back pressure and torque
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New User
Joined: Jan 2005
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back pressure and torque
Please explain to me about back pressure and torque.What is the best set-up for exhaust and air flow so i can keep my low-end torque for 4wheeling,as far as k&n air filters and cat-back exhaust,to whether or not to us headers,etc.All input is appreciated.Thanks
Lead Driver
Joined: Jul 2003
Posts: 7,406
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From: Wheatland, Mo
The more flow, the more power. If you allow more in, allow more out. K&N Air filters are nice, but better to go with the full air intake replacement so you get cooler air in. There are also other companies that make performance air systems, Air Raid, air hog, Just do a lookup on the net,
Junior User
Joined: Apr 2003
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ACtaully, it's more air OUT, then more air in. If you have spent combustion gases left in the chamber, you will never get a good Volumetric efficiency. You have to get the old gases out before you can get fresh air in.
Exhaust and then intake is typically the best order (unless the stock design is really restrictive...sometimes for noise)
All that said, some motors need a bit of back pressure to get power across the board. Mainly these are 2 stroke motors. Short of running straight pipes out the hood like draster, you want the least restictive exhaust you can get. This equates to high flow rate with less backpressure and lets the motor breath easier.
Torque is equal to force time the lever arm. a very simplistic and incomplete view: to increase torque you can either change the gearing to a shorter one in (3.05 to 3.55 or what not) use smaller diameter tires, or change out the crank for more throw (most motors today will not allow this due to clearance issue). OR, you can increase the force being applied to the piston. This can be accomplished with a chip (better tuning or Air to fuel ratio for more power), getting more of the burnt gases out of the combustion chamber, or getting more fresh air into the chamber (the extreme of this is a turbo or super charger).
One last note, you can change the peak torque of you engine and still not have a more powerful motor. You really want to look at the area under the torque curve over the rpm range of the motor. If you look at a street bike like a Ninja 600, it produces no torque until about 7000 rpms, and the curve is very peaky and rise and falls very fast, not something you want in a truck engine. Anyways, if you just more torque from one place on the curve and transfer it to another (which happens with some intake and exhaust mods) you really didn't gain any area under the torque curve, but if you moved it to a higher RPM (more to the right) then you increase your HP. (HP = torque X RPM/ 5252....so torque and HP are always equal at 5252 RPMs.)
Clear as mud?
Exhaust and then intake is typically the best order (unless the stock design is really restrictive...sometimes for noise)
All that said, some motors need a bit of back pressure to get power across the board. Mainly these are 2 stroke motors. Short of running straight pipes out the hood like draster, you want the least restictive exhaust you can get. This equates to high flow rate with less backpressure and lets the motor breath easier.
Torque is equal to force time the lever arm. a very simplistic and incomplete view: to increase torque you can either change the gearing to a shorter one in (3.05 to 3.55 or what not) use smaller diameter tires, or change out the crank for more throw (most motors today will not allow this due to clearance issue). OR, you can increase the force being applied to the piston. This can be accomplished with a chip (better tuning or Air to fuel ratio for more power), getting more of the burnt gases out of the combustion chamber, or getting more fresh air into the chamber (the extreme of this is a turbo or super charger).
One last note, you can change the peak torque of you engine and still not have a more powerful motor. You really want to look at the area under the torque curve over the rpm range of the motor. If you look at a street bike like a Ninja 600, it produces no torque until about 7000 rpms, and the curve is very peaky and rise and falls very fast, not something you want in a truck engine. Anyways, if you just more torque from one place on the curve and transfer it to another (which happens with some intake and exhaust mods) you really didn't gain any area under the torque curve, but if you moved it to a higher RPM (more to the right) then you increase your HP. (HP = torque X RPM/ 5252....so torque and HP are always equal at 5252 RPMs.)
Clear as mud?
Posting Guru
Joined: May 2002
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From: Chicago Area
Just like to add to the very good descriptions, that horsepower keeps increasing just with RPM's until the point that the torque is decreasing faster than the RPM is increasing. That's why the torque peak is always lower than the horsepower peak. And, while torque and horsepower happen to be numerically equal at 5252 RPM, without seeing the curves, its just an interesting tidbit.
Also, depending on valve timing, it is theoretically possible to have so little back pressure that the incoming charge is partially removed before combustion. That is why some have experienced a loss of low end "grunt" with a less restrictive exhaust. I have to assume that the factory engineers do a good job in matching the cam profile to the converter and muffler back pressure, but they have other design goals and restrictions than you may have personally.
Still, I would recommend that you get a "major" manufacturer's cat back or header system, rather than just trying to lower back pressure by guess. And you may have to replace the cams to get significant increases in power this way.
Also, depending on valve timing, it is theoretically possible to have so little back pressure that the incoming charge is partially removed before combustion. That is why some have experienced a loss of low end "grunt" with a less restrictive exhaust. I have to assume that the factory engineers do a good job in matching the cam profile to the converter and muffler back pressure, but they have other design goals and restrictions than you may have personally.
Still, I would recommend that you get a "major" manufacturer's cat back or header system, rather than just trying to lower back pressure by guess. And you may have to replace the cams to get significant increases in power this way.
Last edited by MrBSS; Jan 20, 2005 at 12:04 PM. Reason: addition
Senior User
Joined: Jun 2003
Posts: 174
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From: Idaho
Here is a lot of theory from one of the guys that does a lot of 5.0 Mustang custom ground cams. His other pages get a lot more into valve events, etc. than you probably want to know, but take a look at this one at least especially in regard to cylinder pressure.
http://www.wighat.com/fcr3/camtruth.htm
In my opinion, start small with the intake side and then move to the exhaust side, with maybe just a cat-back. I have seen more than one Mustang slow down at the track when too many of the exhaust side components were opened up. It kills the low (semi low, anyway) and midrange which is what gets the stock 5.0s down the track.
At the same time, opening the intake stuff up too much can kill the volocity of the incoming air and hurt low end as well. Its a balance. With an engine, you can't have power all over the rev range. If you increase it in one place, you typically decrease it somewhere else. You have to pick a range and work there.
Variable valve timing and Vtech and things of that nature work to help the situation, but without that, you are kind of stuck.
Remember, power is a result of torque which is a result of cylinder pressure over time. Or how much and for how long the force is applied to the piston at a given RPM.
http://www.wighat.com/fcr3/camtruth.htm
In my opinion, start small with the intake side and then move to the exhaust side, with maybe just a cat-back. I have seen more than one Mustang slow down at the track when too many of the exhaust side components were opened up. It kills the low (semi low, anyway) and midrange which is what gets the stock 5.0s down the track.
At the same time, opening the intake stuff up too much can kill the volocity of the incoming air and hurt low end as well. Its a balance. With an engine, you can't have power all over the rev range. If you increase it in one place, you typically decrease it somewhere else. You have to pick a range and work there.
Variable valve timing and Vtech and things of that nature work to help the situation, but without that, you are kind of stuck.
Remember, power is a result of torque which is a result of cylinder pressure over time. Or how much and for how long the force is applied to the piston at a given RPM.
