Max exhaust port areas
- Thread starter StepVino
- Start date
Big B NRG
Well, this is awkward
Don't answer if you don't know :X
Making it wider then "necessary" will give you a worse gasreaction. That's what I know for sure. Offcourse your pistonring will always wear out faster (the bigger the faster, even if it's alittle bigger).
On the otherhand, you might gain some torque over the line. But I don't think it stands up to the downsides of having a bigger/wider port.
Crisp gasreaction has mine preference over alittle more torque.
Thanks Big B. By gas reaction you mean the combustion,
or the flows in/out after exhaust opens? (or both?)
After taking a closer look at changing timing, and calculating
areas, I noticed that in some cases (stock cylinders with very
low port timing), it's difficult to raise the exhaust port by removing
material from the top, then making it as wide as possible at the top,
and keep area down where it should be.
If max size is important, then raising the barrel (to meet intake timing)
seems to do the trick. The exhaust roof needs less grinding, giving you
a deficit of area that you can use to widen the port at the top.
PS Bounkass is paying me back for my comment on the in-cylinder
camera.. we are now even!
or the flows in/out after exhaust opens? (or both?)
After taking a closer look at changing timing, and calculating
areas, I noticed that in some cases (stock cylinders with very
low port timing), it's difficult to raise the exhaust port by removing
material from the top, then making it as wide as possible at the top,
and keep area down where it should be.
If max size is important, then raising the barrel (to meet intake timing)
seems to do the trick. The exhaust roof needs less grinding, giving you
a deficit of area that you can use to widen the port at the top.
PS Bounkass is paying me back for my comment on the in-cylinder
camera.. we are now even!
Big B NRG
Well, this is awkward
Thanks Big B. By gas reaction you mean the combustion,
or the flows in/out after exhaust opens? (or both?)
After taking a closer look at changing timing, and calculating
areas, I noticed that in some cases (stock cylinders with very
low port timing), it's difficult to raise the exhaust port by removing
material from the top, then making it as wide as possible at the top,
and keep area down where it should be.
If max size is important, then raising the barrel (to meet intake timing)
seems to do the trick. The exhaust roof needs less grinding, giving you
a deficit of area that you can use to widen the port at the top.
PS Bounkass is paying me back for my comment on the in-cylinder
camera.. we are now even!
With gasreaction I mean both, which translates in fast throttle respons.
One way to raise the timing without making the exhaustport bigger (higher) is raising the cylinder and make it shorter at the top.
Offcourse there are other ways to get a crisp and fast throttle respons, but the width and the height of the exhaustport sets the trend.
tiba_karotsu
OVERREV TUNING
the good side about porting to make the timings higher is that you can also play with roof angles but i see the trend as high roof angles/low port timing, a couple of years ago most cylinders have low roof angles/high port timing & flat roof angles on most high end racing cylinders of the time prove that.
Interesting how design trends flow back and forth between
new and old ideas. Bell's book shows old style ports pointing up,
but new (1983) styles flat across piston. There was a low port
timing trend in the 60's, when state of the art was reducing
crankcase volume.
I get the feeling that you can do pretty much whatever you want,
as long as cylinder and pipe each comlement what the other is doing.
new and old ideas. Bell's book shows old style ports pointing up,
but new (1983) styles flat across piston. There was a low port
timing trend in the 60's, when state of the art was reducing
crankcase volume.
I get the feeling that you can do pretty much whatever you want,
as long as cylinder and pipe each comlement what the other is doing.
I prepare most cylinders this way by raising the cylinder to put the transfers close to the duration I prefer then cut the exhaust port.
If you go to wide on the exhaust the bottom end power will be lost.
If the port is the correct size closest to the piston then sometimes different shaping techniques can be applied to the tunnel part of the exhaust port to make the exhaust port perform more efficent .
tiba_karotsu
OVERREV TUNING
2stroke is never finished. There are a zillion ways to reach different things. That's what makes it fun
i totally agree with you, 4strokes are an evolutionary dead end, it is the poppet valves that limit it's potential. there is only so much you can play with valve angles, sizes, ports etc. the only way for a 4stroke to make more power is by revving it higher which shortens it's service life really quick.
Aloha, I have read all the previous posts here and have comments and maybe some questions...
1: I have seen that an exhaust port that is widest and flatest on the top give the strongest exhaust pulse into the expansion chamber and more power.
2: The strongest cylinders seem to have the most exhaust escaping at the top of the port, ie Evo, wide bridge port. No single rounded exhaust port will produce much power. Of you look at the carbon buildup in the exhaust port, you will find the cleanest area on the top and so the fastest exhaust escaping there.
3: My Malossi Replica with single exhaust port and two auxillary ports gives much better power than a single exhaust port because of the larger exhaust area opening. (time/area). My Metrakit sp2 is mostly the same as the Malossi Replica, but has a larger main port and much larger auxillary side ports and gives more power than Malossi.
But I think were it gets very difficult when you make the transfer angles different with larger exhaust port, you will loose the charge short circuiting into the exhaust and get less power. This is where a lot of research and development and several ruined cylinders will come into play.
frank
1: I have seen that an exhaust port that is widest and flatest on the top give the strongest exhaust pulse into the expansion chamber and more power.
2: The strongest cylinders seem to have the most exhaust escaping at the top of the port, ie Evo, wide bridge port. No single rounded exhaust port will produce much power. Of you look at the carbon buildup in the exhaust port, you will find the cleanest area on the top and so the fastest exhaust escaping there.
3: My Malossi Replica with single exhaust port and two auxillary ports gives much better power than a single exhaust port because of the larger exhaust area opening. (time/area). My Metrakit sp2 is mostly the same as the Malossi Replica, but has a larger main port and much larger auxillary side ports and gives more power than Malossi.
But I think were it gets very difficult when you make the transfer angles different with larger exhaust port, you will loose the charge short circuiting into the exhaust and get less power. This is where a lot of research and development and several ruined cylinders will come into play.
frank
IkKe00
Well-Known Member
Your point 2 isnt correct, its because thats the first place when the mix can escape (when the piston comes down) so its quite logic that there will be allot more pressure on that little spot.
/edit nevermind your correct but what I wrote above is the explanation for that
/edit nevermind your correct but what I wrote above is the explanation for that
Big B NRG
Well, this is awkward
That depends on what kinda engine you are running I think. On scooters I haven't seen negative results in bottom end power. Maybe with automatic (chain driven) engines.
Or do you mean it has "trouble" getting up to revs. That I agree.
I first measure all timings stock, then do some calculations to timing and blowdown and count in the parts that will be used (carb/exhaust/cartervolume), also I take in to matter for what purpose the engine will be used (cross/race/sprint/street).
Big B NRG
Well, this is awkward
2stroke is never finished. There are a zillion ways to reach different things. That's what makes it fun
:winner:
The weird thing is, that allmost every racer with 125cc or up uses a single port + aux. ports. You never see T-shaped exhaustports on that.
A fact is, the middle of the exhaustport (where the bridge is) is the most effective part of the port.
If you want power, it's better to adjust angles then adjust timings. The timing affects when it starts flushing, but the angle affects how it flushes and since where are talking about gas dynamica, we are talking about the "HOW" and in less way the "WHEN"
The weird thing is, that almost every racer with 125cc or up uses a single port + aux. ports. You never see T-shaped exhaustports on that.
Yes that seems why that single + boost ports are better than T shaped even if they have more area. (look at how crazy big Fabrizi main and boost ports are). Figuring out angles I think is beyond the average tuner with only a couple of cylinders to play with. But as we all know porting is where ALL the power is on a 2 tak.
frank
Yes that seems why that single + boost ports are better than T shaped even if they have more area. (look at how crazy big Fabrizi main and boost ports are). Figuring out angles I think is beyond the average tuner with only a couple of cylinders to play with. But as we all know porting is where ALL the power is on a 2 tak.
frank
Your point 2 isnt correct, its because thats the first place when the mix can escape (when the piston comes down) so its quite logic that there will be allot more pressure on that little spot.
/edit nevermind your correct but what I wrote above is the explanation for that
In my mind then the center is where "all the action is" and is more critical at the first opening and get as much exhaust pressure out (widest opening) before the inlet/transfers open. --- taking most advantage of the blow-down time. Then the correct angles for keeping the top of the piston cool and keeping the most charge inside the cylinder and not short circuiting out the exhaust.
Also is not loss of torque when going exhaust wide is because the exhaust pulse becomes much stronger and the expansion chamber makes the power a much narrower power curve? And so more HP but less torque, "more pipey"
I wonder how a cylinder would work if it had another large exhaust port across at the position of the boost ports?
frank
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