1956 camshaft

[Joe L.]

Re: 1956 camshaft

I think any difference in specs is just a matter of how GM listed them. My understanding is that the ...097 cam lobes are the same as the ...077 lobes. The only difference is that the 097 does not have the notch in the rear journal for the early SB oiling system.

I have the engineering data for the ...097 lobes - lift in inches to five decimal places every cam degrees, so if you have specs for the ...077 we can compare.

Duke

Duke------


The specs I have for the 077 are as follows:

intake lift-----0.404"

exhaust lift---0.413"

intake duration------287 degrees

exhaust duration----287 degrees

The duration specs I have for the 097 cam are the same. However, the lift specs are different.

[Duke W.]

Re: 1956 camshaft

I've see those lift numbers before, but I have always questioned their accuracy. They are the same as the ...354 camshaft assembly.

The gross lobe lifts in the 1963 Corvette Shop Manual are .2625"/.2665" and these jibe with the data on drawing no. 3736098. (Unfortunately I've never seen drawing no. 3734078, which would be the authoritative source for the '56 version of the Duntov cam.) Multiply these by the "design" 1.5:1 rocker ratio and you get .3937/.3997". Actual valve lifts are about .370/.370" due to valve clearance and real rocker ratio behavior.

Duntov and Chevrolet marketing material often refer to these cams as the "high lift", but that is a misnomer. Duntov needed more revs without any new parts in the rest of the valve train. By decreasing lift slightly and adding duration, acceleration was reduced, which allowed the additional 1000 or so revs before valve float.

Of course, it's possible that the original '56 design was redrawn with the slightly lower lift, but I've never seen that stated in any source. Duntov's design was literally done on the back of an envelope by today's standards, but it did the job he wanted it to do, and my understanding from various published stories about the design is that the lobe data never changed. The slight difference in actual lift would not have a measureable effect on engine output.

Both ...098 lobes have the same eccentricity above the clearance ramps and are symmetrical. The only difference is that the exhaust clearance ramp is .004" higher to allow additional room for exhaust valve stem expansion.

Duke

[Tom P.]

Re: 1956 camshaft

I presume everyone here knows that a 265 block can be easily and quickly modified so that there is no need for a notch in the rear journal of the cam.

[Duke W.]

Re: 1956 camshaft

...never heard that, before. From previous disussions of the early SB oiling system it appeared to me that oil flow to the lifter galleries is intermittent.

Duke

[Duke W.]

Re: 1956 camshaft

I've see those lift numbers before, but I have always questioned their accuracy. They are the same as the ...354 camshaft assembly.

The gross lobe lifts in the 1963 Corvette Shop Manual are .2625"/.2665" and these jibe with the data on drawing no. 3736098. (Unfortunately I've never seen drawing no. 3734078, which would be the authoritative source for the '56 version of the Duntov cam.) Multiply these by the "design" 1.5:1 rocker ratio and you get .3937/.3997". Actual valve lifts are about .370/.370" due to valve clearance and real rocker ratio behavior.

Duntov and Chevrolet marketing material often refer to these cams as the "high lift", but that is a misnomer. Duntov needed more revs without any new parts in the rest of the valve train. By decreasing lift slightly and adding duration, acceleration was reduced, which allowed the additional 1000 or so revs before valve float.

Of course, it's possible that the original '56 design was redrawn with the slightly lower lift, but I've never seen that stated in any source. Duntov's design was literally done on the back of an envelope by today's standards, but it did the job he wanted it to do, and my understanding from various published stories about the design is that the lobe data never changed. The slight difference in actual lift would not have a measureable effect on engine output.

Both ...098 lobes have the same eccentricity above the clearance ramps and are symmetrical. The only difference is that the exhaust clearance ramp is .004" higher to allow additional room for exhaust valve stem expansion.

Duke

...from Jerry Burton's Duntov biography, page 203:

"Duntov knew a thing or two about camshafts, both from his own racing days with Talbot engines and from his Ardun overhead valve conversions. He had a configuruation in mind that just might work. So he ordered two camshafts built that used the same profile as his Ardun Ford camshaft. The design order was written up on July 31, 1956. In fact, he gave the Ardun specifications, written in Metric, to Fred Frincke, who then had them translated into Engilish measurements. The cam had less lift than the factory high-performance cam, but it lifted the valves earlier".

Engineer Fred Frincke worked with Duntov over the years on many engine and engine related design projects.

About 20 years ago, a decade before I was able to obtain the detailed lobe data on OE cams, I had some conversations with Ed Iskendarian about the Duntov and other Chevrolet cam designs. (He asked for a copy of the lift crank angle diagram I had taken off an LT-1 cam, and I made him one.) Isky said that Ed Winfield was a proponent of "soft action" cams, and he thought Duntov may have gotten the idea from Winfield.

Duke

[Tom P.]

Re: 1956 camshaft

...never heard that, before. From previous disussions of the early SB oiling system it appeared to me that oil flow to the lifter galleries is intermittent.

Duke

Toooooooooooo easy.
In the rear cam bearing bore of early blocks (ie 265), there are two oil holes. The rear cam bearing for a 265 has two holes that are to be positioned over these two holes. As the cam turns, and the notch aligns with the two holes in the bearing, it opens a momentary passage for oil between the two galleys. Well, just pull out your handy dandy Dremel with a carbide rotary file and cut a groove in the bore between those two holes. In 283 and later blocks, a groove was machined around the full circumference of the bore, which then connected those two holes together, as seen below.