Thread: Cam Help Please

Tweet

Hey guys, I posted this elsewhere, I just don't know where. Ineed the intake lobe centerline for a GMPP part # 10185071 cam I have called GMPP and they can supply all the numbers but this one. I need to finish degreeing this cam.
Thanks for being here for guys like me.

peace Bud

Hey Denny, thanks for the numbers. I have all of these. I need the intake lobe centerline.

According to info on the Sallee Chevrolet discussion board, here are the timing points at 0.004" tappet lift....

ZZ4: Seat-to-seat (@.004)
I: 24 BTDC, 66 ABDC
E: 75 BBDC, 28 ATDC

With this info, we can do a little calculating........

Adding 24 plus 66 plus 180 would yield an intake duration @ 0.004" of 270 degrees. Adding 75 and 28 and 180 would yield an exhaust duration @ 0.004" of 283 degrees.

Half the intake duration (135 degrees) added to the intake opening point would put the intake centerline at 111 degrees ATDC. You have to deduct the intake opening point of 24 from 135 to get to TDC, then it's 111 degrees to the centerline.

Half the exhaust duration (141.5 degrees) deducted from the exhaust closing point would put the exhaust centerline at 113.5 degrees BTDC. You have to deduct the exhaust closing point of 28 from 141.5 to get back to TDC, then it's 113.5 degrees to the centerline.

Adding the two centerlines together and dividing by two would yield an actual lobe separation angle of 112.25 degrees.

Now that you have the centerlines, you can also figure intake and exhaust opening and closing points @ 0.050" tappet lift.

Half the intake duration @ 0.050" is 104 degrees. Deducting 104 from 111 leaves the intake valve opening after top dead center at (7 degrees ATDC). 111 degrees ATDC is the same as 69 degrees BBDC, so we'll use up 69 getting to BDC, then go another 35 degrees past BDC to establish the 0.050" intake closing point at 35 degrees ABDC. 69 plus 35 equals the half intake duration of 104, so this proves the problem.

Half the exhaust duration @ 0.050" is 110.5 degrees. 113.5 degrees BTDC is the same as 66.5 degrees BBDC, so we figure back 66.5 of the 110.5 degrees to get to BDC. Going backwards another 44 degrees gets us to the exhaust opening point at 44 degrees BBDC @ 0.050". Adding 44 and 66.5 gets us back to the 110.5 to prove the problem. Going the other way from the exhaust centerline by 110.5 degrees puts us at 3 degrees before top dead center, so the exhaust valve closes at (3) BTDC.

To summarize 0.050" timing:
Intake opens (7) ATDC. Intake closes 35 ABDC.
Exhaust opens 44 BBDC. Exhaust closes (3) BTDC

Here’s the specifications from the “Chevy Power” Manual

GM Performance 10185071 Hydraulic Roller Tappet

“This hydraulic roller tappet is used on the 5.7-liter H.O. 350 ZZ3 & 4 engines. The duration at lash point in degrees (intake/exhaust) is 275/280; duration at .050" tappet lift (intake/exhaust) is 208/221; and maximum lift with 1.5:1 rocker ratio (intake/exhaust) is 474/501. Valve lash is zero/zero and lobe centerline is 112 degrees.”

Remember that the Intake Lobe Centerline is an imaginary line that passes through the camshaft rotation axis at the point of maximum lift of the intake lobe.

Intake Lobe Centerline, or the point of intake maximum lift, refers to the distance in crankshaft degrees from the cylinder's Top Dead Center point to the maximum lift point of the intake lobe. This is usually measured as degrees after Top Dead Center. This figure changes when the cam is advanced or retarded. As you advance the cam, this number will get smaller, as you are opening it fewer degrees after Top Dead Center. Retarding the cam will make this number larger, as you are opening it more degrees after Top Dead Center.

Exhaust lobe centerline, or the point of exhaust maximum lift, is typically expressed in crankshaft degrees before Top Dead Center. As you advance the cam, this number will get larger, since you are opening it more degrees before Top Dead Center. Retarding the cam will make this number smaller.

The average of the intake lobe centerline and the exhaust lobe centerline should equal your lobe separation. 112 degrees is your intake lobe centerline on this camshaft.

Hope this helps,
Glenn

The 112 is the LSA number. Most of Gm's will sit between 106 and 108 on intake centerline.

I'm a bit slow thinking some of this through and I just noticed that the techinspector and denny gave you all the math. They are the masters and I'll certainly defer - but we've come to the same conclusion +/- .25-.3 degrees. (these guys are really good, eh?)

Regards All,
Glenn

Denny, the overlap @ 0.050" isn't 10, it's negative 10. (-10)

And the LSA is 112.25, not 112.3. Your software is rounding off to the next higher number.

Originally Posted by DennyW

Actually, I rechecked, and my software did say 10, not -10.

Do the math. If the intake doesn't open until after TDC and the exhaust closes before TDC, there is negative overlap.

Overlap, definition: number of crankshaft degrees during which both intake and exhaust valves are open.

If the exhaust closes before the intake opens, then there can be no overlap.

Originally Posted by DennyW

I did the math with my software. Check out picture 2...at .050, and see what it came up with.

The problem is the way you're entering information into the program. I entered it into my DynoSim by changing the primary timing figures to 0.050, not using seat to seat as primary, then entering -7/35/44/-3 and it spit out MINUS 10 degrees overlap.

Originally Posted by DennyW

Well, there you go...Found the problem. I like to just use the dial gauge, and degree wheel much better.

Yep, maybe this software is over priced for even as good as it does. The answers should be the same, no matter which way you go. Another fault I would say...

The fault is between the keyboard and the chair. You have to give it the correct data to compute from. Crap in, crap out.

No, I wouldn't expect any chop from a 112.25 LSA and negative overlap @ 0.050". Should provide a good smooth idle with lots of manifold vacuum to run accessories such as power brakes.

Originally Posted by techinspector1

The fault is between the keyboard and the chair. You have to give it the correct data to compute from. Crap in, crap out.

Ouch! Back in my IT days we used to call this PEBKAC (problem exists between keyboard and chair)

BTW: less overlap -> no overlap -> less "choppy", no?

-Chris

Originally Posted by skids72

Ouch! Back in my IT days we used to call this PEBKAC (problem exists between keyboard and chair)

BTW: less overlap -> no overlap -> less "choppy", no?

-Chris

From an edit of post 18....
"No, I wouldn't expect any chop from a 112.25 LSA and negative overlap @ 0.050". Should provide a good smooth idle with lots of manifold vacuum to run accessories such as power brakes."

Chop, however, is created at the intake closing point. That's the only place in the entire 720 degree cycle where there's enough cylinder pressure to push the mixture back up the intake tract to disrupt and confuse the signal at the venturi with the intake valve still open. This is not my thinkin', it's from Ed Iskenderian. I trust his thinkin' a whole lot more than I trust my thinkin'.

^^^^ agreed...

You Guy's are both good guys, and we all make mistakes.
And I wish I knew as much as either of you.
Back when I was taking chemo I went over to get
my friend to help me figure something out on my car.
He started laughing at and said are you kidding me you
taught me that, he thought I was fooling around.
Problem was I had a brain fart weather it was the drugs or
just being tired I don't know. I just had to laugh it off
because I knew he was right. Anymore i think I'm stuck in
one big fart. HE! HE! HE! Gotta laugh what else can I do. Kurt

Hey guys, thanks for all the replys. I thought for a moment there we were going to have to seperate Tech and Denny. I am learning to degree cams and using the intake lobe centerline method is the only way I know right now. This cam is being used in a beautiful '86 GMC short bed daily driven 357c.i. with Patriot 195 aluminum heads, Holley street dominator/650 cfm 4150 carb, Hooker Comp. 1.625 headers. I want a nice streetable truck
Thanks again for being here for guys like me.

peace Bud