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Graph chart help needed

918 views 10 replies 4 participants last post by  Boy1980 
#1 ·
Would anybody on here Be able to tell me why my sled Engine MXZ 600 would just drop bhp like this on a Dyno graph run with jetting perfectly fine The engine keeps the RPM same but as going up through the automatic clutch Gearing seems to dip engine noise and lose this much bhp while on the rev counter it stays the same around the 8000
 

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#4 · (Edited by Moderator)
I assume this is not a crankshaft mounted dyno.

The rpm stays the same but the track is accelerating and sucking HP from what the dyno is recording.

The clutches keep the rpm the same or close to the same as the clutch drives the track faster the track is either slipping on the dyno or it is power being used to accelerate the track.

is this running on a drum?

When the clutch finish shifting the acceleration demand lessens.

Ill bet if you could watch the clutches shift this is occurring during the shift.

This is why when I see these nice smooth dyno curves form pipe builders I know they drew them with a red pen.
 
#6 · (Edited by Moderator)
See if he has a handheld tachometer or a readout showing drum speed. It is not physically possible to drop that much power and not have something really wrong with the engine.

I saw his dyno online. That thing is meant for huge HP and you are trying to measure 80 hp.

I bet its track slip when the clutches shift. There could be some resonance where the lugs are bouncing wildly at that speed .

Its odd that you make 80 hp at 5200 but then 75 at the top. The track is not gripping the roller under acceleration and at the top of the rpm range.
 
#8 ·
A rubber cleated track on a knurled steel roller is horrible traction.

The bottom scale is speed. I cant see but I think it looks like MPH.

It insinuates it is increasing which would lead you to believe traction is not lost. But what is missing is time. That drop in power could have taken extra time to get the drum though the track vibration and it accelerated but much more slowly as the clutches shifted.

evidence of track slip is also in the fact that there was more hp at 5200 than at top rpm. The track was slipping and never transmitting full power.

The dyno looks like an eddy current dyno which can pull constant load versus an inertia dyno that uses a big drum as a flywheel.

When you hit 6500ish you are probably making the highest torque an slip between the track and the drum occurs.

Its not a fault of the dyno but the fact it is not really designed for snowmobiles.

If you go to Land and Sea you will see they dont use that kind of dyno for sleds.
 
#10 ·
Okay thanks very much for your reply I have spoken to straight line performance and he recommends the best thing to do is to fit a AIM Computer So I have all the data to see exactly what is happening
This is something I have seen a on most engines with Rave valves some a lot more radical than others, The engine will build constant power to a point were the Rave valves open then a quick power drop then a constant build of power the electric valves are worse than the mechanical valves! It is worse if the valve opens to soon!
 
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