#6

[SS90]

Found some interesting little wash marks on the piston crown adjacent to each rear stud and coinciding to the rear edge of each rear transfer port. The cylinder head also has an interesting carbon free area to the rear.



The ports are definitely lower than stock. Even with the piston at BDC the piston is masking the port by at least 2mm. Going to have to have another measure of that for sure.

While it is hard to tell from pictures, I can't see too much wrong with the scavenge patterns.

Assuming heaps here, I take it (from what you write in regards to lower port timings) that some material has been removed from the cases to lower the port timing.

This is something I do reasonably often (on certain cylinders), and, as you have found, the piston then obscures the exhaust port by a few mm's at BDC.

I do it to get certain cylinder blowdown times closer to suitable, and as such, have to raise the exhaust port to compensate (a design flaw)

First I was quite worried about this, but it turned out the gains from the correct blow down times MOre than ofset any losses I precieved I would get.

I am guessing who ever designed your engine (wobbly?) has designed it as a whole unit, and a process was followed.

Looking at your scavenge pattern/pipe design, it would be my opinion (I think you mentioned previously as well) that you are running a larger crankcase volume ( with a ratio some where around 1.3 or so)

Such enviroments (lower primary compression) seem to assists even older designed scavenge patterns in being more "efficient", also combined with the now lower port timings.

As a rough example (and VERY VERY VERY big generalisation) using such set up methods I would expect to have exhaust duration around 191 Deg and transfer duration around 130 or MAYBE 131 Deg.

Is that somewhere near what you have?

Also what are your blow down times?

Maybe as you suggest it would be worth revisiting your port timing, and as a test, space up the cylinder to get the port unobscured and degree that as well.......... just as a comparison......... correct blow down times are dependent on operating RPM's (exhaust port duration) if the duration has been lowered, but the blowdown time is the same, then there is a good chance it is now "mismatched" (not enough blow down time) which will have the effect of a great torquey engine (sound like what you have) that has a narrow peak power delivery, which falls off abruptly.

IF I am correct (I'm not saying I am) then you may well need to space the cylinder up to get the blow down times to match the exhaust port duration (at the cost of low end torque sadly)

OR...... you could raise the exhaust port height a few degrees........ and leave the transfers where they are....

In my opinion, the first thing I would do is measure the blow down time, and base the exhaust port duration you want on that time calculation.

I wouldn't be overly worried about the shrouding of the exhaust port (as strange as that sounds!)

[speedpro]

As measured originally with the stock crank the time area and blowdown values were wrong. As it happened both exhaust and transfers needed to be lowered approx 1.4-1.5mm to sort it all out. So I dummied it up with the new crank and the ports opening at more or less the same crank angles and then took material off the bottom of the barrell to lower the ports 1.5mm. It does need to be checked properly again as I have reassembled it with the short stroke crank and longer rod. Port timings are now roughly 190deg for exhaust and 120deg for transfers. It's on the bench at the moment for a few checks.

Gotta fit new fork seals as well.

[SS90]

As measured originally with the stock crank the time area and blowdown values were wrong. As it happened both exhaust and transfers needed to be lowered approx 1.4-1.5mm to sort it all out. So I dummied it up with the new crank and the ports opening at more or less the same crank angles and then took material off the bottom of the barrell to lower the ports 1.5mm. It does need to be checked properly again as I have reassembled it with the short stroke crank and longer rod. Port timings are now roughly 190deg for exhaust and 120deg for transfers. It's on the bench at the moment for a few checks.

What I don't understand is how your crank angle can be the same if you have changed the rod length and stroke...... short stroke plus longer rod = less dwell!

I am of the opinion that would alter the transfer duration somewhat to the "short" side.

190 is pretty cool....but 120 deg for transfers? I'm not sure that is correct!

My experience is that larger crankcase volumes require significantly more transfer time are than 120 deg, but I am happy to be proved wrong on that!

I may be getting ahead of myself, but from what I read I see an engine that can't breath correctly over say...... 9000 RPM fitted with a pipe that looks like it want's to work best between something like 6500 and 10000 RPM.....

[speedpro]

I originally installed this barrel and measured the port open & closing angles. I then calculated how much to raise or lower the barrel so that the ports opened & closed at exactly the same angles as with the original crank. I then skimmed the amount needed off the bottom to set the barrel 1.5mm below where the ports used to open & close with the original crank/rod. This motor does have more primary compression than the total new one. These crankcases haven't been relieved inside and the reed valve is mounted up much closer to the bore.
I do realise that with the new crank the way I have modified it the end result is not precisely what Wobbly wanted to happen. Hence my comment about it needing to be checked properly again. As for the transfers, the primary transfers have been widened about as much as you can and of course the angles have been altered. One of the things I really want to check is the angles.
The motor is good, though a bit down on power. It revs nice and doesn't fall flat. At the last meeting where I regularly followed an FXR or two I was making less gearshifts than them and it wasn't running out of revs where the FXRs were regularly hard on the limiter. Just now and again I was wanting a bit more grunt out of corners but taking a leaf out of F5s book I found the clutch was handy for sorting that out.

[SS90]

OK,

Interesting with the transfer port times you have how it doesn't fall flat in the upper RPM..... It would be fair to assume that Wobbly has designed the pipe well.

It could well be this engine needs to be set EXACTLY to what he planned in order for it all to work "as designed", but I am sure you are aware of that.

I don't have much (any) experience with the MB100 engine as far as tuning goes, but making an assumption that you have a 6 speed MB50 box in there, I am surprised that you have to slip the clutch, sure the final gearing ratio has as much to do with that as anything, but it just suggests to me that your transfer timing is simply not correct (I'm not being antagonsistic here)......... 120 Deg I have never seen with an exhaust of around 190....... personally the shortest duration I have seen on anything "approaching" a tuned small cc two stroke is about 126 deg.

Like F5 Dave said ages ago, it's silly to throw port durations around willy nilly, but just to be sure, is it really 120 Deg? I realise you said you want to recheck it and it may be different, but have you ever seen a performance two stroke with such short transfer duration?

I'm quite interested if it is!

With your blowdown time, while I realise that there is no "one size fits all", but I think from what I see that less than 30 deg is in order.....which will sacrifice some peak power, but will put it over a broader area.....and from what I can tell/guess from your pipe design/port timings is what Wobbly had in mind.

It would be interesting too see what is measure out to be!

[F5 Dave]

I would have thought (now I'm throwing around port timings) that most GP type bikes ran somewhere near the mid 30s blowdown, but things of course get complicated comparing powervalve engines. I've run up to 38-39 bd on high speed engines, but really while it was good for power it did tend to make them somewhat too peaky, even for a 50.

Yes those do sound some low timing values, but I don't know what sort of revs you were intending, I'd previously assumed fairly high. Obviously you can make up for some wilder blowdown timings if your tr area is greatly improved & still efficient, but the short ccting must become an issue at some stage as the ports get crammed close together.

[speedpro]

Being extra careful and measuring the port timing 5-6 times I get 187deg exhaust and 123deg transfer. The barrel is securely bolted down.

I still haven't measured the angles as I had a few beers and watched the Warriors pound the Raiders.

[bucketracer]

Being extra careful and measuring the port timing 5-6 times I get 187deg exhaust and 123deg transfer. The barrel is securely bolted down.

Trans 123deg duration = trans open 118.5deg after top dead center (TDC).

Ex 187deg duration = ex opens 86.5deg after top dead center (TDC).

Therefore blowdown is 118.5deg - 86.5deg = 32 deg.

32 deg seems in the ball park of 28-36deg.

Blowdown time area depends on the exhaust port width/shape engine capacity and rpm.

Blowdown time needed is influenced by the pipe. Section 6 on the Bimotion web page talks a bit about it. http://www.bimotion.se/

I dare say someone with some experience could do quite a bit by juggling the blowdown and expansion chamber diffuser/diffusers to get the power delivery they want.

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[SS90]

Being extra careful and measuring the port timing 5-6 times I get 187deg exhaust and 123deg transfer. The barrel is securely bolted down.

I still haven't measured the angles as I had a few beers and watched the Warriors pound the Raiders.

That's really interesting.....

From my experience those sort of durations don't seem to match what I (assume) your expansion chamber design is optimised for.

Treading on a dangerous path (again) my experience tells me that such durations are more suited to a pipe with quite a different diffuser to what I can asscertain you currently have.....

This isn't a slight on anyones work by the way.

Also, again with my experience the set uo that is required for port time areas of that sort, is a primary compression ratio of about, (letter bombs to the usual address please) say, 1.4:1 (which would also require a different scavenge pattern)

so 187 exhaust and 123 transfer does actually compute (in my book), but I am of the opinion that for such a set up, the exhaust is not quite suitable.....

Having said that, I have no idea of teh pipe specs, but the diffuser is somewhat long (as is the urrent way of thinking.

as silly as it sounds, I think a spacer is required under the cylinder!!

[SS90]

32 deg seems in the ball park of 28-36deg.


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for the times that speedpro currently has, I recon something closer to 25 deg would be in order (requireing the exhaust port height to be raised.....which wold lower the trapped compresion ratio, requireing machining of the cylinder head......)

The devils circle starts again.......

where did you come up with the figures of 28 to 36 deg for those port timings Bucketracer........ my experience tells me that that is way too high!

[TZ350]

Trans 123deg duration = trans open 118.5deg after top dead center (TDC).

Ex 187deg duration = ex opens 86.5deg after top dead center (TDC).

Therefore blowdown is 118.5deg - 86.5deg = 32 deg.

32 deg seems in the ball park of 28-36deg.

From memory Honda RS125's and the early Suzuki RM125's had timings like:- Trans opens ATDC 116deg and ex opens 86deg ATDC and blowdown = 30deg.

Currently my bike makes 20rwhp and the Ex opens 83deg and trans open 114 ATDC, blowdown = 31 deg but with a smaller exhaust port its time area is less.

To make meaningfull comparisons between engines we really have to compare the BMEP's. HP torque rev's are fun but the real comparitor is BMEP.

BMEP-PSI = Average Cylinder Pressure in PSI

Two Stroke -- BMEP = HP x 6500 / L x RPM

Four Stroke -- BMEP = HP x 13000 / L x RPM

L = Displacement in Litres (80 cc = .08 Litres) (700 cc = .7 Litres)

So 27.1*6500/0.125*7800 = 180.7

BMEP of a 27.1hp 125 at 7800rpm is 181psi.

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[SS90]

I would have thought (now I'm throwing around port timings) that most GP type bikes ran somewhere near the mid 30s blowdown, but things of course get complicated comparing powervalve engines. I've run up to 38-39 bd on high speed engines, but really while it was good for power it did tend to make them somewhat too peaky, even for a 50.

Yes those do sound some low timing values, but I don't know what sort of revs you were intending, I'd previously assumed fairly high. Obviously you can make up for some wilder blowdown timings if your tr area is greatly improved & still efficient, but the short ccting must become an issue at some stage as the ports get crammed close together.

Thats interesting with those blowdown times (assuming it is for your RG Fiddy)........ clearly that will increase your peak ) at the expense of range), but I remember you mentioned that RG fiddys arent partial to high RPM's....... I have found around 40 deg is suitable for around 14000RPM, have you found the same?

[bucketracer]

where did you come up with the figures of 28 to 36 deg for those port timings Bucketracer........ my experience tells me that that is way too high!

I can't remember, possibly one of those old school books, Blair gives a guide to transfer and exhaust durations, a bit of simple arithmatic and you can deduce the blowdown degrees required, not saying 30ish is right, just what seems familiar.

25deg, its interesting to have your opinion.

I expect that the blowdown deg's is closely related to the blowdown time area and diffuser section of the expansion chamber.

I can see improvements in chamber design and effective exhaust port width/area reducing the number of blowdown deg's required.

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[bucketracer]

Something for you Speedpro, I went looking for references to blowdown degrees required but got side tracked by this article on measuring ports, and I was particularly impressed by how they measured port angles.

www.kartweb.com/TechArt/2Stroke/chapter2.doc

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[SS90]

I expect that the blowdown deg's is closely related to the blowdown time area

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Ermmmm