ZeeOSix

See attached photo for some cam definitions. I guess you could measure across the cam 90 degrees to the lobe, but you'd have to be careful you're not on the lobe ramps. You could do it both ways and compare to see if it gives the same answer.

Again, remember you are after the base circle radius, so you'll have to divide the measured diameter by 2.

Patrick G

iTrader: (14)

This is correct. Just measure the sides of the cam perpendicular to the nose of the cam lobe then divide by two. This gives you the base circle radius. Armed with this info, you can then determine the lift of the cam by measuring the tall part of the cam (top to bottom) and subtracting the base circle radius (times 2) from that figure. The net lobe lift is your result. Multiply the lobe lift times your rocker ratio and that's your valve lift.

z99ls1

iTrader: (12)

Ok so if I get my stock cam and measure the sides of the cam and devide by two I will get my stock cams base circle? And than I can take my aftermarket cam and do the same thing and however much smaller my base circle number is on my aftermarket cam will be how much longer of a pushrod I will need correct?

PREDATOR-Z

Theoretically Yes. But in real life, you take a pushrod-meter tool and measure live on the motor.

z99ls1

iTrader: (12)

I don't see why you say IN REAL LIFE. When someone is just changing there cam with stock heads that have never been removed it should be as as accurate as any other way I'd think. I understand it is not going to work as soon as you put on different heads or a different head gasket, but alot of people just do a quick cam swap with stock heads that could use this method.

PREDATOR-Z

Because REAL LIFE is much more than just numbers on paper

z99ls1

iTrader: (12)

But if you take out the stock cam measure it and measure the aftermarket cam and go by that it is going to be accurate.

PREDATOR-Z

Approximate YES, accurate, NO.
There are other parts in the valvetrain you have to take account for.

Patrick G

iTrader: (14)

What PREDATOR-Z is trying to say is, measuring will ensure that you install a pushrod with the exact preload you need. For example, lifter preload varies some from engine to engine because of production tolerances. Your pushrods may be in at .095" preload in your engine and might be in at .080" in another engine and .105" in another. If you measure the stock cam base circle, then the aftermarket cam and determine that you need a 7.425" pushrod to get you to the same preload as stock, you're pretty much guaranteed to be at the same .095" preload as stock.

But let's say you really wanted to be closer to .070" preload. By measuring, you'd know that your pushrods were .025" too long and a 7.400" pushrod would be closer to ideal. Your buddy with .080" preload might as well leave the 7.425" pushrods in because they're only .010" too long, but the guy with the .105" stock preload would need perhaps a .050" shorter pushrod to get closest to .070" preload. Like Predator said, theory is theory and the math works, but measuring will ensure that you get the lifter preload you're wanting to achieve.

LS1FREEK

iTrader: (1)

I agree with you Zach, if you do the base circle measurements, it should be 100% in spec of the stock setup, obviously. For someone who doesn't believe this way would give you the right PR length, is then saying that GM didn't install the correct length PR's from the factory.

Edit- Pat beat me to it lol

LS1FREEK

iTrader: (1)

Oh, and I also agree with Patrick lol

ZeeOSix

Here's another wrinkle in this whole discussion. If you use a pushrod length tool, you also need to know exactly which lifter you have and what the total plunger travel is on that specific lifter part number. As you can see in the attached table, LSx lifters varied throughout engine production. So, "technically" you would either have to know exactly which lifter part number was in your engine or also accurately measure the lifter's plunger travel as part of the measurements to determine correct pushrod length.

If you don't change anything in the system but the cam, then using the base circle radius difference between cams to determine the right pushrod length seems so much more straight forward. And yes, it is dependent on if the factory designed the valve train system to put the lifter at mid travel ... but I guess I'd trust them on that, and also trust that GM made all the components close enough to each other to not worry about lifter preloads being too different between all 16 lifters.

According to the table, the OEM lifters in my 2002 Z06 only have 3.20 mm (0.126") of total plunger travel and need to be preloaded at 0.062". Without knowing this detail, a pushrod length measuring tool is useless if I thought I had 0.166" (4.22 mm) total plunger travel.

bmf5150

Measure for sure,mine was different size than what they suggested!

PREDATOR-Z

Put in the time guys, much better results that way. It is paying attention for those details that can make or brake a combination and also why 2 identical combos could have very different power outputs. Not to mention that all those measurements increase reliability of the setup for miles of trouble free power.