squarehead

iTrader: (16)

The following specs are from a cam report:

Centerline 109.25
.05 Lift C/L 109.68
Duration @0.50 Intake 228.18 Exhaust 231.67
LSA 113.25

The cam has 4 degrees of advance.

readings from measurements of plus and minus 0.05” from max lift are

84 and 150

So (84 + 150) / 2 = 117

Should I advance or retard the cam from the centerline indicated on the report which is 109.68

or should I advance or retard the cam from the intake duration @ .05 and divide that by two: 228.18 / 2 approx 114 ?

When a cam has gound in advance, does it only advance the intake lobe?

vettenuts

iTrader: (4)

Do you have the intake valve angles (ABDC, etc.)?

squarehead

iTrader: (16)

The following instructions are why I am confused.

In Comp Cams “Camshaft Installation and Degreeing Procedure”, in step 12 it states “Reference back to your cam spec card and see what the recommended intake centerline is.”. But what is the recommended intake centerline, the intake duration at 0.05” divided by 2 or according to my report “.050 Lift C/L 109.68 ATDC”?

If you look at the COMP CAM card for the LGM G5X2 cam card referenced in this thread https://ls1tech.com/forums/generation-iii-internal-engine/327734-cam-guide.html the intake duration at .05 is 232 so of course half of that is 116. However it also states “THESE SPECS ARE FOR CAM INSTALLED AT 109.0 INTAKE CENTERLINE”. Perhaps they mean “THESE SPECS” are the duration numbers following the phrase I just quoted.

So is the recommended intake centerline for the LGM G5X2 cam 116 or 109?

squarehead

iTrader: (16)

Here are the valve angles on the cam report.

Centerline 109.25 ATDC
.05 Lift C/L 109.68 ATDC
Duration @0.50 Intake 228.19 Exhaust 231.67
for Lift at 0.050 Dur 228.19 open 4.42 BTDC close 43.78 ABDC

The measurements I took were at 84 ATDC and 150 ATDC

KCS

iTrader: (20)

Well, if you got 84 and 150 before and after max lift on your intake lobe, then 117 is your intake centerline. If the cam card says to install it at a 109 intake centerline, then the cam needs to advanced 8 degrees.

Using the timing set, this also advances the exhaust as well which should be retarded the same 8 degrees as the intake.

This is a cam profile:



The left curve is the exhaust, the right is the intake, and the vertical line in the center is obviously TDC. If the centerline of the intake is 117 degrees away from TDC, and neds to be 109 degrees, then it needs to be brought back towards the left closer to TDC (or 0) reducing the distance between TDC and the centerline.

If the intake is 8 degrees too far away from TDC, then the exhaust centerline is going to be 8 degrees too close to TDC.

Moving the profiles to the left moves BOTH centerlines (intake closer, exhaust further) and makes BOTH valves open and close sooner, so thats why it is called advance.

Hope that helps a bit.
Kent

KCS

iTrader: (20)

Its the 109. As far as I know, all cam manufacturers use max lift as the centerline instead of half the duration. They used to use half the duration back when cam lobes were symmetrical, but now with the ramps' rates differing between opening and closing, max lift is now the standard.

squarehead

iTrader: (16)

Are cam positions out of alignment as much as 8 degrees? It seems some adjustable sets do not adjust that much. On a side note this is an LS2, I just thought this was more of a generic question and there would be more visiblity in this section.

My cam has 4 degrees of advance ground in...so if you were using the 1/2 duration 228/2 - 4 degrees of advance it is close to the .050 Lift C/L of 109.68....so I'm wondering are the intake lobes just advanced with respect to the exhaust lobes?

btw I also believe the LGM G5X2 also has advance ground in which may explain the difference between the 1/2 duration number and the centerline.

JPH

iTrader: (2)

Pay more close to your valve events (IVO, EVC) and not so much about the centerlines. Centerlines can be OFF, but the IVO, IVC, EVO, EVC are still where they need to be and that's what you should pay more attention to IMO. Most cam lobes, atleast Cranes have offset ground into the lobes, and that can make the centerlines look off, but the VALVE EVENTS are exactly where they need to be. It doesn't hurt to have a little more advancement in the cam then you want, to take in place of HIGH RPM chain stretch.

squarehead

iTrader: (16)

How do I pay more attention or verify this?

Below are the intake valve events at different lift points from the cam report.

LIFT DUR OPEN CLOSE AREA
0.006 279.91 29.61 BTDC 70.31 ABDC 29.38
0.02 255.39 17.82 BTDC 57.57 ABDC 29.24
0.05 228.19 5.52 BTDC 43.7 ABDC 28.79
0.1 199.26 9.89 ATDC 29.15 ABDC 27.62
0.15 176.01 21.42 ATDC 17.43 ABDC 26.27
0.2 153.34 32.7 ATDC 6.04 ABDC 24.09
0.25 128.8 44.94 ATDC 6.25 BBDC 21.52
0.3 99.8 59.5 ATDC 20.8 BBDC 17.2
0.35 58.85 79.84 ATDC 41.3 BBDC 10.47

JPH

iTrader: (2)

You need a DEGREE WHEEL for one, and a DIAL INDICATOR. IVO @.006 29.61 (rounded to 30*) and IVO @.050 5.52 (rounded to 6*). Most cam companys will round up/down on the "cam card", but not the doctor report.

squarehead

iTrader: (16)

I have been using Comp Cams directions along with a degree wheel and dial indicator. They recommend placing the dial on the spring retainer, so I divided the travel displayed on the indicator by 1.7 For me the 0.006 measurements are fairly difficult to take perhaps too much valvetrain slop. At any rate my readings are not close to the cam report.

JPH

iTrader: (2)

put in on the pushrod, and not the retainer. You got apsorbtion and deflection going on, and that's why your readings are off I bet.

squarehead

iTrader: (16)

That is good advice. I did try that once but once the lifter reached it max point it stuck up there I guess I have too much lube around that area.

KCS

iTrader: (20)

If the intake lobes were advanced in relation to the exhaust lobes, you'd have a smaller LSA.

KCS

iTrader: (20)

What offset is ground into the lobes?

vettenuts

iTrader: (4)

Is anyone else getting 114.61 for the intake centerline position based on 0.050 lift at the lobe?

Goofed, see below.

squarehead

iTrader: (16)

I have been dividing the duration at 0.050 by 2 and adding to the opening event to arrive at the centerline....so 228.19/2 + (-4.42) = 109.675

for lift at 0.100"

190.26/2 +9.89 = 109.52

the cam has 4 degrees of advance gound in

squarehead

iTrader: (16)

4 degrees advance

LSA 113.25

vettenuts

iTrader: (4)

5.52+180+43.7 = 229.22

229.22/2 = 114.61 -5.52 = 109.09

I goofed, didn't correct for BTDC number after dividing in my prior post.

I am struggling on how you can be off by that much, 8 degrees.

JPH

iTrader: (2)

Using a different LOBE family. Like taking a BBC lobe, and grinding it for an LS1 lobe. In simplist terms.