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Post subject: (my) pipes > lowering the revs
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ief  Powerband Master Joined: 27 Jan 2004 Total posts: 782 Location: netherlands |
ok, these are my pipes, based on the 31k header and behave pretty much the same as 31k sigars up to 8000 rpm, then they go like hell up to ? (probably 11,500 rpm)
http://i-de-groot-1.speedlinq.nl/uitlaat-ief2.JPG
(43 schould be 41 btw)
That's all nice but a crank killer so i want to change them.
no i've found out that the 4l header has the same dimentions but is 4 cm longer, that would change the tuned length and the pb schould start earlier. On the other hand, i'm changing the volume of the pipes and that might give poorer performance (?)
i was wondering what any of you think although i know that theory and practise not totaly agree all the time. (and there are more then one theory)
i know, i could just dyno them as they are, mod them and dyno again but i can't weld so has to be done by someone else and he is a very buisy man as i understood (pretty hard to find a decent welder here as well  )
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Posted: Tue Jun 03, 2008 6:19 pm
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iefPowerband Master Joined: 27 Jan 2004 Total posts: 782 Location: netherlands |
somehow i was afraid of this... 
anyhow, if one wants to modify thier stock pipes to a nice performing crank killer > here u are 
dimentions are inside btw.
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Posted: Tue Jun 03, 2008 9:43 pm
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KX3 Mmm, Fully Synthetic! Joined: 02 Oct 2006 Total posts: 1383 Location: Québec |
From my understanding, add some lenght to the header lower the effective rpm. Theory only... I'll try to shorten standard F2 pipes
later this summer as I want to have a good understanding of what's going on with my bike
While on the crank killer subject, why nobody could make a 13k rpm crank as in every modern sport bike?
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Posted: Tue Jun 03, 2008 10:33 pm
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| xen-uno Joined: 27 Jul 2005 Total posts: 845 |
Big difference between what you can do with a multipiece pressfit crank running on needles and balls vs a 1 piece forged crank running on plain bearings. I'm sure a reliable 13K + could be achieved, maybe by running SiN ceramic rolling elements (if the steel elements are a weak point which I doubt). More important would be to supplement the press fit by welding or by some mechanical locking arrangement. Whatever the case, it may be an expensive one shot deal.
There is probably a rpm ceiling point above which an expansion chamber looses it's effectiveness no matter what the design. Yeah it's spinning faster, but power is actually decreasing. Anyone know what the early 90's F1 500's redlined at?
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Posted: Tue Jun 03, 2008 11:22 pm
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KX3Mmm, Fully Synthetic! Joined: 02 Oct 2006 Total posts: 1383 Location: Québec |
| xen-uno wrote: |
There is probably a rpm ceiling point above which an expansion chamber looses it's effectiveness no matter what the design. Yeah it's spinning faster, but power is actually decreasing. Anyone know what the early 90's F1 500's redlined at? |
If I understand correctly, the chamber's resonant frequency to maximize an engine at 10k should be 166hz (or 216hz at 13k). Quite low for a "musical device".
This leads me to think that, ultimatly, the limit of max rpm wouldn't lie in the chamber design... but I'm probably wrong!
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Posted: Wed Jun 04, 2008 1:11 am
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Bare All-gear Wheelie! Joined: 28 Jan 2006 Total posts: 1986 Location: Vancouver Canada |
Dunno about an Ex chamber's "Rev Cieling" not too long ago.. before Turbines became ubiquitous in RC Jet models.. OPS Ducted fan engines were routinely turning 26,000 revs with their Pipes.
Yes, a wee bit different engine./ setup.. .but less so than one would initially think.
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Posted: Wed Jun 04, 2008 2:33 am
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| xen-uno Joined: 27 Jul 2005 Total posts: 845 |
Right ... because chamber size scales with displacement. The ultimate limit is the speed of sound which is what I was getting at. At some point, the piston is opening/closing the exhaust port too quickly for the rebound pulse to make it back. The chamber on the model engine works at extreme rpm's because it is a hell of alot shorter than a bike chamber, for instance. You need a certain volume for a given displacement, so I guess that's why some 2 stroke ATV chambers have such huge bellies ... because they're shorter for high rpm use (shorter = less time needed to create a rebound pulse and less time to traverse back). Kind of throws out the rev ceiling idea as you might be able to design pipes around it, but the powerband would be even more knife edged plus keeping it from grenading would be a challenge. Anyhow ... that's my theory.
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Posted: Wed Jun 04, 2008 5:18 am
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KX3Mmm, Fully Synthetic! Joined: 02 Oct 2006 Total posts: 1383 Location: Québec |
Are you telling us than piston speed is quicker than speed of sound?
I'm dumb as fu*k but I'll ask forever 
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Posted: Wed Jun 04, 2008 7:11 am
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Spike Daily Burner Joined: 25 Aug 2005 Total posts: 227 Location: Swiiiiiitzerland |
Speed of sound = in the 330 m/sec region
Max speed for a piston is in the 25 m/sec.
After that the weight of the piston multiplied by the speed is too much for anything to stand it.
You'd need an adaptable pipe (changes in length) to have an effective pipe at the revs you're talking about. What Xen is talking about is probably more the fact that the pulse is becoming increasingly short and is not as effective in a 2stroke motor (the scavenging/intake business) Could be different with an injection…
Making pipes is really the art of consensus, you can't have everything all the time.
Yu might want to add length to your header (ie increase overall length) to lower your max rpm. Short header make for revvy pipes.
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Posted: Wed Jun 04, 2008 8:44 am
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| jmw |
| KX3 wrote: |
From my understanding, add some lenght to the header lower the effective rpm. Theory only... I'll try to shorten standard F2 pipes
later this summer as I want to have a good understanding of what's going on with my bike
While on the crank killer subject, why nobody could make a 13k rpm crank as in every modern sport bike? |
You can, just don't expect it to last that long unless you can apply modern tollerances to the old crank.
This means many hours balancing the whole assembly and a bit of weight loss within the crank itself.
What you end up with it an engine at odds with itself, the thing can spin like hell and survive but the bike doesn't really feel right in so far as it feels like its ready to eat itself and you with it.
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Posted: Wed Jun 04, 2008 12:33 pm
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| xen-uno Joined: 27 Jul 2005 Total posts: 845 |
No ... I'm not saying the piston is faster than sound. The exhaust port is open for ~ 0.003 seconds at 10,000 rpm (167 rps). Sound at STP would travel 1 meter in that time (0.66m at 15,000 rpm). Thus the chamber would have to be shorter to work well at a high target rpm.
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Posted: Wed Jun 04, 2008 1:36 pm
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