MPG optimised header ideas

[Nigel_S]

Originally Posted by Madact

So, another question comes to mind... can a ram horn header only get away with this much bend because the 'ram horn' itself is a constant, steady turn? Or is having straight sections between the bends a good thing?

My understanding is that you are a fair bit better off with long straights and fairly sharp 90 degree bends. If nothing else, it is easier to get the correct length since the flow will not go around the centre line of all those varying angle bends, but I think it is better for maintaining the gas speed too.

What is the reason for the megaphone collector when it is going into a cat anyway? Do the lengths change when using a megaphone since the gas will be slowing?

Why did Honda put the engine backwards? With my MG it is naturally the correct length since it starts off in the wrong direction and then comes back under the engine, you just need 2 90 degree bends plus straight pipe and the Ys, although it is hard to fit the flexi if you go for the version with long primaries.

[Madact]

Originally Posted by Nigel_S

What is the reason for the megaphone collector when it is going into a cat anyway? Do the lengths change when using a megaphone since the gas will be slowing?

Now that you mention it.. Looking a bit closer into megaphones, they seem to be used in two ways:
1) If used without an exhaust pipe (race applications) it's used to substitute for a tuned exhaust length, for a short outlet to the air
2) If used with an exhaust pipe *without* a cat, it can be used to decouple the header from a 'wrong length' exhaust which would cause detrimental resonance

So AFAICT there's little benefit if either your exhaust is the right length, or you're using a cat. Ricer points maybe

Eliminating the megaphone takes the end assembly from
16" megaphone + 2" for step down / flange = 18"
to
3" for the end of the Y merge + 4" flex pipe (will have to check how much length a 4" flex *actually* adds...) + 1" flange fitting = 8"
... which will definitely make things easier - thanks!

Originally Posted by Nigel_S

Why did Honda put the engine backwards? With my MG it is naturally the correct length since it starts off in the wrong direction and then comes back under the engine, you just need 2 90 degree bends plus straight pipe and the Ys, although it is hard to fit the flexi if you go for the version with long primaries.

The D series engine in the civic has the exhaust ports at the front - the shortest / natural header layout (down & back) has about 37-39" (depending on cylinder you measure) from the exhaust valve to the cat flange, which is no problem and even allows a little 'wiggle room' in the header design.

The K swap is when a K series engine (which are larger displacement than the D series) is swapped into the car (which isn't an OEM configuration) - this engine, fitted into a 6th gen civic, has the exhaust ports at the back instead, hence the odd length problem that the k-tuned ram horn header addresses. (NB I don't plan on doing a K engine swap )

[Nigel_S]

Originally Posted by Madact

+ 4" flex pipe (will have to check how much length a 4" flex *actually* adds...) + 1" flange fitting = 8"

Not sure how much you know about flex pipes but some advice if you don't know:

If you have the room then I suggest a 6" one, I fitted a 6" to mine and it's a nice length, gives decent flexibility and stretchability, the 4" work but they are a lot less flexible.

"would be flex tube with an interlock liner." - mine is an interlocked one in full stainless, far superior to the standard corrugated ones, the inside is essentially smooth and the correct diameter throughout - no need to worry about it affecting the results.

[Madact]

Slightly off-topic, but... whoa. Here's some skill in action.

https://www.youtube.com/watch?v=v8mMQbEkr8w

[iveyjh]

That looks like my shop!!!

[Madact]

Originally Posted by Nigel_S

My understanding is that you are a fair bit better off with long straights and fairly sharp 90 degree bends. If nothing else, it is easier to get the correct length since the flow will not go around the centre line of all those varying angle bends, but I think it is better for maintaining the gas speed too.

Found this reference Bends, flows and pressure drop in single phase flow

It's for steady flow though, of course, but it implies that, for a steady-state flow at least, wider curves are always better than narrower ones if you have the length to spare (and boy, do I have the length to spare...). Also, looking at the bend loss coefficient graph, it looks like "once you start a turn you may as well keep going" - the coefficient for 180 degrees, for example, is always less than twice the coefficient for 90 degrees. If you *need* to turn 180 degrees, a 180 degree bend is definitely better than 2 90 degree bends with a straight section, if the pipe length in both scenarios is equal... this implies that a continuous ram horn bend would also be better than a 'trumpet' style arrangement.



Also, odd mathematical thing, but for a given angle of bend, the difference between the length of the inner edge and the length of the outer edge is a constant regardless how sharp it is... consider a circle of diameter D (the inner edge) and a circle of diameter D+Pd (the outer edge - Pd is the pipe diameter). The difference in circumference is pi*(D+Pd) - pi*D = pi*(D+Pd-D) = pi*Pd.

Considering the limits, an "instant" 90 degree bend would intuitively be a worst-case scenario, regardless of whether we're considering pulses or steady flow, while a sufficiently wide bend is indistinguishable from straight pipe. Again intuitively, if we increase the radius starting from zero, things will get better for a bit. Now, it's possible that
(a) there's a region where things improve as radius increases
(b) if (a) is true, there may be a radius that flows [i]better[i] than straight
but (b) seems implausible and (a) seems unlikely given we know what happens in the limits. Also, if (a) were true, there would be a 'local minima' ideal radius somewhere that was better than a radius which was a bit smaller or a bit bigger, if that were the case I suspect it would get a notable mention in physics classes and fluid dynamic texts

Could be wrong about this, and dynamic behaviour of pulses is complex enough to possibly invalidate some piece of reasoning here, but I think it's correct.

Now, there *is* a tradeoff in the steady state here as per the graph if you're trying to optimise for least resistance and don't care about length - if you can save some pipe by doing smaller radius bends, it's probably worth it, though from the graph, the region where bend radius < 1.5D is a region you don't want to be.

[Madact]

So, the 'traditional' style ram horns don't have anywhere to go (as you can see, they project waaay too far in all directions), but I've come up with a much 'swoopier' layout that seems to do the trick - I think this one might be the winner similar total turn as a ramshorn (i.e. 180 degrees more than the minimum possible), but with much gentler curves...

[Madact]

And just a quick update - based on some more internet research and talking with an actual mechanic, I'm revising the 'normal operation' temperature ranges down to approx. 950F for low rpms (VTEC-e ranges), 1100F for mid-range rpms, and 1300F for high rpms (power VTEC ranges). The magic number which fits in the overlap of operating regimes for my engine with these temperatures is 25.5" (or 51" for the 'long' system). Hopefully a more accurate estimation, and helpfully shaves off a few inches

[Madact]

Some curly bisimoto headers for inspiration

[freethink]

Originally Posted by Old Mechanic

I'm amazed that no header manufacturer has not built or mass produced a tubing header for the VX. I'm not sure but think I read here that the CX uses the same manifold design. With the onset of cracking issues and the cost of factory replacements, close to $500, it would seem that there would be a market for a tubing header type aftermarket replacement.

The only rational reason for not producing one is that there just aren't that many of them (original CXs and VXs left to justify the start up costs, but even that makes little sense when all you need to do is get the intial setup for the computer to bend the components and weld them together.

Is there something I'm missing here?

regards
mech

There are tons of cheap D-series tubular headers available on ebay that will fit the VX. You might need a DX cat and midpipe to make it fit though.