h a l f b a k e r yOn the one hand, true. On the other hand, bollocks.
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This one requires a fair amount of engine and mechanical knowledge to understand, so bear with me. I'll try and explain it as simply as I can.
It's a known performance modification to engines to increase the rocker arm ratio. This results in a higher valve lift and duration without having to change
camshaft profiles.
Varying the rocker arm ratio means that at lower RPMs, you can have a lower ratio, meaning that the valves won't have as much lift or duration. This is a desired quality that gives better low end torque.
At higher RPMs, the ratio could be increased, resulting in increased valve lift and duration. This allows the engine to breathe better and produce more high end power.
Essentially, the way that this could be accomplished is to mount the pivot point on some kind of track that would enable it to move back and forth underneath the rocker arm. The whole mechanism could be controlled via some source in the engine, i.e., vacuum or oil pressure. I know there are many different variable valve timing systems out there (VTEC, i-VVT, etc), but this seems a lot simpler than changing camshaft profiles or computer-controlling valves. Hopefully someone out there will be able to understand everything I just said and comment on it!
Hotrocker
http://www.hotrocker.com
Variable Ratio Rocker Arms [half, Sep 22 2002, last modified Oct 04 2004]
[link]
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How, exactly, would the variability be carried out? When performance builders "increase the rocker arm ratio" as you mention, how do they do it? What is it about the rocker arm replacements that is different than the rockers that they are replacing? |
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When you envision this, are you seeing the pivot point (towers) move somehow? If so, would it be possible to have the rocker arms stayed somehow so that their main structural attachment is not the pivot point and then move the towers, the pivot point, about some percentage of the length of the rocker? |
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Imagine if you could "cage" the rocker arm, maybe with raceways at each end, so that they are free to move in Y and then the towers--pivoting at their base where they meet with the cylinder head casting--are attached with pins to another raceway slot in the rocker arm such that when the towers are acted upon to pivot by, say, a simple cam mechanism, the top of the tower, the pivot, can slide to and fro. The main camshaft profile remains the same but the effective lift is increased or decreased. The "simple cam mechanism" would likely need to be more along the lines of a Desmodromic action so that it could both push and pull on the towers and maintain a constant tower position. |
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Combined with the very high valve spring rates needed to keep things from floating, I am imagining that the forces at work here are tremendous. The lower pivot of the tower would have to be incredibly strong. |
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Finally, I do wonder though if the profile of the cam, the duration, needs to be changed to properly accomodate the (effective) higher lift. It seems to me that when a high performance camshaft is ground, more is done than merely increasing the lift of the profile; the shape of the profile is altered as well and a variable rocker arm ratio would have no effect on the lift duration of the profile. |
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The idea of the rocker arm ratio is simply to act as a multiplier of the cam lobes. For instance, if the cam lobe specifies a lift of .25", and you have a rocker arm ratio of 2.0 (this doesn't exist, but for example's sake), the effective lift of the valve will be .5". Thus changing the ratio results in changing the multiplier. |
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Because of this, I don't think the camshaft profile will need to be modified in any way, because you are simply amplifying the existing profile by varying amounts. Granted, it's possible that a new profile would have to be designed in order to maximize the effect of the variable pivot. |
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Bristolz: I was envisioning some sort of moving tower. I like your idea of having the rocker arms suspended independently of the tower, but like you said, there are structural issues that have to be addressed here. When performance shops modify rocker arm ratios, they actually replace the existing rockers with ones with different geometry. Stock ratios are on the order of 1.1 to 1.25, performance ratios can get as high as 1.8 or 1.9. |
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Of course, this system as stated would only work on pushrod/lifter based valvetrains...overhead cam setups would need a different method in order for this to work. I happen to be partial to those good ol' pushrod Chevy small blocks... |
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With the name [vetteboy] I naturally assumed you to be a Mopar fan ;-) |
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I can't think of a way to do this that wouldn't make a squeal like cricket wings on a hot day. |
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It's been done, there was an article in HOT ROD a few years back, if i can find it i'll post the issue. |
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PS there's a company building variable rockers for SBC engines and by now possibly other engines |
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//How, exactly, would the variability be carried out?// Like this. (link) |
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"The variable ratio rocker arms can operate with ratios from as little as 1-to-1 and as big as 1.9-to-1"
"The Hotrocker gives the equivalent of multiple camshafts in an engine" |
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"You can read Jim McFarland's article on the Hotrocker in the May, 2000 issue of Hot Rod magazine" |
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has he gone? that's a pity. |
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Geez, I wished I'd done a search rather than blather on like that. |
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[bristolz]: I rather enjoy your blathering. It's generally well informed, educational, informative and rarely poetic. ;) |
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it all sounds a bit complicated...
Some times a more technical approach is more simpler. Where physics and geometry become a headache its better to have some controllable system.
For OHC and DOHC type valveheads this rocker arm system doesn't apply. Although many such valve heads have cam followers, they act directly on the valve. There is still room, of course, for some kind of leverage system there, which is what VTEC, NEO-VVL and other variable lift systems in xOHC heads do this already.
It'd be better to have electronicly controlled hydralic actuators that work on the valve and have complete control of augmenting the cam profile and allowing the valve to follow a infintely variable range of motion. This could be done by a mechanically variable rocker arm with leverage that can adjust on the fly DURING the valve opening cycle. |
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I have been thinking a bit about variable valve timing on OHC set ups. It seems to me that given a large rocker sitting over the valve more than one cam can be "in play" at any one time. The valve is always going to follow the cam with the proudest profile. |
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In fact the rocker only needs to be able to accomodate two cams at a time to be able to change between multiple cams on the fly... (But I think this is how the honda one works so I didn't bother to post a new idea). |
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