Camshaft timing question

[55Post]

I am installing a Cam in a SBC. The cam card says:
Adv. dur.-- 288/299
@ .050--- 250/261
LCA--- 112 Int CL--- 108
Timing @ .050": Int--- 14/56 Exh--- 63/17
Lash--- .020/.022

My cam program says with a 108CL: Int--- 17/53 Exh--- 66.5/14.5

I called the cam supplier and he says: Int--- 21/49 Exh--- 70.5/10.5

I want to install this cam advanced 8deg for N20 operation.
What the h*** numbers do i use?

[rookie]

I use a degree wheel and the intake centerline method.

It looks like they are giving you .050 #'s

(depending on where they measure ie: .06 .020 seat)


My two programs show a 288 intake seat to seat,

one shows, Open 40 BTDC and Close 68 ABDC, if installed on 104 ceterline

This one shows exh EO, at 89.5 EC, BBDC at 29.5 ATDC

the other shows, IO at 39 BTDC and IC at 69 ABDC, if installed on 105 ceterline

It looks like your supplier is accounting for lash???

[automotivebreath]

I agree with rookie, degree to lobe centerline.

For eight degrees advanced, degree the intake lobe to 104 centerline, yes?

To calculate the valve opening and closing points with the cam in this position you
will need to know if the camshaft was originally ground "advanced" to the 108 CL
listed on the cam card. Some grinders put them straight up at 112 and others grind
the cam to 108 centerline.

[Stan Weiss]

Originally Posted by 55Post

I am installing a Cam in a SBC. The cam card says:
Adv. dur.-- 288/299
@ .050--- 250/261
LCA--- 112 Int CL--- 108
Timing @ .050": Int--- 14/56 Exh--- 63/17
Lash--- .020/.022

My cam program says with a 108CL: Int--- 17/53 Exh--- 66.5/14.5

I called the cam supplier and he says: Int--- 21/49 Exh--- 70.5/10.5

I want to install this cam advanced 8deg for N20 operation.
What the h*** numbers do i use?

I do not see a problem. You ran numbers for a 108 ICL - 4 degrees advanced the manufacture gave you numbers for 104 ICL - 8 degrees advanced

[55Post]

Stan,

My issue was that the #'s given by the camshaft supplier were based on a 108CL and my cam card says 14/56 and 63/17 on a 108CL.

My program shows 17/53 and 66.5/14.5 on a 108CL. It's true if my numbers are correct it should be 21/49 and 70.5/10.5 on a 104CL.

I'm going to put the Intake CL at 104 and go with it.

Thanks for the input guys.

[Stan Weiss]

Sorry I missed the 14/56 63/17
that is LCA 112 - ICL 111 250/260
The ICL is none of the above and the exhaust is 1 degree short.

[55Post]

my buddy and I pulled out my rusty/trusty degree wheel. Using my hex adjust timing set I put the Intake at 20/50 @ .050". According to my calcualtions(They may not be correct but what the h***.), this should put the cam 7deg. adv. and the CL at 105.

That's my story and I'm sticking to it.

Thanks for all ya'lls help

[automotivebreath]

To double check, turning the engine in its normal rotation; zero your indicator at
maximum lobe lift, then reverse the rotation of the engine until the lifter has dropped
about 0.050 before maximum lift. Slowly rotate the engine until the indicator is 0.020
before max lift and record the reading on the degree wheel, continue past maximum
lift until you are back at the 0.020 point on the other side of the lobe and record the
reading on the degree wheel. Add the two figures together and divide by 2, this will
give you the intake centerline.

[BlackCat13]

Originally Posted by 55Post

I am installing a Cam in a SBC. The cam card says:
Adv. dur.-- 288/299
@ .050--- 250/261
LCA--- 112 Int CL--- 108
Timing @ .050": Int--- 14/56 Exh--- 63/17
Lash--- .020/.022

My cam program says with a 108CL: Int--- 17/53 Exh--- 66.5/14.5

I called the cam supplier and he says: Int--- 21/49 Exh--- 70.5/10.5

I want to install this cam advanced 8deg for N20 operation.
What the h*** numbers do i use?

This card says 112 LSA installed at 108 ILC, right? says INT 14/56 @ .050
If you want it installed more advanced at 104* = then you degree to 18/52 @ .050.

The intake specs to open at 14 BTDC and close 56 ABDC, so you want it to open sooner = 18 BTDC and close sooner, 52 ABDC.

The exhaust takes care of itself because it is ground in the shaft.

[Nick Campagna]

This may be a fine point, but why worry about "centerline" when you know the point of max lift ? granted, on a symmetric lobe, +/- .020 from max lift can define the centerline, but so what ? if we take symmetric lobes to be a limiting case of a general lobe, the point of either max lift, "even split" duration pre/post max lift, or any other measure may not be important. valve opening and closing are important, along with point of max gas velocity in or out of the cylinder. I just need to know why some worry about centerlines when open / closing points on I & E seem to be more important. Could someone please explain beyond valve piston clearance issue ?