ok i didnt read most of the stuff that was written recently but this is why a little backpresure is good.

with piston engines the exhaust valve has to push inside the chamber which already has a great deal of pressure in it. backpressure sort of equalizes the pressure so that the engine doesnt have to work as hard to open them. of course once the valves are open, geater amount of airpressure inside the chamber thrusts out exhaust. this accompanied by the exhaust stroke of the piston actaully creates a short vaccum pulse which draws out most of the exhaust. as was mentioned before (i forget which magazine. but one did a test between just headers, the x pipe, and the h pipe. the motor put out around 15 more hp (i think) with the h and x than just straight pipe. keep in mind this engine was already putting out over 400bhp so gains would be much less on most engines.

rotary engines dont require backpressure because they have no valves to open.

i skimmed the last few posts so this may have no relavence, but turbos dont need back pressure on the outgoing side (this would slow down the turbine). but it can be slightly beneficial on a piston engine before it hits the turbine. (see above)

i never really delved into this matter too much so if there are minor mitakes in what i said feel free to correct me, but basically what i said is correct.

Justin

 

Ummmm no, the open motion of the valves are mechnically operated by the camshaft, and do not need any help via pressure opening or closing, the exhaust valve spring which closes the valve has open prssures in the rage of 200 lbs plus, the rating of the spring has more to do with keeping the follower or lifter stuck the profile of the cam lobe, not overcoming any kind of pressurein the manifold or the engine itself...Max

 

Have a read of this about backpressure. It was written by Julian Edgar, editor of Autospeed e-zine www.autospeed.com

Another furphy that has widespread currency is the concept that engines need back-pressure. Simply, there is no properly tuned engine where increasing exhaust back-pressure causes an improvement - in power, torque or fuel economy. One of the reasons that this idea has gained support is because when people change their exhaust they seldom check the air/fuel ratio or re-map the ignition timing to once again give optimal performance. For example, some MAP sensed cars drop substantially in power with a large exhaust fitted because they are then running lean.

Atmospherically inducted cars that use a tuned length system to improve cylinder scavenging (via extractors, for example) are sensitive to exhaust diameters within the tuned length part of the system. This means that the maximum effect of exhaust pulsing may come from an exhaust system that is small enough that some exhaust back-pressure is developed. However, that is a quite different concept to saying that engines "need" exhaust back-pressure! Turbocharged engines require as big an exhaust as possible, with the same applying for naturally aspirated cars once the tuned length part of the exhaust is passed.

Few tests have been done that clearly show the affect of changing back-pressure. Most muffler and exhaust comparison tests change more than one parameter simultaneously, making the identification of exhaust back-pressure as a culprit difficult. However, Wollongong (Australia) mechanic Kevin Davis is one who has done very extensive testing of varying back-pressure on a number of performance engines. These range from turbocharged Subaru Liberty [Legacy] RS flat fours to full-house traditional pushrod V8's. In not one case has he found any improvement in any engine performance parameter by increasing exhaust back-pressure!

The tests came about because Kevin has developed a patented variable flow exhaust that uses a butterfly within the exhaust pipe. He initially expected to use the system to cause some back-pressure at low loads "to help torque". However, he soon changed his mind when any increase in back-pressure proved to decrease torque (and therefore power at those revs) on a properly tuned engine! What increasing the back-pressure does do is dramatically quieten the exhaust.

One of the engine dyno tests carried out by Kevin was on warm 351 4V Cleveland V8. Following the extractors, he fitted a huge exhaust that gave a measured zero back-pressure. Torque peaked at 423 ft-lb at 4700 rpm, with power a rousing 441hp at 6300 rpm. He then dialled-in 1.5 psi back-pressure. Note that very few exhausts are capable of delivering such a low back-pressure on a road car. Even with this small amount of back-pressure, peak torque dropped by 4 per cent and peak power by 5 per cent. He then changed the butterfly position to give 2.5 psi back-pressure. Torque and power decreased again, both dropping by 7 per cent over having zero back-pressure!

And if you still believe that exhaust back-pressure improves performance, simply block off part of your exhaust outlet and see if your car goes any faster!

 

Those guys are most likely peddling 6" exhaust pipes to the general public, the dyno doesn't lie, if I can find that car craft mag from last year, I will scan and post it, it may suprise you but an atypical 350 chev built moderaltly lost power across the whole band once the exhaust got beyond 2.5 " in diameter, adding low restriction drag mufflers over street mufflers dropped performance. Check Borla's site, they made alot of 1/4 mile cars faster by adding mufflers..
Who knows what his "butterfly valve" did to flow and turbelence within the pipe. David Vizard has published numerous books and articles on exhaust system design, he has a come up with the number of 200/ft/sec for minimum for exhaust gas velocity, any less than that, and exhaust scavenging comes to a standstill as the there is no longer a low pressure wave following every exhaust pulse, the same happens when header primary tube size is increased..Max