Issue with degreeing my cam

[Mopud]

Hi gents,

I was degreeing my cam today but a couple of the readings i was getting didn't quite add up.

I've been following the crane cam instructions (http://cranecams.com/?show=article&id=3) to degree my comp cam 68-231-4. I would follow Comp's instructions however they only offer the intake centreline method which is useless to me at this point as I don't even have the cylinder head back from the machine shop yet.

The cam specs are here http://www.compcams.com/Company/CC/cam- ... id=10&sb=0, but to summarize it to suit this post:

Intake Opens -4 BTDC
Intake Closes 30 ABDC
Exhaust Opens 42 BBDC
Exhaust Closes -8 ATDC

I get pretty much spot on to the readings for intake close and exhaust close as written above, maybe half a degree to one degree out.

However for the intake and exhaust open the reading comes in at 4 ATDC and 8 BTDC.

To explain more clearly i've taken a few pictures. Please forgive the quality, it was dark and the flash was just reflecting off the wheel.

Intake Open


Intake Closed


I can easily take note of the max lobe lift for both intake and exhaust on the dial indicator and I have checked and double checked my TDC over and over again with the same results. Are my readings correct? If they are i would have expected Comp Cams to say 4 ATDC and 8 BTDC on the cam card. The way they write it appears to act like a double negative.

Can someone please clarify for me or advise as to where i might be going wrong? Admittedly my dial gauge doesn't reach the lifter so i've hacked up an old pushrod to act as an extension, it seems to be accurate enough.

Regards,

Mopud

[SilverXJ]

That actually looks right. The ATDC and BTDC is a bit hard to under stand when using negatives.

-4 BTDC would actually be 4 ATDC
-8 ATDC would be 8 BTDC

You don't need the head to do the centerline method. Do it like you are doing now, just measure differently.

[Cheromaniac]

The cam specs are here http://www.compcams.com/Company/CC/cam- ... id=10&sb=0, but to summarize it to suit this post:

Intake Opens -4 BTDC
Intake Closes 30 ABDC
Exhaust Opens 42 BBDC
Exhaust Closes -8 ATDC

According to my calculations that's 206 degrees intake duration, 214 degrees exhaust duration, intake centerline 107 degrees ATDC, exhaust centerline 115 degrees BTDC, so lobe separation angle is 111 degrees.
Looks like you've got it spot on.

[Mopud]

Thank you gents, i'm happy that it lined up so well because I don't have adjustable timing gear and I don't feel like messing around with the crankshaft snout.

It's confusing (stupid) how they write the negatives, but i guess there's some logic behind it beyond my understanding.

Onwards and upwards.