Forum whore
Yup, up to a point. If the rear cone is too steep, the return pulse can turn into a shockwave before it reaches the cylinder and then you will have less of everything.Originally Posted by wobbly
Al, you next question will probably be 'how steep is too steep?'
I've tried to incorporate the optimum rear cone ratio in my FOS exhaust concept. You can go shallower if need be, but going steeper won't do much good.
Moped rider
ok thanks for the replies.
i built two pipes a fat and a thin one with same percentage header and diffuser 32 68 same tuned lengths.
thin one has 90mm long belly and stops revving dead at 8500 tuned length was supposed to be 8800
fat one has 73mm long belly and revs to 10000.
fat ones belly = 4.8 times the start diameter of just under 25mm. ive never seen one as fat as that. 118mm. the rear cones angles are very sharp, im surprised it works.
im making 3 more pipes . i will reduce the angles and hence belly width will come down.
i'd love to try another FOS pipe but ive welded up my 29mm long cylinder duct to narrow ever so slightly from my single ex port area equiv 25mm to 24.5mm exit in accordance with wobblys duct formula. and using the fos calcs X = 25.8 and D1 wants to be 26.6mm as opposed to my 24.8
so ive got my age old problem of cylinder port exit not adhering to the FOS formula.
heres the two pipes ive made and road tested. the fat one works well apart from at 6k its misfiring. the thin one works well for a cruising pipe which i wanted but has no over rev. which i thought i didnt want but now i realise i would like a little.
with the fat one i wanted to use angles seen on 125/250 bikes pipes but my small start diameter doesnt allow such large angles due to simple geometry unless my belly gets obscenely fat.
Forum whore
Al , you are making it all way to complicated for a shitbox road bike.
The skinny pipe has way too steep rear cone angles for a cruiser , and the FOS design is worse , but I guarantee that is overscavenging the hell out of the cylinder due
to the steep diffusers.
With the little info I have here is a super simple layout that will work , if the TL is correct from the other two designs.
The diffuser is simply divided into 2/3 - 1/3 lengths and the belly diameter a guess to get a shallow rear cone to fit easily.
I have no idea if the 15mm exit is correct for the projected bmep.
Ive got a thing thats unique and new.To prove it I'll have the last laugh on you.Cause instead of one head I got two.And you know two heads are better than one.
Moped rider
lol shitbox road bike, i think mickey mouse hurts the bikes feelings less.
neither of those pipes - fat and thin were FOS. i just stuck to those 32 68 percentages. but went sharper angles on the fatty.
what i couldnt understand is the fat ones much sharper angles allowed over rev but the thin ones stopped over rev dead.
the thin one is a successful frontside cruiser pipe. no good for over rev.
but the fat one isnt much down on frontside yet has a better peak and loads of over rev.
yet i thought both the excessive volume and sharp angles would work against it?
massive thanks for that pipe design wobbly, . ill make that pipe plus a slightly thinner version of the successful fat one and id love to make an FOS pipe but my hurdle is my duct diameter of 24.5mm which follows your converging wobbly duct dimensions and not the D1 of 26.6mm according to the FOS calcs
as a builder i'd be tempted to just either blend it in slowly over the headers full length or deviate where i thought it would harm it less. but i accept that neither of those would adhere to the FOS recommendations and might be a total waste of time.
once again thanks for the help and advice.
re over complicating i just wanted to throw the kitchen sink at it.
ive always made single stage header diffuser and rear cones with a parallel belly. 3 angles in all. and been scared of complicating the maths and extra welding. but now ive done it its nothing to be scared of.
fatty
Scooter boy
I am using an 2 stroke engine that is motocross based. The ratios are not ideal for top and the ratios are not very close. Does anyone have any tricks or ideas on how to find different gears that will fit my shafts or some other cunning plan? Not sure if there is a website with shaft sizes and centres out there?
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No8 Wire Extraordinaire
Al, all I see there is heaps of room for a radiator and an auxiliary turbo water pump. Some time with a hacksaw. . .
Don't you look at my accountant.
He's the only one I've got.
Forum whore
Al , a fat pipe will have way less wave amplitude as it enters the rear cone section. Thus what would be considered a very sharp rear cone of say 24* on an 80 Dia
belly section , would hardly develop any reverse pulse energy on a 118 Dia.
Thus the thin pipe would have no overev , and the fat pipe would act like it had only a 20* rear cone ie better frontside and overev power, by default.
Ive got a thing thats unique and new.To prove it I'll have the last laugh on you.Cause instead of one head I got two.And you know two heads are better than one.
Low personal B.M.E.P.
Welcome to the group on here who are struggling with the same problems as you. Make, year and model will help with any advice.
Scooter boy
KTM 65sx 50cc
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Scooter boy
1. 2.846
2. 2.125
3. 1.722
4. 1.429
5. 1.217
6. 1.083
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2 smoke lover
Admittedly they are more road ratios than race ratios
Compare Pornography now to 50 years ago.
Then extrapolate 50 years into the future.
. . . That shit's Nasty.
Low personal B.M.E.P.
But still six where some of us less fortunate are looking at four....Luxury.
Husaberg's the small KTM research expert. Away from home atm at the Burt Munro.
Fair weather rider
Frits.
Would it be possible to display your latest version of FOS exaust concept ?
Pristine Knee Sliders
Here: #39051
Forum whore
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