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Old 11-20-2014, 04:18 AM   #21 (permalink)
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Originally Posted by Ecky View Post
Also, as was mentioned before, take a look at Mazda's design. They were advertising that a carefully designed header was crucial in letting them use extremely high compression.
Had a bit of a look at it (see autozine.org article)... basically the upshot seems to be that better scavenging allows higher compression ratios without knocking (very useful info! ). As I'm planning a mini-me head swap soon with the option of a mild compression ratio increase, this is very good to know, this might persuade me to actually build a header

The header geometry itself seems to be a standard 4-2-1 design, but the innovation seems to be that they've coiled the tube around the cat to warm it up faster, and also allow the longer length to fit in the engine bay given an exhaust-ports-at-the-back layout.



Now I'm dubious about the "warms the cat up quicker" claim, it seems to me that the amount of radiation / convection heating at startup would be tiny compared to direct exhaust flow, but maybe that extra few percent gets them over some EPA-mandated "is your cat this fast to warm up" line . In any case, I reckon just having a ceramic-coated SS manifold should be sufficient to do the same trick.


Last edited by Madact; 11-20-2014 at 04:38 AM..
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Old 11-20-2014, 07:17 AM   #22 (permalink)
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Originally Posted by Piotrsko View Post
Couple of notes: is there room for the length you'll need under the car? Probably the prime reason OEM still makes cast headers.
Well, I thought I might be able to get 80" before the cat... but I just did some measurements, and - bwahahahaNO. Based on not moving the cat, I have 18" from the cat flange to the centreline of the current bend, 15" from centreline of the exhaust up to the port centreline, 7" from port centreline to the top of the motor (which I'll call the max. vertical centreline for a ramshorn - there's room above but not much), 6" and change from the flange to the AC radiator, and 10" width to work with between the AC pump and where the gearbox bulges out - though I should be able to fit a pipe down the side pretty easily.

Straight pipes under the engine, with a bend up and then simple ramshorns give a max. pipe length to the cat. flange of about 18" (under the engine) + 15" (up to the level of the ports) + 4.5" (up to the start of the ramshorn) + 8" (180 degrees bend at 2.5" centreline radius) + 7" (90 degree bend at 4.5" radius) + 3" (start of the ramshorn to the port) = 55.5, give or take. About 26" short ah well, that's what drawing boards are for, huh?

Might be able to squeeze a bit more out with some clever routing, of course. Or I could just loop everything around like a tuba, would make trying variable geometry easier I guess

Good news though, I measured the exhaust ports on a D head, they're 39mm x 28mm with a spacing of 84.5 (roughly - I wouldn't advise setting that running in a CNC mill ). Which gives a port cross sectional area of 989mm, equivalent to a 1.4" pipe. So 1.375" primaries might work, with careful port matching on the header side. Also 1.625" OD tubing can be used for primaries and still leave room to jam an equal diameter pipe between them, which opens up possibilities.

Last edited by Madact; 11-20-2014 at 08:54 AM..
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Old 11-20-2014, 09:26 AM   #23 (permalink)
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Originally Posted by Madact View Post
So, the trouble with primary diameter… the actual exhaust ports on the engine are oval 1.75” x 1.3125”, which is similar in area to a 1.5” ID tube - the above equations suggest anything over this is a waste of time and ideally smaller would be good, but we don’t want a ‘step’ in the wring direction, so 1.5” ID it is.

The primary length to the first set of Ys (our ‘P1’) is then the ‘P’ from the 5800 rpm calculation - 32”. Of course, many sources are quite insistent on the ’15 inch’ thing, and it would be nice to take advantage of anti-reversion effects, so we may as well put a step there, out to 1.625” ID, which is the next standard tube size.
Why do you not want a step in the "wrong direction"? I think it is only going to be a problem if you have long duration race cams...

A 32" primary is excessive and will miss out on some wave effects which could give extra scavenging between pipes, I think that should be the entire length of the header - a lot of people who write up and explain these equations, including professionals, don't understand them!

15 inch is about right depending on the speed of the exhaust flow down the pipes, going a few inches either way will just change the rpm at which you get best effect.

The most important decision is the pipe diameter, this controls the speed at which the exhaust gasses flow down the pipe, the thinner the pipe the faster they go and the faster they go the more inertia they have and the bigger the vacuum they can then pull behind them when the cylinder empties which in turn means more exhaust gas gets pulled out of the cylinder leaving less work for the piston to do in pushing out the remaining exhaust gas. If the piston does work to remove the exhaust gasses then that uses fuel, if the exhaust gasses do it then it is done for free. Essentially you want the pipes as thin as possible without them being so thin that they block the flow, the target speed is normally half the speed of sound, taking into account that the speed of sound is a lot faster at exhaust gas temperature, the reason people try to keep the exhaust gasses hot is to maintain a consistent speed of sound, although most people don't realise that. Once you get passed the half the speed of sound the exhaust gasses start having difficulty flowing and that gets worse as you approach the sound barrier.

If you want it to work well at low throttle then you need small diameter pipes to keep the gas flow speed up, the pipe lengths should then match those of a race header as the timings of flow and wave reflections will all be the same. You just need to make sure that the pipes are big enough for the flow to remain not excessive at full throttle and full rpm.

At least that is how I see it, others understand it in other ways...
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Old 11-20-2014, 11:21 AM   #24 (permalink)
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Nigel, thats some good information! Its got to be tough to calculate the speed of sound in a medium that is shedding heat and becoming more dense as it travels through the tube. On my Beetle header I loose so much heat before it gets to my turbo its hard to get it to build its full boost potential. It takes tens of seconds under acceleration to build the header heat for the turbo to convert to the last 4 lbs of boost. I have Jetcoated headers but I'm going to wrap them and put aluminum around that to keep oil off of the wrap. So many variables to contend with. You have the amount of fuel being consumed adding changing temps in the exhaust flow and the temps already soaked into the header and the speed of the airflow around the header pulling heat out too.
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Old 11-20-2014, 12:05 PM   #25 (permalink)
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If you are going to venture into header design, do yourself a favor and purchase Pipe Max ( PipeMax36xp2 ). I am unaffiliated, but it is highly regarded at Speed Talk forums, by guys that live and breathe this stuff. From dirt track to Nascar to 4000 hp pro drag cars, there's a lot of big engine builders on there that know there stuff, and trust the software.

Header design is a lot more than target an rpm and pick a diameter/length. Everything from camshaft timing & overlap, to EGT and engine displacement effect what the "best" exhaust is. Collector design is very important as well, be it a straight diameter/length, or getting into merge collectors with taper and volume coming largely into play.


And yes, you don't want to step down to a smaller area than the exhaust port. However, it's not completely unheard of to taper down to a smaller primary to keep velocities up, or form the tube end to fit inside the exhaust port to reduce cross sectional area. A step will cause an unwanted reflection wave back into the port, but a proper taper (<7 deg) will not cause issue. In most cases primaries are larger than the exhaust port, and a step in that direction will not cause so much turbulance, and will help break up the reflected wave from the end of the primary, which can fight low rpm reversion on engines with cam timing favoring higher revs.


The more I try and learn about header and intake design, the more I realize I know nothing. And getting it "right" the 1st time, without a dyno, is a pipe dream. Not saying you can't do something to make improve over stock, at least on older cars. But there's a reason GM will run hundreds of simulations before ever making a part, then test a few dozen different iterations.


Found this one interesting, primary "venturis". Supposably spread the torque curve out, and came in at a lower rpm, with no loss of top end power.
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Old 11-20-2014, 12:12 PM   #26 (permalink)
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Quote:
Originally Posted by kafer65 View Post
Nigel, thats some good information! Its got to be tough to calculate the speed of sound in a medium that is shedding heat and becoming more dense as it travels through the tube. On my Beetle header I loose so much heat before it gets to my turbo its hard to get it to build its full boost potential. It takes tens of seconds under acceleration to build the header heat for the turbo to convert to the last 4 lbs of boost. I have Jetcoated headers but I'm going to wrap them and put aluminum around that to keep oil off of the wrap. So many variables to contend with. You have the amount of fuel being consumed adding changing temps in the exhaust flow and the temps already soaked into the header and the speed of the airflow around the header pulling heat out too.
Well if you use the energy in the exhaust gasses to drive a turbo then the energy isn't available for extraction of the remaining exhaust gasses so the piston has to do the work instead using extra fuel. That's why a turbo engine of the same capacity as a NASP has lower fuel efficiency. It means that the design of the headers is not so important since all you are really interested in is driving the turbo sufficiently to give the boost while causing as little backpressure and thus extra work for the pistons as possible. Sounds like yours is a semi-turbo - trying to do both!
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Old 11-20-2014, 12:15 PM   #27 (permalink)
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Many thanks Nigel, you've brought up a bunch of stuff I hadn't really considered, and spurred thoughts about a bunch more

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Originally Posted by Nigel_S View Post
Why do you not want a step in the "wrong direction"? I think it is only going to be a problem if you have long duration race cams...
I was under the impression that a step down in diameter would reflect part of the initial positive pressure pulse back into the cylinder, as well as restrict flow more than a step in the other direction?

Quote:
Originally Posted by Nigel_S View Post
A 32" primary is excessive and will miss out on some wave effects which could give extra scavenging between pipes, I think that should be the entire length of the header - a lot of people who write up and explain these equations, including professionals, don't understand them!
I'd be right with you (as would the various formulae I've found) if I was designing for best scavenging at say 5800 rpm, but I'm looking for a torque boost closer to 2400rpm too . Though it looks like I may have to settle for less overall length (and torque boost at higher RPMd) due to space constraints.

Another nice thing about pulse resonance is that you get harmonics, so if there's resonance at frequency X from a long pipe (assuming closed pipe resonance, i.e. a restriction / reflector) you'll get a similar (albeit slightly weaker) effect at frequency 3X, so it's 'as if' you had a resonator 1/3 the length when you hit that frequency... not sure that will help of course

Quote:
Originally Posted by Nigel_S View Post
15 inch is about right depending on the speed of the exhaust flow down the pipes, going a few inches either way will just change the rpm at which you get best effect.
Do you know whether the 15" thing is about pressure pulses or flow? I haven't been able to find any explanation of this figure, apart from a couple of old quantitative parametric studies done ages ago on 351ci V8's, and those didn't attempt to answer the 'why' of it .

Quote:
Originally Posted by Nigel_S View Post
... taking into account that the speed of sound is a lot faster at exhaust gas temperature, the reason people try to keep the exhaust gasses hot is to maintain a consistent speed of sound, although most people don't realise that.
Not to mention that the speed of sound is also dependent on pressure, the size of the pipe (sound travels faster in a pipe than free air, and varies with diameter), and the fact that the speed of sound is measured relative to the medium, meaning sound travels faster going down the tailpipe than it does going back towards the engine . And of course pressure pulses travel at the speed of sound while mass flow is much slower, and positive pulses and negative pressure pulses react in different ways to steps in different directions, as does flow... Goodness knows I don't have my head around all of it yet...

Quote:
Originally Posted by Nigel_S View Post
If you want it to work well at low throttle then you need small diameter pipes to keep the gas flow speed up, the pipe lengths should then match those of a race header as the timings of flow and wave reflections will all be the same. You just need to make sure that the pipes are big enough for the flow to remain not excessive at full throttle and full rpm.
Well, if say, you're running at half the rpm that the race header is tuned for, then the engine will be half as far along opening and closing valves etc. compared to where the race header is intended to deliver the reflections at... I can't see that being a good thing.

Another smaller effect I just realised, it even that to get the equivalent (single-tube) reflection times at part throttle I believe you need longer pipes - for a single pipe t(reflection) = t(going down the pipe) + t(coming back) = d / (v + v1) + d / (v - v1), where d is pipe length, v is the speed of sound and v1 is the speed of flow. If your full-throttle flow v1 is 1/2 the speed of sound v, then for a quarter the flow rate the pipes need to be about 30% longer for the pulse to return in the same amount of time - that's quite significant. Looking at the graph, I was thinking that maybe an even lower speed might be useful to keep part-throttle tuning consistent, if that's desired - 1/3 the speed of sound would keep the resonant frequencies within 10%, for example. Of course that would only apply to part-throttle at high RPMs. Interesting implication there too, in theory of your header is tuned for say 5500rpm and you're running at 6500 for example, backing off on the throttle might actually bring the header back 'into tune' by slowing down the exhaust flow... the relative effect is larger the higher the flow velocity, and would have more impact on a header with a narrow power band effect, could be a useful insight.
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Old 11-20-2014, 12:32 PM   #28 (permalink)
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If you are going to venture into header design, do yourself a favor and purchase Pipe Max ( PipeMax36xp2 ). I am unaffiliated, but it is highly regarded at Speed Talk forums, by guys that live and breathe this stuff. From dirt track to Nascar to 4000 hp pro drag cars, there's a lot of big engine builders on there that know there stuff, and trust the software.
Cheers for that, I've been looking at finding some more comprehensive software. Could be handy to get closer to the ballpark than the formula approach, and it looks relatively affordable, I might just get it. Really what I'd like though is something which actually does FEA of the gas in the pipe, so I can 'touch' it and try unconventional things - something like like WAVE (but it's about $500) or the GasDyn software developed by the Politecnica de Milano (but I don't think they even *sell* that one, I suspect you have to have a partnership with the university)...

Quote:
Originally Posted by adam728 View Post
Header design is a lot more than target an rpm and pick a diameter/length. Everything from camshaft timing & overlap, to EGT and engine displacement effect what the "best" exhaust is. Collector design is very important as well, be it a straight diameter/length, or getting into merge collectors with taper and volume coming largely into play.
Indeed - rpm and resonant frequencies isn't a bad place to start in figuring out a ballpark total size, of course but yeah, there is indeed a lot more to it, as I'm discovering.
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Old 11-20-2014, 01:12 PM   #29 (permalink)
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I was under the impression that a step down in diameter would reflect part of the initial positive pressure pulse back into the cylinder, as well as restrict flow more than a step in the other direction?
I asked because I didn't know. I suspect a pipe a little smaller than the port would be better for best economy, I would position it off centre so that the main flow didn't hit the step though, I haven't looked at your head but the main flow out of the port is unlikely to be down the centre of the port.

A step up in diameter creates a reverse wave which encourages flow which is why some manifolds have extra steps but the normal explanation for the header being wider than the port is to prevent exhaust going back into the cylinder during overlap on long duration cams which I don't understand but I guess has been found true by experiment. If you don't have long duration cams then there may be no issue.

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Originally Posted by Madact View Post
Do you know whether the 15" thing is about pressure pulses or flow? I haven't been able to find any explanation of this figure, apart from a couple of old quantitative parametric studies done ages ago on 351ci V8's, and those didn't attempt to answer the 'why' of it .
Both are important but I believe flow is by far the most important. You need to work out how long the initial pulse of exhaust gas takes to travel down to the end of the primary at the exhaust flow speed, as it passes the other primary in its pair it will create a strong vacuum in that pipe via the venturi effect (same way airbrushes suck up the paint), the vacuum then takes time, dependent on the speed of sound, to act on the exhaust valve for that pipe and start to suck the exhaust gasses out of that cylinder, it continues sucking until the exhaust flow from the first cylinder slows, thus the time period of the exhaust flow needs to be taken into account to avoid wasting suction but sucking at the time it will have most effect. The same happens as the exhaust reaches the end of the secondary but there it sucks on both primaries of the other pair. The timings all get very complicated, then you have to add in the wave effects!

Most explanations are way off and conflict with each other, then I am sure some experts give the wrong explanation intentionally so that nobody else can work it all out!
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Old 11-20-2014, 01:19 PM   #30 (permalink)
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Cheers for that, I've been looking at finding some more comprehensive software. Could be handy to get closer to the ballpark than the formula approach, and it looks relatively affordable, I might just get it. Really what I'd like though is something which actually does FEA of the gas in the pipe, so I can 'touch' it and try unconventional things - something like like WAVE (but it's about $500) or the GasDyn software developed by the Politecnica de Milano (but I don't think they even *sell* that one, I suspect you have to have a partnership with the university)...
I think the latest PipeMax does some simulation rather than just calculations as the earlier versions did. Don't know the details and most of these things are always used for full throttle on race cars so may need a lot of brainpower to come up with some results you can trust!

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