A, B or "touring" cam?

[Jim Brierley]

Originally Posted by Gene F

I get it that lift means the valve travels higher, and gets more breathing. But what is the duration (in plain english)?

Seat-to-seat duration is about 254 crankshaft degrees on a stock B cam, 260* on the .339" lift cam. Duration on a 260* performance cam will out perform the same duration on a stock cam because the rate of lift is greater. The .050" figure is a random figure used by many grinders because the rate of lift may vary as done by different grinders, .050" tends to give a better idea of how the cam will run under racing conditions. Too much cam in a stock or near stock engine can actually hurt low-end power, so stay on the mild side for street (touring) use.

[nkaminar]

Everything about camshafts can be found here: https://www.tildentechnologies.com/index.html. Thank you Terry.

[Gene F]

Originally Posted by Flathead

Duration is how long the valve is off of it's seat, Usually measured after a certain amount of initial movement of the valve, like .050 after opening to .050 before fully closing. Measured in degrees of crankshaft rotation.

Thanks

[Gene F]

Originally Posted by Jim Brierley

Seat-to-seat duration is about 254 crankshaft degrees on a stock B cam, 260* on the .339" lift cam. Duration on a 260* performance cam will out perform the same duration on a stock cam because the rate of lift is greater. The .050" figure is a random figure used by many grinders because the rate of lift may vary as done by different grinders, .050" tends to give a better idea of how the cam will run under racing conditions. Too much cam in a stock or near stock engine can actually hurt low-end power, so stay on the mild side for street (touring) use.

Thanks

[JayJay]

Well, after all that, it appears I have a stock Model A camshaft. three bearings (as is the block), eight lobes, not nine. Before I pulled it out of the engine I put a dial indicator on it and found the lobe lift is 0.284"-0.291". Since a Model A cam starts out at 0.302" and a Model B starts out at 0.334" (see https://www.fordgarage.com/pages/camshaftspecs.htm), I suspect I have a stock Model A shaft that has worn lobes. I'm going to do some more detailed measurements tomorrow before I send it off to be reground, but at first blush this appears to be a good shaft.

And the babbitt (isn't this how we decided to spell it) appears to be in excellent shape, both rods and mains. Interestingly, there were no shims on the mains, so it will be interesting to plastigage it to see what the clearance is.

[J Franklin]

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My late 30 engine had an early 5 bearing cam in it when I got it and it was a screaming machine. Got it to 70 once for a very short burst on the freeway.

[ursus]

Originally Posted by J Franklin

My late 30 engine had an early 5 bearing cam in it when I got it and it was a screaming machine. Got it to 70 once for a very short burst on the freeway.

I'm curious about that because I was told that the original 5-bearing cams gave slightly more power than the later 3 bearing cams, but with a less smooth idle.

I wonder how well the shorter 5 bearing cam bearings (especially the center bearing) hold up in a later engine block?

[J Franklin]

I think there was more power, It definatly wanted to go faster than otheres I have driven. It was very happy at 55MPH.

[nkaminar]

Depending on the cam, the power may be produced at a higher rpm. Even the "touring cams." An engine with Babbitt bearings and unbalanced crank may not be able to stand the higher rpm, like 3,000. (I like to capitalize his name in honor of the early work that he did that enabled the production of many engines and accessories. Plus my spell checker seems to like it that way.)

[JayJay]

Originally Posted by nkaminar

(I like to capitalize his name in honor of the early work that he did that enabled the production of many engines and accessories. Plus my spell checker seems to like it that way.)

Spoiler alert: most spell checkers will let anything with a capitalized first letter pass as a name. Mine likes "Babbitt" but not "babbitt", seems to still have the latter confused with "rabbit".

[J Franklin]

Rabbit bearings sound like the would go "Lickity split"!

[nkaminar]

Wabbitt bearings. “Kill the Wabbit! Kill the Wabbit! — Elmer Fudd

[Pete]

Originally Posted by nkaminar

An engine with Babbitt bearings and unbalanced crank may not be able to stand the higher rpm, like 3,000.

Well, we know they will last at least 500 miles at 6500 rpm twice a lap. They were crudely balanced though.

[Benson]

2900 rpm is no problem with a properly rebuilt babbitt engine.


My friend drove his car regularly on long tours at 65 MPH (that is about 2900 RPM).


At 35,000 he pulled one shim (one rod/main cap at a time) on each main and rod bearing.


Each time the crank was locked.


He replaced all the shims and put another 35,000 miles on it and no problems at 70,000 miles.


The same re-builder did my engine and at 25,000 it is still working just fine.

[Jim Brierley]

Originally Posted by ursus

I'm curious about that because I was told that the original 5-bearing cams gave slightly more power than the later 3 bearing cams, but with a less smooth idle.

I wonder how well the shorter 5 bearing cam bearings (especially the center bearing) hold up in a later engine block?

It seems that early cams don't wear as much, but are ground to the same dimensions. There were three 5 brg cams, the first had all 5 ground to the same size, #2 had #2 and #4 ground smaller to ease assembly in the 3 brg block, #3 had them just rough turned on a lathe.