I have had different point of views on this subject.

With all things being equal, what is the effect of reducing back pressure by the use of headers, high flow cats or larger pipes.

Does it lean or richen the fuel mixture? How does the computer react to lower back pressure and what does it do?

Thanks,
TB

 

I am not sure how it affects your A/F ratio, but it will cause a noticable reduction in low-end torque, however it should add power up top.

 

That's interesting and I have noticed that but is that drop in torque caused by the richening of the fuel mixture or is it something else?

TB

 

I'm only going by what I was told and what makes sense to me:

At higher rpm's, the engine benefits from lower exhaust backpressure when pushing the spent exhaust gasses out the exhaust valve. It basically doesn't have to "push as hard" to get the spent fuel and gasses out.

At low rpm's, the piston doesn't need as much help to push out spent gasses, and actually some backpressure helps builds cylinder pressure on the compression stroke.

Or something like that...

I don't think it affects A/F. And the computer doesn't measure backpressure anyway, only A/F... I think.

Case in point, the only difference between my truck last year and this year is a new race tune, FTVB and high flow cats, and I've gained almost 3mph trap speed on comparable heat/humidity days at the same track vs. last year.

 

I have a question for those that say back pressure affects performance. Why dont top drag racers use the back pressure theory? If back pressure makes tq or hp why dont you see pro-stocks, dragters or funny cars with some back pressure device?

 

The reason you see devices like "Zoomies" and short headers on dragsters is to move the power band up in RPM. If you're making so much power on the launch that you can't get it to the ground, why not kill some of that low end torque and replace it with high RPM horsepower?

 

I gained low end torque with longtubes and hi flow cats.........go figure

 

I don't know, but I do know that the engine is simply an air pump, the more air that goes through it, the more power it makes.

 

<snip>Wrong screen name used</snip>

 

You can’t compare gasoline applications with nitromethane applications. Nitromethane does not burn as quickly as gasoline. In fact, there is not enough time to burn all of the nitromethane between when the spark plug fires and when the exhaust valve opens. So the engine is pumping still-burning nitromethane into the exhaust pipe. That's why you see flames shooting out of the exhaust of a drag-racing car. And at stoichiometric 1.7:1 air/fuel mixture (for nitro), the flame front measures 7,050 degrees F. With that data, trying to produce around 8,000 horsepower and getting it to the rear wheels in just 15/100ths of a second – I don’t think you want too much in the way.

Does that make sense?

--Steele

 

people who loose power when adding headers and highflows are the people who don't check their a/f after mods and get retuned if neccessary. I dont' see how people are loosing more low end torque because that where the headers seem to show more of a gain is under the peak of the curve. Headers tend to cause the compuer to richen the mixture from what i am told so if you are running richer than before, that could be where the power loss is.

 

That's my question. Has anybody confirmed this first hand and measured the difference in A/F? Why is that mixture richened due the the reduction in back pressure, if that's the case?

TB