ESE's works engine tuner

[TZ350]

But still, if I hear dinamik simming 47 and making 34 then what's the use ai?

I think your right, changing things like port timing is wrong but varying the combustion and I guess delay and other related variables is the way to get the simulation to reflect your engines reality at the crank.

Although EngMod2T is just a simulation and not real life on a dyno, it does suggests the outcome of any changes and direction to take, and EngMod does this very well. But sometimes I have found myself tickling the simulation to get a bigger hp number, with changes that can't be made in real life.

Below is Wobs view.

Combustion Efficancy, I vary depending upon the projected quality of the elements.

Running high com on Avgas with a toroid, in a square engine its up around 92%
Low com on pump with crap chamber shape and oversquare, its down around 82%.
Then lastly I move it around a bit if I have what seems to be too much power, but the curve shape is on the money.
It seems to be a fudge factor that can be used to move all the power up and down, it doesn't change the shape.

Re the computer sims believability factor, I have done so many trips to the dyno and got exactly what the sim said I would that its simply not an issue any more.
Only when reaching the outer limits, such as trying to model an RSA, that depends so much on alot of info we still havnt seen yet ( such as the PWM curve for the powerjets etc),then its a case of crap in = crap out.

[TZ350]

Chambers offers a virtual Chocolate Fish to the first person who correctly identifies what this cylinder came from and nope, not a lawn mower.



Low wide inlet port, transfers and exhaust are just drillings.



The carbon build up clearly shows the combustion gases have been back flowing down the transfer ports. More so on the exhaust port side and its instructive how far the carbon extends down the port wall, apparently this happens on part throttle at high rpm.

It seems that at high rpm and part throttle the pressure in the cylinder is higher than in the crankcase when the transfers open and hot burnt combustion gases back flow down the transfers. Part throttle detonation is the current big issue for our 30+hp air-cooled engine. Retarding the ignition on part throttle is only part of the solution.

The above Quote links to a detailed discussion by Jan Thiel about this back flow phenomenon and its effects on high performance 2-stroke reliability.

[crazy man]

a chain saw

[F5 Dave]

Well I'm glad you did the larger carb experiment. That is not what I would have expected either, but it underlines my mantra of 'test everything'. I have been surprised many times fitting stuff that is going to be better & isn't. For instance I had a 24mm carb for my 50 as well as a 26 & a 28. All Mik flatslides. I made a new inlet & it was 26mm at the manifold.

Of all the carbs the 28 was the best. The 24 which I was hoping would be my kart track setup wasn't as good but almost matched the 28 in overrev. the 26 was worse all over no matter what jets were fired at it. Clearly the slope may be dependant on the air bleed, but you should be able to get some of the curve to match, like peak.

[Drew]

a chain saw

I thought that too, but how do you run a chain saw at part throttle/high rev?

So my guess is, radio controlled plane.

[TZ350]

a chain saw

We have a winner

I thought that too, but how do you run a chain saw at part throttle/high rev?

Yes, good point ... not sure.

[TZ350]

Well I'm glad you did the larger carb experiment. That is not what I would have expected either ... For instance I had a 24mm carb for my 50 as well as a 26 & a 28.

Interesting about the different carbs you have tried on the 50 and that the 28 was the best, where I would have gone for the 24.

[Drew]

Yes, good point ... not sure.

It's bugging me for some reason.

Only thing my less than optimum mind can come up with, is that if the governor on a chainsaw works the same as a lawn mower by oscilating the throttle butterfly quickly when the revs get up. I suppose that is part throttle.

[dinamik2t]

Below is Wobs view.

An addition on those, but only from 'sim life' testing:
Ceff will impact the overrev. Lower the factor and it has less power/rpm in overrev. Also, higher Ceff setups, are more sensitive to ADV and have higher cylinder temperatures.

[crazy man]

We have a winner






Yes, good point ... not sure.

yum chocy fishie

[TZ350]

It's bugging me for some reason.

Only thing my less than optimum mind can come up with, is that if the governor on a chainsaw works the same as a lawn mower by oscilating the throttle butterfly quickly when the revs get up. I suppose that is part throttle.

All the Still chain saws I have had something to do with didn't have a governor, just a trigger throttle. Although you didn't hold it on part throttle much, it was more like all or nothing but I guess all the momentary times on part throttle all add up.

For a chain saw I suspect you also get this blow back when its maxed out and won't rev any higher because there is insufficient blowdown time area to support any more rev's.

[HPbyGD]

How about just taking all the vent lines from the carb and either apply a small pressure or a small vacuum to them to get an indication of which way your jetting needs to go ? It is a lot faster then stopping the testi to change jets !

Gary

Having managed 29hp with the taper bored 24mm OKO I wanted to see what difference a bigger carb might make. The taper bored OKO is 31mm at the engine flange so I bored another OKO out to 30mm at the slide and 31 at the flange.

Set the 30mm carb up with a main jet one size bigger than the 24 and ran it up. It started and warmed up ok but when I tried to move up through the gears it wouldn’t accelerate.

Anything over half throttle and it just sat there going Blarrrr, sounded rich but I was caught out thinking that the other night. If it was lean I would have expected it to just cut out, it kept going but no matter what I did with the throttle it just would not pickup, sounded flat and rich.

The taper bored 24 had a 145 main, the 30 a 150 so it’s hard to imagine the 30 needing a main smaller than the 24. ???? ran out of time to look into it. I didn't think of it at the time but I should have pulled the power jet feed line off. Killing the power jet would have quickly told me if it was rich or lean.

Kel and I now think the flat sound to the exhaust is due to the top exhaust port chamfer blunting the exhaust note as the port is no longer cracking open suddenly, is that a good thing ???.

[ief]

Oeps, seems I 'lied' a little, I was under the impression comb eff. was @ 0.83 but it was set @ 0.87, that cost me about 3 hp (so 38.5 now) but I presume I can make up for it by working on the exhaust once again (all ready have made about 70 versions/ adaptations )

Other then that I could see it as being pretty ok-ish, comb file is mostly wobs work with exception of the ignition curve as is the temp file.

I found most of the positive changes became; as wide as much tr, little longer tr duct, slightly shallower angles, lowered exhaust and aux (from what i had) shape of the inlet was good for one kw (!) and reducing the carb length and longer intake (83.5 degr if memory serves) on the RV.

And of coarse fine tuning the exhaust...

Anyway, I got some more playing to do wit 0.83 comb. eff. See if I can get it back to 40+

[FastFred]

The carbon build up clearly shows the combustion gases have been back flowing down the transfer ports. More so on the exhaust port side and its instructive how far the carbon extends down the port wall, apparently this happens on part throttle at high rpm.

I think it also happens on full throttle at high rpm when the blow down becomes to short for the rpm and the pressure of the remaining combustion products in the cylinder push into the transfers and prevent the inflow of fresh mixture. Adjusting the blow down time area is a little trick that can be used to govern the top rpm (speed) of the chain saw, lawn mower, moped or scooter etc.

... the governor on a chainsaw works the same as a lawn mower ...

Drew may be right, chainsaws and lawn mowers et all have simple built in governors.

[Yow Ling]

I think it also happens on full throttle at high rpm when the blow down becomes to short for the rpm and the pressure of the remaining combustion products in the cylinder prevent the inflow of fresh mixture. Adjusting the blow down time area is a little trick that can be used to govern the top rpm (speed) of the chain saw or Moped Scooter etc for a simple self governor.



Drew may be right, chainsaws and lawn mowers et all have simple built in governors.

The pipe probably helps govern the revs too, I think microlights use the pipe to cap the rpm as well