potterc32

iTrader: (1)

Just got a Comp Cams xtreme energy cam kit. After opening everything up, I noticed the diameter of the lobes are smaller than the original cam that came out of my SBC. The grind numbers on the cam match the kit, and the engine was the 195 hp GM crate motor that you see in jegs and such.

Is this normal? Do they just grind them smaller on the "heel" or base circle of the lobe? I could see how that would make sense if you can just adjust your rockers down further. Everything else about the cam fits the engine, right length, cam journals, and distributor gear. I've never looked into this before and couldn't find any answers online.

Anyone see this before?

Orr89RocZ

iTrader: (20)

Yup probably a different base circle on the cam

potterc32

iTrader: (1)

So nothing to concern myself with?

Orr89RocZ

iTrader: (20)

Probably not. Can check by taking dial calipers to overall lobes and then to the base circles to get an idea of the lift.

AlkyIROC

A cam lobe can only be so high. It needs to stay lower than the journals to slide it into the block. To increase lift, the base circle is lowered. Big cams need a small base circle to get the extra lift. All blank cams have the same diameter cam lobes when they start out. As the cam is ground, the base circle keeps getting smaller until the desired lobe lift is reached.

Of course with a smaller base circle, you need longer pushrods to keep the same valve train geometry.

2005Impalla

You might need longer push rods

potterc32

iTrader: (1)

This cam is advertised as not being a "small base circle cam". I marked the top of the valve stems with a marker, and it looks like the rocker arms still contact in or very near the middle of the stem. Is there anything else I can do to check and see if I need longer pushrods?

2005Impalla

Nope, did you rotate the motor. You want to look at the wear pattern and if it is centered your good. Larger lift cams some times require shorter push rods so if its a lot more lift maybe they compensated my making the base of the cam a little smaller. If the rockers are good than your fine.

potterc32

iTrader: (1)

Yeah I turned the motor over like 5 or 6 times after marking the stems. (Got tranny fluid all over my shoes in the process, that was exciting!! haha) Lift on the new cam is 487/490 compared to the original ~390/~390 (sbc 350 GM crate engine). So it is considerably more lift but nothing wild over .500" or something.

AlkyIROC

Wear pattern in the center of the stem is great but proper geometry gets the smallest wear pattern even if it's slightly off the center.

As the rocker does a sweep across the stem while the lifter is going over the cam lobe, the rocker will leave a witness mark on the stem. The smaller that mark is , the better the geometry is even if it's not dead center on the stem. The rocker must go full travel from no lift to max lift and back to no lift before you can check the witness mark size and position.

With hydraulic lifters, they should be pumped up with oil and/or the valve springs should be swapped out to lightweight springs so they don't compress the plunger in the lifter while doing the sweep. With solid lifters, it's a lot easier.

While at full lift with the valve springs you will be using, measure the distance between the coils. If you can, also try to see how far away the bottom of the retainer is from the top of the valve seal. You wouldn't want to install a bigger cam only to find out the heads won't take the extra lift.

Damon

What Alky said is exactly right.

When they say it's a "small base circle" cam they mean EVEN SMALLER than the already smaller base circle required to get the cam through the journals.

Night rider327

iTrader: (13)

The base circle changes as the lift goes up.

The std. sbc cam journal size is 1.868"
To find base circle size you subtract the lobe lift (not listed valve lift) twice from journal size.

Lobe lift is without the rocker ratio factored in. (Listed lift / listed rocker ratio = lobe lift)

Base circle will change with the profile of cam and can range from around .850" to 1.228"

AlkyIROC

If you want to see a small base circle, you should see the lobes on the 0.800" lift cam in my BBC.