance mod, exhausts are the most steeped in secrecy and myth.). Given the nature of how the engine works, backpressure would never be a constant. if it is X at 3000rpm, its going to be twice X at 6000rpm, until it gets so bad the car simply cannot rev anymore because no fuel or air is able to get into the cylinder. Explain how thats ever a good thing?

In simple terms, the amount of power an engine can produce, is directly related to how much air and fuel it can get in, and how quickly it can get the exhaust gas out of the engine. By applying backpressure, all you are doing is PREVENTING it from expelling all the gas it possibly can - therefore you are putting a burden on performance (or outright hamstringing it).

Like i've said previous, if i am wrong, tell me why. With a well backed up bit of science...

I'm not the one clutching at straws here.. I have ample amount of resources here that will back me up... you have yet to provide one.

[TERRA Operative]

But Cosmo Dude, still you fail to provide any concrete evidence, and you argue against references we provided with your own opinion.

Untill you can give us correct evidence of your claims, all your posts are nothing but heresay.


BTW, 2 strokes don't use backpressure. The cavity in the resonator is tuned to provide a scavenging effect, much like in a 4 stroke.
See here:linky
Ooh, what's that? I do believe it's scavenging! Well I'll be...

[Cosmo Dude]

This thread is based on a question regarding a street going road car produced for use on Australian roads post 1983.

True/False

[TERRA Operative]

Which has evolved to the discussion of whether 'backpressure' is beneficial to the increased production of power from said car, or any car for that matter.

True.

Have you got any references backing up your claims yet? I am interested to know if I and the other members rebuffing your claims are correct or not.

[LordWorm]

Originally Posted by Cosmo Dude

This thread is based on a question regarding a street going road car produced for use on Australian roads post 1983.

True/False

True.
But the laws of physics apply in equal measure to everything. You cant just get new laws of physics because its a road application not a race application.

You, like so many others, are confusing exhaust velocity with backpressure.

Small tailepipe = high velocity, which = power. but it will only flow so far before the gas starts moving the other way - that is backpressure... and that is bad.

as a rule of thumb, a 2.25in mandrel bent exhaust system will allow you to flow the engine to 200flywheel horsepower before you start pushing the exhaust in the wrong direction (you've dumped too much exhaust in the system and its starting to clog up).

There are other things that will give backpressure without velocity (such as restrictive crush bends, cats and muffs etc). Thats why performance modders go with straight through mufflers, high flow cats and mandrel bends, and why racers ignore the cat and mufflers all together in alot of cases. The persuite of velocity with no backpressure.

You'll get backpressure in a standard car. Crush bent system, moulded/formed manifold, overly restrictive mufflers in order to get emmissions down. Just because it rolls off the factory floor like that doesnt make it the best possible way it can be done (if it did, none of us would mod - ever).

Getting back to the headers/collectors - in a correclty designed header, the point of highest velocity is the collector - which is why you cannot positivily effect power beyond this point... if the gas isnt traveling fast enough out of the collector, it never will.

[Cosmo Dude]

Settle down.

I have an opinion that differs from yours and I have had three days of fun leading you on.

You have provided some useful information but it is based on a premise that I personally don’t agree with and in some ways could be considered contradictory.

No hard feelings

Cosmo

[TERRA Operative]

Ah, so we have a post whore on our hands.

But why support an incorrect theory that has no scientific backing, that even you cannot back up. I mean, damn, you can't even provide a single reference.

Time for you to go away.

[LordWorm]

Originally Posted by Cosmo Dude

Settle down.

I have an opinion that differs from yours and I have had three days of fun leading you on.

You have provided some useful information but it is based on a premise that I personally don’t agree with and in some ways could be considered contradictory.

No hard feelings

Cosmo

No hard feelings taken.

The reason i posted so much info about it was to inform the missinformed. I'm by no means a super exhaust expert, but i have taken words from those who are, and read lots of interesting stuff about how it all works - learnt alot along the way. Like you, I had the same feeling abotu backpressure. The more of it the more torque, the better the car runs, no chance of valve burnthrough blah blah blah.....

But in the face of hard facts, it really doesnt stack up. Going back and reading pages of backpressure proponants, you start to see they are speaking gobbledgook that'd make you believe that exhaust technology is some black art of alchemy with no real science behind it - because try as they might, they cannot explain why backpressure is good, how to tune it, how much is required, and what exactly it is meant to be doing.

In an NA application, backpressure is the enemy of power... there are far better ways to get power at desired RPM (by increasing velocity through means other than small pipes).

when you start talking FI...its a different kettle of fish...something I know very little about.... TERRA Operative should be able to help out here.. but i'd assume the principle would remain the same (even though you are compressing the air in the combustion chamber so who cares if a little bit of residual exhaust gas from the last power stroke is there...).

Do some research - have a look around. The maths etc behind it are in plain sight on the net.... If once you've seen all the arguments you are still thinking backpressure is a way to go... by all means, i'll be waiting here with potato in hand =)

[TERRA Operative]

In turbo applications, the turbo provides a restriction to exhaust flow, hence the reason for 3" or even 4" straight through exhaust systems.
The fact that the charge is being forced into the engine negates the need for scavenging, so the only concern is getting the exhaust away from the turbo as fast as possible.

Scavenging, when working correctly, can almost be viewed as a form of 'backwards boosting' in that instead of pushing the new charge in, you are sucking it in.


Meh, I'm getting bored of explaining the same concept in so many different ways. I think this thread has run it's course for me.

[LordWorm]

Originally Posted by TERRA Operative

In turbo applications, the turbo provides a restriction to exhaust flow, hence the reason for 3" or even 4" straight through exhaust systems.
The fact that the charge is being forced into the engine negates the need for scavenging, so the only concern is getting the exhaust away from the turbo as fast as possible.

Scavenging, when working correctly, can almost be viewed as a form of 'backwards boosting' in that instead of pushing the new charge in, you are sucking it in.


Meh, I'm getting bored of explaining the same concept in so many different ways. I think this thread has run it's course for me.

sucking it in? sucking it out you mean =)

[Cosmo Dude]

Originally Posted by TERRA Operative

Ah, so we have a post whore on our hands.

But why support an incorrect theory that has no scientific backing, that even you cannot back up. I mean, damn, you can't even provide a single reference.

Time for you to go away.

Terra,
I don't believe that I was wrong. I have a different opinion.
Lately I've been a little grumpy with my posts but generaly I don't post without a reason.

[twilightprotege]

you can still get a too big of an exhaust for a turbo system....because of oh oh, not that v word again....

anyone that wants maximum power needs to have an exhaust system just large enough to flow the required exhaust gas at the rev limit. and that goes for FI and NA. JUST large enough exhaust.

[LordWorm]

Originally Posted by twilightprotege

you can still get a too big of an exhaust for a turbo system....because of oh oh, not that v word again....

anyone that wants maximum power needs to have an exhaust system just large enough to flow the required exhaust gas at the rev limit. and that goes for FI and NA. JUST large enough exhaust.

Yes
but if you plan to mod beyond your exhaust, dont get just big enough, else you'll need to change it everytime you mod =)

Just big enough for your final planned output i'd say =)