Quote:
Originally Posted by freethink
I have a cheap ebay header on my DX, and i gained power all through the rpm range. The best part is the increase in low-end power due to the longer primary tubes. I can now drive at two to three hundred rpms lower without bogging the engine. Lower RPMs should theoretically equate to better MPG.
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Nice - cheaper the better, I've heard, as if you go cheap *enough* they start using realistically sized pipes instead of shiny oversized ones (that choke the engine by reducing gas velocity) "for the JDMs"
I'd love to know the diameter of your primaries - got a caliper handy?
Also, if the article on the new Mazda (earlier this thread) is reliable, you may also be able to safely increase your compression ratio (and hence efficiency) a bit, using the same grade fuel - might be worth considering a thinner head gasket if the head ever needs to come off.
Quote:
Originally Posted by freethink
I would stay away from "ram horn" headers, as they're meant to increase peak HP at the expense of low-end torque. Formula One engineers get away with using these because they keep the engines revved up, thus having no need for low-end power.
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The rams-horns I've seen do seem to be designed for that, but AFAICT that's not a feature of the rams-horn itself. I think there's two different reasons rams-horn headers get used (in racing or other contexts):
1) To allow the pipe length required for a tuned exhaust where 'total tailpipe' length is limited.
2) To allow feeding a turbo with a minimum of sharp bends, and also have enough primary length to reduce 'cross talk' between the cylinders.
On a side note, conventional wisdom seems to be "there's no scavenging effect in a turbo engine" - I happen to disagree
there's more back pressure from the turbo, but there's also more intake pressure from the compressor, so the in-cylinder dynamics will be pretty similar to NA apart from somewhat elevated pressure / density / viscosity, and turbo or no turbo, if you can get a bit more air-fuel mix into the cylinder, it should make more power. A rams horn eliminates sharp bends for a fairly decent down-pipe distance, and you should get at least some of the inertial part of the scavenging happening, if not the tuned-length effects you get on an NA engine...
Quote:
Originally Posted by freethink
If you're looking to dump tons of money and time into building a prototype MPG header, look into hydroformed head pipes. Gas flow dynamics are similar to Aerodynamics inside of bent pipes. Round is not always the best shape for gas flowing through a bend. Check out Micron motorcycle exhaust systems. They're engineers had a very good understanding of gas flow dynamics.
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I doubt I could get hydroforming consistent enough myself to be better than mandrel bends, and I'm not looking to drop enough money to pay someone who can
. I might give sand bending a try (the price is right
, and I like learning new things), but I think fabrication will be a welder, grinder, hammer and dremel job...
Those Micron 'serpent' systems look amazing though. The 'flat' bends in particular are interesting, as they should decrease or eliminate Dean vortices... however, that degree of flattening without doing bad things to the cross sectional area profile would take a
lot of precision.
I've been thinking about a slightly D shaped cross section - with the 'inner' side flattened slightly - but again, reliable fabrication is an issue. Sand bending around a rigid form might do this slightly, which would be one of those fortuitous side effects, but I'm not sure that amount would make a difference. Perhaps... using an oversized pipe and bending
without sand? Accurate preservation of cross sectional area would still be an issue. The most reliable way to do this cheaply I can think of would be to take a 'good' sand bend and use rollers to flatten it to spec - the rolling stage would also make the bend tighter, but you could compensate for that fairly easily by doing a few tests first.
All that said though, for my purposes I think I'll just go for nice consistent constant cross-section bends. I doubt the gains would be all that huge, and I'm better off keeping it simple
