[freethink]

Originally Posted by Madact

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.



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...



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

My ebay header primaries measure 1.66" or 42mm.

Let me clarify the purpose of 'ram horn' headers
1) to make the primaries equal in length. Again, this gives best peak power, but reduced low-end is a consequence of this.
2) to keep a turbocharger oriented in the OEM position, so OEM oil/coolant lines can be retained, along with use of commonly available exhaust down pipes.
3) underhood space limitations.

I would say 'ram horn' headers do not have minimal bends. They have a lot more bends actually and obviously. Perhaps the bends are smoother, but none the less, there are more bends. I would also say the only exhaust system component used to tune resonance is the header. The rest of the components have different purposes. The cat and muffler are for emissions, and the rest is just conduit for transporting exhaust gases to the rear of the vehicle.

I would happen agree with you about there being scavenging in a turbocharged vehicle. Just because there is more positive pressure between the piston and turbine blades doesn't mean there is no scavenging effect. The exhaust pulses are still moving at very high speeds. Remember turbochargers spin at speeds of up to 150,000 rpms. Exhaust gas still has mass and therefore enertia.

As far as D-shaped primaries go, i wouldn't go crazy, but perhaps i might try making the inside radius form slightly wider and slightly oval shaped. Perhaps using a little less sand might do the trick. I wouldn't use oversized pipe though; gas is compressible, so it's not necessary to keep cross-sectional area the same throughout the entire primary. Enertia will compress the gas to the outside of the bend allowing it to squeeze through the smaller area of the bend.

Getting a little off topic here, if you're interested in going further down the rabbits hole, take a look at a technique called Intake Velocity Porting. It involves making your intake ports smaller and more aerodynamic, giving you better volumetric efficiency throughout the whole rpm range. It was developed by a guy named "Motoman" for use on motorcycles, but the same principals apply to any 4 stroke engine.

Hope this helps. Looking forward to seeing a finished product.