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Edited Date/Time
2/2/2016 3:47pm
ML - can you find out more info on the following seen on the bud racing engine.
https://p.vitalmx.com/photos/stories/2016/01/28/full_BUDvsPC_11_of_35_7…
It's clear to me the head has been welded (kawi head really needs floor lifted more) as well as highly modified in which the external look is much different than the OEM (or pc). My guess just by looking is the head is using finger followers instead of buckets like a KTM uses.
Just a question that could be neat to find more information on if they don't seal their lips entirely.
https://p.vitalmx.com/photos/stories/2016/01/28/full_BUDvsPC_11_of_35_7…
It's clear to me the head has been welded (kawi head really needs floor lifted more) as well as highly modified in which the external look is much different than the OEM (or pc). My guess just by looking is the head is using finger followers instead of buckets like a KTM uses.
Just a question that could be neat to find more information on if they don't seal their lips entirely.
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It would take a damn novel to write why - but it's my simple opinion ALL dirtbike four strokes (well performance ones) should be a finger follower for a huge host of reasons.
The bucket becomes a weak link in a hurry when chasing HP and RPM
The bucket diameter sets how fast the valve can lift via the cam profile. On modern high end four strokes - most cams are right to the limit of the bucket diameter.
Sadly - the bucket and cam only meet over the width of contact patch to the cam - the rest of the bucket is dead space....only there because it's easy to produce something round (vs say a square or rectangle) and a corresponding female side in the head to match it. This is un needed weight. So often you may find more power with a larger bucket diameter and more aggressive cam profile - only to loose RPM ceiling ability and valve train stability.
Secondly - the load from the cam opening the bucket is applied over the distance of that bucket - so it must be strong enough to not break. This is a challenge. For me - buckets are often a weak link - prone to cracking or shattering. OEM kx250f buckets with aggressive riders commonly break on stock engines.... Sourcing higher quality, stronger, and CONSISTENT batches of buckets is a real challenge on any engine. The OEM buckets are usually quite good on yamaha - but often DLC coating them is a positive.
Thirdly - keeping the bucket from wanting to twist sideways in it's bore in the head - and designing the head to take this side load requires material around the bucket - making valve spacing or bore size and large bucket diameter become a conflict of interest. The design of the heads' bucket bores has to be correct with proper chamfers and design features that eliminate stress concentrations...this is really important when pushing the RPM envelope.
When the new yamaha 250f came out we were very early into development. The yamaha has small bucket diameters vs the kawi. One of my first all out attempts was a big bucket head - machining new bucket bores (must be ultra precise) along with larger buckets and a cam profile to make use of the buckets. But yamaha didn't leave much extra material around the OEM buckets - and going larger proved to be unreliable - with the head prone to cracking.
Finger followers can allow less mass - a properly shaped cross section of the follower for excellent strength to weight, and almost no limitation to cam aggressiveness - assuming the valve system can take the extra acceleration. I do think the ktm design "side loads" the valve more than a bucket does, as one negative.
To my knowledge - F1 and moto GP are mostly finger follower systems to allow the 18000-20000 rpm windows and lobe profiles used.
I have not designed a valve train from scratch and gone through all the durability testing that OEMs do. There may be many things the Japanese OEM's know about buckets vs finger followers that steers them to buckets.
Hope this was at all useful...anyone wishes to add or detract by all means I'd like to hear more too.
Someone in an earlier post mentioned valve angles are changed. I was under the impression that in the 250 class valve angles had to remain the same as OEM spec.
Post a reply to: Bud KX250f