MPG optimised header ideas

[freethink]

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.

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.

Sharp bends are the enemy of a properly designed header.

[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. Unfortunately I haven't been able to track my mileage since the install due to speedometer problems. I did have to perform a small amount of porting in the flanges to correct the cheap manufacturing design, but definitely worth $46.
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.
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.

[ultimx]

I was waiting for someone to post bisi motos header. A main point to also think about is a good collector design. Burns stainless which I believe Neal posted comes to mind since u like tigging. Also slower speed also helps out for mpg, I decreased my speed by 5mph and gained 2 or 4 mpg, I can't remember over the same trip all things being equal. Also for tuning purposes, you might want to convert to obd1 as there are many tuning solutons out there, one cheap one being chrome. Or hondata for better option. Your initial design I was curious how you were going to close your hood but the last mockup looked promising. I eveuntally was going to swap the same 3 stage vtec engine into my car but other projects have put a hold on that. I cant wait to see your header design and final gains as to the design. Also, I didssee you mensioned using a adjustable fuel pressure regulator on your obd2 engine, you should try and get as close to 50-55psi on it as possible as Honda injectors atomize fuel the best at that pressure. That should be set if going back to obd1 and you can adjust the fuel tables to what you want in open loop as well as closed. Just fyi.

[Nigel_S]

Originally Posted by freethink

Sharp bends are the enemy of a properly designed header.

Sharp bends certainly hurt performance but you have to define "sharp"!

A 2D CLR bend is normally considered adequate to not hurt performance and the sharper the bends the longer the straights and I think long straights are good for pulling a vacuum behind the exhaust pulses. However I don't have any data or links to sources to prove that or say conclusively what's best so I'm not going to argue about it...

[Nigel_S]

Originally Posted by ultimx

A main point to also think about is a good collector design.

The angle the pipes join in the Y pieces is rpm dependent with Formula 1 exhausts having an unbelievably high angle, so for low rpm with long exhaust pulses you will want a very shallow angle.

[Nigel_S]

Originally Posted by Old Tele man

cos(Φ) = (r2/r1)^4

Evolution is more concerned with preventing blood clots than making efficient use of the inertial energy of gasses flowing at mach 0.5...

[adam728]

Originally Posted by Old Tele man

...true, but headers designed for *economy* certainly are NOT operating with gasses at mach 0.5 either.

Nor are they flowing thru flexible tubes, an incompressable fluid, etc.

I really fail to see how to compare the two.

[Nigel_S]

Originally Posted by Old Tele man

...true, but headers designed for *economy* certainly are NOT operating with gasses at mach 0.5 either.

If they are designed for economy then they should be, oversized tubes with slow gas flow definitely hurt performance except at very high revs.

[Nigel_S]

Originally Posted by adam728

Nor are they flowing thru flexible tubes, an incompressable fluid, etc.

I really fail to see how to compare the two.

Actually, a lot can be learned from studying what evolution has come up with in blood vessels and the reasons behind those decisions, but when you get into the details of join angles you have to remember that blood clots kill and evolution doesn't favour designs that increase the chances of death, so you need to understand the reasons behind the designs before you can decide if they are useful for other purposes.

[Madact]

Yikes, this thread really took off

Originally Posted by freethink

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.

Good suggestion, unfortunately it's illegal in many, many places to change the cat position. Luckily (a) I have a manual, Australian model with the cat under the car near the transmission and (b) the law here only prevents moving the position or affecting the operation of the cat, neither of which will be a problem (I'll be leaving the cat right where it is - which is why I was looking at rams horns - and quick warmup should be assured using stainless and/or coated primaries)

Originally Posted by ultimx

A main point to also think about is a good collector design. Burns stainless which I believe Neal posted comes to mind since u like tigging.

Originally Posted by Nigel_S

The angle the pipes join in the Y pieces is rpm dependent with Formula 1 exhausts having an unbelievably high angle, so for low rpm with long exhaust pulses you will want a very shallow angle.

Yep, I've been doing a bit of research there, and I'm thinking around 10 to 15 degrees (centrline angle, each side), and also having the incoming tubes take on a slight 'D' shaped profile on the approach rather than just being cut and welded, producing both a slight venturi effect (the cross sectional area decreases when you flatten one side) and a cleaner interface where the pipes meet.

Originally Posted by ultimx

Also for tuning purposes, you might want to convert to obd1 as there are many tuning solutons out there, one cheap one being chrome. Or hondata for better option. Also, I did see you mensioned using a adjustable fuel pressure regulator on your obd2 engine, you should try and get as close to 50-55psi on it as possible as Honda injectors atomize fuel the best at that pressure. That should be set if going back to obd1 and you can adjust the fuel tables to what you want in open loop as well as closed. Just fyi.

I think I'll be running the stock ECU etc. for regulatory purposes in the end, unfortunately. I'm hoping worst case, it runs a little lean in open-loop mode on account of the displacement - I think it should be fine in closed loop - though it may even have some learning feedback from the closed loop into the open-loop to compensate for various things, I'll just have to see how it goes at that stage.

Originally Posted by Nigel_S

Evolution is more concerned with preventing blood clots than making efficient use of the inertial energy of gasses flowing at mach 0.5...

Originally Posted by Old Tele man

...true, but headers designed for *economy* certainly are NOT operating with gasses at mach 0.5 either.

Originally Posted by adam728

Nor are they flowing thru flexible tubes, an incompressable fluid, etc.

No offence to Old Tele man (it's interesting stuff all the same), but I'm going to have to agree with Nigel and Adam here, on a few points:
1) The design criteria for an artery branch is very, very different from the design criteria for a collector merge.
2) The quoted formula seems to give results in the 50-70 degree range for the branch, which looks really wrong...
3) Flexible walls may make a difference, and while for most aerodynamics problems people on this forum are interested in we can assume incompressible flow (like blood, for instance... whoa, that sounds creepy), that's not the case for exhaust gasses.
4) All I've read so far indicates that approximately mach 0.3 (average) is a good target velocity for scavenging - much higher, Poiseuille’s law breaks down completely and transsonic effects & flow resistance start to choke the engine, much lower and you lose the inertial effects.
5) Unless the Reynolds number is similar, you can't use the same optimisation results even if the objectives are the same. Blood vessels are (from a quick web search) in the region of roughly Re = 2000, exhaust flow is (by some quick calculations) more like Re = 50,000 to 100,000. This puts blood flow near the upper limit for laminar flow (makes sense, as this is more efficient), while exhaust systems are very firmly in the turbulent regime, even if they weren't disturbed by valves, gaskets, bends and merges on their way out. The large difference means that a lot of the 'rules' change, and optimal geometries certainly go out the window when you have this kind of change of regime.