Intake Center Line

Lonnie Cox · 06-24-2011, 09:36 PM

My question is if your intake center line is set at 108 and should be 102 would you have less clearance piston to valve? I think we found our problem being down on power. I now am worried about clearance. Should I be?
Thanks to all of you for helping.

Ed Wright · 06-24-2011, 09:44 PM

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

Bob Bender · 06-24-2011, 09:54 PM

Quote:

Originally Posted by Ed Wright

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

x2

Bob Gullett · 06-24-2011, 10:54 PM

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

Ron Gusack · 06-24-2011, 11:56 PM

Quote:

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 · 06-25-2011, 12:55 AM

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 · 06-25-2011, 08:50 AM

Quote:

Originally Posted by Ron Gusack

And the exhaust clearance will be less.

Of course.

Greg Reimer 7376 · 06-25-2011, 11:00 PM

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 · 06-26-2011, 06:59 AM

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 · 06-26-2011, 11:52 AM

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.

06-24-2011, 09:36 PM	#1
Lonnie Cox Junior Member Join Date: May 2010 Posts: 88 Likes: 0 Liked 0 Times in 0 Posts	Intake Center Line My question is if your intake center line is set at 108 and should be 102 would you have less clearance piston to valve? I think we found our problem being down on power. I now am worried about clearance. Should I be? Thanks to all of you for helping.

06-24-2011, 09:44 PM	#2
Ed Wright Veteran Member Join Date: Mar 2005 Location: Sand Springs, OK Posts: 8,132 Likes: 896 Liked 389 Times in 168 Posts	Re: Intake Center Line Your intake valve would have more clearance at 108 than 102. __________________ Ed Wright 4156 SS/JA

06-24-2011, 10:54 PM	#4
Bob Gullett VIP Member Join Date: Mar 2002 Posts: 1,069 Likes: 41 Liked 414 Times in 92 Posts	Re: Intake Center Line http://www.compcams.com/Technical/FAQ/LSAproperties.asp __________________ 68 Chevy11 327 F/S

06-25-2011, 12:55 AM	#6
Alan Roehrich VIP Member Join Date: May 2006 Location: Murfreesboro TN Posts: 4,913 Likes: 995 Liked 1,035 Times in 271 Posts	Re: Intake Center Line 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. __________________ Alan Roehrich 212A G/S

06-25-2011, 11:00 PM	#8
Greg Reimer 7376 VIP Member Join Date: Nov 2008 Location: Glendora,Calif. Posts: 1,111 Likes: 164 Liked 649 Times in 208 Posts	Re: Intake Center Line 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.

06-26-2011, 06:59 AM	#9
Jim B Member Join Date: Dec 2008 Posts: 100 Likes: 0 Liked 1 Time in 1 Post	Re: Intake Center Line 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. Last edited by Jim B; 06-26-2011 at 07:09 AM.

06-26-2011, 11:52 AM	#10
Adger Smith VIP Member Join Date: Dec 2001 Location: Texarkana Ark/TX Posts: 2,348 Likes: 434 Liked 677 Times in 261 Posts	Re: Intake Center Line 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. __________________ Adger Smith (Former SS)