why is back pressure good?

i have a 81 kz1000 that has kerker exhaust on it. i want to remove the baffle from the muffler to make it louder. but i have heard that some back pressure is a good thing. i was woundering why that is? And what will be the difference if i do remove the baffle?

I doubt you'll notice any difference in performance. Exhaust is a whole mess of fluid dynamics.

If we just opened the valve to the air, it would work and be very loud.

If we add pipe, we can use the momentum of the prior exhaust pulse to help the next one along. Done properly things will be quieter and we reduce the pressure in the cylinder for the charge stroke. This momentum generating pipe is where folks get the idea that 'back pressure' is needed.

Once your head pipes hit the collector, the rest of the system is there to quiet, to couple the mass flow out the end of the pipe, and to give a pleasing tone. If the tone is not pleasing to you, you may remove the baffle with no ill effect on performance.

Well, here's my limited understanding of this:

Engines work by air flowing from the intake, through the piston chambers, and out the exhaust. In a non-turboed (read non-supercharger, NOS, etc...) engine, the air flows into the piston chamber via a vaccumm.

Here's what happens.

1. A door opens in the piston which leads to the air intake. As the piston moves there is a vaccumm in the chamber which must be filled. The only thing to fill it is the outside air.

2. The door to the air intake closes, and a door to the exhaust opens. At the same time a HUGE explosion happens in that chamber. Which things explode the expand. The only place for the expanded gas to go is out the exhaust door, so it does.

So technically, back pressure ISN'T a good thing. In the perfect solution you would have a constant flow of gases from your intake, through your pistons, and out your exhaust.

So I'm with you. I want to know why having a bit is good. Cause from my understanding of how the combustion engine works back pressure doesn't make any sense.

Sorry to differ Duck, but back pressure is needed for tourqe. When you have an open header that actaully increases horse power(top end), as the motor wil flow much more freely, but you will lose torque. HP= how fast you can go, Torque= how fast you can get there. You will most definitly notice a huge difference in take off speed. (torque) Try it, it won't hurt anything unless you try to use the motor to stop you(downshifting) You will hear loud pops. This is bad for the motor as you could break or bend the valves.. Also the size of the exhaust matters too.. If you have a car with 2" pipes and you want more horses, you free up the exhaust by putting on 2 1/2" pipe. You will lose some torque but if you have enough Cubic inches you won't notice it. If you have a smaller motor you will lose possibly enough torque that you can't even break the tires loose.. You can also go the other way by putting smaller pipes on to increase your torque, but you will loose valuable HP, (Top end) The motor can't breathe at high RPM's. That is why race cars have open headers, they are always running top end and they have the cubic displacement that torque is not as important.

Post edited by: vach, at: 2007/05/21 22:15

Duck wrote:

I doubt you'll notice any difference in performance. Exhaust is a whole mess of fluid dynamics.

If we just opened the valve to the air, it would work and be very loud.

If we add pipe, we can use the momentum of the prior exhaust pulse to help the next one along. Done properly things will be quieter and we reduce the pressure in the cylinder for the charge stroke. This momentum generating pipe is where folks get the idea that 'back pressure' is needed.

Once your head pipes hit the collector, the rest of the system is there to quiet, to couple the mass flow out the end of the pipe, and to give a pleasing tone. If the tone is not pleasing to you, you may remove the baffle with no ill effect on performance.

well said...

I believe also in there somewhere you need to keep the regular air temps away from the superheated exhaust valves because the sudden change in air temp which = cooling will burn a valve .

I assure you you will notice it. I do it all the time with my car and bike... I have a 283ci punched out to 301ci with 2 1/2" inch pipes, headers and flowmasters. The little motor is in a 66 Impala, a pretty heavy car. Not enough torque because of small ci displacment. I can crack the tires loose only with the mufflers on the car, (torque). When I take the mufflers off, the car won't crack the tires, but it screams on the upper end. My kz 650 same thang my friends.. Do some more research and you will find that restricting the exhaust in ANY way allows for more torque(back pressure). Opening it up in ANY way (reducing back pressure) gives you more upper end.. Garunteed. Just try it you will see.. It's more noticeable on smaller motors. If you have enough HP OR torque you my friends might not be able to tell the difference as to what is happening..:blink:

HP = Tq x rpm / 5252

So, increase Tq = increased HP.

Open headers shift the torque higher in the rpm band, thus the increase in HP.

More restrictive exhaust shifts the torque lower in the rpm band , thus less HP.

Torque is a measurement.
HP is a calculation.

Typical big block hot rod magazine build, 600 hp, 650 tq.

Typical high hp diesel truck motor, 600 hp 2100 tq.

HP is all in the rpm

Thanks for the formula Steel. I was trying to say that in an idiotic way.
I have bent valves running open headers using the motor to stop the car. The backfires are ignition before the valve is completly closed. You may or may not need heavy duty valves and springs. I learned the hard way. I wouldn't recomend letting the bike backfire when stopping with open headers as it is very stressful on valves and springs.. Sorry fellas, I had a very bad day..

I am running a Kerker with no baffle, I personally like it but it did cause a bog in the midrange. WiredGeorge gave me a lot of advice on jetting and clip position. It's all good now but it did require some changes. I love the sound so it was worth it to me.

Ron

pull the baffle out of the kerker and put end cap back on,best of both worlds;)

vach wrote:

The little motor is in a 66 Impala, a pretty heavy car.

I remember we used to call them "Lead Sleds" because of the weight but we still loved them. Got any pics of your little 66? My dad had a sweet little 62.My buddy "Slink" had a cool 66 Caprice SS/327 with bucket seats etc.

Post edited by: pumps, at: 2007/05/22 00:42