FZRSupport: the FZROnline Archive

Hy guys! I did my own muffler. So as YG says the length of the exhaust must be kept in order not to do major adjustment to the carbs and keep some low end torque. Can someone please tell me what is the length of the stock exhaust in the sections that are marked with green and orange?

I measured my 96 stock can and it's appro. 54 cm & 78 cm

thanks skunkwork . I appreciate it!

skunkwork wrote:I measured my 96 stock can and it's appro. 54 cm & 78 cm

measured my 92 stock can . 54 cm long & 78 cm from tip to beginning of the joiner pipe.
TG to stop from burning valves etc be sure to mimic the diameter of the pipe as well if it gets too small the heat is retained too large and no back pressure to stoke the fuel in the cylinder head.

yamaha_george wrote:

skunkwork wrote:I measured my 96 stock can and it's appro. 54 cm & 78 cm

measured my 92 stock can . 54 cm long & 78 cm from tip to beginning of the joiner pipe.
TG to stop from burning valves etc be sure to mimic the diameter of the pipe as well if it gets too small the heat is retained too large and no back pressure to stoke the fuel in the cylinder head.

Four strokes don't use backpressure to keep fuel in the cylinder head, they use exhaust (and intake) valves for that.

olie05 wrote:

yamaha_george wrote:

skunkwork wrote:I measured my 96 stock can and it's appro. 54 cm & 78 cm

measured my 92 stock can . 54 cm long & 78 cm from tip to beginning of the joiner pipe.
TG to stop from burning valves etc be sure to mimic the diameter of the pipe as well if it gets too small the heat is retained too large and no back pressure to stoke the fuel in the cylinder head.

Four strokes don't use backpressure to keep fuel in the cylinder head, they use exhaust (and intake) valves for that.

Err so why did Yamaha design platted pipes & EXUP valves ?

YG is correct about the exup and its purpose.
the muffler i built has the interior diameter of 52mm and the linkage pipe has the same diameter too. so it's all good.
thank you for the measurements YG. My actual end can is 200 mm long, has the outside diameter of 85 mm and the inside diameter (where the gasses pass) of 52 mm. It's a moto gp replica end can and it's made out of fiberglass. It has a unique sound. I'll take some pics next year (LOL) and post them. Happy New Year! everybody!!!

YG is IMHO incorrect about the EXUP; it is not about controlling backpressure, but about gas velocity and reversion. By throttling down under low-flow conditions, it maintains the exhaust gas velocity above a point that can turn around and flow back into the chamber, contaminating and diluting the charge.

DonTZ125 wrote:YG is IMHO incorrect about the EXUP; it is not about controlling backpressure, but about gas velocity and reversion. By throttling down under low-flow conditions, it maintains the exhaust gas velocity above a point that can turn around and flow back into the chamber, contaminating and diluting the charge.

Don,
I think you just made my case for me :-}
The idea with platted pipes & EXUP valves and their like is to use the exhaust gases reversing to stuff the new charge of fuel mixture back which "leaks " in to the exhaust pipe during valve overlap (both exhaust & inlet being both open ) into the combustion chamber.

If there was no back pressure what would cause the exhaust gas change direction & not go straight out to atmosphere ? Atmospheric pressure is MUCH less since the engine has a compression ratio which is compared to it

YG,

You're thinking of 2T expansion chambers, shining examples of which can achieve supercharging equivalent to approximately 7-8psig of boost. 4T pipes have far less volume to accumulate fresh charge, and fractional pressure pulses resulting in greatly reduced supercharging effect; they are much more simply extractors. The issue comes in at low revs; a large-bore pipe designed for max power at high revs has much lower gas flow at low rpm than a smaller-bore pipe designed for mid-range. As the positive (reversion) pulse is generated, it tries to turn the flow around (which in a 2T chamber is a GOOD thing), and stuff spent gasses back into the chamber.

The EXUP prevents this by maintaining the gas flow above some empirical velocity that can't reverse in flow; anti-reversion cones do it by diverting the reversing mass into the corners and away from the actual exhaust port.

"Backpressure" interferes with flow OUT the pipe. In the case of a 2T chamber, this maintains a higher density of gasses (including fresh charge) in the chamber to be stuffed back into the cylinder. In the case of a 4T pipe, it just gets in the way.

Well Don all I can say is that Mr.Watanabi from YMCo was telling me porkie pies when i asked him about the Platted pipes and EXUP may not have been good in translation some place :-{

Didn't you know - enthusiasts ALWAYS know more than the OEM engineers!

yamaha_george wrote:

DonTZ125 wrote:YG is IMHO incorrect about the EXUP; it is not about controlling backpressure, but about gas velocity and reversion. By throttling down under low-flow conditions, it maintains the exhaust gas velocity above a point that can turn around and flow back into the chamber, contaminating and diluting the charge.

Don,
I think you just made my case for me :-}
The idea with platted pipes & EXUP valves and their like is to use the exhaust gases reversing to stuff the new charge of fuel mixture back which "leaks " in to the exhaust pipe during valve overlap (both exhaust & inlet being both open ) into the combustion chamber.

If there was no back pressure what would cause the exhaust gas change direction & not go straight out to atmosphere ? Atmospheric pressure is MUCH less since the engine has a compression ratio which is compared to it

I disagree.

The purpose of the exup valve is not to stuff whatever leaks into the exhaust back into the cylinder. The exup valve helps to maintain flow of gasses in the direction of the inlet flow. As Don said, this is achieved by increasing the exhaust velocity through a variable area chamber in the exhaust, not by increasing backpressure.

The main difference here is that in a two stroke the intake charge is pressurized, and has a tendency to leak out of the exhaust during overlap. In a (non boosted) four stroke, the inlet is at negative pressure and if there is exhaust overlap the piston moving downwards in the bore will actually suck exhaust back into the cylinder (a form of EGR). The exup valve helps prevent this from happening by using the momentum of the air inside the exhaust, thus preventing the cylinder from sucking in the diluted charge.

Pretty much what Don said, just in different words.

Read here: http://www.fzrarchives.com/ipb/index.ph ... opic=48932

olie05 wrote:

yamaha_george wrote:

DonTZ125 wrote:YG is IMHO incorrect about the EXUP; it is not about controlling backpressure, but about gas velocity and reversion. By throttling down under low-flow conditions, it maintains the exhaust gas velocity above a point that can turn around and flow back into the chamber, contaminating and diluting the charge.

Don,
I think you just made my case for me :-}
The idea with platted pipes & EXUP valves and their like is to use the exhaust gases reversing to stuff the new charge of fuel mixture back which "leaks " in to the exhaust pipe during valve overlap (both exhaust & inlet being both open ) into the combustion chamber.

If there was no back pressure what would cause the exhaust gas change direction & not go straight out to atmosphere ? Atmospheric pressure is MUCH less since the engine has a compression ratio which is compared to it

I disagree. Read here: http://www.fzrarchives.com/ipb/index.ph ... opic=48932

Did you read this article? as I find that the paragraph on "tube talk" quite enlightning since it states my case about keeping the fresh charge from leaking out thro the exhaust by using back pressure.

The section marked "under pressure " seems to say what I said the the platted pipes are used as Don says like a slide trombone effect at controlling the pulses of pressure wave (back pressure his words not mine) over a greater rev range since all exhausts are compromised.

Going on to the section "throttled" it states that the valve(EXUP) controlled the pressure wave ie the back pressure that was so useful at keeping the fresh fuel charge over the piston not out thro exhaust since as I stated there is an overlap on the valve timing cams .

I can still hear Mr Watanabe san telling our sales people all this and the glazed look in their eyes LoL

The article states that during overlap (1) the negative pressure wave helps to fill the cylinder by sucking gasses out of the cylinder through the exhaust valve. (2) The following positive pressure waves (is this what you call backpressure?) help to slow the momentum of the fresh air to keep it from going from intake valve directly out of the exhaust valve. This last part on it's own would not help the case of a naturally aspirated 4 stroke engine, since the intake charge would have maximum velocity at the end of the intake valve event, long after the exhaust valve has closed. However, I could see how it would greatly help in the case of a two stroke.

Basically I'm saying that (1) is what they designed the EXUP valve for and (2) is an added benefit, but not the primary purpose.

Did you read the article?