web cam #73 and rocker geometry

[woodman]

Hey guys, does anyone know off hand if swapping a stock grind cam with a #73 grind web cam will affect rocker geometry? I wouldn't know how to deal with it if it did, so if it does, stock grind it is.

[chrisreale]

I think you might have to use different push rods. Jake Raby has talked about this in the past. Do a search here or at shoptalkforums.com

[Zeke]

Rocker geometry is a function of lift. With a lot of lift, the rocker pushes the valve down so far that it ends up pushing on the edge. The rocker is centered in it's tarvel to compensate. I don't know the lift of the #73, but I will guess that if it is under .450, you might not have much to worry about. Lift is not the only thing that cams can accomplish. Duration and overlap determine the characteristics of the engine as well. You are going to keep your FI, right? You need a cam that is designed for FI.

[woodman]

Right, keeping FI. #73 lift - 0.426

I guess i could use a little interpretation.
Haynes says that the stock cam specs are as follows:
intake open @ 12 deg btdc
closes @ 42 abdc for duration of 227 deg
exh open @43 bbdc
closes @ 4 atdc for duration of 227 deg

I've also seen that the valve lift is .385"

[Jake Raby]

The 73 grind is a "Generic cam" and I base several of my grinds from their base, however even being so mild it is the hardest to get the geometry correct on!!!! most of my more aggressive cams are easier to set up than a 73, partially due to the ramp rates. The 73 is a nice grind, and don't let a little work stand between you and the cam you desire and will work best for you...conquer the challenges and stick with it!


ANY cam change requires the rocker geometry to be reset!! So does the following.

-Head resurface
-cylinder shim change
-removal of stock head gaskets
-cylinder length change (the aftermarket cylinders are .040 LONGER than stock OEM 039 cylinders (FYI)
-shorter/taller valve installed height
-shorter/longer valves
-valve seats recut, thus plunging valves further into the seats

(Just suck it up and do it every time the heads come off, no matter what cam you have)


The stock cam absolutely sucks,(in every respect, especially cylinder head temps) I NEVER use one in any engine.

The rocker geometry must be verified everytime you assemble the engine in my opinion. it is THE biggest area of concern in the top end assembly of the TIV engine. Neglect it and kiss valves and guides goodbye, sometimes in less than 1K miles.

It sucks, but has to be done. I learned this the hard way.!

in 99.9% of applications the stock pushrods are NOT the optimum length for the geometry to be "happy"

Good luck, don't overlook it, or you'll end up a statistic in no time flat.

[woodman]

Thanks for chiming in Jake, and giving me all that homework!!
Yeah, you're right. I agree that it's better to go all the way or not at all, right?!

[jamcleod]

interesting, could you explain a little about how & what to check for and how to remedy problems....

[Zeke]

Originally posted by jamcleod
interesting, could you explain a little about how & what to check for and how to remedy problems....

Oh man, explain a little. Not gonna happen. I think an older issue of Hot VW's has Jake procedure to do this. This is the only area of engine building that I have someone else do for me. According to Jake, you may end up with different lengths of push rods in the same engine!

[woodman]

Milt, Do you think George at European can handle this task? I'm taking most everything that needs machining over to him next week.

BTW, hows your car running?

[woodman]

A quick google search resulted in this, plus a gazillion others. Just a very basic intro to understand what it involves. (I hope I've not mispoken.) All I know is that I don't know nuthing...

http://www.kymcovw.com/Media/pdfs/rocker_geo.pdf