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vtm666 Regular Location: Melbourne Registered: June 2004	Re: FS: Brand New 2 x Stainless Steel Cannons $130 each Sun, 05 December 2004 00:46 Autospeed wrote: 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! Julian Edgar editor@autospeed.com [Report message to a moderator]

	Subject	Poster	Date
	FS: Brand New 2 x Stainless Steel Cannons $130 each	vtm666	Mon, 27 September 2004 01:33
	Re: FS: 2 x Stainless Steel Cannons $130 each	Madm0nk	Mon, 27 September 2004 06:08
	Re: FS: 2 x Stainless Steel Cannons $130 each	vtm666	Mon, 27 September 2004 09:25
	Re: FS: 2 x Stainless Steel Cannons $130 each	dim51m	Tue, 28 September 2004 01:04
	Re: FS: 2 x Stainless Steel Cannons $130 each	vtm666	Tue, 28 September 2004 01:14
	Re: FS: 2 x Stainless Steel Cannons $130 each	driftymcgee	Tue, 28 September 2004 13:12
	Re: FS: 2 x Stainless Steel Cannons $130 each	vtm666	Tue, 28 September 2004 13:30
	Re: FS: 2 x Stainless Steel Cannons $130 each	vtm666	Wed, 29 September 2004 05:48
	Re: FS: 2 x Stainless Steel Cannons $130 each	vtm666	Thu, 30 September 2004 05:26
	Re: FS: 2 x Stainless Steel Cannons $130 each	driftymcgee	Fri, 01 October 2004 05:41
	Re: FS: 2 x Stainless Steel Cannons $130 each	vtm666	Fri, 01 October 2004 05:59
	Re: FS: 2 x Stainless Steel Cannons $130 each	vtm666	Sat, 02 October 2004 14:59
	Re: FS: 2 x Stainless Steel Cannons $130 each	vtm666	Thu, 07 October 2004 00:06
	Re: FS: Brand New 2 x Stainless Steel Cannons $130 each	vtm666	Fri, 08 October 2004 02:36
	Re: FS: Brand New 2 x Stainless Steel Cannons $130 each	vtm666	Sat, 09 October 2004 00:46
	Re: FS: Brand New 2 x Stainless Steel Cannons $130 each	vtm666	Thu, 14 October 2004 14:14
	Re: FS: Brand New 2 x Stainless Steel Cannons $130 each	justcallmefrank	Thu, 14 October 2004 14:15
	Re: FS: Brand New 2 x Stainless Steel Cannons $130 each	vtm666	Tue, 19 October 2004 12:23
	Re: FS: Brand New 2 x Stainless Steel Cannons $130 each	vtm666	Sat, 04 December 2004 11:20
	Re: FS: Brand New 2 x Stainless Steel Cannons $130 each	nick2b	Sat, 04 December 2004 23:11
	Re: FS: Brand New 2 x Stainless Steel Cannons $130 each	vtm666	Sun, 05 December 2004 00:31
	Re: FS: Brand New 2 x Stainless Steel Cannons $130 each	vtm666	Sun, 05 December 2004 00:46
	Re: FS: Brand New 2 x Stainless Steel Cannons $130 each	nick2b	Sun, 05 December 2004 02:03
	Re: FS: Brand New 2 x Stainless Steel Cannons $130 each	vtm666	Sun, 05 December 2004 02:37
	Re: FS: Brand New 2 x Stainless Steel Cannons $130 each	nick2b	Sun, 05 December 2004 02:53
	Re: FS: Brand New 2 x Stainless Steel Cannons $130 each	vtm666	Sun, 05 December 2004 03:10
	Re: FS: Brand New 2 x Stainless Steel Cannons $130 each	vtm666	Tue, 14 December 2004 12:19
	Re: FS: Brand New 2 x Stainless Steel Cannons $130 each	FKN16V	Wed, 15 December 2004 10:01

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