Author Topic: Piston skirt cut v inlet port drop?  (Read 2029 times)

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Offline John Orchard

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Piston skirt cut v inlet port drop?
« on: October 31, 2022, 08:45:39 PM »
Sorry for all my posts, I love the experience on here :-)

So ...... Rather than cut 6mm of the inlet skirt of my TS125 piston, surely dropping the inlet port the 6mm should give the same effect on performance, while still keeping the strength & stability in the piston in the bore .... no?

I've gone to a VM32 so the inlet port needs to be opened substantially, it's a '77 model with the reed into the crankcase, although the reed cage has been opened-up with a carbon petal.

My thoughts also consider keeping the piston-port smaller to help keep up flow inertia.

Maybe I'm a bit crazy trying to re-invent the 1977 wheel ;-)
« Last Edit: November 04, 2022, 08:32:21 PM by John Orchard »
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Offline PeterC

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Re: Piston skirt cut v inlet port drop?
« Reply #1 on: November 01, 2022, 08:47:59 AM »
If you drop the inlet 6mm then what is the total duration? Are there any comparisons with other engines?
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Offline John Orchard

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Re: Piston skirt cut v inlet port drop?
« Reply #2 on: November 01, 2022, 10:05:22 AM »
If you drop the inlet 6mm then what is the total duration? Are there any comparisons with other engines?


The total duration is the same as if the piston is cut?
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Offline sleepy

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Re: Piston skirt cut v inlet port drop?
« Reply #3 on: November 01, 2022, 10:13:28 AM »
The change in duration would be the same but if you want to know what the actual change in degrees would be it depends on what the engine stroke is, the length of the conrod and how high the port is to start with. A 6mm drop in inlet port would normally be considered quite a radical change.

Offline John Orchard

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Re: Piston skirt cut v inlet port drop?
« Reply #4 on: November 01, 2022, 11:08:40 AM »
The change in duration would be the same but if you want to know what the actual change in degrees would be it depends on what the engine stroke is, the length of the conrod and how high the port is to start with. A 6mm drop in inlet port would normally be considered quite a radical change.


:-) So far I've lowered the bottom edge of the port 10mm and cut 4mm off the skirt :-)
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Offline sleepy

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Re: Piston skirt cut v inlet port drop?
« Reply #5 on: November 01, 2022, 12:27:59 PM »
Lucky it's a TS your starting with. If you did that to a TM125 or an RMm or s and it would even run without adding a read valve.

Offline John Orchard

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Re: Piston skirt cut v inlet port drop?
« Reply #6 on: November 03, 2022, 08:57:42 PM »
After investigation, I found a number of tuners suggested that due to the inlet being made larger from lowering the bottom of the port, the increase in time & area had more of an effect than just cutting the piston, which only increases time. It is suggested, which I tend to agree with, that lowering the port floor 3mm is approximately equal to 4mm off the skirt.
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Offline djr

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Re: Piston skirt cut v inlet port drop?
« Reply #7 on: November 06, 2022, 07:54:05 AM »
I am probably wrong,
 but if that engine has part of the reed straight into the crankcase , isn't that part of the inlet open all the time/360 degrees duration ?
Obviously depending on some vacuum opening the reeds.

In which case, then cutting the piston may have no effect, but opening up the port MAY increase flow.


Offline John Orchard

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Re: Piston skirt cut v inlet port drop?
« Reply #8 on: November 06, 2022, 11:22:26 AM »
Those PP/case reed engines do not have a very large reed block, I'd imagine that in a 'high port inertia' situation (read; high rpm), when the small reed block is not large enough to take advantage of all that flow, the incoming flow will overpower the start of the downward travel of the piston.
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