ESE's works engine tuner

[wobbly]

Muhr , yes the atomiser shroud in your hand.
I did a back to back with the long and short variant , thinking the short one would create less turbulence in the small 30mm bore.
But from memory I had to make a 3 main jet change to get identical egt , and bugger , the power curve didnt change at all.

[Muhr]

Can not say that my experience has been different, however, may be useful in some applications

[sniemisto]

I was trying 24mm and 28mm PWK carburetors on dyno. I ended up with both producing identical EGT 580C to end of 6th gear run. They did that with identical 120 main jet.
28mm was better in top and over rev (photo enclosed, solid line is 28mm). With 125 I got almost same power as 24mm with 120.

On open road testing I usually end up to EGT 680C with long gearing. For some reason 28mm was going really high, up to 760C. I needed to jet up until 128 main until I ended up with 680C.
With both both jets I lose power if I try to turn throttle back from WOT.

Why bigger carburetor is making hotter EGT in open road, but still acting same as smaller in dyno??

28mm is original Keihin from KTM SX and is having “2t cut” in atomizer shroud. 24mm is chinese YSN coby and it has lower 4t type atomizer with uncut straight end.

Wobbly mentioned that atomizer lenght affected on main jet sizing. Could we be dealing on related thing here?

Bike is Derbi 50cc with FOS pipe having 14mm stinger restriction. I measure EGT 150mm from piston face.

Regards,
Sami




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[wobbly]

The problem to deal with first is the huge difference in egt on the dyno and the road .
This in short means the dyno is not representing the real power curve of the pipe , not even close with 100*C difference in egt
But the other issue is that any engine that can survive being run at 760*C has serious tuning issues , probably way too much ignition retard in the overev.
This means you are choosing the main jet to kill exhaust temp - not make power , but the dyno never even gets to that temp , so who knows what the issue really is ?

[sniemisto]

Thanks Wobbly for your reply. I think I understand.
When I’m not able to get enough egt for good power in dyno, I end up over retarding it in over rev. That produces too high egt in open road testing and that sould be corrected with advancing until temp is again low enough, not with jetting up. I should then change pipe dimensions if I’m not happy with power after that.

I haven’t measured egt in typical kart track we are racing, I assume temp is lower with shorter gearing and not that much overrev. That should be good thing to test. Anyway problem is still there.

I like testing bike with long gearing overrev run because we have one meeting every year we are running on really long straights. Somehow piston is doing great with 4 hour race and 680C. On 760C test I stopped straight after I saw it. I’m not 100% sure if my egt gauge is reading right.

I run 16 degree advance on max power 12300 rpm, it goes to 5 in 14000 rpm and stays there. It can be seen in open road that egt starts rising after 13000 rpm when advance is dropping under 10.

Our inertial dyno is not able to get real egt and power curve of pipe. Maybe some wrapping during dyno could fix this problem a bit? Or this type of heat buildup is not replicating real situation?


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[wobbly]

The first issue is to get the dyno inertia/gearing such that the run time is long enough to replicate the acceleration rate at the track.
680*C is fine if the engine was designed to be run at that temp , with the fuel suiting the compression.
But it has been my experience that 680* as a pipe design temp is only possible on 110 octane if the compression and timing are correct.
15* at peak power is absolutely right ( for good fuel at 680* - what is it ? ) and kart tracks always favour high compression for off corner drive with plenty of turns
and usually only one long straight.

But having the egt climb to 780* past peak power when held up there is a function of 3 elements.
Too much retard - I would be looking to drop to say 10* - 1000 rpm past peak , then flat line it.

Secondly , to prevent having to jet up to stop temp runaway , the main air corrector needs to be reduced in size - this then means a smaller main can be used to get best peak power
at the correct temp the pipe was designed around , and the egt wont keep on rising. If the corrector and the retard are correct , then the engine will make peak power at the correct egt
and it will hold this egt into the overev. The smaller corrector keeps the fuel curve tracking the best power bsfc number more accurately.

Lastly compression , the fact the engine survived 780* simply means the compression must be too low - even given the severe retard curve.
Severe retard can be used if the pipe Lt is purposely made too long , to help mid power ( also if the longer Lt gives better superposition in concert with the chosen EPO timing ).
And then the long pipe is made to rev well past its designed rpm with retard generated header temp - but in this case an electronic power jet works even better.

[sniemisto]

Thanks a lot, there’s many good points I can proceed with.
Engine has 13,5 compression and running on 98 octane with 5% ethanol.


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[Muhr]

One question, are you on target with the rest of the engine as you are with stinger reduction / exhaust?
If you have an efficient exhaust but perhaps not as efficient supply of fresh gas?
As for compression, I use for example 8.2cc head on a 125cc with 98 octane does not say much. I think that RON is not very interesting, your MON value will determine, where I assume that you have 87.5 at a gas station in Finland with 5% ethanol. But you should at least be able to run a 3.3-3,2cc head

[sniemisto]

Muhr, I’m not at target.
I’was about to mach my egt figures with EngMod and real life to produce new pipe dimensions. Then I faced this too high temp issue.

There is also many thing I plan to do differently on next cylinder. I have too high aux exhaust ports (main 192,8 - aux 192), STA is still blowdown limited, too low axial angles.. + other small things.

I have fresh power valve cylinder to start modificate for new spec, but it has waited to properly match my EngMod model to current situation. It now appears that it still need these dyno / egt issues solved before I’m there.

Thanks for your reply, 3,2cc head is thing that I like to test also.


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[sniemisto]

I adjusted ignition curve like wobbly suggested and keeped carb as it was with big 128 main jet. It felt goin same as usual on the road test, 106km/h @ 14100 rpm.

I tried to do 2-5 gear accelerations and brakings replicating race track. EGT stays quite nicely in 580-600C, same as dyno is capable of. Still with long 6th gear pull it went up to 675C, but took longer time.

Edit: There is air correction hole in carb frame, located in same passage as idle air screw. It is 0,6-0,7mm (no correct size wire to accurately measure). I put piece of gasket there to block it. It produced quite same EGTs during test ride. Bike felt accelerating better, need to dyno it if there is changes to see.
I didn’t try to jet down at this point as EGT is still high.

[philou]

14mm stinger is very small.
For a 50 cc I never go below 16 mm

[wobbly]

I believe now we have more info , and I have slept on it , the underlying issue is not directly the swept compression , but the dynamic compression.
This factor is affected by the Delivery Ratio , and the scavenging plus trapping efficiency %.
If any of these three are compromised then the real trapped compression ratio is low.

In this case , changing just the ignition curve , dropped near 100*C , out of the long run temp , proving that that element was seriously wrong for the pipe length and tune.
Getting to just under 600* for the short sprint track simulation is low, so for that scenario it needs jetting down.

I would say that as something else is badly affecting the dynamic compression , that you could go up a whole point in swept compression , lean it down and make a good gain in power.
Of course fixing the real issue has got to be the plan.
Be interesting to know what the Mach is in the stinger nozzle , if the bmep is low due to the low dynamic com , then a 14mm stinger may suit this engine just fine as it is now.

[F5 Dave]

Just a point on measuring compression on a 50.

Measure Measure Measure. When you start getting consistent results then start writing them down. They are so small the difference of 0.sodall of a cc makes a big difference.

I would grease the ring, run it up to the top, wipe, lock flywheel, put on head, 5w fork oil, burrette , fill till 2nd thread to account for spark plug.

Then wipe. Repeat again and again. Then again. Of course sunken spark position complicates matters.

[wobbly]

Better way to do it is to use a flat plate of clear plastic.
Using a thin wipe of grease glue the plate to the head surface and using ATF fill it to the top of the threads.
Clean it up and repeat turned over with the plug in , and a plate with a hole in it.
The different readings give you exactly the assembled plug thread volume.
Then if you use ATF you dont need any grease around the ring , as all this does is fill up the volume between the ring and the bore.
The only downside , but this applies to any oily fluid , is you must leave the burette sitting for at leat 15 to 20 minutes , as the dropping fluid level leaves around 0.5mil on the glass bore
dependant upon the speed of pour.
The very reason I finally bit the bullet and got raped buying a digital lab burette - a used one from an American medical auction house still cost me 400 USD.
Dead cool way to do it and accurate to 0.01cc.

[Muhr]

Muhr, I’m not at target.
I’was about to mach my egt figures with EngMod and real life to produce new pipe dimensions. Then I faced this too high temp issue.

There is also many thing I plan to do differently on next cylinder. I have too high aux exhaust ports (main 192,8 - aux 192), STA is still blowdown limited, too low axial angles.. + other small things.

I had not guessed that BD was your biggest limitation.



I usually draw the volume in the computer and confirm with burette. here is a 50cc with a 2cc dowel