cam timing ?

[rod]

cam timing figures show the cam is ground at 4 degrees advance.
my questions are: if i install the cam straight up [0] is the cam now advanced 4 degrees or is it 0. if i physically move the cam to 4 degrees advance, is it now 8 degrees advanced or 4 degrees advanced?
i've got all the other numbers figured out and under control except this on issue.
thanks
rod in AZ

[Tom keedle]

if you installed it straight up, wouldn't it be 4 degrees retarded?
or maybe I'm the one retarded...

[Greg Reimer 7376]

Several contradictory terms collide in a discussion of camshaft timing and event nomenclature.The actual reference point that determines where the cam'is' in an engine is actually the #1 cylinder intake valve lobe centerline. If a cam card specifies that the cam is "in" at say, 104 degrees, that is the 'straight up' position. Straight up doesn't mean that the two OEM timing marks are pointing at each other.That could mean anything.Such factors as error in the position of the crankshaft keyway,error in the crank sprocket keyway location,wear in the chain, error in the position of the cam sprocket, etc.etc. are all variables that could stack up in a variety of ways to result in many different outcomes.That is why a thorough understanding of how to degree a camshaft according to the manufacturer's instructions is essential.The figure given at the manufacturer's spec is the equivalent of "straight up",no matter how you get it. That might require an advanced key, a retarded key, or a lot of different things. The 104 degrees ATDC figure, if you put the cam in at, let's say,102 degrees ATDC,would be 2 degrees advanced. If it checked at 108 degrees ATDC,it would be 4 degrees retarded. The dial indicator and the degree wheel are what matter, and all this is for naught if true #1 cyl.TDC isn't accurate or unknown. This has an awful lot to do with the reason one man's car flat out flies, yet another car with identical parts doesn't run nearly as well. Back in the day, I built a 409 Chevy, and it only went 14.80's at about 100. It just didn't pull. After much tuning, i observed that fuel clouds could be seen hovering above both carburetors at wide open throttle when going through the gears.I got lucky, set the engine up at TDC, removed the front cover and the cam was one tooth retarded.Re-installing the cam, pulling the distributor and installing it where it belongs,(40 degrees total) resulted in an instant low 13 second car at 107 mph with no other changes. Cam timing means a lot and it has to be right. Good luck making it work. Just start with the cam grinder's reccommendation printed on the cam card,then experiment from there as a starting point.

[rod]

greg. thanks for your time and thoughts. i will set the high point at 108 and then advance it the 4 degrees the desk top dyno shows and see what happens.
rod in AZ

[Aubrey N Bruneau]

Rod, maybe I'm over simplifying this, but I think you can get it from reading the "event" timing figures on your cam card ?
An example, with simple numbers out of my head ( this is a single pattern cam ):

Intake opens 20 degrees before TDC... closes 40 degrees after BDC
Exhaust opens 20 degrees before BDC... closes 40 degrees after TDC

You can see that this cam would have 60 degrees of overlap.
Now, if it were ground by the manufacturer, at 4 degrees advance ( a nasty habit these days ), the cam card would read like this:

Intake opens 24 degrees before TDC... closes 36 degrees after BDC
Exhaust opens 24 degrees before BDC... closes 36 degrees after TDC

The 60 degrees overlap remains unchanged... it just occurs 4 degrees earlier in the crankshaft rotation.
Tends to make an engine "feel" stronger, usually creating more "grunt" at low RPM... invariably at the expense of high RPM horsepower ( my experience anyhow ).

So, if your parts are accurate, chain good ( I doubt anyone would be installing a cam with a sloppy chain ), installing the cam, with the timing set at the zero or "straight up" position, will result in your valve timing being in what is known as a 4-degree advanced position.

As Greg stated, to verify this, it is always recommended to degree the cam, using the timing set that will be in use in the engine.

[Greg Reimer 7376]

Well,from one 409 guy to another, does the term "straight up", etc. actually mean anything? The only relevant figure If you're assembling an engine, is the intake valve lobe center, which is specified on the cam card. If the cam is in the right place, all other figures are a moot point. The duration and overlap figures are nice to know and indicative of the torque and horsepower expectations of a given engine/cam combination,but have nothing to do with the intake valve lobe center figure. Usually, when building stocker motors, the rate of opening/closing is most critical at or about .125" lift, because that seems to be the point of the piston to valve clearance issue. It's worse on a 283 as opposed to a 350, because of the quicker piston movement because of the longer stroke.Sometimes it's necessary from that point of view to have to move cam timing back and forth a few degrees in order to achieve proper operating clearances. One fact that is invariable--the laws of physics in relation to colliding parts due to improper operating clearances are certainly costly to a racer's budget.

[Greg Reimer 7376]

When I first got into this Stock Eliminator form of insanity,I bought a two tone blue 66 Chevy Bel Air 2 door post car from Tony Janes.It had the 283,220 horse combo,a 1.98 low gear PG, 12 bolt with 5.57 gears, and it became time for a rebuild. Now, Div.6 has one of the finest automotive minds I ever met in my life, and a real 283 wiz in the form of Cal Method, so I depended on him for some good advice.I had noticed that the exhaust valves had been refaced to the point that you could practically shave with them.No margin whatsoever.This car was built long before the cam and spring rule revision, so obviously somebody had to do what he had to do.Intakes were OK, so they got a refreshening(TRW OEM replacement valves). I put pistons and bored it to the next oversize,(040"),and with the new pistons, I had all kinds of piston to valve clearance. I then took a new set of exhaust valves, with the new uncut valve faces,(nice and narrow, and they had a rather thick margin. Needing another .030" or so clearance, I ran the valves over to a friend's place,where he had a matal lathe, so we chucked up a valve and could safely face .040" off the face of the valve, thinning it down, and retaining the narrow face. I took the valves to the post office and using the digital postal scale, determined the new doctored exhaust valve weighed in at exactlu 6 oz, whereas the uncut purely OEM replacement valve out of the box weighed 6.2 oz. That's a two ounce, 1.5" diameter washer not going for a ride in that engine. The engine seemed to like it, as RPM wasn't a problem.We then removed an additional .020 from the intakes, and felt safer, since two combustion chamber volumns were at the minimum already. Then, the spring rule changed, so I went in with new springs at the unheard of tension of 125 on the seat and 250 open( K motion 700 springs- still a good deal). This car, however barely ran the index on a good day. I left at 3500 against the converter, shifted at 7400, went through at 7800 at 98 mph, never quite got the hang of it. I sold the car some times later, and found out years later it had way too loose of a converter in it, and the engine never had the beans to pull it in high gear, let alone lock up the converter. In retrospect, the idea of turning an engine that high with cast pistons, stock valve train and two-piece valves is rather terrifying. With today's parts, the head work allowed, the valves we use now, the rods, intakes, etc, this car would have gone nearly 1 second under what that one ever did. Amazing, the evolution of a species.