cam timing ?

[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 !

[Alan Roehrich]

The intake will rarely hit so long as the piston to valve clearance is the same as piston to head clearance, or greater. The exhaust is an entirely different deal though. Even at 0.070" or so we see evidence of the exhaust valve touching the piston.

The next thing you could look at for your combination is a set of 4-2-1 stepped headers. With a really good set, you can use a smaller exhaust lobe, as much as 6 degrees less duration.