Someone Teach Me About Cams... Single Pattern vs Dual Pattern

[gt350hr]

66Fairlane,
      Andy ,
         Using the drawing above  you can see the lobes are separated. That is of course the LSA and the lobes are ground to a specific "fixed" spacing. The only way that can be changed ( very slightly and not always) is to "regrind" the lobes.  What can and does change is the "positioning" of the cam to the crank. I hesitate to use the common "timing" term as guys somehow think that is related to ignition timing which is totally independent.  Cam timing numbers ( on the cam card) are calculated in a very simple way. I will use an easy example.
     Cam duration at .050 is 260 ( on both) and lobe separation is ground to 110.
     Intake opening is determined by taking half of the duration ( 130)  and subtracting the LSA ( 110) so the intake opening on the card would be 20 BTDC.
     Intake closing is determined by adding the intake opening number ( 20) to 180 ( to get to the opposite side of the lobe) and subtracting that from the total duration at .050 leaves the intake closing number of 60.  20+180+60 = 260 , the total duration. The "events" can change but the numbers always add up.
   "I" calculate the exhaust numbers in the opposite way by getting the exhaust closing ( smaller number) first and then the exhaust opening number ( larger).
     Cam advance can be tricky. In the above example advancing it 4*s  would change the intake numbers to I.O. 24 and I.C. to 56. The exhaust is calculated as if it were retarded ( using my method) and those new numbers would be E.O. 64 and E.C. 16 .  This  is because of the "ICL" change from 110 to 106 and the "ECL" change to 114 ( 106+114=220 Divided by 2=s 110 the ground in lsa.
      I hope that simplifies it a bit and removes some of the "mystery" out of camshafts.
       Randy

[66FAIRLANE]

Hi Ross & Randy

Just been able to get back to this. Thanks for the explanation. Got it now.

Where I was going wrong was my understanding of the term 'straight up'. I did not realise it meant ICL & ECL being equi-distance from TDC. I always just thought it meant installed as per cam card recommendation. Thus I could not work out how you could grind advance in. Makes sense now.

[scott foxwell]

Hi Ross & Randy

Just been able to get back to this. Thanks for the explanation. Got it now.

Where I was going wrong was my understanding of the term 'straight up'. I did not realise it meant ICL & ECL being equi-distance from TDC. I always just thought it meant installed as per cam card recommendation. Thus I could not work out how you could grind advance in. Makes sense now.

You and about 99% of the rest of the engine building community.

[blykins]

Hi Ross & Randy

Just been able to get back to this. Thanks for the explanation. Got it now.

Where I was going wrong was my understanding of the term 'straight up'. I did not realise it meant ICL & ECL being equi-distance from TDC. I always just thought it meant installed as per cam card recommendation. Thus I could not work out how you could grind advance in. Makes sense now.

You and about 99% of the rest of the engine building community.

Hahahaha whatever dude.

[babybolt]

I've wondered for awhile about the old factory cams, some of which Holman Moody still sells.  All are dual pattern and must have been derived by extensive engine dyno or track testing by Ford or Holman Moody.

Specs in link:

http://www.holmanmoody.com/cams.html

[blykins]

Hi Ross & Randy

Just been able to get back to this. Thanks for the explanation. Got it now.

Where I was going wrong was my understanding of the term 'straight up'. I did not realise it meant ICL & ECL being equi-distance from TDC. I always just thought it meant installed as per cam card recommendation. Thus I could not work out how you could grind advance in. Makes sense now.

You and about 99% of the rest of the engine building community.

FWIW, these guys get it......don't know who they are, but they get it.

http://www.camcraftcams.com/degreeing-a-cam/

"A cam ground with a 108 lobe separation and advanced 4 degrees will have a intake centerline of 104 and exhaust centerline of 112. Advancing it another 2 degrees will result in a 102 intake centerline and a 114 exhaust centerline. Adding the intake and exhaust centerlines together and dividing by 2 will give you the lobe separation. The same cam installed straight up (with no advance) will have a 108 intake centerline and a 108 exhaust centerline.
Note that straight up refers to lack of advance. It does not mean, “ We lined the timing marks up and installed the gears”. When I ask where the cam is installed and get “straight up” for an answer then I know the cam was probably not degreed in properly."

I checked Lunati's website...................they get it too.................

And from Jay Allen (Camshaft Innovations)......

"Straight up is when the cams intake centerline and exhaust centerline are the same. Hence, the intake centerline is also the same as the camshafts LSA as well. This can occur at any position on the crank gear. The camshaft does not rotate on its own. The position of the crank gear becomes irrelevant. "

Jay gets it.

[TorinoBP88]

Decide right from the get go if gas mileage is important? Of so stay close to stock.

Of not, have fun, but don't get the biggest one possible.