KR-1 (B): "Basically it uses a pair of secondary exhaust ports which open slightly earlier than the very conservative main port."
KR-1R wrote:KR250 EXdur (BBDC ABDC) ** SCAVdur (BBDC ABDC)
B. ........186° (93 93) ** 126 (63 63)
C. ........186° (93 93) ** 126 (63 63)
D. ........188° (94° 94°) ** 126° (63° 63°)
KX125 ........EXdur ** SCAVdur
90 (50.6) ......187° (93.5 ..) ** 131 (65.5 ..)
91(50.6) ....... low speed 187° high speed 191° (L93.5 H95.5) ** 131 (65.5 ..)
the 1990 KX125 was 'rated' @ 38.5PS and used similar F3 KIPS with a bridged exhaust
191° is 84.5 ATDC and 25.9mm main exhaust with 26.8mm subs (187)
^^^ too high
A 50.6 stroke against 54.5mm i have tried to compare like for like using crank angles
A page with the port map dimensions from RS/TZ over multiple years
Some Table Data
From the table note the
Sequential opening of A then B then C transfer ports (B follows A by 2.5°, C follows B by 1.5°)
The vectors of A B and C are directed centre rear of the piston crown to build a pocket of charge as separate as possible from the burnt outgoing exhaust gases.
Note also the
1mm width of divider between A and B transfers .
Look at widening the B transfer on the KR to match the 1mm spec.
Dont narrow the divider between B and C (boost) where the ring end gap runs, instead add filler to smooth out the flow blockage/step of the electrofusion peculiar to KR casting
Perhaps the sub-exhausts should be left alone al together - if they are made wider then the gudgeon hole causes a short circuit between the A transfer and sub port. Perhaps why some karts have triangular subs.
a conclusion you could make without doing a simulation model is...
the standard KR-1 exhaust opens too late ~91.5deg ATDC (the fuel is burnt, work done, the piston slowing on stroke) - should be nearer to 82-84° (192 duration) = 25.7mm ATDC
the standard transfers flow too early (not enuf blowdown - © 2Tinstitute) opening just 25° after the exhaust port.
By raising main exhaust roof alone, this increases the delay (blow down) to 31° after exhaust which is much closer the 32° A-transfer timing found on eg the H0NDA RS125/250.
Start with a conservative main and see how it responds by artificially raising it with thicker base gaskets.
because the KIPS system has no yamaha spool PV or guillotine power valve arrangement you WILL sacrifice low down power if you port to a high exhaust. You will be making a motor with less powervalve benefits.
There has been some vigorous debate here on height of subs in relation to main. The Subs are lower than the main exhaust on the YZ125 and lower on the TZ over many years. Referring to the original 1988 port map of the KR-1, the only reason they are higher than the main is this was a road bike (conservative)
In race tune the main was always meant to be raised - doing this tunes peak power to >11,000 rpm (11,500-11,750 rpm Formula 3 tune) when the OEM transfer heights make more sense.
these timings are only a fraction of the story.
Like on a four stroke, knowing the cam timing is only part of determing how it might perform.
It doesnt tell you the lift or overlap or the size of valves, the shape of ports, compression ratio and everything else that adds up to making most power.