Intake Center Line

[Ed Wright]

Your intake valve would have more clearance at 108 than 102.

[Bob Bender]

Originally Posted by Ed Wright

Your intake valve would have more clearance at 108 than 102.

x2

[Bob Gullett]

http://www.compcams.com/Technical/FAQ/LSAproperties.asp

[Ron Gusack]

Originally Posted by Ed Wright

Your intake valve would have more clearance at 108 than 102.

And the exhaust clearance will be less.

[Alan Roehrich]

If you are now moving your intake center line to 102 from 108, to correct your problem, you will decrease intake valve to piston clearance.

[Ed Wright]

Originally Posted by Ron Gusack

And the exhaust clearance will be less.

Of course.

[Greg Reimer 7376]

Advancing the cam decreases the intake piston to valve clearance and increases the exhaust piston to valve clearance.Retarding the cam that much increases the intake piston to valve clearance,and decreases the exhaust valve piston to valve clearance. Also, advancing the cam increases torque and reduces peak horsepower.Retarding the cam decreases torque and increases peak horsepower.Wouldn't it be great to be able to use a variable cam timing device that allows you to leave the line in at 98,and cross the finish line in at 108?Also, the weight of your car can affect where the engine wants the cam. My K/SA 68 Chevelle hardtop might just want it somewhere else than my O/SA 68 Chevelle wagon.Those two cars weigh 490 pounds different.

[Jim B]

I don't know your application but you may want to invest a couple of minutes and leak the engine before a trip back to the track. In a minimum valve clearance built engine running the cam 6º retarded from the checking centerline will reduce exhaust piston to valve clearance beyond the safety margin and the piston may wind up closing the valve. It may not apply to you but it's just a thought.

[Adger Smith]

Lonnie,
You got some very good answers in this thread.
I would never consider moving the cam around, very much, to get PTV clearance. Depending on the lobe profile & rocker ratio you only get between .005 to .008 clearance (change) per degree of cam movement. Some more, some less....
Another issue with retarding the cam is the ex valve will open at a later point in the cycle. A late opening of the ex valve can be a problem when trying to produce power at high rpm. At high RPM it can cause lost power by what is called pumping losses. At High RPM the late opening causes the Ex to loose some of the blow down affect and the piston has to pump the spent ex gasses out. In real bad cases there can be ex gasses trying to go back into the intake runner when the intake valve opens. That really kills power.

[Greg Reimer 7376]

In the last days of the muscle car era-1970-1972, the envirodemons flexed their muscle and enacted things that tried their hardest to destroy the American car. (The '73-77 era just about did.) If you drove a,say,a '69-71 Nova,350 4barrel SS, it ran strong, was pretty responsive, and got reasonably good mileage. The '72 engine came out, and there was a distinct drop in power. Same engine, around 9:1 compression,same trans,gears,etc,where was the loss coming from? The '72 and later cars,irreverently referred to as'smog motors', had a lot of small differences. First was ignition timing-around 4 degrees initial with less mechanical advance available in the distributor,down from around 8 degrees, but the cam had more exhaust lift. A quick check of the NHRA blueprint specs verifies this, but the exhaust valve opened late, and closed later-right into the intake cycle. Combined with that, was a real restrictive exhaust with built in back pressure.Anybody remember the vacuum cleaner noise in trucks and vans with the screenlike grid over the end of the tailpipe? The tubing diameter was small, and the reason for all that was to cause a small amount of exhaust to be trapped in the cylinder and be an inert substance effectively reducing NOX emissions. I remember q-jet cars bogging really badly when the secondaries opened on WOT acceleration, there was no place for that extra fuel,air,and exhaust to go.Next year the infernal EGR valve came out, and the problem was worse. Some of those cars were practically undriveable when the engine was still cold and the choke opened rather early.Top speed in a '72 was about 90-100,no matter what you did to it. The proper way to fix those later cars was to junk the exhaust system,put on a good set of headers with a dual exhaust, advance the cam about 8 degrees,(or replace it with an earlier cam),re-curve the distributor,and get someone who knows quadrajets re jet the thing and set it up correctly.Made all the difference in the world! The same thing happens in a race motor to a lesser degree with a late closing camshaft.