ESE's works engine tuner

[wobbly]

I believe the big radius at the bore centre line has two effects.
Firstly is to improve the bulk blowdown flow at low opening angles of the piston controlled port orifice, due to the gas attaching early to the roof.
Secondly the exiting wave amplitude is lowered and smoothed out by the gradual ( instead of a sharp edge ) port opening.
This would improve the scavenging action bandwidth, but the peak value is probably then raised as well,simply because the port opens earlier when the pressure above the piston is greater.
Thus you get the best of both, a wider effective scavenging action, combined with a higher peak value, making the pipe work better over a wider range.

The big radius can only be applied to a port with the chrome removed, it then has to be replated to permit the ring to run on a hard surface as it bulges out into the port - unless of course you have an iron bore..

[TZ350]

If you only added a chamfer to the existing port, outflow will improve ...

Chamfer and a little enlarging of the transfer port entry, no other engine or ignition changes at all and after changing the main jet from 135 to 145 the power on the dyno is up, from 27 to 29.

... outflow will improve, wasting less energy in turbulence and sending more energy the pipe. It will also have raised the exhaust timing, opening the port when cylinder pressure is higher; another reason for the pipe to perform better.

I believe the big radius at the bore centre line has two effects. Firstly is to improve the bulk blowdown flow at low opening angles of the piston controlled port orifice, due to the gas attaching early to the roof. Secondly the exiting wave amplitude is lowered and smoothed out by the gradual ( instead of a sharp edge ) port opening. This would improve the scavenging action bandwidth, but the peak value is probably then raised as well,simply because the port opens earlier when the pressure above the piston is greater.

Thus you get the best of both, a wider effective scavenging action, combined with a higher peak value, making the pipe work better over a wider range.

OK so the top of the exhaust port chamfer is a good thing.

I have met people who claimed they could tell just by listening whether an engine had auxiliary exhaust ports or a T-port. All I can hear is whether the silencer is still silencing or needs to be repacked.

This sound change is not a subtle thing that only I can hear, even people here in the workshop who have no real interest in tuning 2-strokes say they notice it, to them my bike now sounds like a FXR150, 4-stroke flat, fat and lazy not the typicall sharply tuned 2-stroke exhaust note.

Funny sounds can turn up if the ignition is borderline. Improving the cylinder filling can be enough to prevent the spark from sparking at the right time every time. A simple test is to close the plug gap to 0.2 to 0.3 mm. If that improves things, you need a stronger ignition.

Thanks for the tip, I will try closing the plug gap and also Wob has pointed out that my Ignitec has two channels and I could connect both of them to the coil for extra umph in the spark department. Wob has also sold me a special plug, it would be wasted trying it now, but when the engines running properly I will try it in a Back to Back dyno run.

The triple port cylinder has Exhaust and Transfer STA's capable of easy mid 30's rwhp so at 29 something is holding it back.

[Haufen]

Chamfer and a little enlarging of the transfer port entry, no other engine or ignition changes at all and after changing the main jet from 135 to 145 the power on the dyno is up, from 27 to 29.

Well then, sounds like a 155 is worth a try!

Attachment 270001

Hi Haufen ,
where did you get this picture from ? Are you casting your own barrel ?

hi Neal,

these are not my sand cores. But it would be nice if they were . I found that picture somwehere on the net, could have been pit-lane.biz. The picture illustrates what I was trying to describe using words (but did not succeed). Look at the curved shape of the main exhaust port floor, there seems to be a reduction in area within the port.

Verstehe nur Bahnhof, Haufen.I am still in a place far, far away (Dresden) and I have no access to my home archive; I only remember that the entry area of the B-ports is a trifle under their exit area. That is not something you should aim at (on the contrary); it evolved historically because Aprilia wished to keep the cylinder stud positions in their original place, so new cylinders would fit old engine cases. The RSA125 should have been free from that restriction because it was designed at Derbi, not at Aprilia. But in the development phase it was decided to fit an existing well-proven Aprilia cylinder, so the funny entry/exit relation remained.

Thanks Frits. What I was referring to was a post from wob a couple (of hundred) pages back where he writes about having the exhaust port area at the exhaust flange about 75% of the value at the bore to gain horsepower.

All modern race engines have a nozzle restriction at the flange face, as big T ports and tripple Exhausts loose alot of velocity going into the header.
Rule of thumb is a 75% of the effective EX area at the flange.
Stepped oval duct into a round flange does work, but I have used a CNC oval to round transition in the spigot for years, as has Honda in A kit, and Aprilia factory engines, this works way better.
Here is a sim showing my new 400cc F3 engine, with and without the spigot nozzle.

And I was wondering if the exhaust port floor of the rsa was shaped the way it is to achieve the same thing. And if it was, if the value was about 75% also, or if another value gave better results.

[F5 Dave]

. . .
Anything over half throttle and it just sat there going Blarrrr, sounded rich but I was caught out thinking that the other night. If it was lean I would have expected it to just cut out, it kept going but no matter what I did with the throttle it just would not pickup, sounded flat and rich.

The taper bored 24 had a 145 main, the 30 a 150 so it’s hard to imagine the 30 needing a main smaller than the 24. ???? . . .

Its very easy to imagine it, same amount of suction but bigger hole. Less velocity, less signal. Best to always start off rich though. It can often catch you.

[2T Institute]

Attachment 270001

Hi Haufen ,

where did you get this picture from ? Are you casting your own barrel ?[/QUOTE]

I got that pic from a Italian contact, his partner has the laser sintering machine to build these casting cores. IIRC he did most of the work for Aprilia. The 'lamp stand' as I called it is the voids in a casting, fits together with the cooling passage core and the outer shell. When placed together and cast voila a APE cylinder.

Wob I radiused a Stihl chainsaw cylinder(a bloody expensive one) revs to the moon with no replate and (touch wood) ring issues yet.

[wobbly]

Yea but Lozza was it a R3,5 and was the port at 70 + % and did it pull 14500,or even closer to the moon.

The curved convex shape of the Aprilia duct corresponds directly with the area increasing due to the Aux ducts entering the main.
Again the idea is to keep the duct volume down.

The 75% area at the cylinder exit is just something I discovered after running hundreds of sims, most of them worked best with an oval to round transition in the flange that started with this area
and the pipe header diameter equalled the total effective area of the ports.
Its been tested and proven so many times now, by so many other people, that it should be the first mod to make to any T or tripple port engine.
Here is a pic of one I have just done, that happened to have a tapered spigot - enabling the Aux ducts to be run all the way down into the pipe.
Check the big rad on the transfer duct/bore edge.

[SwePatrick]

The taper bored 24 had a 145 main, the 30 a 150 so it’s hard to imagine the 30 needing a main smaller than the 24. ???? ???.

This isn´t an exact knowledge actually.
This is directly related to the airspeed through the carb.

Higher speed draws more fuel from main jet(all others too infact)

Example, on my build.
I had 200 in main jet(keihin jet) inside my Mikuni TMX 40(bored out 38mm).
that one had problems with overflowing so i changed to an TM36.
I set that one up exactly like the 40mm just jetted it down to 185, but engine ran so rich i had to jet it down all the way to 160.

Of course there is an area change, that´s why i jetted it down to 185 to compensate for that.
When ending up on 160 i thought it can´t be making any power as it´s alomst the same as my 88cc build, that one has 145 in main.
But when testing it,, it still makes HUGE amount of more power than my 88cc build.

Here in Sweden we also say that if you are having to jet your carb insanely big your pipe is wrong.

With alls this said, i just wanted to state that jetting can be mindgobbeling, and just ignore it, follow your gutfeeling.

Rgds
Patrick

[F5 Dave]

. . .and just ignore it, follow your gutfeeling.. .

and the testing results.

[speedpro]

OK, I do have a big carb at 34mm on a 100cc motor. It has a 430 main, proven on the dyno to make best power. Someone else can check with Wobbly whether the pipe is right.

[teriks]

This sound change is not a subtle thing that only I can hear, even people here in the workshop who have no real interest in tuning 2-strokes say they notice it, to them my bike now sounds like a FXR150, 4-stroke flat, fat and lazy not the typicall sharply tuned 2-stroke exhaust note.

My 250sx recently did just that, a new plug with correct gap cured it. -Just to add to Frits thoughts on the ignition perhaps beeing the cause in your case.

[FastFred]

This sound change is an interesting one as by all accounts its making as much power or a little more so its not an engine that's gone off song.

[teriks]

This sound change is an interesting one as by all accounts its making as much power or a little more so its not an engine that's gone off song.

Imo thats well in line with

Funny sounds can turn up if the ignition is borderline. Improving the cylinder filling can be enough to prevent the spark from sparking at the right time every time. A simple test is to close the plug gap to 0.2 to 0.3 mm. If that improves things, you need a stronger ignition.

Witch makes sense since the electric breakdown strength (or resistance to sparking) of a gas is ~proportional to it's density (that is not the case for the breakdown strength along the insulator surface though).

[SwePatrick]

OK, I do have a big carb at 34mm on a 100cc motor. It has a 430 main, proven on the dyno to make best power. Someone else can check with Wobbly whether the pipe is right.

Sarkasm?
Anyway.. i figure you are running mikuni jets..
If so,, just a dumb statement again.
as Keihin measures in 1/100mm
and Mikuni some 'flownumber'

How should i express myself?,, 'Geez' maybe? *lol*

Rgds

[dinamik2t]

Check the big rad on the transfer duct/bore edge.
Here is a pic of one I have just done, that happened to have a tapered spigot - enabling the Aux ducts to be run all the way down into the pipe.

Extra nice Wob!
(the path from C to B tfr entry is 'aftermarket'? This was also in the KTM 50 cylinder, wasn't it? it's a reed valve trend?)

In the tripple port exit, do you calculate .75 area with or without the aux 'cheeks'?
The old RSW drawings Frits provided, have the main oval ~39x31.5, plus a R11 for the aux's. While the 'main' exit is about .75, the total 'main+aux' exit exceeds that number quite much.

[Frits Overmars]

Chamfer and a little enlarging of the transfer port entry, no other engine or ignition changes at all.

No kidding. Man, it's a different engine now!