Quote:
Originally Posted by theaudir8fan
Mike@GTM, most of what you say make sense, but i can't help to pose a few questions.
1, on calculation what you say makes sense, but under heat either iron or aluminum is extremely hot, so with the piping being so hot already the exhaust flow should have a minimal reduction in velocity considering the temperature of surrounding pipe is under extreme heat. Also wouldn't this issue be easily addressed by thermal coating and heat warping the said pipe which would prevent the loss of heat.
2, the exhaust pulses that you mention is true, but since the headers aren't equal length, as in the case with your twin turbo kit, wouldn't the pulse from cyl 1 interfere with pulse from cyl 3 as the pulse from cyl 1 travels down the manifold towards the turbo inlet while the pulse from cyl 3 is trying to exist the exhaust valve from the heads into the manifold itself. This logic would also apply to cyl 5 and right bank. So from the turbo point of view, the propeller would recieve one large pulse from the combination of pulses from cyl 1,3,5 and have a long pause until the next big pulse is generated.
Please note that I'm not trying to start a forum war, just trying to learn something, your reply would be greatly appreciated  |
I'll toss my 2 cents in and try to help out if I can.
In response to question #1: Thermal coating and wrappings are beneficial in keeping the heat where you want it. The problem is that tubing cannot hold the heat in as well as a cast manifold. Simple thermodynamics will prove that the pipe, given the material and wall thickness, will cool much more rapidly and will radiate heat at a greater rate than a cast pipe. The cast manifold has a greater density and can both insulate and maintain the temperature in a much more stable way. Cast manifolds can also handle the stresses imposed by heat cycling far greater than SS piping which can grow/stretch at different rates depending on pipe and weld quality. As long as the casting is done properly, the rate of growth and expansion can be predicted. Another point I would like to make is that thermal coatings are expensive by them self and heat wrappings are notorious for contributing to a higher rate of rust/corrosion.
Cost is also a HUGE, unmentioned (as far as I could tell) part of this equation. Building a proper set of manifolds whether tubular or cast is expensive from a production point of view. Creating a proper cast manifold like what GTM or Greddy uses is far more cost effective when pricing a kit out, not to mention reproducible time after time. Everyone has to make money and I am sure GTM and Greddy did their cost analysis to realize that doing a true tubular manifold would have probably priced their kits outside of an acceptable price point.
Even with jigs, fixtures, pre-bent sections and so on. Tubular manifolds are expensive to create and require a skilled welder/fabricator hours and hours of time welding. For sake of argument, even if a machine was employed to weld the hundreds of welds, the cost of the machine itself would have to be absorbed into the production costs of the manifolds themselves.
In response to question #2: There is way more science here than I can explain. The pulsing is not as severe as you make it sound. There is also something to be said about how the ports are designed in the manifolds and how each cylinder's exhaust gases are introduced into the exhaust stream. Smoother transitions and alignment will lessen the pulsing that you are referring to even in a log type manifold. Something else to keep in mind is that being equal length by itself IS NOT ENOUGH and the end of the equation in an equal length manifold design inorder to have proper scavenging. Other critical parts of this includes: pipe diameter and the way the collector is designed. A great "appearing" set of manifolds with an improperly designed collector or pipe diameter that is too large will null any gains made due to the fact that the exhaust stream will stall out and will not merge together efficiently.