Boostaholic

iTrader: (4)

Can someone help me understand the relationship between backpressure and torque?

Also, I don't understand why a motor under load at the same rpm would generate more exhaust gas than a motor not under load?

dscott

I'll take a stab, but someone else will have to tell you if I'm right or not.

Very simple example:
Underload: on the highway - floor it.
Not under load: sitting in garage in neutral (being an *** to the neighbors) - floor it.

under load, the motor is working harder, dumping more fuel into the combustion chamber - more exhaust gasses.

Not under load, it doesnt take much fuel at all to turn a free spinning motor - less fuel and gasses.

That has to be a step in the right direction, isnt it?

Boostaholic

iTrader: (4)

The fuel amount should be based on the air going through the maf I would think, and the intake stroke should suck the same amount of air under load or not

cyclerage

iTrader: (59)

Well for 1, you would only have to hold your throttle open lets say 2% for the motor to run at 2500rpm in neutral, now depending on the load, you could be holding you throttle open 30% to achieve 2500rpms. The more you open your throttle, the more air/fuel will be alowed into the motor and once burned(expaned) leaves you with more exhaust. Thats why a car is louder driving at full throttle that if you rev it in neutral.

crainholio

iTrader: (2)

Now if you want to get deeper into this, and understand why engines tend to lose low-rpm torque when you reduce exhaust backpressure, go read "Scientific Design of Exhaust and Intake Systems". It's all about cylinder filling and how you use the energy of the exhaust impulse.

Real page-turner!!

crainholio

iTrader: (2)

...which explains why there's a throttle butterfly to manage the amount of air that is allowed into the intake, and therefore into the cylinders.

Put a load on the engine, and you have to open the throttle more to maintain the same RPM. More throttle means more air/fuel being allowed into the cylinders, which means a larger quantity of reagents being combusted, which results in more exhaust products.

Seriously, drop the $20 or so and read the book I mentioned previously. It's great stuff.

Pro Stock John

iTrader: (34)

My loose view on this stuff is based on dyno info.

The more backpressure you have the more the tq will go up but the hp will go down.

MNR-0

Backpressure is often a misunderstood term. For high RPM applications you want minimal backpressure. For street applications you want some backpressure.

Backpressure to some is nothing more than a restriction in the exhaust. To me it means increased velocities at lower RPM which aids the extraction of exhaust gases via a vacuum effect. All exhausts flow and all exhausts create a vacuum after the exhaust pulse - its the exhaust design that determines how use of those exhaust pulses are optimised.

A smaller exhaust will provide increased velocities at lower RPM and so will generate more torque in the stock power range. A larger exhaust will favour the upper RPM range at the cost of the lower register.

As a general rule of thumb, the best all-rounder is 2 1/4" on an unopened 5.7l. A stroker will favour 2.5" and turbos/SC 3".

hc_performance

iTrader: (4)

I would say more like 2 1/2" on stock or some minor bolt-on's and 3" for heads, cam, strokers or Superchargers. And for me at least 3 1/2" to 4" for turbo applications. Hard to flow too much air for turbo's. But that's just me.

Another_User

But if the cause of the backpressure also causes poor flow it doesn't really help, for example; non-mandrel-bent pipes.

SouthFL.02.SS

Usual scenario on our cars.
Cutout closed (more backpressure): Less peak TQ, but TQ available at lower RPM.
Cutout open (less backpressure): Higher peak TQ, but at a much higher RPM.

Boostaholic

iTrader: (4)

Some good info here, thanks for the input.