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.

[Aubrey N Bruneau]

No doubt, Greg... in our Stock class, moving the cam around, in order to achieve clearance, is all too often the case. Makes it even more crucial to degree in your cam, check clearances. I run mine 4 degrees retarded. My exhaust lobe is not the excessively violent action ( minimized seat timing ) compared to that of my intake lobe, so that, combined with the lighter exhaust valve, makes it easier to control. I have my valves "sunk" approx .050" deeper than spec ( creates somewhat larger head comb chamber cc's ), in order to achieve my whopping .035" piston-to-valve clearance on the exhaust. Intake is at around .070", so it's fine. Why insist on retarded ( maybe it comes naturally ? ) ?
I just couldn't ignore the 22+ HP and 200-300 RPM increase on the dyno.

[Greg Reimer 7376]

I'll bet you miss one shift and you're DONE!! It seems that the P/V spec you shoot for in a stick car is around .100".In my 327/automatic Chevelle,I've shot for .050" intake, .060" exhaust.One thing we don't always grasp is that the engine's actual running clearance with the actual race springs is considerably more than on the stand with soft test spring. Was there any consequential engine damage due to the drive line breakage at Mission last year? I almost think that having a ready-to-use spare set of heads ready in an emergency would be a good thing. Problem with a 409 is the cost of a pair of good #690 cores. Get the Edelbrock head approved while you're at it!!

[Alan Roehrich]

Wow, 0.035" is tight. Real tight. I'd want double that, minimum, even on an automatic car. If you haven't done it, I'd seriously consider cutting 0.035" off the valve face, if it leaves you with enough margin. A bent exhaust valve would not be my worry. I'd think that might hit hard enough to break an exhaust rocker, and if it does, I hope you don't have Shubeck style lifters.

[Aubrey N Bruneau]

oh, I KNOW, guys !... no where NEAR ideal, or even somewhat comfortable.
Valves... basically have no margin left now. This .035"-.038" on the exhaust, is all that could be available. When I disassembled last year, I certainly found distinct marks on the pistons, where the exhaust valves give them a thorough "polishing" ! Intake at .070" ? No signs of anything at all.

Greg, I DO have a set of 583 heads that I hope to have ready for next season. These 690's on my car, got the %&#$ beat out of them last year... well one anyhow. Smashed the intake seat, and put a 3/4" diameter hole in the combustion chamber surface. Had it welded by Edmonton Cast Iron Repair, then a couple seats and surfacing by Kenny Gilmour in Calgary. Great work by everybody, but I'm not real comfortable with such an extensive repair.
When I stripped the ring gear on the line in Mission last year, it was no issue to the engine. I don't have a two-step, so I was just bringing up the RPM, and engaged at around 5500-5800. Got out of it instantly. At sea level, the car goes through the traps at 7200 RPM, and I have a 7400 chip in the 6AL.
It was the greased track here in my home town last June, that caused the multiple hits on the rev limiter, which is REALLY what caused the split retainer on num 4 intake valve.

Alan, I have the Performance Research coated tool steel lifters, and surface hardened camshaft. No problems AT ALL ! I must admit, some kinda ridiculous price, but pretty damn good stuff.
Never hurt anything on the exhaust side. It's always been the intake. The lobe, I believe, is more suited to a long stroke, "torque" engine, that doesn't see much past 6200. Takes some pretty outrageous spring pressures to control it.

Hope all this talk is giving Rod something else to worry about !