Whats better? Why? How does it all work?

Just so you know I am asking for a subaru (coz I figure all the physics is the same in Japan as it is in Germany).

Is there a noticable difference in power or torque between the two? What sort of things should I look out for if I'm designing a new system?

BTW, the motor is around 2.5l, redlines at 6500rpm and puts out around 160hp stock (flywheel).

Exhaust gas like all things weighs something. When traveling through a pipe gas can have momentum, such that as it moves along it pulls along air behind it. Much the same as syphoning fuel from a fuel tank with a hose.

With merged collectors the idea is that as all the pipes come together at one point. As only one exhaust valve is opened at a time there is flow exiting through the pipes in a pulsing but steady stream.

Individually the momentum of the gas exiting the exhaust valve and travelling out the exhaust pipe also helps scavenge the combustin chamber for exhaust gasses, and at the same time due to camshaft overlap it also helps brin in the first part of the intake air/fuel mix. When these gasses traveling through pipes combine at a collector, the scavenging affect is multiplied at the collector.

At a certain engine speed this scavenging affect is at its peak, and it is at this RPM that the engine will produce its peak torque. If you match your camshaft to match that RPM your engine will be very efficent.

Tuning that RPM point is a combination of matching your carburation or throttle bodies with your intake and exhaust valves, head porting, camshaft and exhaust header size. Then you get into tuning the diameter of the merged collector.

This in part explains why one engine with brand X camshaft makes more power than another engine with the same camshaft. Some engine recipes work really well and others are just bad combinations that sort of work.

Based on what you've said with the engine being 2.5lt and redlining at 6500 i'd say 40mm or 1 5/8th inside diameter tube, with a merged collector starting at about 50mm (2"). At the end of the merge you want as little a restriction as possible, so a 2.5" muffler or better.

Cheers,

thanx mate. that's what I was wanting to know.

i assume the primaries need to be as close to equal length as possible. is that correct?

As close as possible but flat 4 engines are very hard to get the pipe exactly equal length. Also make sure that the pipes are in the firing order as they all meet into the collector in a clock wise direction. Prolly one of the first mistakes noticable on a lot of exhaust systems. But get it right and there is noticable power gains to be had:thumb

a subaru head out flows most others on the market.
for a honda or subaru we have been running 1 3/4 and 1 7/8 primary ,2 1/4 secondary with 2 1/2 and 3 'extractor's back.
dyno sheet's have been teling us these are some of the best comb's.cat's and muffler choice can make or brake a nice combo also.
some subaru and honda heads flow over 300 cfm standard.you can't suck it in if aint getting it out.
some subaru's come with 1 5/8 id standard.
the problem with the bigger diametre pipe is noise.
as for the equal length,there is no point having one straight pipe and the other one with 3 x 90' bends to match the length.some common sense must be used.as long as your not going nut's with the bends to match length,close enough will be fine.

Is this the same for turbo's? I've seen a lot of turbo's with unequal length pipes. Does the turbo also act like a vacuum pump and draw exhaust gas out of the chamber? Is this maybe why you can get away with it on a turbo?

so many questions.....

Turbo's work a lot diferently. But also, Yes in a perfect world you have match length headers for turbo use as well, but what is more important with turbo setups is getting the heat to the turbine as quickly as possible. Most cases this means short uneven lenghts.

The next factor is that there is always severe space limitations with turbo's so its a case of what you can get to fit most of the time, or you run the piping where it isn't going to burn the hell out of your paint work.

Some turbo headers will get that hot on high HP applications that the headers can become transparent. The header on my previous 2332 engine was wrapped in header tape. It actually has glass beads on it from where the header wrap was melted onto it.

Back pressure in exhaust systems from the turbine housing inducer to the head can be as much as double the intake boost pressure. There is no scavenging. This opens up some explanation into the diferences between turbo and NA grind cams, and overlap, but lets not go there.

Ideally you have as large as possible dump pipe off the exhaust side of the turbine as well. This pressure drop accross the turbine allows the hot gas to expand rapidly and spin the turbine up. Having too restrictive exhaust systems on turbo cars makes them laggy, and restricts top end horsepower. Thus the reason for those 4" exhaust systems you see on the jap imports, like my other toy.

Cheers,
Ian

[ Edited on 9-5-2007 by 2443TT ]