What's up with these cams?

[Mashermoto]

I have a set of cams from a 1978 Standard that are very different from my set of cams from a 1979 Special. I'm wondering if one set is other than stock.



The cam profiles are radically different. The 78 is on the left and the 79 is on the right. The 78 has a much more gradual ramp on the front and back sides of the lobes, which looks like the valves would have a longer duration around the peak of the lobe. In comparison the 79 has little ramping on either side. It is much more triangular looking near the peak of the lobe.



As you can see here, the marking are very different also. The 78 on top has a flat ground place on the shaft with the identifier stamped on it. The 79 has the identifier in raised characters which are part of the casting. The 78 intake is identified as a X3 and the exhaust is a 9-5. The 79 intake and exhaust are both identified as 3H5.

Here are the lobe diminsions:

Stock spec - Intake 1.449 Exhaust 1.429
78 - Intake 1.447 Exhaust 1.429
79 - Intake 1.448 Exhaust 1.448

So the 78 is within OEM specifications for both cams, but only the intake is in specifications for the 79.

Does anybody recognize these items

Does anybody know how one set would perform over the other in regard to low, mid-range and upper power curves

[TopCatGr58]

Hey Masher,

Either your 79 Special isn't actually a 79...have you checked the engine/frame # with the guide in the tech tips on matching year/model with serial #??

Or....someone has already been in there and put 80 cams in your 79, because the 3H5 designation is from the 80 year model!

The earlier year model cams were supposed to be a bit hotter, the 78 engine being the strongest, but then changes were made due to EPA and such, and engine internal designs were changed also...larger intake valves, different piston/head domes....that's why 9.0:1 comp in the later models, vs. the 9.2:1 in the earlier models! HTH?

I can't keep track of the other specs, like best for low/mid/top end? T.C.

[Mashermoto]

TopCat,

You must be right...I don't have the frame, but the engine number starts with 4RO, which implies this is a 1981 SH.

I'm considering using the 81 cams. I just got off the Web Cams site. Their information says a longer duration cam profile (78 cam) will produce output at higher RPM. Therefore, if I want more low end output, the 81 cam would be better.

Lobe centers are another factor...tighter lobe centers produce more low end output. But who knows what the lobe centers measure. I'll check that out when I get the top end built

Man...how blessed we are with decisions when modifying engines