Macadoo

Hey fellas, I'm confused about something. I have a fair understanding of camshaft geometry as far as lobe separation, duration, and height. But I see a lot of talk of folks needing shorter pushrods for their new cams. Doesn't this negate the effects of a taller lobe? Or is it because they bought too aggressive of a grind? Or maybe I actually don't understand camshaft geometry, which is entirely possible

joe_padavano

Unfortunately, it's this.

Pushrod length has nothing to do with cam lift. All it does is correct rocker geometry so that the rocker tip correctly contacts the tip of the valve stem throughout the travel of the rocker. A cam with more lift causes the rocker to pivot through a larger arc, which means that the tip of the rocker travels inboard-outboard on the valve tip over a longer distance. If you simply increased cam lift and kept the pushrods the same, the rocker tip would be in the same place on the valve tip at zero lift, but then would be further outboard at max travel. Shortening the pushrod recenters the rocker tip over the total travel.

Now, this ASSUMES the two cams have the same base circle. Really high lift cams sometimes use a reduced base circle, since the maximum absolute lobe lift (dimension from the cam centerline to the tip of the lobe) is limited by the I.D. of the cam bearings (the lobes need to pass through the bearings. Reducing the base circle of the lobe increases total lift of the cam, but again screws up rocker geometry. This may require a LONGER pushrod to correct geometry.

jag1886

I don't claim to know a lot about cams but I know a lot of aftermarket cams are ground a smaller base circle, that could be one reason.

RandyS

In this scenario, is it correct to assume that the heads have adjustable rockers? What should/could be done with fixed height rockers? (yeah, I know, change 'em).

Macadoo

Oh sure, I shoulda' thought of that. So it's a ratio between base height and lobe height and where the arc centers, makes sense. And I hadn't thought about the limitations of the bearing ID.
Thanks Joe

Lars

if your base circle is smaller, and your headgasket is thicker, you could be loosing lifter preload... = ticking. MR efi should know the cam's basecircle. how much was milled on your heads?

Macadoo

I was looking at the paperwork and it doesn't say. Seems like it should but it doesn't. I wasn't too impressed by this shop. I'll call Monday and see if they keep records.

joe_padavano

Please read my post above. A SMALLER base circle will usually require a LONGER pushrod to make up the difference, but the ultimate pushrod length depends on both base circle and total lobe height.

Nope. The correct pushrod length will allow fixed rockers to work just find. Adjustability is nice to accommodate variation in head gasket thickness, head milling, and valve stem height without resorting to different pushrods, but more often than not, the uninformed will simply crank the adjustment and screw up rocker contact pattern with the valve. If measured correctly, a custom pushrod will allow stock fixed rockers to work just find. Of course, getting custom pushrods every time is not practical, thus the preference for adjustable rockers.

cutlassefi

Who's shop?

Normally most aftermarket flat tappet cams with conservative lifts (.400-.500) will have similar base circles. There is normally some room between the diameter of the 4th cam journal (that's what dictates overall lobe size) and the top of the lobe so significantly reducing the base circle isn't necessary with conservative lifts. On a mild cam the base circle rarely varies more than .020 or so from stock. Remember even if you go from .400 to .480 valve lift you only need the lobe to grow .050 due the 1.6 rocker ratio.

Macadoo

I wasn't speaking about your work EFI, sorry if I gave that impression. I was speaking of the machine shop I took my heads to. They didn't even hot-tank the heads, just degreased them. They came highly recommended and it was essentially a large facility packed with heads and blocks so they are definitely used.
I'll try and call tomorrow and see if they have records about my millwork. Although it sounds like I most likely don't need to worry.

jag1886

Originally Posted by jag1886 View Post
I don't claim to know a lot about cams but I know a lot of aftermarket cams are ground a smaller base circle, that could be one reason.
Please read my post above. A SMALLER base circle will usually require a LONGER pushrod to make up the difference, but the ultimate pushrod length depends on both base circle and total lobe height.
PICK PICK PICK

captjim

Along with other factors such as milling of deck and heads along with head gasket, valve stem height, etc. Bottom line; pushrods are relatively inexpensive. Assemble the engiine, then measure using an adjustable pushrod and order a set of the proper length. This isn't rocket science, it is how most engines are built.

Lars

you are learning it's not "bolt some stuff togeather and it should work" like ripples in a pond, its all connected.