ESE's works engine tuner

[wobbly]

As Neels says you have to use some judgement to get the actual flow width correct.
Using the example he gives in the help file, if we call the bottom port an A port with a heavily sloping front wall, the flow width will be very close
to the original flow width as he drew it .
If we call the top port a B port with a hook, its effective flow width will be very close to the chordal as he drew it.
The A port may in fact be much wider chordally, but because of the entry geometry the actual flow thru that port will be reduced by the cosine
of the entry angles - exactly the same as the software reduces the effective area by the cosine of the axial upward angle of the duct.
Where there are two distinct entry angles, use your judgement to get what would be the real flow width.

[peewee]

wobbly how did you go about calculating the entrance ? the engmod help file doesnt seem to be real specific about this.

did you just use chordal width x height from case to cylinder sleeve, like the green arrow ?

or did you use something more like a flow width as the red arrows show. red arrow A + red arrow B x height of case to cylinder sleeve ?

[peewee]

actually if you used a chordal width you would probly do something more like this , with the opening width split into 2 parts to give designation for the A port and B port. then simply green arrow A x height. green arrow B x height

[wobbly]

Depends upon the cylinder design.
In the case of the Yamaha you are working on the septum divider is very tapered, so the effective entry area is pretty much as you say for each port - the two green arrows times the height.
But in an Aprilia where the septum is very wide, the effective duct acts much more like the angled flow widths times the height.
Then there is the extra entry coefficient added by the shape of the inner bore wall.
The length is the important factor, the entry area/exit area ratio does not make a big difference at all, except where you do not have the transfer duct volumes as part of a measured input.
I recently ran a test on this out of interest - with everything kept constant ( including the case vol ) having a 1:1 entry/exit ratio and a 1 :1.5 ratio made virtually no difference.
This ties in with my assertion that all of the mixture transferred into the cylinder is sitting in the ducts, there is no actual flow from the case into the cylinder when the transfers are open.

[Grumph]

This ties in with my assertion that all of the mixture transferred into the cylinder is sitting in the ducts, there is no actual flow from the case into the cylinder when the transfers are open.

I don't disagree with you....How do you see the classic MZ style 3rd transfer at the rear fed by a window in the piston ? Is this the exception ?
Is it worth looking at Joe Ehrlich's boost ports again - if room can be found for them ?

[husaberg]

What is this bike Husa?

Its been done with 2 watercooled MB50 cylinders by Phil Bird. Actually GPR own it for last 20 yrs

I've looked at RG cases & wondered about a Vtwin conversion that might just fit in the RS chassis. Would be light for a twin which the GT isn't + GT is 5 speed.

Yes, I have seen pictures of it somewhere that Scott posted a while back.



RG V twin .... that could be possible, light, good gearbox.

Heavy 5 speed GT, thats why I thought it might be a good candidate for cutting the gear box off and making a CVT out of it.

Gpr100
Suzuki GT125 bottom end modded MB50 cylinders water cooled engine made by...(dave or mike mentioned it) Phil Bird

[wobbly]

The duct length is very short in the MZ style thru the piston feed entry, but this is probably offset by the fact that the axial angle is so steep the
effective area at the bore is tiny - thus the actual flow cc is tiny as well..
But even that small duct has to be filled from somewhere, and the hole in piston,into the duct works well compared to non at all.
I worked at EMC building pipes in the early days of the Rotax 256 , and the idea of a hole in the piston, lining up with a hole in the boost port duct made a couple of Hp in the twin.
We tried doing the same thing with all the transfers, spending hours grinding holes in the bores, but sadly this made less power.
Later on in the Rotax development it was worked out that the boost duct entry was badly shrouded, and once this was fixed along with a 1/2 moon cutaway in the piston skirt
was introduced, power went up another couple of Hp over the hole in piston idea.
Joe's boost ports were a quick fix for crap scavenging in a piston port, and really were just a band aid - but as he was very good at ,Yamaha paid a fortune to him for the idea to be used in
the twin 125 Yamaha that really was a very good little engine in its time.
Yamaha paid Joe another fortune for the dumb offset combustion chamber patent, that only got into production for 1 year in a TZ.
Then they finally realised that having no squish on one side, and huge squish velocity on the other ,actually caused detonation - someone at Yamaha Corp must have felt really stupid/and or got fired.
But Dr Joe went laughing all the way to the bank - then started taking loads of cash from USA outboard people, again mostly bullshit technology, but hey I dont want to overly deride those
that taught a young Kiwi plenty about the workings of a racing 2T.

[senso]

Wobbly, one question regarding the transfer ducts sizing, you say/have discovered that the relation between the entry and exit areas is not a critical value, but what about the ducts volume, since the engine only charges the cylinder with what is available in the ducts, having small ducts with reduce volume can limit the engine total power?

Being more specific, I have a 54x43mm cylinder(yes I know, its over-square as hell, but to mitigate that I will use a 45mm stroke), that cylinder has a total volume of 84cc in the ducts of the A and B transfers, being the C feed directly from the reed-cage intake.
I usually make then a bit bigger, but much because there is only 4mm of aluminium to seal the cylinder against the crankcase, but enlarging them makes the duct outer walls practically vertical, what would be better, to have a bit more volume to feed the 100cc cylinder or try to maintain the curvature?

The boyessen ports are also tiny, I can only bore them to 6mm, more than that must be made oval.

Some pictures to make the question more clear:

[wobbly]

When I said that the length is the important factor, not the in/out area ratio, I should also have added that the duct shape that sets the scavenging
efficiency of the port combinations more important than even the length.
The duct length in the sim includes the area below the cylinder deck, so in reality the volume you quote will be way bigger than the swept volume.
So to answer the question,keep the curvature and make the Boyesen ports oval to ensure you dont start to become intake area limited with the bigger cc
as well as the higher bmep.

[F5 Dave]

Back to the 256 shrouding issue, I've ended up reversing my skirt (ooer) for more support (I went from having virtually no skirt with centre cut out and mooned piston to full piston & centre protrusion). But I've got the stub in the middle, I guess if I'm charitable like an RS Honda. Case reed mod to MB100.

this lightly encroaches into the reedblock stream but so be it. The Boost entry is now filled in on both sides from this picture, but it is shrouded from the centre to the duct, but not from the reed as much (but with 90*turn). The Rotax being rotary valve I guess this would be an issue more than a case reed valve? Or should I be looking at a hole?

[TZ350]

What an 85% RG50 exhaust port looks like.

We are in untried territory now, I have some new pistons coming, and it will be interesting to see how the extra 5% goes.

Timing, Ex opens 84 atdc, Trans open 121 atdc and as wide as possible, In opens 90 btdc with Suzuki case reed.

[F5 Dave]

Better put a real careful chamfer on that and smooth it super well. Even then the piston might pop out the port every 2nd revolution and up the bore alternate ones. Maybe you've created a four stroke?

[jonny quest]

I recently did some dyno testing on a newer Yamaha YZ125. They have a hybrid type flat top piston with a 12 degree angle about 3mm wide on perimeter with head to match.

I installed a dome piston with exact same edge height, modified a head to RSA style to match the dome, and used same compression ratio as flat top setup (15.5:1).

The stock flat top made a full 1 hp more. The only advantage dome setup had was a flatter peak curve.

Both pistons were cast OEM variety, the domed piston was from a CR125 (very similar to RS125).

Just curious if anyone has found more power with a flat top piston?

[TZ350]

Better put a real careful chamfer on that and smooth it super well. Even then the piston might pop out the port every 2nd revolution and up the bore alternate ones. Maybe you've created a four stroke?

I am hopeful it won't behave like a typical four stroke and shit itself, never the less I won't be standing to close when it gets run up for the first time on the dyno.

[F5 Dave]

If it survives pull the head off and check for scratches above the port. Uncharted waters but certainly got the area mine was begging for but I was only game to push 75%. Spend some time. I have a cigarette oilstone for final smoothing but you'll need to go hard to start with, its sharp as pictured.