Exhaust volume - change of plans

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Channel Hopper
Posts: 1310
Joined: Thu Sep 23, 2004 5:26 pm

Exhaust volume - change of plans

Post by Channel Hopper »

This is more for the techi people than the aesthetically minded.

Having started the reassembly of the engine/frame components, and noting the final appearance of the exhaust cans from the thread on the titanium stuff from Japan, I have the following dilema.

I planned to use some expansion pipes originally fitted to a KH250, as these were exceedingly light, however they have been finished with some very small end cans, non removable. Additionally (the main point for a second opinion) is the volume of the pipes is designed for an 80 cc piston and I am thinking these would have some issues in letting the 400 (now a 367cc) rev out.

I also have a set of H2 expansion pipes- dont ask - sitting here and these are still lighter than the standard pipes, but with a more pleasing end can which can be packed with replacement fibreglass.

Question is the volume of the pipe and how this is related to the characteristics. I am assuming that the power of a cylinder is directly proportional to the amount of fuel/air burnt on each stroke and therefore proportional to the amount of exhaust gas chucked out.
The power output of the standard NS is about the same as the earlier KH750, so I am under the impression the H2 pipes are closer to the ideal needs of the NS engine.
Should I be swayed by other factors such as length of pipe, cross section of the downpipe (or of the expansion chamber at a particular point), or is it a complete secret known only to the likes of TS and the Figaroli boys with their snail horn designs, and not to be worried about on a tight budget ?

The front downpipes are no problem to fabricate (as long as I can find the right flanges for the CR barrels - no ATAC to worry about but the elusive triple bolt clamping - the rear barrel is remaining NS, and I have a manky centre pipe which can be cut down to make the right downpipe retaining the U-bend) , but the issue of the volume of the two stroke pipe is something I have no experience of.

As Ive said earlier the 'full endcan' look is already one reason for the change, so the deciding factor is, are small 250 pipes better for the engine than the much larger 750 ones ?

Any ideas ?

Channel Hopper
Posts: 1310
Joined: Thu Sep 23, 2004 5:26 pm

Post by Channel Hopper »

I suppose I could always find some aluminium pipe/bean cans to rivet onto the end of the 250 pipes, but thats not the point.

VinCBR900
Posts: 34
Joined: Sun Nov 14, 2004 6:53 pm
Location: Near Loughborough, UK

Post by VinCBR900 »

There are many places on the web which tell you how the various sections of the expansion chamber affect power, and Im sure you know about this already, so wont go into this here.
If teh tuned length of the pipe is right for your porting, and the cross sectional area is also right, then there is no reason why the pipe shouldnt work between engines.
However, the most important point on pipes is teh gas temperature, since this affects the speed of sound hence the pulse velocity. Too narrow pipes with not enough CSA and the pipe will be too restrictive, so the temperature will go up and the pipe wont work until at high engine speed. If the singer is too small for example, the result is the gas gets too hot i.e. too restrictive, and the pistons melt.
I dont know about teh H2 bipes since it depends on when they were designed. If they are modern designs then give it a go. If they were 1970s designs then Id say forget it.
Try to use pipes which have the most diffusor tapers - teh section after teh header - since they work over the bradest RPM range.

Pipe design is still a case of suck it and see. The most complex formulae dtill need the pipes cutting and experimenting with the get the most optimum setup.

So I would try the H2 pipes if they are modern personally.

Cheers, Vincent

Channel Hopper
Posts: 1310
Joined: Thu Sep 23, 2004 5:26 pm

Post by Channel Hopper »

Thanks

As you might have gathered, nothing I have here is modern. Its all here as a result of the hoarding aspect in me, something I inherited from my parents.

The H2 pipes are almost certainly early 80s, the 250 ones are probably 90s construction, but not in keeping with any development specifically for high states of tune, except the expansion aspect and the weight saving over the originals (which were heavy).

Ill take some pictures and post them in the new section

Channel Hopper
Posts: 1310
Joined: Thu Sep 23, 2004 5:26 pm

Post by Channel Hopper »

And after measuring the dimensions of the two types of pipe, the larger ones are only about 10% bigger, so Ill stick with plan A (for now ) :shock:

Cant attach pictures in the test section, so heres the URLs of the pictures from a site I work on

Weight per pot is bag of sugar (1kg) plus 1Ib 8 oz, or 3.7Ibs (in new money thats 5kgs for the complete system)

Additional saving as ATAC is also removed

http://www.satellites.co.uk/php-bin/for ... entid=2052

http://www.satellites.co.uk/php-bin/for ... entid=2051

VinCBR900
Posts: 34
Joined: Sun Nov 14, 2004 6:53 pm
Location: Near Loughborough, UK

Post by VinCBR900 »

After a very demanding signup, I managed to see your pictures.

The pipes do look rather dated - they are single stage diffusor with cross sectional area in the belly. They look like they are designed for a highish speed engine, which makes sence since a KH250 has short pistins and stroke and could rev relatively high?

Modern designs have much larger voluems to get larger pulse effects - look at the fat pipes on snowmobiles or on late 1990 250s like RGV250 or KR1.

I know you are after performance at a price but I wouldnt want to fit pipes desuigned maybe 10 years ealrier in place of the NS400 pipes, even though they may be lighter. Fitting different cans and junking teh ATAC should give you a weight saving anyway, and if you are really persistant you can cut them open and remove the double skins too.

What about fitting some pipes from a KR1?

Cheers, Vincent

Channel Hopper
Posts: 1310
Joined: Thu Sep 23, 2004 5:26 pm

Post by Channel Hopper »

I had considered going to an off road meetup and try to blag the cast offs from riders using 125s. If they were Honda CR riders I would also get the matching exhaust flanges, saving further effort in the works department.

I know the exhaust shape does make a lot of difference, however any reduction in power using these exhausts would be cancelled by the gain of power with the new top ends/ updated carbs.

If I was going for outright power, Id have a go at shoehorning the SC28 engine presently on the washing machine into the frame. It doesent weigh that much more than the NS lump, and I could junk the two stroke tank and pump. (and should last a bit longer).

Channel Hopper
Posts: 1310
Joined: Thu Sep 23, 2004 5:26 pm

Post by Channel Hopper »

And Ive just found this page which gives me the information I was looking for.

http://www.vf750fd.com/blurbs/stroke.html

I might even have a go at cutting my own pipes at some point

VinCBR900
Posts: 34
Joined: Sun Nov 14, 2004 6:53 pm
Location: Near Loughborough, UK

Building own expansion chambers

Post by VinCBR900 »

I have experimented with building my own pipes in the past.

I wrote some software to help design hydro formed pipes, available at

http://www-staff.lboro.ac.uk/~elvpc/progs/progs.html

There are also programs available from

http://super-cross.com.ar/Software.html

The more tapers in the diffusor first expanding taper of the exhaust, then the wider the powerband can be.

I have some software for 4 stage diffusor pipes too.

Cheers, VIncent

Channel Hopper
Posts: 1310
Joined: Thu Sep 23, 2004 5:26 pm

Post by Channel Hopper »

What a coincidence, I was on that university site earlier today looking at the SC28 parts as Im pulling one apart as I type.

Lovely engine

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