Cris

What is the standard base circle diameter for a SBC cam? I've looked everywhere and cannot find a consistent number.

SWCDuke

There is no "standard". Each OE lobe/application has its own base circle dimension.

For example, the LT-1 cam inlet lobe eccentricity is identical to the L-72 lobe, but they each have different base circles.

Duke

Cris

Thanks Duke:

Wow, that's surprising since the base circle is one component in push rod length. Was there a typical range for the base circle diameter? I've seen numbers from 1.03 to 1.125".

SWCDuke

The higher the lobe lift, the smaller the base circle, so what they were probably designing for is a target range of rocker angle at maximum lift.

For example, the 30-30 cam with gross lobe lift of 0.32336" (0.306336" above the clearance ramp top) has a nominal base circle radius of 0.59974". The L-79 cam lobe has a gross lift of 0.29807" and the nominal base circle is 0.61708".

The Duntov cam has the least gross lift, 0.26250"/0.26650" (0.25450" above the clearance ramp top for both) of all the cams of that era and the largest base circle of 0.64865" nominal radius. All others are between the Duntov and 30-30 as is their lobe lift.

The base circle tolerance is +/- .001" from nominal.

Don't know where you got your data, but mine is from the GM drawings.

Duke

Cris

Sorry Duke, my numbers were from aftermarket cam manufacturers.

I would think that the bigger the better for base circle, especially with an aggressive lobe profile. I like your theory about shrinking the base circle as lift increased to keep the max lift rocker angles roughly the same with stock push rod lengths.

My interest in this comes from plans to build a 4" stroke small block. Engine builders say the most that will fit is a .875" base circle cam. That's a long way off from the 1.25" ish numbers you are citing. Seems like the cam grinders would be a little handcuffed on how steep the ramps could be with such a small base circle.

CFI-EFI

The camshaft interference is with the connecting rod. The stroke of the crank determines the location of the rod, but the profile of the rod is a major factor in the clearance. SOME rods won't clear a standard cam in a 383, but with a little grinding, almost all will. Every combination of crank, rod, and camshaft is different. Beware of "pat" answers and be prepared to deal with whatever pops up.

RACE ON!!!

gkull

Buy the stroker rotating kit. They have done it enough times that they know what rods work without grinding them.

I've never had a problem with Crane SBaseC roller cams of .930 dia.

Cris

CFI-EFI, gkull:

I guess I should have filled in the details. (I was thinking of saving this for a separate post.) The .875" base circle comes from engine builders using Eagle cranks and Eagle H-beam rods. Eagle has just introduced an H-beam rod called their "extreme stroker rod". I have talked to Eagle, but they only have one rather sketchy example of someone using these rods with a 4" stroke and a 1.05" base circle cam.

Bottom line, I believe a larger base circle is better. Whose rods give the best clearance (vs strength) in a 4" stroker application?

GOSFAST

We have a Comp Cams (.875 base) in an engine we had bolted to the dyno last weekend, for testing. First pull 800 HP on a 421 C.I. SB. with a 4150 carb. This one is not a Pump-Gas engine though. This unit has a 3.875 stroke with a set of custom "Howards" alloy rods. The rods cleared both the cam and the pan rails (we did notch the rails, but didn't break through the casting) with a 'bone-stock" #511 400 SB casting. We we're even suprised that everything fitted. We don't usually build this type unit based on a stock casting. When you order a cam properly matched for this application, you may not have a choice to "base-circle". I say this because the specific lobes we ordered to make the most power could not have been put on anything other than the .875" circle. Thanks, Gary in N.Y.
P.S. Using any decent steel rod should not be too much of an issue.

Cris

"I say this because the specific lobes we ordered to make the most power could not have been put on anything other than the .875" circle."

Gary, I don't understand this. I would think you could put more lift on a larger base circle cam. Reducing the base circle restricts the lift and ramp profiles doesn't it?

GOSFAST

When you're trying to get the maximum lift at the valve (on a SBC platform), I'm talking between .800" and .900" lift, you need the cam as small as possible on the "heel". It's opposite of what you're trying to picture. Right now, believe it or not, it's hard to actually get them to grind what we really need to maximize the power. For instance, I need a bigger cam than what we've been using with respect to Retro-Hydraulic Rollers. They can't give us big enough grinds it seems to get past 800HP on a 9.75:1 C.R. short deck (9.800) BB to run on pump-gas. There are physical limitations. Let me put it another way, if you make the backside of the lobe bigger, you'll have less room on the topside to incorporate the proper lift. If the base (circle) measures 1.500" (these are numbers only for comparison) and from the heel to the toe it measures 1.750", with a std. 1.5 rocker you'll only have .375" lift at the valve, now leave the 1.750" number in place and "tighten" the base (circle) to 1.300" and now you've got .675" at the valve. Hope it's a little clearer. Thanks, Gary in N.Y.
P.S. We'd like to see higher lifts, maybe in the .900+ range.

Cris

Gary, thanks. I think I understand. Your issue is the cam lobes clearing the cam journals. SBC journals are 1.868" in diameter or .934" in radius. This means your lift (at the cam) plus the base circle radius cannot exceed .934".

You've got some pretty extreme examples. But even in my mild case of a .6" lift cam and 1.5 rockers, I find the base circle diameter will be limited to 1.068 max!

comp

interesting info

gkull

Why would you believe that? The highest lift SBC that i have ever personally owned or worked on was @.760 net lift. It's was done with a Crane or other .930 base circle. Cam strength has also never been an issue. 240#closed in sprint car motors turning mid 8000 rpm and power steer, Mag and fuel pump is driven off the back of the cam. The rear of block is open for attachments to the cam because you don't have a starter or flywheel.

I drive around a 4 inch stroker small block (Motown) The smartest thing to do is just get the tall deck raised cam aftermaket block and then don't worry about clearance even if your using 6.250 big steel rods that can live at the 1200 hp level.

Cris

gkull:

I was not considering strength issues when I said a larger base circle is beneficial. I believe a larger base circle (theoretically) can support a more aggressive cam profile. This is because there is a limit to the ramp rates the lifter sees. A larger base circle allows the lifter to get up to full lift quicker (in cam shaft degrees). To visualize this think of a very large base circle diameter say 6" and a cam lift of .4 inches. In this extreme example, you could pop open the valve in a very short cam period and even have a flat profile on top of the cam.

Now draw a base circle of .25 inches in diameter. You'll see you are really hamstrung trying to get .4 inches of lift with a practical ramp profile.

How much real world difference would there be between a .875" base circle and a 1.05" base circle? I don't know, but I'm sure going to ask the next time I talk to a good cam engineer.