Janbros' 2-stroke excel for ports, heads and pipes

the FOS pipe is not "the best possible pipe". the pipes work, but it is only a concept that Frits created to have a starting point for "novice tuner's".
Wobbly's duct is created after many many many simm's and engines built and is a proventhing . So if the FOS does not correspond with the Wobbly-duct, it is not a discrepancy, it just means that Frits' concept is close but not exact.
I keep the Wobbly-duct as is and adjust the following diameters. you can decide to make them all smaller so the angles stay the same, or you can make each diameter bigger in steps so that the relations to each other stay  the same but you end up with the FOS belly.

best is to make first some straight pipes and experiment a bit. a straight pipe can be made much easier and fast and after each test you will be able to close in on the best SoS-value (enter the rpm of max power from the dyno in the spreadsheat and than adjust the SoS to get the same length again)
when you have the correct SoS, you can start playing a bit with dimensions, shorter/longer headers/rear cones while maintaining the total length and optimize the pipe, and than make a curved one that will fit the bike. and than realize that a curved pipe does not work the same as a straight one ...

what is sos value? i dont think im ever going to progress from a novice tuner. i was searching this forum and seeing your posts from 8 years ago and wondering what you now use for your sky pipes? a pal almost talked me into buying torqsoft, it has good angle adjustments for the header that would save me an hour or so doing it by hand but i read on here that it is just another blair "rip off". i find the pipes using blair are ok. but then i wouldnt know any different. i have always wondered if transfer timing comes into the design of a pipe. blair and jennings and the FOS pipe ignore this. but then im sure jan said the suction wave wants to peak at bdc. which doesnt need any transfer info. im also wondering in order to get a better pipe i should use actual temperatures of the pipe in use and for each section. not just a vague figure for the full length? or does it happen so fast its insignificant. one reason i will not spend a lot of money on software is that i know my porting will always be less than perfect so any software simulation would give a false impression. the cylinders i am using have flawed transfer tunnels designed around stud spacing and asthetics rather than performance. ie off the shelf airsal polini italkit etc 50-70cc. i am usually single port exhaust and i reckon thats been left behind in development stakes.

if i build a straight pipe then i am changing things it then becomes guesswork and there are a million chnges you can do either in combination so it would take a lifetime to get things spot on. and how do you know if all the variables but one make a great pipe but then adjust that one missing variable and it throws everything out? in life i am a struggler and if things can go wrong they do go wrong. so for me it would be impossible. i dont have a dyno. i use my pals. but he charges and that would cost me more than engmod2t twice over to try and get the perfect pipe.
so a pipe that is good enough would do me. i know that a few uk reknown pipe makers use torqsoft.

JanBros a écrit:

the FOS pipe is not "the best possible pipe". the pipes work, but it is only a concept that Frits created to have a starting point for "novice tuner's".
Wobbly's duct is created after many many many simm's and engines built and is a proventhing . So if the FOS does not correspond with the Wobbly-duct, it is not a discrepancy, it just means that Frits' concept is close but not exact.

Jan, as you know, my approach is based on the blowdown area of the exhaust port while the Wobbly-duct is based on the total exhaust port area, which is too big in most cylinders. Wobbly agrees that starting from the blowdown area is the better approach, but if you are stuck with the existing port area, his duct gives good results in practice.
Mixing the two approaches will raise a lot of questions, as you are now experiencing  [Vous devez être inscrit et connecté pour voir cette image]

frits you will have to excuse my ignorance, im guessing blowdown area is the area of the exhaust port above transfer opening. so where should i use this figure in calcs? using it as my starting "ex port area for wobbly duct and pipe construction?
according to janbros excel my port effective area is 496mm² = effective diameter of 25.1mm
blowdown area is 255.3 mm² efective diameter 18mm.

so my wobbly duct goes to 90% of this blowdown area then back up to 100% for the pipe to connect to?

using full area duct diameter measure 25 -22.5 - 25
using blowdown area duct diameters measure 18 = 17.1 - 18

i am using 90% of the area and not 90% of the diameter which would give wrong numbers.

aljaxon a écrit:

frits you will have to excuse my ignorance,  im guessing blowdown area is the area of the exhaust port above transfer opening. so where should i use this figure in calcs? using it as  my starting "ex port area for wobbly duct and pipe construction?

In my FOS exhaust concept there is no need to enter the blowdown area; I did the work for you. And if you try to enter it in the Wobbly-duct, you are mixing concepts and neither Janbros nor Wobbly nor I can help you with that; you will have to do your own development there.

i had some time so worked out some values using FOS
my effective port areas diameter is 25.1mm   178 duration.

rpm - - -   x - - -   lmax - - -   D1    

7000 - - 18.6 - - 1118.8 -- -- 20.2  
8000      19.9         979         21.86  
9000      21.1         870         23.4  
10000    22.25       783.2       25  
11000    23.3        712           26.45  

so if i want 9000 rpm my Lcyl (or wobbly duct end diameter is 23.4)  is that near enough? wobbly says duct exit is same as port area diameter  25.1mm  
if i use X as per fos then my wobbly duct is reduced to his recomendations by 1.7mm which is quite a bit.

my cylinder tract is 29mm so the rest of the wobbly duct will be part of the pipe.  ie the first 62mm to get 91mm in total.
ive made one and added cooling fins as to waynes advice and it goes quite well.  the camera is making the first join look worse than it is.
[Vous devez être inscrit et connecté pour voir cette image]

@aljaxion : I'm almost as novice as you are. all I did was collect all the info Frit and Wobbly have given over the years and assembeld it all together in one spreadsheet

JanBros a écrit:

@aljaxion : I'm almost as novice as you are. all I did was collect all the info Frit and Wobbly have given over the years and assembeld it all together in one spreadsheet

and a great job youve done JanBros.

searching these forums in 2015 i see you are asking the same questions as me seeking info on pipe design. i wonder what you found worked for you?

aljaxon a écrit:

searching these forums in 2015 i see you are asking the same questions as me seeking info on pipe design. i wonder what you found worked for you?

for aircooled 50cc moped's that rev to max 10.000 :
SoS only 450-475
bigger belly (usualy 6% bigger)

thanks and can i ask what calcs or software you have found best? and did you incorporate a "wobbly" type duct i stumbled across luc foekema a while back and wondered if his paid stuff would be any good compared to FOS.
for my mobylette i worked out sos to be 430 reverse engineering a pipes peak power on the dyno.
for my puch i want a slightly wider power band due to the wide apart ratios. and a wider belly gives sharper angles and narrower power band? so id want smaller belly? yet your sky doesnt have gears?

small Christmas present, changed the exhaust page, new links in the first post.

The links do not work, greetings from Argentina.

new links in first post

Creo que hay errores en la hoja datos, estimo que las formulas correctas serian las siguientes:


Deck - Aux Open
=SI('Exhaust New'!$N$4=0;"";$AN$22-'Exhaust New'!$BW$307)
=SI('Exhaust Old'!$N$4=0;"";$F$38-'Exhaust Old'!$BW$307-'Exhaust Old'!$W$51)


Aux  bottom vs BDC
=SI('Exhaust New'!$N$4=0;"";$AN$22-$E$44-'Exhaust New'!$BW$308)
=SI('Exhaust Old'!$N$4=0;"";$AO$22-$F$44-'Exhaust Old'!$BW$308)

new links on first page due to update Frits's FOS concept

I think there are errors in the data sheet, I estimate that the correct formulas would be the following:

Deck - Aux Open
=SI('Exhaust New'!$N$4=0;"";$AN$22-'Exhaust New'!$BW$307)
=SI('Exhaust Old'!$N$4=0;"";$F$38-'Exhaust Old'!$BW$307-'Exhaust Old'!$W$51)


Aux  bottom vs BDC
=SI('Exhaust New'!$N$4=0;"";$AN$22-$E$44-'Exhaust New'!$BW$308)
=SI('Exhaust Old'!$N$4=0;"";$AO$22-$F$44-'Exhaust Old'!$BW$308)

[Vous devez être inscrit et connecté pour voir cette image]

Hi,
you are correct about the Exh New Deck-Open Aux. J4 needs to be N4
the Exh old bottom vs BDC : BW308 is the same as AQ520, but changed it anyway.

new link in first post.