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Snake Oil Decoding: Rocker Arms
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pjalau
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Joined: May 04, 2006
Posts: 487

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PostPosted: Fri Sep 23, 2011 10:39 am    Post subject: Snake Oil Decoding: Rocker Arms Reply with quote

I have seen a lot of snake oil that will "enhance engine performance". (I get spam emails about enhancing performance too, but that's about a totally different thing).

Lighter (insert part here), magnetized doohickeys, etc etc etc. Since my mechanical engineering degree is still a few years away (about 60 last I checked) I figure I would unleash the awesome power of the samba and have someone de-mystify something that I have been discussing offline.

Forged Rocker arms and increased ratios.
These guys:
http://www.mamotorworkstv.com/vw/product/forged-rocker-arm-assemblies-125-ratio_305028/

First question:
I understand that the height of the valve may have some effect on engine performance in that it will allow more air/gas in and more air/gas out. Bigger bang = more HP (usually)
What I don't understand is the ratio calculation, and what effect it has on HP. Can someone explain it to me like I'm a 12 year old?

Second Question:
Since I'm looking into rebuilding yet another engine, is there a reason (other than price) NOT to get these? What I am really asking is: are these yet more snake oil? Why or why not?

Much thanks in advance.

-Peter (the clueless and too lazy to study mechanical engineering.)
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telford dorr
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Joined: March 11, 2009
Posts: 3547
Location: San Diego (Encinitas)
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PostPosted: Fri Sep 23, 2011 11:26 am    Post subject: Reply with quote

A little out of my area of expertise, but:

All late model VW rocker arms are 'ratio' rocker arms. The only difference is the ratio. Stock is 1.1:1, IIRC. This means that the valve moves 1.1 units of length when the push rod moves 1 unit. This is done to minimize the movement of the lifters, push rods, and rockers and still get adequate valve opening. The less distance these components have to move, the less energy they require to accelerate, and thus the faster they can operate for a given amount of valve spring force. On the other hand, because of the ratio, more force is applied to the lifters and push rods to move them.

Several things must be considered in valve train design. First is valve to piston clearance under all operating conditions. Lack of clearance = bent components. Second is adequate springs. You must have enough spring to keep all components in constant contact with each other (so that they don't slap), but not so much that these same components (including the springs) fail under the load. More spring = more force to turn the cam = more hp to operate the valve train. Third is the requirement of appropriate cam ramp design such that the components accelerate and decelerate as slow as possible, while still providing the needed valve lift, duration, timing, and operating frequency, so as to minimize impact loading and wear. You want to softly set the valve back onto its seat, not slam it.

So, in short, if you're going to use other than stock components, you become the engineer with the task of determining if your component choices will work together. For instance, if you use 1.4:1 ratio rockers, the cam must be designed to work with them. You just can't slap ratio rockers on any cam and expect it to work properly - the acceleration ramps would be too radical. [That said, I hear that 1.25:1 works fine with the stock cam because the acceleration and deceleration ramps on the stock cam are extremely conservative. You still need to address the spring rate, bind, and valve-piston clearance issues.]
_________________
'71 panel, now with FI
'Experience' is the ability to recognize a mistake when you're making it again - Franklin P. Jones
In theory, theory works in practice; in practice, it doesn't - William T. Harbaugh
When you're dead, you don't know you're dead. The pain is only felt by others.
Same thing happens when you're stupid. - Philippe Geluck
More VW electrical at http://telforddorr.com/ (available 9am to 9pm PST)
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