I was reading in ateupwithmotor the other day and came across the Z/28 article. It was good, but I commented that the camshaft duration must have been wrong. The author responded (he's really good at this) and verified that his information was correct. I even checked it out at Summit (Comp Cams has OEM replacement cams for engines like that) and he's right. Sort of. I know that different manufacturers, OEM or aftermarket, have different definitions of what "open" is. The standard the aftermarket has adopted is to measure when the lift reached 0.050". This is often repeated alongside the "advertised" number.
Example: Comp 12-908-9, which is a huge roller type cam for drag racing. The advertised duration is 300/308 and the 0.050" duration is 264/270, around 37 degrees of duration pre 0.050"
Example 2: Comp 11-604-5, which is a huge mechanical flat tappet for drag racing. Adv 294/304 and 0.050" is 256/266, a difference of 40 degrees or duration pre 0.050".
Enter the DZ302 cam, Comp 12-107-3. A large by huge massive cam which surely gave up power because it was too big, even at 9 or 10 grand on that small of an engine.
Adv 346/346, 0.050" 254/254, a difference of 92 degrees
Here is my thinking: A lot of our thoughts today which can be considered myths evolved from inferior technology and materials in the heyday of early racing. Ideas like short strokes, dry hops and straight front axles worked well back then, but not for good reasons that are applicable today. Perhaps Chevrolet was trying to make this engine last like a race engine should be closing those valves oh so slowly? The 254 is still a very big cam for a 302, so maybe the 346 ridicularity is just because of those opening and closing rates which would compensate for weaker pushrods, valvesprings, heavier retainers and valves, etc. What say you? It's easily the biggest cam comp sells for a small block, besting the next biggest by 20 degrees, so something is up. If that's so, it's probably an awesome cam for endurance racing.
Old cams were designed around the springs and tech available. I ran a 302 cam in a 287 back in high school. 30-30 worked better than most other grinds of that day. Over the years I saw more than one spring seat cut into the coolant passage ruining the casting (or cracking through the thin area). Today's roller grinds, longer lighter valves and valve trains are the only way to go.
some of that lead-in could also be due to the manufacturing equipment available to grind cams at the time. i took a cam design class taught by Scooter Brothers of Comp Cams many years ago, and one of the takeaways i got was that they can design all sorts of cool profiles but can't always grind them.
perhaps they were bleeding off some compression to make the engine easier to start? i mean, the stock CR on the DZ was 11:1.
perhaps they had to open very slowly from adv to 050 so they didn't crash valves into pistons?
perhaps they had to ramp them up and down gently to control seat bounce with "reasonable" valve spring stiffness? it still had to survive the warranty period, you know.
dammit, i was typing same time as andy. i knew i'd just sound like a copycat if i didn't get in before the master. 
You don't really specify enough information to make a judgement here. Whats the LIft? The lobe separation angle? The timing?
Looking purely at the duration, I've run that much and more duration on a 289 with great success - On the track. It's wasn't very streetable, making all it's power above 6k RPM, but for a track engine it was perfect.
If you've ever heard a Z-28 running, it does have a very radical cam in it. 254 degrees would surprise me at all. And after looking it up, the duration was in fact 254 degrees. This engine made peak hp at 6700 rpm, so you need to take that, and compression ratio into account also.
The stock DZ 302 cam was a dog until about 4200rpm and would spin to 9,000rpm. It was a mechanical cam and worked a lot better in larger engines (400ci) than most people realized. Remember the DZ engine was a very special engine when it was built, with the dual carb setup the engine made 400hp even though it was rated at 290hp.
FOUR HUNDRED HORSEPOWER FROM AN OE 1968 GM ENGINE!
AngryCorvair wrote:
perhaps they had to ramp them up and down gently to control seat bounce with "reasonable" valve spring stiffness? it still had to survive the warranty period, you know.
This gets the 'likely' vote in my opinion, which was said by Andy previously as well. It's just amazing that they had that much lead in.
I am very curious to hear more about the cam manufacturing process, that's a very good point as well. I might check out some of the other muscle series cams (the ones that are intended to be replicas of the originals) to see if they are similar in their adv/0.050 ratios.
John Brown wrote:
The stock DZ 302 cam was a dog until about 4200rpm and would spin to 9,000rpm. It was a mechanical cam and worked a lot better in larger engines (400ci) than most people realized. Remember the DZ engine was a very special engine when it was built, with the dual carb setup the engine made 400hp even though it was rated at 290hp.
FOUR HUNDRED HORSEPOWER FROM AN OE 1968 GM ENGINE!
You're right, of course, but a modern super stock spins faster than that with much less cam.
I think our suspicions are correct. I looked up the spec for a Ford 289 from the same "factory muscle" series, and the duration spread is somewhat ridiculous as well. Adv 283/261, 0.050" 228/227. A difference of 55 for intake and only 34 for exhaust.
Neat - how far we've come, eh?
My opinion..... it also had to do with it being older type rockers... making it easier on the valve stem since it is a solid lifter cam, they may have decided that too steep a ramp could adversely affect the valve stem - possible mushrooming of the stem if the rocker "hit" the stem too hard
Ramp angles are also somewhat restricted by diameter of the lifter foot; Chevy's "can't" run the ramp angle of an AMC, for instance, when we're talking about flat tappet cams because the diameter is smaller causing higher PSI pressures on the cam load for the same ramp angle with the Chevy. An AMC, meanwhile, can run nearly the same lobe profiles as a roller Chevy or Ford but with a flat tappet.
pres589 wrote:
Ramp angles are also somewhat restricted by diameter of the lifter foot; Chevy's "can't" run the ramp angle of an AMC, for instance, when we're talking about flat tappet cams because the diameter is smaller causing higher PSI pressures on the cam load for the same ramp angle with the Chevy. An AMC, meanwhile, can run nearly the same lobe profiles as a roller Chevy or Ford but with a flat tappet.
True, but the other cam I mentioned as a comparison is the same technology for the same engine, just newer.
I'll have to call a freind who was at GM at that time but as i recall he told me the "spin tron" or strobe light showed the valve violently slaming and bouncing so bad the keepers would rip out of the valve so they gave it a more suttle ramp.
Also- a flat tappet mechanical lifter cam has quietening ramps ground into it- Basically, a very shallow ramp to take up the valve lash before the lobe actually opens the valve.
Its why the only meaningful number for duration is the .050 number.
Streetwiseguy wrote:
Also- a flat tappet mechanical lifter cam has quietening ramps ground into it- Basically, a very shallow ramp to take up the valve lash before the lobe actually opens the valve.
Its why the only meaningful number for duration is the .050 number.
That's why I compared to another flat tappet mechanical, for that exact reason. Obviously it does matter, because there had to be a reason Chevy made such a wacky cam.
Comparing advertised duration is a complete waste of time, unless you know where it was taken from. Don't forget the lash used with these grinds, too.
Ignore the advertised duration numbers. It is pretty much a waste of time.
Imagine a street car, from the factory with 254 degrees duration at .050" in a 302, with a 4 gear and 4.10 final drive.
My dad had an early Z28 when I was a kid. Those were the days.
