Mark Allanson
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| posted on 13/2/08 at 01:08 PM |
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Manifold design
I am thinking of making my own manifold, doing a 4:2:1 design. The general concensus on here is that the primaries must be the same length, but I
found this BMW manifold on ebay and its design doesn't even try to keep the equal.
What happens if they are not the same length? Does someone come and tell you off?
 
Rescued attachment a61b_1.JPG
If you can keep you head, whilst all others around you are losing theirs, you are not fully aware of the situation
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Mr Whippy
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| posted on 13/2/08 at 01:18 PM |
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It’s the gas pulses flowing down the pipe, ideally one after the other. If the pipes are of different lengths then at some point in the rev range two
or more pulses will arrive (where the pipes merge into one) at the same time which in theory would slow the flow down. I don't know how much it
really matters, personally I’m not to keen on them as these tubular exhausts make a right racket unlike the cast manifolds.
You have off course to run the engine with no manifold on (after 11:30 pm) for pure fun. You can usually see right into the engine through the valve
gap with all the flames inside, I love that. Next thing you'll find yourself doing is picking up a stick to poke in there  yeah lets see how
hot it is, oh it's the small things in life...
[Edited on 13/2/08 by Mr Whippy]
Fame is when your old car is plastered all over the internet
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muzchap
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| posted on 13/2/08 at 01:20 PM |
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I'm that the Impreza RB5 with the Prodrive Plus Pack exhaust had 2 primaries longer then the others which gave it the incredible growl and bark
- it truly sounded like bits of the engine were being mangled and thrown out the exhaust - it scared the shite out of people standing around during
the day!
I'd hazard a guess that equal primaries are a safe bet, but professional tuners will know how to adjust the primaries to give different engine
performance/noise characteristics.
HTH
M
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If you believe you're not crazy, whilst everybody is telling you, you are - then they are definitely wrong!
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dhutch
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| posted on 13/2/08 at 01:27 PM |
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I can understand the reasoning, as you say, all the cylinders fire at diffrent, equally spaced, times around the cycal. And that is how you want the
exhast gasses to join, idealy. In order to keep the backpresure constant, and as low as possable. etc etc.
- That said, if your plumbing it up in 2inch ruddy drainpipe anyway, does it matter. Gasses are compressable anyway, and it will proberbly sort
itsself out.
- I dont know. Does anyone have any proof, dyno printouts with two diffrent manifolds? What about the exhast note?
How do you mesure the lenght anyway? String lenght along the center line of each pipe?
Daniel
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iank
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| posted on 13/2/08 at 01:27 PM |
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quote:
Originally posted by Mark Allanson
What happens if they are not the same length? Does someone come and tell you off?
Absolutely, that and you'll restrict the power and torque you can produce.
There's a lot of variables involved but as I understand it:
The cylinders of the engine spit out a plug of hot gas into their header, this travels down until it hits the collector and wanders off down the
exhaust. So you have a series of these plugs coming down the headers slightly timed apart.
If they collide in the collector (which they can if headers are different lengths) it causes back pressure at certain RPMs which is bad, but better
since they create suction behind them if the headers are the same length at some RPM they actually suck the next plug faster in the other header which
helps to empty the cylinder quicker giving the possibility of more power with a cam that can take advantage.
This is also why you tie 1-3 and 2-4 together as the pairs of a 4:2:1 system rather than 1-2 and 3-4 (or 1-4 and 2-3)
The above is horribly simplified no doubt, but reality probably involves lots of maths and computer simulation at which point just glue some pipes
together and see what happens (which is the way a lot of the manifold companies no doubt work, those that don't just copy other peoples designs)
--
Never argue with an idiot. They drag you down to their level, then beat you with experience.
Anonymous
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jambojeef
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| posted on 13/2/08 at 02:01 PM |
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I wouldnt worry too much about the BMW manifold you have found.
If its at all a recent one it will be from the v8 engines and so you have the cylinder firing order split across the two banks making it difficult to
adapt to your 4 cylinder.
Have you got a copy of 4-stroke performance tuning by Bell?
All good stuff and explained very clearly in there.
Geoff
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r1_pete
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| posted on 13/2/08 at 02:32 PM |
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quote:
Originally posted by iank
This is also why you tie 1-3 and 2-4 together as the pairs of a 4:2:1 system rather than 1-2 and 3-4 (or 1-4 and 2-3)
This does of course depend on the firing order.
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twybrow
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| posted on 13/2/08 at 02:41 PM |
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That book is a great read (although quite hardcored in places). Pick it up and answer all the questions you could ever dfream of....
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DarrenW
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| posted on 13/2/08 at 03:39 PM |
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Doesnt vizard quote some good lengths to aim for on a pinto manifold as well? I seem to recall my manifold, supplied by Mac#1, is based on the vizard
sizes as far as is possible in a 7.
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grub
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| posted on 13/2/08 at 04:57 PM |
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well thats me f---ed then
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Phil.J
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| posted on 13/2/08 at 05:01 PM |
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Exhaust pulses travel down different diameter pipes at different speeds, and Ducati exploit this fact to get around having to package primaries of the
same length when cylinders exhaust from different places on the engine.
I suspect (but don't know for sure) that primaries of the same volume will do the same job as primaries of the same length.
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sebastiaan
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| posted on 13/2/08 at 06:51 PM |
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Nope, the speed of sound in the exhaust gas is the determining factor. Same temperature in the headers = same speed of sound = same length needed.
Diameter only influences gas velocity, not the velocity of the pulses (as these travel at the speed of sound...) Get the primarier big enough in
diameter and there won't be any pulse tuning effect left... Read up on standing or stationary waves and you'll soon get it.
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madteg
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| posted on 13/2/08 at 07:40 PM |
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manifold
Fit a turbo then it dont matter
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muzchap
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| posted on 13/2/08 at 07:54 PM |
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quote:
Fit a turbo then it dont matter
LOL That's a VERY good point - although I was too chicken and opted for an S2000
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If you believe you're not crazy, whilst everybody is telling you, you are - then they are definitely wrong!
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DanP
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| posted on 14/2/08 at 01:13 AM |
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There is a pretty useful calculator here: http://www.headerdesign.com
you will need to register to access the calculator, for my V8 the lengths I needed for a peak RPM of ~5500 came out at 1metre (which I was a bit
suprised at), you can play with the config and see how the lengths change depending on the RPM you want peak HP at.
BTW have a look at the manifold on a BMW M3 if you wanna see if they worry about equal lengths.
Dan
[Edited on 14/2/08 by DanP]
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