Offshoreonly.com - Cam degree calculation questions

Offshoreonly.com (https://www.offshoreonly.com/forums/)

- General Q & A (https://www.offshoreonly.com/forums/general-q-20/)

- - Cam degree calculation questions (https://www.offshoreonly.com/forums/general-q/373011-cam-degree-calculation-questions.html)

Quote:

Originally Posted by mcollinstn (Post 4802436)

Depends.
using + and - on valve events can cause confusion. It is always more "accurataccand prevents confusion and math errors to list them as

X degrees BTDC
X degrees ATDC
X degrees BBDC
X degrees ABDC

Here's my math from your numbers. Correct me where I'm misinterpreting your readings.

your intake opens at 2 ATDC (START COUNTING)
turn 178 to BDC
It closes at 47 ABDC

That is 178 + 47 = 225 duration.
centerline would be 112.5 + 2 = 114.5 ICL

your intake opens at 2 BTDC
Turn 180 to BDC
it closes at 47 ABDC

That is 2 + 180 + 47 = 229 duration
Centerline would be 114.5 - 2 = 112 5 ICL

As far as how cams are ground, and how timing sets are marked, I've seen them every way imaginable.

If you use a mercruiser part number cam and a mercruiser part number timing set, you MAY be safe to install straight up,, but I've even caught that kind of stuff being off (due to supplier changes).

My rule of thumb..
If it is a Japanese/Korean/German car, and you are using oem parts,, then trust the marks..
If it is ON or FOR an American auto engine,, then pull out the piston stops (for the TDC mark on the balancer), the degree wheel,, and the indicator (for the valve timing).

Quote:

Originally Posted by TomZ (Post 4802440)

His cam came out of my stock 1998 502 MPI. It’s definitely a factory Merc/GM cam.

Because it comes from Tom's motor, I trust him and my methods aren't perfect is why I'm going through all this. I need to correct my notes for next time.
The negatives are where I threw you I know. Intake was atdc and needs to be a negative. Minus a minus becomes a positive. That's why you see me write it one way in one formula and different in another. Confusing. You're right. I'm consolidating different people's methods until I've got it fine tuned. I'm pretty confident in my measurements and calculations now.
Thanks to everyone that stuck around and saw me through till I got it right. Not trying to close this down if anyone has anything to add.
Billk, still would live to see what you come up with.

I still say you are installed almost "straight up" rather than the +6 or +9 that Merc specs them. Again, I believe ICL of 109 is where it's "supposed to be".

https://cimg2.ibsrv.net/gimg/www.off...6f708c4abe.jpg

Quote:

Originally Posted by mcollinstn (Post 4802468)

Let's say I was, and I probably am, how would you install the cam? I mean physically from step one. On mine everything except the timing set is stock. Dot to dot leaves me 2° retarded, it seems like I'd have to advance it back to where it caused all my problems to get the 109. Is that pic something you made using my measurements?

Those are your measurements.
If your measurements are accurate, then your cam is 1 degree advanced.

I keep reading that Merc specs that cam on a 109 ICL which is 5.5 degrees more advanced than you've got it.
Offset bushings (for the cam sprocket) can be used. A bushing that offsets the cam 2 degrees equates to 4 crank degrees..
Lower sprockets with multiple keyway can be used.

It doesn't hurt my feelings where you decide to put your cam,, but I do want you to be sure that you know where it is.

On a single engine cruiser, you don't really want to soften the midrange torque in exchange for a little more up top. Or at least I wouldn't think so.

Now, the way I understand it, you never knew where the cam was originally installed. This time you're gonna know exactly where it is.

I don't know if your heads are bolted down and torqued yet, but I do hope that once you get the cam where you want it, that you run clay snakes across a piston and bolt on a head (using an old gasket) install some lightweight testing valvesprings, oil up and bleed the lifters and drop em in, pushrods, zero lash the rockers and slowly turn the motor over at least 2 full rotations. Watch the lifters and make certain that they don't collapse any (which is the reason you use test springs and bleed the air out of the lifters..).
Then (and only then) will you know your true piston to valve clearance when you measure the thin sections of the smashed clay snakes.

Quote:

Originally Posted by mcollinstn (Post 4802476)

My goal is to install it correctly. If it's supposed to be an ICL of 109°, that's where I want it. My problem right now is wrapping my head around the fact that as it stands now I need to install it so far advanced from dot to dot. I get stacking tolerances and poorly made parts will cause you to have to move things around a little, but 8°? Is it me or does that sound like a ton?
I just got my head back last night with a new valve seat. They won't go back on permanently until I get this right and I've checked the PTV correctly.
So for me to move the ICL from where it is at 114.5° to 109°, am I correct in saying I need to move my adjustable crank sprocket another 5.5° advanced? That would have me putting it at 8° how it's marked.

https://cimg3.ibsrv.net/gimg/www.off...c6db4bccd2.jpg

I would think as long as you have the opening and closing events at .050 right you would be good to go. Then check the center line.

To add to my last post, this is what's confusing me with this cam. My Lunati cam was ground with 4° advance. I had to install it dot to dot. Not 4° advanced with the adjustable timing set.
here are my calculations when I did that. Cam card and my hand written notes. Where you see two numbers written for IVO etc, one is from the card and the other is my measurements.

https://cimg3.ibsrv.net/gimg/www.off...b348417d86.jpg
https://cimg8.ibsrv.net/gimg/www.off...2005cc0d1c.jpg

I put a Lunati hrdo roller in the 427 for the Vette, it was ground 4* advanced and I installed it straight up as per the instructions

Looking at your numbers you are very, very, very close. Your set up of the dial indicator could be off a little.