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lower the rev-limiter

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It seems to me that very high rpms are almost always hard on the valves. Is it possible to lower the rev limiter maybe 500 to 1000 rpm? I know this will slow the bike but it may be OK for trail riding. Does anyone think it would help? Maybe put in a switch..............

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500-1000 rpm!!!!! 😢

wud it even run ???

the idle of my ttr125l is 1500 if im not mistaking

if u want to save ur valves jus ride below rev limiter and check your valves once or twice a year

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Yes, lowering the rev limiter would save the valves among other things, but how much, I am not sure. I would think the dyno would answer your question. I am getting my '05 Montesa/Honda HRC RTL-250, 4-stroke trials bike at the end of this week. It looks a lot like the CRF engine as far as the profile and cylinder shape, but it has normal rockers w/tappets for the single cam, 4 valve head. That shows that the lower RPM engine would not need the bucket/shims/special coatings., ect. Maybe I am way off base, but you might be on the way to solving the enduro/HS guys issues w/the valves not lasting a season. I/ve had my X almost a year now, and no issues yet. I ride on the bottom to mid in the woods only. I would not have a clue as to how one would build a rev limiter. Clint

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The stock rev limit is what the factories consider "safe". These motors will turn several thousand RPM more with a programable ignition such as the Vortex. Or less if you so desire. tdub

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Check out the dyno graph here, peak power is at 11000 rpm's for the R model.

Certainly no need to rev to 13000 for a lot of reasons. The most important being power output is dropping off at that point . graph.jpg

😢 KLX

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All of this is a band-aid. The problem is the valve and seat materials, not the rev limiter. Lowering it may buy some time...maybe, but the condition will surface.

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no valve or oil issues on my bike

Race it in hare scambles and do a lot of trail riding, and even some moto.

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the x cams are harder on the valve train, i'm seeing more x failures on here than any other engine! The reason is because of the shorter 222 duration it has. even though the lift is only 296" it throws the valve a little harder @ 11 or 12 k rpm.The revs really need to stop on those @ 11k rpms.The longer duration R profile is easyier on dropping valves!

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the x cams are harder on the valve train, i'm seeing more x failures on here than any other engine! The reason is because of the shorter 222 duration it has. even though the lift is only 296" it throws the valve a little harder @ 11 or 12 k rpm.The revs really need to stop on those @ 11k rpms.The longer duration R profile is easyier on dropping valves!

I've wondered about that myself. Have you guys measured the maximum valve opening and closing velocities on an X at 12,000 RPM versus an R at 13,000 RPM?

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So, does anyone know how much work it would be to lower the limiter to 11000 rpms? Cost?

It would be a lot cheaper to simply shift before you get to the limiter. That's what works for me. I think most of us here would agree that these bikes don't get much more, if any more power after 11k. Just like any four stroke, they seem to have their best pull in the mid to upper mid range IMO.

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Can anyone direct us or explain this valve action in technical terms or illustrations?

Thanks as this makes some sense but I would just like to draw a better picture in my mind 😢

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I've wondered about that myself. Have you guys measured the maximum valve opening and closing velocities on an X at 12,000 RPM versus an R at 13,000 RPM?

Down @ my dads shop we have a Spin Tron.What that is is a spinning set up engine that you can analyze the valve movement/control etc....

I ran this thing for about 5 years and figured a few things out about valve crash!With the optical follower i could pin point problems real fast, so what i'm seeing on the X cam is a max velocity thats higher than the R profile.Its great to 10000 but after that it's crashing the train we call it!

I'm building a smaller version Spin machine now for MX engines, it will be the first one i know about thats built to analyze the single cyl . 4 valvers.

Then i'll throw on a CRF head and spin it with all these cams and my cams etc.... to see whats going on to the train @ 10/11/12 k and pin point the problems.

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http://www.compcams.com/Technical/TimingTutorial/

Try this link (sorry Dan I know it's the "OTHER" guys) it explains the cam pretty well. The trick to the cam is of course to open and close the valves, but the profile of the lobes needs to be such that in closing the valve quickly when you have significant lift, it must slow the velocity of the valve so it lands softly on the seat or you get the beat up valve and seat syndrome. Dan can probably elaborate more on it and maybe give a link to their site that explains it better. When it comes to cams the Crower family know what they are talking about. 😢

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Down @ my dads shop we have a Spin Tron.What that is is a spinning set up engine that you can analyze the valve movement/control etc....

I ran this thing for about 5 years and figured a few things out about valve crash!With the optical follower i could pin point problems real fast, so what i'm seeing on the X cam is a max velocity thats higher than the R profile.Its great to 10000 but after that it's crashing the train we call it!

I'm building a smaller version Spin machine now for MX engines, it will be the first one i know about thats built to analyze the single cyl . 4 valvers.

Then i'll throw on a CRF head and spin it with all these cams and my cams etc.... to see whats going on to the train @ 10/11/12 k and pin point the problems.

I didn't know for sure, but I suspected all along that you guys did a lot more than just weld and grind a cam. Given this level of R&D I think your cam prices are more than reasonable. BTW, I don't expect to get your all of your data for free, but I'd be very interested in what you can share. Thanks!

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When a valve is closing,the spring in its closed up state will exert it full pressure and the designated cam contour will be followed.As the valve approaches the seat,however,the spring having opened out,will be at its weakest.It is at this point that good valve gear and cam design show up,because it is possible-given adequate spring tension-for the valve to be lowered right onto its seat with gentleness by a well designed layout,thus with a minimum tendency to bounce off again(valve float).Assuming Honda knows how to design a cam then the springs are mostly responsible for returning the valve gently to the valve seat as they are responsible for making the valve follow the cam profile.A simple switch to stronger valve springs will not always solve the problem as another design limit may be reached such as the spring wire becoming so thick the spring coilbinds. IMO if the engine cam is working well at lower rpms and not at higher rpms then the springs are a little weak and should be upgrade to eliminate/reduce the valve float.A cam system can be complicated from an engineering standpoint because it consits of one or more rods ,gears,springs and levers all which are elastic(have flex)Each member has mass which is distributed according to its geometry(hence a desired light as possible valve train).Various kinds of friction and damping will be present because of relative sliding,air and fluid film(oil) resistance. Because of this alot of time in CAM design has to be spent in the shop testing outcomes.Pro Circuit has addressed this problem by offering very expensive light titanium valve(low mass) with and improved coating(hopefully) that will be more resistant to wear(see dirt in carb thread) and an improved valve spring kit.

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Because of this alot of time in CAM design has to be spent in the shop testing outcomes.

Agreed. You can model all day long, but the SpinTron is where the rubber meets the road in valve train stability analysis. I'm happy to see Crower taking this approach and I look forward to any results they can post.

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