Toymods Car Club Inc: Tech & Conversions => Exhaust Myth... Fact or Fiction?

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oldcorollas
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January 2003

Re: Exhaust Myth... Fact or Fiction?	Wed, 09 February 2005 08:47

mm, i follow that, but i always think of it in terms of cam/s.. i bet the first time you heard that "engine needs backpressure" it was from a V8 guy who had a really big cam... the overlap of a cam works by allowing the intake and exhaust valves to be open at the same time. ideally, there is a low pressure pulse arriving at the exhaust valve (produced by length tuning of the manifold) as it is nearly closed, and this helps to, a) suck more waste gas from the cylinder, and b) lower the cylinder pressure when the intake valve opens. the problem is that a given overlap works best in a fairly small rpm range. too high rpm means that the overlap is not large enough, and there is not enough time for the exhaust to assist cylinder filling. too low rpm means that the intake and exhaust valves are open at the same time for too long, such that there is little benefit from the exhaust, and also allows the fresh intake charge to be sucked out through the exhaust valve... ideally, you want the exhaust negative pulse to hit the exhaust valve bwfore the intake valve opens, and a positive pulse to hit the exhaust valve _JUST_ as the fresh intake charge reaches the exhaust valve, as this maximises the benefit of the low exhaust pressure pulse, and minimises intake charge loss. now.. if you have a motor that revs to 5000rpm.. and a cam with 300deg duration, you have a serious problem. a cam like that is _meant+ to work at sayyy.. >5000 or >6000rpm. if you never reach that rpm, then you will always lose fresh charge out thru the exhaust, and thus you lose the benefit of the exhaust pressure depression. now, if you have a too large cam and you are losing fresh intake mixture out the exhaust, what do you do? of course you restrict the exhaust, such that you _INCREASE_ the pressure in the exhaust manifold, such that it is above the residual cylinder pressure, so that when the exhaust valve is open, the fresh charge will not escape as much past the exhaust valve. this will be effective at increasing power and economy, as you are keeping more of the air/fuel where it should be.. so basically the long and the short of it is, if you have a free flowing exhaust, you should choose your cam carefully so that it matches your desired rpm range of operation.... OR, if you have chosen too large a cam for your desired rpm range, fit a restrictive exhaust so that you keep more mixture in the cylinder. the former will give better power and torque overall, and the latter is a band-aid (elastoplast?) patch for a poor choice of camshaft so it is not really a myth.. it is based on experimental fact, but is is also based on poor engine design and to answer your next question: the reason you have an exhaust (apart from not burning your engine bay) is so that you can have a negative pressure pulse reflected by the exhaust manifold so that it reaches the exhaust valve at the correct time to suck more exhaust out of the cylinder. this occurs because when the positive pressure pulse (fromt he exhaust valve opening and letting exhaust into the manifold) reaches a junction, a negative pressure pulse is reflected bacwards. by choosing the correct configuration (ie 4-2-1, 4-1, 6-2-1 6-1 etc) AND the correct pipe lenghts from primaries, secondaries and collectors, you can increase the VE significantly (around 10%?) over an untuned exhaust.. the other aspect is that as well as 'good' rpom regions, there will also be 'bad' rpm regions where it performs worse (but not much worse over untuned). you can also tune for 1st reflection, or 2nd reflection etc... read "the scientific design of intake and exhaust systems" by P.Smith. Cya, Stewart [Updated on: Wed, 09 February 2005 08:52] [Report message to a moderator]

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