Carl Walichnowski
Active Member
Good question which I didn’t even think of.
What to remove while porting??
- Thread starter Carl Walichnowski
- Start date
like to know b4 I decide to do it. I mean I got extra rings but I'm hitting 35 cruising 40 full throttle as is. I would like to know if I gatta break in at 20 again
Carl Walichnowski
Active Member
You may want to create a thread asking that same question, I’m sure there’s a lot of MBers who are porting for the first time.
FurryOnTheInside
In Memoriam
If you want to change the height of the ports then you should really print out a degree wheel and stick it to some cardboard so you can use the figures for the "port durations" that a few members have posted. As there are differences between engines the degree wheel gives a truer comparison, and degrees of duration is what actually matters!
But you'll probably be safe if you only lower the bottom of the intake by up to a few millimetres and only raise the exhaust port by 0.8 or 1mm. Raising the exhaust port will increase the top rpm and you can certainly go further than 1mm but it is said to reduce your low rpm torque so your acceleration might be a bit slower. This advice obviously ignores the fact that the stock exhaust duration may vary.
Of course if you want to leave the heights stock and maybe come back to that later that will be fine too!
Widening the top half of the exhaust port and the lower half of the intake port is a very common modification.
I think the only limit is that you must leave some cylinder wall to support the piston rings. I read that the intake mustn't be wider than the position of the ring gaps, but the piston never gets that far down the cylinder, so idk about that. The maximum exhaust port width is certainly limited by how much of the rings can be unsupported before they bulge out into the port and catch on the top of the port as the piston moves up the cylinder.. But based on the figures I've seen on other threads that limit is high enough to not matter for now.. No way you'll be brave enough to get near that sort of width on your first go.
I made my ports somewhere in the middle of the widths had seen posted in various threads and I am sure I'll be experimenting more, seeing how wide I can go before I either start losing power or lose confidence to keep widening them (probably the latter). I am not sure if I ever actually posted my port widths but they are just conservative and fairly average so there wasn't much point. I just used the shapes that were there and made them more of a trapezoidal shape.
We shouldn't follow one another sheepishly anyway IMHO, or it will stifle creativity and take the fun out of modding and seeing what you can get out of your engine.
Cylinders are inexpensive to replace and what's the worst that can happen, that it won't be as good as you were expecting?
As long as you finish (bevel/round) the edges of the ports by rubbing around the edge with a bit of wet n dry paper wrapped around your finger. You don't want to catch a piston ring on the edge of a port. Other than that I think you might as well just get stuck in and enjoy the experience of exploring without too much rigid guidance.
I mainly want to cut away some of bottom of my piston to match my intake side. really didn't want to go threw the break in period again if I had to change my rings. I have the rings I got a whole engine that i bought on eBay it's a black one that I got my money back cause all the person did was stay paint it and it's a bad job at that. chain will not go on the drive he told me to keep the engine. so spare parting it if needed.If you want to change the height of the ports then you should really print out a degree wheel and stick it to some cardboard so you can use the figures for the "port durations" that a few members have posted. As there are differences between engines the degree wheel gives a truer comparison, and degrees of duration is what actually matters!
But you'll probably be safe if you only lower the bottom of the intake by up to a few millimetres and only raise the exhaust port by 0.8 or 1mm. Raising the exhaust port will increase the top rpm and you can certainly go further than 1mm but it is said to reduce your low rpm torque so your acceleration might be a bit slower. This advice obviously ignores the fact that the stock exhaust duration may vary.
Of course if you want to leave the heights stock and maybe come back to that later that will be fine too!
Widening the top half of the exhaust port and the lower half of the intake port is a very common modification.
I think the only limit is that you must leave some cylinder wall to support the piston rings. I read that the intake mustn't be wider than the position of the ring gaps, but the piston never gets that far down the cylinder, so idk about that. The maximum exhaust port width is certainly limited by how much of the rings can be unsupported before they bulge out into the port and catch on the top of the port as the piston moves up the cylinder.. But based on the figures I've seen on other threads that limit is high enough to not matter for now.. No way you'll be brave enough to get near that sort of width on your first go.
I made my ports somewhere in the middle of the widths had seen posted in various threads and I am sure I'll be experimenting more, seeing how wide I can go before I either start losing power or lose confidence to keep widening them (probably the latter). I am not sure if I ever actually posted my port widths but they are just conservative and fairly average so there wasn't much point. I just used the shapes that were there and made them more of a trapezoidal shape.
We shouldn't follow one another sheepishly anyway IMHO, or it will stifle creativity and take the fun out of modding and seeing what you can get out of your engine.
Cylinders are inexpensive to replace and what's the worst that can happen, that it won't be as good as you were expecting?
As long as you finish (bevel/round) the edges of the ports by rubbing around the edge with a bit of wet n dry paper wrapped around your finger. You don't want to catch a piston ring on the edge of a port. Other than that I think you might as well just get stuck in and enjoy the experience of exploring without too much rigid guidance.
I just saw some videos where people are cutting the bottom of the piston for more power. I like power lol and speed. I'm cruising solid in low to mid 33-35 I did hit 39 40 at one point but need to get better brakes (should be here tomorrow) those speeds and nothing but cruiser brakes ain't cutting it.
Street Ryderz
Well-Known Member
As Furry said you can go pretty wide with the ports safely the only limitation is the flange area and stock stud spaceing,I've tried to run 31mm across on the exhaust at the flange area and it leaves no materail to seal properly and the header had to be indented for the nuts to clear on the studs or even for the round heads of a allen bolt. 27mm seems to be the biggest that works best and still fits the stock spaceing.So if the flange area is 27mm across and lets say it's 20mm in height that's an equivalent dia of 26mm and the window should be just slightly smaller and since the window height is only 14-15mm unless it's been raised,you can still make the windows width 27mm and have the sides flush and straight with the flange area.At this width there is no ring catch to worry about again as Furry said just make sure the edges are chamfered and it's all good!The intake is the same thing and should have an equal or no more than 2mm smaller dia or equivalent dia than that of the exhaust port.
FurryOnTheInside
In Memoriam
I don't think you'll have to break in like the first time all over again, and you can use the same rings. It is often said that break in is all about riding slowly, but as Jaguar explained it's high load that seats the rings, accelerating hard or riding uphill, and that's pretty much all I do anyway.
Cutting a notch in the bottom of the intake side of the piston skirt is equivalent to lowering the bottom of the intake port. It extends the intake duration.
I would rather switch to a reed valve intake before lowering the intake much, because the piston ported intake is "symmetrical", it is open when the piston is descending (and its a wonder that the piston ported engine works at all, lol) so there's a limit to how much duration you can have before it spits lots of a/f back out of the carb. Reed valves allow a much longer duration (and are quieter) because they block that reversion.
But to stay on topic. I am sceptical about the reasons given for cutting the piston skirt. I'm not saying that a little longer duration isn't worth exploring, but I see that the skirt notch is recommended for reasons that don't particularly add up imho lol.. The notched skirt clears the top of the intake.. Okay but at that moment the piston is no longer ascending anyway.
I haven't been in the hobby long enough to prove that widening and slightly lowering the bottom of the intake port is more effective than skirt notching, but it makes sense to me.
I have a "crappy time" black ebay engine too, but even after doing several of the most popular mods mine has only been up to 8000rpm / 37.27mph so far, so you're doing well!
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FurryOnTheInside
In Memoriam
Oh I totally forgot about breaking through to the studs! And I forgot all the actual figures lol.
I don't use the black engine it's a complete but unusable engine. but i have picked a part of it here and there. I bought my engine I use from pension company out of Massachusetts.I don't think you'll have to break in like the first time all over again, and you can use the same rings. It is often said that break in is all about riding slowly, but as Jaguar explained it's high load that seats the rings, accelerating hard or riding uphill, and that's pretty much all I do anyway.
Cutting a notch in the bottom of the intake side of the piston skirt is equivalent to lowering the bottom of the intake port. It extends the intake duration.
I would rather switch to a reed valve intake before lowering the intake much, because the piston ported intake is "symmetrical", it is open when the piston is descending (and its a wonder that the piston ported engine works at all, lol) so there's a limit to how much duration you can have before it spits lots of a/f back out of the carb. Reed valves allow a much longer duration (and are quieter) because they block that reversion.
But to stay on topic. I am sceptical about the reasons given for cutting the piston skirt. I'm not saying that a little longer duration isn't worth exploring, but I see that the skirt notch is recommended for reasons that don't particularly add up imho lol.. The notched skirt clears the top of the intake.. Okay but at that moment the piston is no longer ascending anyway.
I haven't been in the hobby long enough to prove that widening and slightly lowering the bottom of the intake port is more effective than skirt notching, but it makes sense to me.
I have a "crappy time" black ebay engine too, but even after doing several of the most popular mods mine has only been up to 8000rpm / 37.27mph so far, so you're doing well!
I don't have many if any mods on mine still got muffler that came with their kit I did cut off the pipe on the inside and that made the bike feel smoother and oddly quieter. I put on a thinner head and a speed carb. I do have to rejet the speed carb and I run with a 36 tooth sprocket. 40 mph was my max.
FurryOnTheInside
In Memoriam
Ah, I was confused, sorry. Is your engine another generic though?
I have a different muffler, the one that's not so easy to cut off the internal tube because there's two of them. I only opened the holes in the bottom baffle plate a bit, but then added 83cm of silicone tube to quieten it so it's probably about as restrictive as stock.
I have a 38T sprocket to try out soon so I'm hoping to break 40mph (but I'm always carrying panniers and I have a trailer to pull, too!). I have a 36T but I thought that will just be a bit much and wouldn't be able to accelerate and climb very well.
With the 42T it didn't feel like it was load that was limiting the speed but just the engine's top rpm with stock exhaust port height.. Well, that and the straight wasn't long enough so I had to brake as soon as I saw the 8--- on the tach, and still nearly crashed it haha.
I think a maximum rpm of 8000 or 8500 is probably better for the lifespan of the cheap bearings than running it up to 10000 anyway.
A 36T sprocket will keep the load on your engine nice and high so that will be good for seating (or re-seating) the rings, I believe.
