Bucket Shim vs Valve Sprung

[Ben Wilson]

Yep, engines with rockers etc. suck! Having cams operating the valves straight up is much better as far as hanging together at high revs goes.

You'd think so, but then why do F1 engines use finger followers?

Basically the geometry of a cam on bucket design limits the ramp rates and lift of the cam. With too much lift, the cam will physically overlap the side of the bucket (one of the big limitations of the 4AG) and with an aggressive ramp rate the cam basically crashes into the bucket instead of lifting it smoothly.

Take a look at this thread for more information than you'd possibly need.

[o_man_ra23]

So unless you are building an engine that is so highly strung it gets rebuilt after every race meet, bucket and shim will give you the best result?? At the RPM a street engine (even a wild street engine) will rev, a good choice of valve springs can do away with the shortfalls of the bucket/shim design and render the ducati design a waste of time for a car engine (motorbike engines rev a lot higher, so Ducati have a reason to use it).

As for the question of shim over or shim under, I would say shim under. Also, you can get self adjusting buckets (basically hydraulic lifters/tappets) such as those found in the RB26DE engine. I dont like anything such as this as the oil galleries are so small that minute amounts of impurities in the oil (the junk that gets through most oil filters) will cause them to block up after a time, and they are an expensive pain in the a-hole to replace. But yeah, shim under has the advantage that the shims are wholly captive, and it would take a valve spring failure or similar to unseat them, whereas shim over bucket the shims have been known to be flicked out by lumpy cams.

[Smokey228]

This was just a general question. But i am building a 1G, and was curious to know...
Thanks for all the info guys, this is quickly becomming a good thread for info

Cheers,
Jase

+rep erupts from the tall odd shaped structure and falls all over the contributors of this thread!

[Smokey228]

So unless you are building an engine that is so highly strung it gets rebuilt after every race meet, bucket and shim will give you the best result?? At the RPM a street engine (even a wild street engine) will rev, a good choice of valve springs can do away with the shortfalls of the bucket/shim design and render the ducati design a waste of time for a car engine (motorbike engines rev a lot higher, so Ducati have a reason to use it).

As for the question of shim over or shim under, I would say shim under. Also, you can get self adjusting buckets (basically hydraulic lifters/tappets) such as those found in the RB26DE engine. I dont like anything such as this as the oil galleries are so small that minute amounts of impurities in the oil (the junk that gets through most oil filters) will cause them to block up after a time, and they are an expensive pain in the a-hole to replace. But yeah, shim under has the advantage that the shims are wholly captive, and it would take a valve spring failure or similar to unseat them, whereas shim over bucket the shims have been known to be flicked out by lumpy cams.

If you were to build a engine which had a shim over and wanted to put some big cams and build it reasonably decent like. Would there be a easy enough way to convert it to shim under? Or would it be more money and trouble than its worth? ie. If you keep shim over with some nice cams and engine work is it going to spit the shim?

Cheers,
Jase

[ed]

find a shim under engine that has compatible parts and swap them in

worth it if your engine is *that* hot... ie big cams big rpm

[jfallen]

Also, you can get self adjusting buckets (basically hydraulic lifters/tappets) such as those found in the RB26DE engine..

RB26 uses solid shim under design, as do the newer RB25 NEO heads

the RB25/20 and CA18's run hydraulic lifters and they are rubbish. I'd ordered a set of solid lifters from JUN for my CA and they coulden't deliver. Nissan realised this and that's why All SR20's run hyro lifters except those found in the GTiR. (Group A regs didn't allow you to change lifter design)

here's the drawing they sent:


anybody able to beat $80 a lifter, keep in mind they cannot be heat treated after machining due to possible warping so need to be milled from sometihng that's got a HRC 58-~63, then nitrided.

Cheers
Jordan

[ed]

if youre serious and want a quote on a custom bucket, i can find out

[takai]

Going on memory here, but i think the 1NZ (not SZ) lifters are a pretty dead ringer for the CA. But they may not have the nipple size you need.

[ed]

yeah, the 1nz is only like 5-6mm or there abouts - a LONG way from 18.8mm

[LeeRoy]

Basically the geometry of a cam on bucket design limits the ramp rates and lift of the cam. With too much lift, the cam will physically overlap the side of the bucket (one of the big limitations of the 4AG) and with an aggressive ramp rate the cam basically crashes into the bucket instead of lifting it smoothly.

Is is possible to machine out the bucket hole and put larger buckets in making it so the cam cant catch the side of the bucket/shim and flick it out?

[Squid]

yep, ducatis have an interesting system, i like it a lot, and it has been applied to car engines occasionally also.

its actually called desmodromic (desmondromic?)

Looove that sweet Ducati music!

you can tell a desmo ducati rider at the lights, he's the one trying to work out if his bike is only making the funny noises it's supposed to be making

Close to a -ve slap!! (joking) there are many funny noises on the duc that it took me a while to get used to!! The desomodroic valves are noisy and produce a lot of noise that had me worried I had fucked the valves!

[takai]

yeah, the 1nz is only like 5-6mm or there abouts - a LONG way from 18.8mm

Ah, bugger, i couldnt remember the nipple size.

[Ben Wilson]

Is is possible to machine out the bucket hole and put larger buckets in making it so the cam cant catch the side of the bucket/shim and flick it out?

Yeah, I've seen it done.

[Joshstix]

For sure I've seen 3S heads modified to take FJ20 buckets. It's a nice upgrade, they're shim under and rather large diameter allowing for some seriously aggressive ramp rates.

[LeeRoy]

So would the benefits of this outweigh the costs cause i cant see something like this being cheap even if suitable buckets could be sourced from another engine for next to nothing. How close did they machine to the ridge end where the bucket is held in place cause towards the center of the head that rim is not too thick maybe giving only 1-2mm of metal that could be taken out.