AMA spec. sheet info from GM Heritage Center

[Bill W.]

This could be a good one for Duke or any other cam experts . In the GM Heritage Center they have all the engine specs for the 65,66,67 BBs. If you look at the cam specs for the 65L78 &,66 L72 they are the same . Then if you check the same spec sheets for the 67 L71 The lift and duration drop way down . I know the parts book sold the same cam for all the solid lifter Corvette BBs (the 3863143 and the 3904362 without the groove) But with the AMA sheets showing so much difference could there have been a little more radical cam used in the 65 and at least some 66s .The Duration drops from 336 to 302 and the overlap dropped from 108 to 80 . I have my original L78 cam from # 16120 and a New GM 3904362 but I have no idea how to check them . The cam from 16120 is pitted or I would reuse it .

[Jim D.]

Re: AMA spec. sheet info from GM Heritage Center

I'm certainly not a cam expert but my SWAG would be that they changed the way the specs. were measured. If you compare advertised specs. vs. the same specs. @ .050 you'll get a large difference like you noted. I'm not saying that's what GM did, but it's a possible explanation.

[Bill M.]

Re: AMA spec. sheet info from GM Heritage Center

This could be a good one for Duke or any other cam experts . In the GM Heritage Center they have all the engine specs for the 65,66,67 BBs. If you look at the cam specs for the 65L78 &,66 L72 they are the same . Then if you check the same spec sheets for the 67 L71 The lift and duration drop way down . I know the parts book sold the same cam for all the solid lifter Corvette BBs (the 3863143 and the 3904362 without the groove) But with the AMA sheets showing so much difference could there have been a little more radical cam used in the 65 and at least some 66s .The Duration drops from 336 to 302 and the overlap dropped from 108 to 80 . I have my original L78 cam from # 16120 and a New GM 3904362 but I have no idea how to check them . The cam from 16120 is pitted or I would reuse it .

They changed the lash from .020/.024 to .024/.028

[Duke W.]

Re: AMA spec. sheet info from GM Heritage Center

I'm certainly not a cam expert but my SWAG would be that they changed the way the specs. were measured. If you compare advertised specs. vs. the same specs. @ .050 you'll get a large difference like you noted. I'm not saying that's what GM did, but it's a possible explanation.

Exactly!!! Chevrolet was not consistent in how they measured timing points and duration, and they never really specified how it was measured. I've seen the engineering drawings and have the lift vs. crank angle data, which is lift, in inches, to five decimal places every camshaft degree. The inlet and exhaust lobes are the same and and both cams use the same lobe on both sides.

The clearance ramp height above the base circle is .012", so if you assume a 1.7:1 rocker ratio, the cold clearance with iron heads should be .020" on both sides.

Duration at .050" lifter rise is 242*, and this, of course, includes the clearance ramps. Duration at .062" is what you use to compare to hydraulic lifter camshafts and that number is 231*. Lash point duration will vary according to the recommended lash, which is why you see differing specs from GM, but the lash point duration I measure from the lift-crankangle data is always a little different than what GM specifies, but I can't be sure they are using the same measurement criteria.

BTW, this same lobe was used on the inlet side of the LT-1 cam, but on a slightly smaller base circle. The exhaust lobe on the LT-1 cam is the 30-30 lobe.

SAE standard J604D calls for measuring duration at .006" valve lift, but J604D did not come along until years after this camshaft was designed. Bottom line is that you cannot compare a mechanical lifter camshaft to a hydraulic lifter camshaft or a mechanical lifter to another mechanical lift cam unless you know the clearance ramp height, and most of the time you don't know that value. Guys bandy about and compare cam data all the time, but they usually don't know the context, which makes comparisons meaningless, especially when comparing a mechanical to hydraulic or one mechanical to another.

Duke

[Bill W.]

Re: AMA spec. sheet info from GM Heritage Center

Duke . I understand what you are saying . Is there a way I can compare my original to my NOS cam . Can the lash adjustment change the overlap and the duration as Bill suggested ? My original has some pitting so I dont think it would be usable . Bill

[Duke W.]

Re: AMA spec. sheet info from GM Heritage Center

"The cam from 16120 is pitted or I would reuse it ."

What's 16120?

All the BB SHP cams have the same lobes and lobe indexing from '65 to '71. The only difference is no rear journal groove on '67-up.

What do you have? How about year, engine option, id number on the original cam and id number on the NOS cam and the part number on the box?

The Federal Mogul CS-165R is built to the GM print, but I don't think it has a rear groove, so a machine shop will have to cut it for installation on a '65-'66 block along with the proper three-hole bearing.

Pitting is an indication of surface fatigue failure, so it's best to replace it.

You can compare them by taking a lift-crank angle data set either in the engine, on a lathe, or a set of V-blocks.

Duke