ESE's works engine tuner

[Peter1962]

The bearings used these days in 2T - MX cranks are all flat construction , silver plated cages, so the quality isnt the issue.
Its the inertial loads of stress reversal at BDC and TDC, and the inability of the cage to accelerate fast enough to prevent skidding ( instead of rolling )
by the needles.
Pressure fed, shell bearings, as used in a 4T dont have these issues, and the bottom ends are pretty bullet proof.
As I said, by giving up service life, the 72mm cranks can be spun to 11,000, but be prepared to change the big end VERY regularly.
I have changed just the cage/needles on a KZ2 crank 4 times with no issues as the rod and pin dont wear at all.
Its the cage scuffing that causes failures, no matter how much, or how good the oil used.

Gas Gas 250 engines used in kart road racing are 68mm square, and they are bullet proof at 11.000 - the normal MX crank are severely marginal.

So -in theory- the WiWa GasGas 250 engine could be a solid base for modification to a tuned MX engine since it is bullet proof at high rpm's ? ( I was thinking for example in supermotard, where high rpm's are necessary). I know it has a balance shaft, which is a good thing. But what I do not know is if his cylinders have a power valve or not.
Wobbly, have you ever worked with this engine ? What would be your assesment ?

[wobbly]

I have only worked with the kart specific engine, that uses the quad cases as a basis and has a special cylinder casting.
The only down side I can see is that it has a T port, but makes very good power in this form for road racing use.

[2T Institute]

So -in theory- the WiWa GasGas 250 engine could be a solid base for modification to a tuned MX engine since it is bullet proof at high rpm's ? ( I was thinking for example in supermotard, where high rpm's are necessary). I know it has a balance shaft, which is a good thing. But what I do not know is if his cylinders have a power valve or not.
Wobbly, have you ever worked with this engine ? What would be your assesment ?

What makes you think that MX 250's are not revved to 10,500-11,000 on an MX track? Motard tracks are very tight usually top speeds are less than 120kmh. What kills the big end in a 250 National kart engine is the length of time spent at those RPMs.

[wobbly]

If you look at the dyno charts of all the usual 250 MX engines, they are miles over the pipe and make so little power past 10,000
I could see no point in reving there at all.

[Jvenni]

Assuming you are describing Aux Port shape ( or Hook Port as the Mericans call them ) the triangular shape used in a hi performance
cylinder is like that for a couple of reasons.
The top edge chordal width is greater than the duct width, but as this upper edge opens first the pressure differential is greatest and we get the maximum flow
ability in the upper 1/2 of the port opening, where the port area matches the duct area for maximum exit velocity.
As the piston moves down past the 1/2 height point, the Blowdown pressure is alot less, thus the flow rate reduces, so we dont need anywhere near as much area to keep up the flow needed.
Then there is the issue of vertical short circuiting from the A port window.
Only having the front corner down near the A port, reduces the tendency for the A ports flow column to do a U turn above the timing edge and exit thru the Aux port.
The same applies to the port linking area presented to the passing small end pin cavity.
The rear corner of the Aux port that is closest to the pin hole, has a much reduced area, and is well separated in distance from the A port, so there will be much less linking time area
thru the pin hole cavity.

Re the real port flow capability.
If we look at the raw numbers of the Aprilia in EngMod is would seem that the Transfer STA is excessive in relation to the Blowdown numbers.
But this is taking no account of the big radius on the timing edge of both the Ex and Aux ports.
Thus the effective Cd numbers for the Blowdown is much improved, and this is accounted for in the scavenging model that was presented in an SAE paper from I think the Uni of Valencia.

Hi all,

Very motivated by the results Wobby has shown I started to grind out the moons in the exhaust duct of the KZ10 cylinder.

During training on the track we saw a drop in exhaust temperature and a loss of overrev. ( Green line ) Out of corner we lost speed. First response was to lean out the engine but this didn't effect the EGT at all. Next step we changed the gear ratio ( shorter ). Temperature was ok, exit ok but we lost on top speed ( Yelloe line ) and a lot of overrev was there. The 'operating window'of this engine has changed to 12.000 to 14.200. Normally we gear within the range of 11.500 to 13.700 but there seems to be some kind of cooling effect within this range now.

As we have lost speed at time I trying to figure out what happend and what I could do to improve on low revs. Shoud I lower the cylinder or extend the exhaust pipe length by adding an insert at the exhaust duct ? Or buy a new cylinder ..






.

[wobbly]

What ignition timing are you running - and what are the head cc.
Also do you have the short factory header manifold.

[Jvenni]

Thanks for replying.

The head cc is at the 13.2 cc ( 11.2 cc without plug hole ) and ignition is set at 1.55. We don't use the short outlet manifold yet but we removed the gasket between head and manifold. Inlet is with the factory short manifold

[wobbly]

Depending always on the track you need to try many combinations of pipe length and timing.
Less timing ( 1.2 ) = more egt and more overev power, this usually compromises the off corner drive ( but this is easily fixed with a 1/2 tooth on the back ).
Depending upon the fuel, you may find that even more timing ( 1.8 ) will give you plenty of bottom end, allowing slightly
taller gear ratio to gain the speed.
The short exhaust manifold also increases the overev power even more, but again what you gain on top you loose down low .
But with the correct combination, the short gearing and the ability to rev harder is always quicker.
Just changing the gear,and reving hard wont be fast unless the whole engine package is set to run that way.
You need to find the combo that works best for that particular track, and especially where the first 5 karts are maybe 2/10 apart, every tiny detail counts.
We regularly rev to 14,8000 with a ton of timing, a short pipe and run at 680*, but you will need really good reeds ( nothing like stock at all ) to get power up at that level.

What I dont understand is that even with the shorter gearing you are pulling way less rpm out of the hairpin,and way less corner speed, so of course it will be slow.
In that case the shorter gearing ( yellow ) should have the exit rpm way higher, and then accelerate faster - but its not,so something else is badly wrong

[diesel pig]

I am considing getting a set of straight cut primary gears made for a road going engine I want to turn into a road racing engine and since it would be quite an involved not to mention expense project.

I would like to ask some questions to clarify my understanding so I don’t make choices I regret later.
I notice factory road racers have slightly lower primary reduction ratios compere to dirt and road bikes for example a racer might have 3 to 1, and dirt & road 3.5 to 1, I thought this may because of the racers higher top speed (more power and better aerodynamics) the factories gave the racers slightly lower reduction ratios to avoid painting themselves into a corner as regards sprocket sizes e.g. stupidly large front sprockets and ridiculously small rear sprockets, is there any other reasons or advantages to making the gearbox spin faster?

This brings to my next question since the water pump on my engine runs off the crankshaft primary gear if I was to change the gearing I would have to change the water pump gear which got me thinking about changing the speed of the impeller as we know better cooling in a two stroke better power but would speeding up the impeller work? Or would I just end up over speeding the impeller and cause it to cavitate?

[wobbly]

The overall gear ratio from crank speed to rear wheel,to achieve the terminal speed you want is the overriding issue.
Of course a road race engine will be reving harder, so this will naturally mean top speed is increased.
Then the torque multiplication factor of the primary dictates how much load is on the clutch - lowering the ratio decreases the clutch load, and increases road speed.
But as power increases you will need usually more plate area or stronger springs, but less so if you have dropped the primary ratio.
Then as you mentioned there is the issue of sprocket sizes.
In general we usually want to be running a small front and rear sprocket combination to keep the chain run close to the swingarm - increasing anti squat.
Then you have to factor in the 1st gear ratio,by dropping the primary you may have an issue getting off the line.
Lastly is the water pump speed.
If you can, its way better to increase the impeller depth to shift more water without changing the speed,as this will be higher than stock anyway if the crank is spun harder.
I did this on the TZ400, with an 8mm wider impeller, using the stock pump gears, and it runs at 50*C in 30* heat - a stock TZ350 would be closer to 70*.
Many companies make uprated pumps for Banshee's and Boyesen make them for all manner of 4T bikes.
One last point is that there is very little difference in efficiency/power between straight cut and helical gears - 2 Hp in 80.
But a straight cut means no side load on the crank, so you can run a roller on the highly loaded gear side.

[marsheng]

We are busy playing with an Ignitech and a KR150. I see very little difference between the power runs between with the ignition set on 12 deg 16 and 20 deg btdc after 7500 rpm. Is the pipe or some other thing affecting the performance, or is it just at 20 deg there is more heat lost in the engine which dosen't really show any negative effects on a short dyno run?

Thanks Wallace.

[marsheng]

The Maico has got a new bad habit. It runs on full for 1/2 a second after WOT and then snapped closed. Bit confused as to where to look. It can't be sucking in that much air somewhere to keep it open on full. Seems to run correctly every where else.

Cheers Wallace.

[marsheng]

I am considing getting a set of straight cut primary gears made for a road going engine I want to turn into a road racing engine and since it would be quite an involved not to mention expense project.

I would have to change the water pump gear which got me thinking about changing the speed of the impeller as we know better cooling in a two stroke better power but would speeding up the impeller work?

My Maico gearbox range from 1st to 5th is 2.2: 1. Originally I had a 440 gearbox internals (I didn't realize I had the wrong set) which was a 3:1 ratio. It was really bad on the track. Gears just too far apart for the 250.

Note this is on a 5 speed 1980 250 but it gives you an idea.

The water pump really only needed a low speeds. Bikes only boil when in traffic or going up a monster hill with a 1 ton trailer.
Once on the open road, you could almost get away with thermosiphon.

See if you can find out what happened to this motor

http://www.kiwibiker.co.nz/forums/showthread.php/127584-Water-cooling-the-Buckethttp://www.kiwibiker.co.nz/forums/sh...ing-the-Bucket

[wobbly]

" The water pump really only needed a low speeds. Bikes only boil when in traffic or going up a monster hill with a 1 ton trailer.
Once on the open road, you could almost get away with thermosiphon. "

Really - tell me then why every stock TZ350 I have ever seen runs over 70* - add an aftermarket thicker modern radiator and if you are lucky
it might get down to 60* on a cool day.
I had to completely change the water routing with a plate welded on top of the cylinder, then CNC an impeller 50% deeper to get the TZ400
down to 50* where it should be.

[JustHarry]

We are busy playing with an Ignitech and a KR150. I see very little difference between the power runs between with the ignition set on 12 deg 16 and 20 deg btdc after 7500 rpm. Is the pipe or some other thing affecting the performance, or is it just at 20 deg there is more heat lost in the engine which dosen't really show any negative effects on a short dyno run?

Thanks Wallace.

another not with the kr150
its using lots of water doing to dyno runs. and also pressurizing the overflow tank to the point where it is blowing a lot of water out... but only at round 7000rpm. round peak torque. i think the head gasket has slipped but is unusual that only blows through radiator at 7000rpm. is this a normal symptom of a slipped head gasket? or could the cylinder sleeve have moved causing a leak?

Cheers Harry