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back pressure equals torque?

Discussion in 'General Tech and Maintenance' started by thehare, Sep 24, 2004.

  1. thehare

    thehare Member

    Ok Im wanting to get a full exhaust made for my car. I want to put a 3 inch on it now so i wont have to do it later & pay for a smaller exhaust now & then pay again to put a bigger one on there. non turbo as of now. I have always been told that backpressure equals torque & that never made any sense so i did some searching and found this:

    Destroying a myth.

    Some say that "an engine needs backpressure to work correctly." Is this true?

    No. It would be more correct to say, "a perfectly stock engine that cannot adjust its fuel delivery needs backpressure to work correctly." This idea is a myth. As with all myths, however, there is a hint of fact with this one. Particularly, some people equate backpressure with torque, and others fear that too little backpressure will lead to valve burning.

    The first reason why people say "backpressure is good" is because they believe that increased backpressure by itself will increase torque, particularly with a stock exhaust manifold. Granted, some stock manifolds act somewhat like performance headers at low RPM, but these manifolds will exhibit poor performance at higher RPM. This, however does not automatically lead to the conclusion that backpressure produces more torque. The increase in torque is not due to backpressure, but to the effects of changes in fuel/air mixture, which will be described in more detail below.

    The other reason why people say "backpressure is good" is because they hear that cars (or motorcycles) that have had performance exhaust work done to them would then go on to burn exhaust valves. Now, it is true that such valve burning has occurred as a result of the exhaust mods, but it isn't due merely to a lack of backpressure.

    The internal combustion engine is a complex, dynamic collection of different systems working together to convert the stored power in gasoline into mechanical energy to push a car down the road. Anytime one of these systems are modified, that mod will also indirectly affect the other systems, as well.

    Now, valve burning occurs as a result of a very lean-burning engine. In order to achieve a theoretical optimal combustion, an engine needs 14.7 parts of oxygen by mass to 1 part of gasoline (again, by mass). This is referred to as a stochiometric (chemically correct) mixture, and is commonly referred to as a 14.7:1 mix. If an engine burns with less oxygen present (13:1, 12:1, etc...), it is said to run rich. Conversely, if the engine runs with more oxygen present (16:1, 17:1, etc...), it is said to run lean. Today's engines are designed to run at 14.7:1 for normally cruising, with rich mixtures on acceleration or warm-up, and lean mixtures while decelerating.

    Getting back to the discussion, the reason that exhaust valves burn is because the engine is burning lean. Normal engines will tolerate lean burning for a little bit, but not for sustained periods of time. The reason why the engine is burning lean to begin with is that the reduction in backpressure is causing more air to be drawn into the combustion chamber than before. Earlier cars (and motorcycles) with carburetion often could not adjust because of the way that backpressure caused air to flow backwards through the carburetor after the air already got loaded down with fuel, and caused the air to receive a second load of fuel. While a bad design, it was nonetheless used in a lot of vehicles. Once these vehicles received performance mods that reduced backpressure, they no longer had that double-loading effect, and then tended to burn valves because of the resulting over-lean condition. This, incidentally, also provides a basis for the "torque increase" seen if backpressure is maintained. As the fuel/air mixture becomes leaner, the resultant combustion will produce progressively less and less of the force needed to produce torque.

    Modern BMWs don't have to worry about the effects described above, because the DME (car's computer) that controls the engine will detect that the engine is burning leaner than before, and will adjust fuel injection to compensate. So, in effect, reducing backpressure really does two good things: The engine can use work otherwise spent pushing exhaust gas out the tailpipe to propel the car forward, and the engine breathes better. Of course, the DME's ability to adjust fuel injection is limited by the physical parameters of the injection system (such as injector maximum flow rate and fuel system pressure), but with exhaust backpressure reduction, these limits won't be reached.

    - Adapted from Thomas V.

    I read that & basically what i understood from it is when you put a exhaust with little backpressure on a stock motor it will basicaly run the car lean cause more air iis running through the motor so all you need to do is just get more fuel into the motor(i.e. turn up fuel pressure, get bigger injsctors, etc...) & it should not loose any torque. right? My motor is not stock either (h22a, skunk2 stage 2 cams, 75 wet-shot, cenerforce dual friction clutch, lightweight flywheel, LSD) So should i be good to turn up the fuel pressure or put bigger injectors on it & still come out with the same torque or what...
  2. handlebarsfsr

    handlebarsfsr Senior Member

    basically, theres 2 parts to the equation of airflow, both intake and exhaust- backpressure and velocity. backpressure is always bad. ideally an i/c engine wants no backpressure. however, as most people know, slapping a 3" exhaust on a stock motor generally hurts low end torque... this is thanks to our friend, velocity. air is a fluid, it has fluid properties. if you know what a syphon is, then thats exactly how the intake and exhaust colums of air work. the faster the column is moving, the more air is sucked in the intake using that vaccume, and the more is pulled out on the exhaust. ideally, you want very high velocity with very low backpressure. obviously this is pretty impossible to achieve across the rev range, as the airflow varies widely in its volume and velocity from idle to redline. choosing an exhaust/header/intake/ intake manifold/cams/head port that all work together to put that mix of velocity and backpressure where you want the maximum gain.
  3. phunky.buddha

    phunky.buddha Admin with a big stick Admin

    That's the key. Just go as large as possible before you run into delamination (turbulence) of the air stream through the exhaust.
  4. pissedoffsol

    pissedoffsol RETIRED

    all techi muble jumbo aside- just get the 3" now, and even if it makes a hair less power now, who gives a shit? right?

    onlything to be concerned with is that a 3" cat back won't fit yoru stock cat... youre gonna need a 3" cat....

    and then that sin't going to fit your header collector... and so on and so forth....
  5. cheese9988

    cheese9988 Senior Member


    If that is true, will certian things like bends in the exauhst of a bored out cat and such cause turbulence?
  6. phunky.buddha

    phunky.buddha Admin with a big stick Admin

    Always- but mandrel bends don't cause much of a problem at all. Barriers to flow like baffles and constrictions like crush bends cause more turbulence than a simple constant area bend.

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