From an engineers perspective whats the connection between back pressure and torque. I hear all the time that an open exhuast will reduce torque but never understand the connection between the two.

 

an open exhaust will reduce torque because there is not as much exhaust velocity at low rpms, where torque numbers are huge. Open exhausts promote high hp, high rpm applications because they flow very well and have good exhaust velocity at high rpm.

 

They are inversely proportional.
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Well, not exactly, but close. You Googled it if you didn't already know the definition, right?

Unles something goes wrong with the mixture or timing when changing from a restrictive (high back pressure) exhaust system to a free flowing (low back pressure) system, torque and power go up when back pressure goes down. It works the other way around also.

BTW, if torque goes down at any rpm so does hp. If torque goes up, power goes up. They are directly proportional (if you throw in rpm and the constant of 5252).

I hope the relationship between torque and back pressure is intuitive to you.

Jon

 

I disagree with some parts of the above post.

An exhaust system that is extremely highflowing will not promote good torque, because at low rpm where you often find your peak torque, the exhaust gases are not flowing at optimal velocity. They are moving slower because there is not enough gas to fully fill the exhaust pipes. Am I explaining this right?

This is equivilent to a highflowing, short runner intake for high hp versus a long runner intake designed for high torque. The longer runners don't flow as much air and create turbulence, which create torque.

EDIT- SO, to an extent, a long exhaust with a little more back pressure can create more torque than a short exhaust.

 

I think the answer to the previous poster lies in the assumption made by Old SStroker. "Unles something goes wrong with the mixture or timing when changing from a restrictive ". This is one of those perfect world assumptions. And he is correct that anytime you increase torque (at a given rpm) you will increase hp (at that same rpm). The previous sentence is true in a static sense, but going to a dynamic view of what's going on, you need to take away the assumption that was made. BTW, I have no clue what the true answer is to the original question.

 

Maybe this will help. The exhaust valve begins and continues to open as the piston is moving downward through the last portion of the power stroke. A very low-restriction, free-flowing exhaust system will dump the pressure from combustion out into the atmosphere. This will happen anyway, but if the exhaust system is somewhat restrictive (and that is a key concept in optimizing the power and torque as old sstroker said) then the combustion pressure will continue to be harnessed longer by placing force on the piston. This results in more torque and HP.

Now, engines that are optimized for torque below 3,500 RPM's (let's say an 8.1 L in a Top Kick Truck that can pull a 30,000 pound load) may have smaller exhaust ports, smaller exhaust valves and exhaust valve events that occur relatively late (as compared to that same engine set up for a dragster).

Steve

 

Scavenging, not back pressure, creates torque. Back pressure decreases power.

 

Yes but different exhaust systems are designed to have a maximum scavenging effect at different RPMs.

A super high flowing exhaust will make less torque than a lesser flowing exhaust because its maximum scavenging effect occurs later in the rpm range.

 

exactly. no one needs backpressure.

 

Also to note:

Changing the length of the exhaust pipe will, to some degree, change the torque curve.

IIRC, the shorter the exhaust pipe (cutout in I-pipe) the higher the peak torque RPM, so it seems like you loose some low end, where in reality, it probably just pulls a little harder up top than it did stock.

Just my .02

 

For the love of god...if nothing else...just tell everyone you know that "back-pressure" is NEVER good for power.....If I hear one more person at a get together say, "I need more back pressure".. I'll go crazy!!

 

the new phrase will be "I need more scavenging!!"

 

ALWAYS, ALWAYS, ALWAYS.
Ideally you want ZERO back pressure ALWAYS. Except with a turbo, duh.
Smaller dia merely creates higher velocity, which improves scavenging.

 

i need more back pressure> looooooool

 

Please re-read post #7.

Thanks,
Marty

 

Why is everyone so dismissive of backpressure? OEM exhaust systems should be designed to create the most power in the RPM range where the customer will most visit.

This infers that the system will constrain the engine after some useable point and backpressure will be innate and important in any well designed street car. As street cars aren't "tached" out all the time.

 

This info on intake runners only tells part of the story;

Intake manifolds runner lengths are determined by a number of factors, including engine flow dynamics and the desired RPM band. Think of an engine as a giant air pump, with air resonating inside back and forth as it tries to make its way through the engine. If the air intake charge runs into a closed intake valve, it will be reflected back into the intake runner and travel back towards the other end of the runner. At the other end of the runner, it is reflected back towards the intake valve and the process repeats itself until the intake valve opens. Intake runner length design attempts to maximize this harmonic effect. You want the reflected air charge(s) to reach the intake valve just as the valve is opening. This provides a “ram air” effect of air and fuel into the combustion chamber.

The problem is that the optimum length of the intake runner varies, depending on where you want the power band to be. The diameter of the runners also comes into play-smaller diameter runners are good for low RPM torque because they create turbulence/better mixing. Larger diameter runners are better for high RPM power because they can flow more air overall.

Exhaust scavenging has many similarities, as the pulsating pressure wave is a factor in designing the proper size exhaust pipe for the desired RPM band and setup.

I agree that backpressure is a bad thing for internal combustion engines, but that doesn’t mean that other factors that effect exhaust scavenging and timing don’t come into play as well.

 

For the sake of supporting your argument, might you have done any testing of back pressure in any automobiles? If so, how, and what have you found.
For the sake of supporting my argument, I have, and know how much 'back pressure' there is in a 'street car'.
So, do tell, how much back pressure do you think they have? And at what RPMs. No speculating, just what you know to be fact.

 

OEM system are also designed for noise surpression.

20 PSI would be lucky.
1.5 PSI is considered optimal by the racer for the street car.

No reason to get shrewd.


http://www.sjdiscounttools.com/otc7215.html
http://alltiresupply.com/p-OTC-7215.html

I doubt 1.5 PSI is the max for a factory vehicle if gauges come for 0-8 PSI. Maybe for racing.

 

Not getting shrewd. Just wanna know if you've ever tested any.
There's virtually no measurable back pressure in a street car, in the lower rpms.
1.5 is not optimal, it is MAX. You want LESS THAN 1.5 at WOT.
Wow!! Almost missed the 20 psi. Thats crazy!! Maybe behind a turbo, but at anything near that, you've got a collapsed exhaust.
Anything over 1.5 psi, and you start loosing HP.