Exhaust back pressure on NA, is it needed?

[HSVGTS215i]

Or is the old backpressure is needed on NA cars a myth?

Is there such a thing as too big an exhaust?

Cheers.

[PBoB]

I don't know about the myth or not but a tuner once said to me, "do top fuel dragsters have back pressure?" Their exhausts are only like a metre long so I wouldn't think so, but they are high HP motors too so anyone wish to give us a definitive answer?

[HSVGTS215i]

haha yeh a top fuel dragsters a bit different to our daily drivers

[Phonic]

Four stroke engines do not need back pressure. But you can go TOO big of an exhaust, in witch case you create back pressure. The idea is too keep a relatively fast gas velocity with a low exhaust gas pressure. Obviously you have to find a balance between the two.

Having an exhaust thats too big, will actually cause back pressure, this is due to the exhaust gas expanding and therefor cooling. The cooler exhaust gas is denser, this slows the exhaust flow. This is why you sometimes loose low down torque with large exhaust systems. At lower rpm there isn't enough flow to keep the right gas velocity for optimum scavenging. This is obviously not a problem at higher rpm.

But if you reduce the piping diameter to increase low rpm gas velocity, you will produce more low town torque, but at high rpm the exhaust will not be able to flow sufficiently again causing excessive back pressure and harming top end power production.

Until someone can create a variable diameter exhaust system, you have to find the right balance in exhaust sizing.

[Delco]

I don't know about the myth or not but a tuner once said to me, "do top fuel dragsters have back pressure?" Their exhausts are only like a metre long so I wouldn't think so, but they are high HP motors too so anyone wish to give us a definitive answer?

A blown alky motor has about 40 psi forcing air in , a scavenging exhaust to boost low rpm torque isnt required or desirable .

[VSSTATO5.0]

Bigger isn't aways better. Dispite what you may have heard having a 3" cat back pipe on a 4 cyl Honda isn't a good idea, unless it's making over 300hp. If the pipe diameter is too large you then run the risk of losing horsepower. No back pressure causes a loss of low end torque. Some back pressure is required. Your goal is to increase the flow, without robbing the engine of the power gain.

An exhaust that creates too much back pressure is called a Restrictive Exhaust. Most OEM exhaust are build as a restrictive type with cost and gas ecomony in mind. A restrictive exhaust hampers performance in two ways. One is that the engine has to work harder to push out the exhaust gases from the cylinders. Second is the engine has left over gases in the camber which then get diluted with new incoming air and fuel mixtures. This can help gas mileage, but performance suffers.

Yet, backpressure is needed to remove gases from the engine. Back pressure creates something of a suction to help remove spent gases when it isn't too restrictive. Too much backpressure and it can't all escape, too little and the engine will pull it back in through the exhaust valves when open. It's a balance your trying to achieve as you increase the power of the engine.

It is usually associated with a popping or burble noise coming from the exhaust, not a backfire. You should expect properly tuned engine to burble or pop when the throttle is close from a high RPM. Think of a top fuel dragster and how the engine at idle makes the popping noise. That engine is creating a lot of backpressure. That is also the reason for the short open headers and with the remaining small amount of back pressure the gases are removed easily. If your car is making loud pops under hard accleration/decleration then that is backfiring.

Backfiring is not a good thing. We've all heard of a car being called a fart can. It's due to the unburnt fuel getting into the exhaust system and then it is being ignited. Commonly it's a small air leak in the exhaust system connections. It can also be due to a lean air fuel mixture or it could even be related to the ignition system. The ignition sequence could be off, or the plug wires worn, and the coils themselves could be bad.

If you plan to upgrade your exhaust then you should look for a system that is .25 to .5 inches larger than the existing stock exhaust piping. This will give you the best increase. Also to consider is what RPM will your car or truck operate at, most of the time. A exhaust pipe with a smaller diameter will produce more low to mid range RPM torque, where as a larger pipe diameter will increase mid to high range RPM torque.

[Dacious]

There is a natural restriction in any tube, even a plain open pipe. In order for exhaust gas to escape it has to push the gas in the tube out of the way.

The answer is the engine is tuned by the maker or tuner for a certain backpressure in every revrange from idle, part throttle, to WOT, so changing exhaust type without retuning it will cost it somewhere. It might be more efficient at WOT, but may run too rich or lean down low. Or vice-versa.

Any car with a street legal, full length underbody exhaust system really has too long a tailpipe for best efficiency - why you see race cars with an exit under the side.

You can tune a motor for zero backpressure - but it may not develop it's optimum torque, hp, fuel economy or emissions. There's a complex interplay between cam lift/duration, bore and stroke and revs. Not to mention flowrates at the intake and exhaust ports. And all engine designs of a given bore/stroke are not equal.

[dyno junkie]

The size and length of the collector is what determines the power/torque output of a given engine combination.
After the gases have been scavenged out of the engine by the
extractors/manifolds the size of the exhaust is not that important.

If you dyno a car through open extractors or cats, with the O2 sensors in place, that's the maximum power that it will make with that combination.

You then build a cat back system that does not reduce this measured power, i.e. if twin 2 1/2" makes the same power as no system in place this may be the best size for the system, you would therefore not get any gain going to a twin 3". However if you fit a twin 2 1/4" system, that is of the same configuration as the other two systems and power improves this indicates that possibly there is a problem with the size of the collectors, i.e. it is too big and is not scavenging correctly.

An examble of this is a set of 4 into 1 extracors with 3 1/2" collectors on a standard LS1 engine.
This is too big and going from a small system such as a twin 2 1/4", with a X pipe that is acting as the end of the extractors collector and helping the scavenging effect, up in size to a twin 3" system will have the effect of reducing power. Most people would wrongly assume that the larger system is the cause of the power loss, however all it has done is unmask a pre-existing fault in the engine combination.

[Covert]

Bigger isn't aways better. Dispite what you may have heard having a 3" cat back pipe on a 4 cyl Honda isn't a good idea, unless it's making over 300hp. If the pipe diameter is too large you then run the risk of losing horsepower. No back pressure causes a loss of low end torque. Some back pressure is required. Your goal is to increase the flow, without robbing the engine of the power gain.

An exhaust that creates too much back pressure is called a Restrictive Exhaust. Most OEM exhaust are build as a restrictive type with cost and gas ecomony in mind. A restrictive exhaust hampers performance in two ways. One is that the engine has to work harder to push out the exhaust gases from the cylinders. Second is the engine has left over gases in the camber which then get diluted with new incoming air and fuel mixtures. This can help gas mileage, but performance suffers.

Yet, backpressure is needed to remove gases from the engine. Back pressure creates something of a suction to help remove spent gases when it isn't too restrictive. Too much backpressure and it can't all escape, too little and the engine will pull it back in through the exhaust valves when open. It's a balance your trying to achieve as you increase the power of the engine.

It is usually associated with a popping or burble noise coming from the exhaust, not a backfire. You should expect properly tuned engine to burble or pop when the throttle is close from a high RPM. Think of a top fuel dragster and how the engine at idle makes the popping noise. That engine is creating a lot of backpressure. That is also the reason for the short open headers and with the remaining small amount of back pressure the gases are removed easily. If your car is making loud pops under hard accleration/decleration then that is backfiring.

Backfiring is not a good thing. We've all heard of a car being called a fart can. It's due to the unburnt fuel getting into the exhaust system and then it is being ignited. Commonly it's a small air leak in the exhaust system connections. It can also be due to a lean air fuel mixture or it could even be related to the ignition system. The ignition sequence could be off, or the plug wires worn, and the coils themselves could be bad.

If you plan to upgrade your exhaust then you should look for a system that is .25 to .5 inches larger than the existing stock exhaust piping. This will give you the best increase. Also to consider is what RPM will your car or truck operate at, most of the time. A exhaust pipe with a smaller diameter will produce more low to mid range RPM torque, where as a larger pipe diameter will increase mid to high range RPM torque.

:rofl: You should have let us known you cut and pasted that from a seller of cheesy car parts on ebay.

[macca33]

Maybe it's his site...:hide:

Cheers

[Ned_Flanders666]

Maybe it's his site...:hide:

Cheers

Lol,

I agree with Covert. He could have at least referenced (cut and paste) where he got it from.

[ringram]

Some guys on here are talking total BS.
Zero back pressure is desired.
What has been alluded too correctly however is optimal primary pipe and header design. This creates pulses of pressure which "echo" back to the port when they hit the collector, or infact the resonator, these low pressure pulses help pull intake charge into the engine and clear out the rest of the exhaust gas. Thats how a header works. If you go too large on the rest of the system you gain nothing, lose some velocity and cost more with more noise. So you need to figure out the most cost effective size and buy that.

If you want to read something more useful than green eggs and ham on the topic take a look here http://www.popularhotrodding.com/enginemasters/articles/hardcore/0505em_exh/index.html

There are forumla's there for calculating pipe and exhaust diameter.
I think I figured out dual 2.5 is good for up to 550fwhp and dual 3" around 720fwhp. So unless you have something more than heads and cam the dual 2.5 with suitable primary header diameter is best.