ESE's works engine tuner

[shnaggs]

So if you are running a solenoid powerjet you can simulate its action more accurately by setting the pipe temp at 450 at say 500 rpm past peak.
It will take some fiddling to find the element that is affecting the power the most past peak.
I have found time and again that all it needs is to pull down the Ex exit area to 75% and add a tapered nozzle out to the 100% header area.
The increased pipe temp doesn't make it " suck " more it increases the wave speed,that in turn reduces the effective tuned length.
You can try adjusting the intake length to get it into resonance in the overev, depending upon the harmonic being used and the room available.

A 250 well tuned will sim accurately at around 85 to 88% C.E

Thanks again Wobbly, I will have to tinker around with that some more.

Damn I am showing my ignorance, makes sense though now that I think about it. Higher temps= the pulses moving faster, ie the pipe effectively gets shorter, not "sucks" more. Thanks for the clarification.

Right now I'm at 82.5% C.E. This is what is boggling my mind, if I raise the C.E. up to around 88% I'm almost at 70hp!!! Either I have totally messed something up, or I have stumbled upon gold. I'm sure it is the first...

I made all my measurement's as accurately as my small brain can. The only part I guessed at was: crank case volume.

[wobbly]

Keep in mind that the sim reads out crank power.
When using a normal street tire or slick you deduct around 12% to 15% to give power as seen on a Dynojet.
ie 70 Hp = 62 RWHp to 59 RWHp = OK for a good 250.
The very best 250 road race engine are getting high 60s at the rear wheel of a kart, that has little wheel inertia so probably mid 60s
if done with a bike wheel on a Dynojet.
When putting a long rod into a YZ250 and using a spacer plate power rises, now who knows if that is due to the rod or the case volume - it works.
What case vol ( with transfers ) did you use.

[peewee]

Indeed, that I believe was my mistake when I did my 100. I think I reduced the volume of the transfers too much. Vinamold showed me what the finish was like where it was hard to see but your cylinder has the luxury of plenty of space.

ill print out some more templates for the outer wall then final check everything with the vina. the entrance probly wont be near as big as it should but this is the only thing I can see to do at this point. I believe the reason the entrance is so small is because it had 140mm rod with 82mm stroke. theres now a spacer plate but the bad initial design hampered what I could do with it

[shnaggs]

Keep in mind that the sim reads out crank power.
When using a normal street tire or slick you deduct around 12% to 15% to give power as seen on a Dynojet.
ie 70 Hp = 62 RWHp to 59 RWHp = OK for a good 250.
The very best 250 road race engine are getting high 60s at the rear wheel of a kart, that has little wheel inertia so probably mid 60s
if done with a bike wheel on a Dynojet.
When putting a long rod into a YZ250 and using a spacer plate power rises, now who knows if that is due to the rod or the case volume - it works.
What case vol ( with transfers ) did you use.

I used 900cc with transfer volume. It was a rough guess and it got me close to 1.6 crank case ratio, which I read was a typical ratio for a mx 250.

This may be a stupid question, but could you in theory make a good pipe/tune for an engine that makes more power then you would really need in your application, and then use the power valve opening to cut the top off the power band, essentially making for a more linear/wider power band???

[TZ350]

... could you in theory make a good pipe/tune for an engine that makes more power then you would really need in your application, and then use the power valve opening to cut the top off the power band, essentially making for a more linear/wider power band???

Keeping things in resonance for longer sounds like a good idea for extending the power spread. But I think lowering the power valve at the top end to flatten the power curve by strangling the top end power by reducing the effective blow down time area is likely to promote detonation. Due to the cylinder being unable to be purged properly and the excess residual hot combustion gasses being forced back down the transfer passages and overheating the fresh incoming air/fuel mixture.

[wobbly]

If I assume you are measuring the case volume with the piston at TDC, then 900-250 = 650.
900/650 = 1.38 CCR.
This is a perfectly normal CCR for a 250MX - 1.6 is completely insane if it was measured or a wild guess.
But as I said, adding a 5mm plate under the cylinder when going to the 130 long rod makes more power.
So a bigger case ,getting closer to 1.3 is a good thing for the highly modded kart race engines.

[peewee]

1.6 would be crazy. seems like one of the engines i have might of been around 1.5 but it was crap from the early 1980s. to get beyond 1.5 or so , would probly really take some misguided efforts. very short rod and a wristpin located near the piston crown

[peewee]

ended up changing out the rod and piston which allowed about a 8mm spacer. never did recheck the volume

[peewee]

sorry guys the edit button seems to be broke but i remembered the rod stroke ratio was originally only 1.7 which i think is part of the reason the case ratio was through the roof and the wristpin was higher than i have ever seen in my life

[wobbly]

You can, in all cases, create a huge amount of extra power, in a particular band, by changing the Ex port timing.
This can, and usually does, badly affect the Ex STA, but if as a consequence the pipe goes into superposition resonance,then this effect is way greater than
the effect of the limited STA.
You can arrange the superposition to occur at any rpm, with the limitless combinations of pipe length and port timing.
Usually the best effect is seen when the residual pressure in the duct rises, as the rpm goes over the peak of the pipe/port - then it will just keep on reving instead
of hitting a wall as the power drops off savagely.

[Peter1962]

Keep in mind that the sim reads out crank power.
When using a normal street tire or slick you deduct around 12% to 15% to give power as seen on a Dynojet.
ie 70 Hp = 62 RWHp to 59 RWHp = OK for a good 250.
The very best 250 road race engine are getting high 60s at the rear wheel of a kart, that has little wheel inertia so probably mid 60s
if done with a bike wheel on a Dynojet.
When putting a long rod into a YZ250 and using a spacer plate power rises, now who knows if that is due to the rod or the case volume - it works.
What case vol ( with transfers ) did you use.

Wobbly, is this specific for the yamaha yz250 engine, or have you seen the same effect on a honda cr or ktm 250 ?

[wobbly]

Same effect on a late model KTM250 using a long rod and plate.
The small case does work best for the MX tuning where they pull out of corners at 4500, simply better throttle response.
But in karts and road racing, the long rod combined with the bigger case works with all the other mods done to increase upper and top end.
But having said that for so called "expert " use on a 250 MX, every man and his dog used the long rod and plate for years in the YZ250 range of engines.

[shnaggs]

Yeah bugger. I did the calc wrong on the crank case. just did a run now with 1.33 volume, now my numbers are much more worldly and pathetic!

I do appreciate you guys keeping me on the straight and narrow.

Wobbly, you talking about the pressure rising in the duct after peak hp...do you mean port duct, or header?

[ief]

But doesn't that imply that raising the ccv to 1:1.6 raises the power to unworldly figures and doesn't that then imply there must be something else wrong?

[shnaggs]

There seems to be a glitch in engmod I think. I am using the 'crank case calculator' to determine my ratio. No matter which option I pick, it automatically selects 'transfer volume excluded from ratio'.

If I enter 900 in 'BDC volume plus transfer volume', hit calculate, I get 1.33. But then it highlights 'transfer volume excluded from ratio'?
Well, didn't I just use it in the volume calculator???

Is this wrong or am I wrong? Lets just say that 1.3 ccv is a good ratio, should I include or exclude the transfer ports in the ratio? About a 3hp difference for me.