RPM Fade

jdtech65 · Mar 10, 2012

This last weekend I was out west at Laramie (8,000-10,000). I had just added SLP MTX 65g weights and a 13.6 Power Addiction head to my 12 800 163 before we left. I started out with 2 grams in the belly and 1 gram in the tip. Previously before I added the head and weights I was running 10-60s out there and hitting 8,000 pretty repeatedly. The first day I would normally see a high of 7700-7800 while climbing. So I figured the next day I would try it without some of the rivets in the weights. I removed both the 2 grams in the belly and the 1 gram in the tip and just ran them empty. That second day then I would start out hitting 8,000-8100 while climbing but it would quickly fade to as low as 7200 usually being around 7500 though. I had cleaned the clutches before this last trip. The belt is still the original with only 500 miles on it. The only thing I really see on it though is 5" of cord missing on one side but its been like that for 250 miles.

My clutch set up so far includes slp mtx 65 gram weights and the slp blue/pink spring in the primary- which is electric start.
The secondary has the polaris black/purple spring with 1 delrin washer in it.

Any ideas on what I should look for to keep the rpm at 8000? Heres a couple pics of my clutches and belt.

pura vida · Mar 10, 2012

change your belt and see if anything changes. mine tends to be really sensitive to belts with pulled cords.

pv

Dartos · Mar 10, 2012

Just to be clear. You stated on the first day you were hitting 7700 - 7800. Was the RPM steady?

If so I would put the weight back in and see what it does for troubleshooting. If that is the only change it seems to me you have got you clutching out of whack but if it continues to from way back down then at least you know that is not the culprit.

crashnbash · Mar 11, 2012

Try a new belt. Sounds exactly like a worn belt, or clutches getting to hot.

modsledr · Mar 11, 2012

If you've got threads pulling then change the belt before you blow it.

Troubleshoot your clutching with a fresh belt.

Bossman · Mar 11, 2012

Several laws of physics are at play in your situation, the most important being pipe heat. The HC head increases the heat in the head and piston, stealing it from the pipe. Decreasing pipe heat lowers exhaust gas speed and pressure which lowers the RPM at which the pipe is most efficient. There is also a possibility that the can does not have as much back pressure as the stock one, which also lowers the ideal operating RPM. You can verify this info by subscribing to DynoTech and looking in the archives for Kevin Cameron's articles on the subject. Jim Czelaka, owner of DynoTech, has also written some interesting stuff on the subject.

All of that being said, you were probably close to peak efficiency at 7800 RPM, as these motors have a very wide torque band. We have drag raced several Pro's with different weights and stock helix's many times. A Pro spinning 7700 RPM will accelerate as well as one turning 8000 RPM, but will not climb as well on a long pull, where the pipe gets hotter than in a drag race.

The photos of your clutches show belt slippage that was taking place when running the lighter weights which resulted in belt and clutch heat. At that time there was not enough load being put on the engine to build pipe heat. I would start by putting the stock can back on and putting the 3 grams back in each weight and shooting for 7850-7950 RPM. Like the other guys said, a new belt won't hurt either.

OnlyPolaris · Mar 11, 2012

Have you made sure your belt to sheeve clearance is the proper spec after installing the MTX weights.

jdtech65 · Mar 11, 2012

The rpms seemed more stable the first day but we didn't climb much the first day due to lack of visibility. I was shooting for 8,000 so I figured I would remove some weight.

The belt side clearance is within spec. It was high with the 10-60 weights but the SLP weights brought it back to spec.

I will try a new belt the next trip and also add some rivets back into the weights. At least now I have the SLP tool so I can at least change the rivets on the mountain now. It was backordered for a while so I didn't have it last trip.

I'm also planning on changing my helix.

I know it would be better to just try 1 thing at a time so I know what made the biggest difference but I might only get out once more this year. Hopefully twice yet if they get some snow and can make it work.

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RPM Fade

jdtech65

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pura vida

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Dartos

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crashnbash

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modsledr

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Bossman

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OnlyPolaris

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jdtech65

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