In a message dated 10/22/03 3:12:57 PM Pacific Daylight Time, wa...@hispeedwireless.com writes:
> Hey all this is why I used to rebuild my VW in my sandrail every year and > now since being converted to the R.S Hoover way of Do it right and maintain > it. I have not needed an overhaul since 97. > ------------------------------------------------------------ Now my hat don't fit :-) ------------------------------------ Actually, it isn't too difficult to get the VW valve train geometry dead-on, it just takes a bit of time. I'm told Santa is bringing me a digital camera this Christmas so I'll be able to post some pictures of the tools I've made for setting up valve train geometry. 'VW Trends' published a rather lengthy article titled 'Dialing in Your Cam' which describes the first stage of the process. Once you've got the cam dialed in and have measured the actual lift at the cam the remaining pieces of the puzzle fall into place. (If you're into engines there's a lot more poop about PORSCHE valve train geometry than you'll ever find for the VW. You CAN set-up a VW properly... but the factory never did. Close enough was good enough.) The whole idea behind setting up the valve train geometry is to find the point of minimum loss in the process of turning an upward push, delivered at an angle of about three degrees, into a downward shove, delivered at an angle of about nine and a half. The two basic principles involved are the fact that the push-rod and valve can only move in a linear fashion (or nearly so) while the rocker arm can only move through an arc, with the input arc typically having a shorter radius than the output (that is, the rocker ratio is 1.1 to 1, 1.25 to 1 and so forth. Stock is 1.1, after-market rockers go up to 1.45 and just to make things interesting, sometimes the rockers actually come pretty close to those numbers :-) (Nowadays, you gotta check EVERYTHING.) The tricky bit is HOW you go about matching the mid-point of the linear push (or shove) to the tangental mid-point of the arc. Think about it for a minute. Push the DOOR open with a broom stick... Broom stick travels pretty much in a straight line but the door has to swing thru an arc. No matter WHAT YOU DO the path traveled by the broomstick will always be LESS than the arc traveled by the door. That is, the broom stick travels on the chord of the arc whereas the door follows the radius of the arc. Bottom line, there's always some loss. Your job is to arrange things so as to give the LEAST loss, which also translates as the MOST motion at the valve. So you find out how much the cam pushes the follower and the follower pushes the push-rod and the rocker swings through an arc... and you raise the rocker up or lower it down so that the mid-point of its arc perfectly matches the mid-point of the linear travel of the valve and you adjust the length of your valve stems (if needed) and the length of your push-rod (ALWAYS needed) and then you run the engine on the test stand for five or six hours and discover everything has changed anyway... :-) Actually, you crank in a bit of Texas Windage when you do the set-up, so that as the engine wears in and you come back and re-torque the heads and tweak the valves, the anticipated amount of wear will have caused the tangental point to be within a gnat's ass of dead on, or so close that it's well within the range of your adjusting screw. (Gnat's ass is a technical term meaning five-tenthousandths of an inch if the wind is from a south and its a Tuesday. Work with me here; you'll pick it up as we go along.) I suppose that's really the tricky part, knowing the amount you have to stay on the high side of spec so that as the engine wears in it runs sweet instead of sour. A lot of that part of valve train geometry won't make a lot of sense because nowadays, with "Volkswagen" parts coming from all over the world, from factories Volkswagen never heard of and without any quality control to speak of, the initial wear-in coefficient is based on guesses and good intentions. But ANY attention you pay to your valve train geometry will usually be rewarded with SOME improvement in your volumetric efficiency. On the other hand, I've seen lots of engines that gave away as much as 25% of their lift at the valve from being thrown together right out of the box. The engine will tell you. Trouble is, most folks don't understand what it sez. --------------------------------------------------------------- Didja see how Mike did the set-up on his Corvair? That photo of the roller-rocker just kissing the valve stem? For VW's I made a jig so I can set a valve at its computed half-height. Then I paint the stem with Dykem and have a scriber on the rocker arm so that when it's set at the proper height it draws a line across across the face of the valve stem. An optical comparitor shows me how close I've come to the center of the valve stem for that particular setting. That probably sounds sorta gim-crack but I haven't found any better way of doing it. Actually, it's not too bad of a method for a guys like me with greasy fingernails instead of engineering degrees because I'm not actually MEASURING anything in quantified terms, I'm simply COMPARING things, peering through a glass to see if the line is too high, too low or just right. Raising the pivot point of the rocker will cause the line to fall a little lower, lowering the tower will raise it. Once you get it dead on you go back and re-run the numbers to see if the input is close enough to tangental. If not, you can use a different valve or a different tower. The height of the tower and the length of the valve stem don't really matter (within reason) so long as the mid-points of the linear travel exactly match the tangental points on the arcs. And keep in mind, you've got four rockers on the same shaft; you're seeking HARMONY rather than an identical numerical value. Then it hums. 'Free' horsepower. More power for the same fuel. Wears at a slower rate. Runs a lot smoother becuase the valve train stresses are more evenly distributed... or something. It also causes you to grow long, blond Surfer God hair allll over your bod-dee. Actually, I was just kidding about that last part. (As he tosses his long, blond, Surfer God hair back from his noble sunburnt brow. Being a native Californian isn't as easy as it looks.) Engineering types get off on what happens when you DON'T pay attention to the valve train geometry, as in calculating how fast the engine will trash itself, or how low the efficiency can fall before EPA will sue your ass back into the Stone Age. The Stone Age is where you'll find all those VW 'experts' who never mention valves at all, telling the Yuppies, "This engine will RUST OUT before it WEARS OUT." Uh, right, John. -R.S.Hoover
