Alan454

Here are some measurements I made over the weekend of stock cam in a 7.4L 330 Hp Gen IV motor. The duration specs in the service manual were way off, but the lifts were very close. You can see the very shallow opening ramps which leads to a high overlap for such a mild cam. It also leads to a late intake closing which reduced the dynamic compression to a pitiful 6.3:1 assuming 117.5 cc heads and -3 cc flat-tops.

I believe that the profiles show significant wear in the transition from ramp to flank.

Hope someone finds this useful ...

Alan

http://www.nacsys.net/boat/merc7.4lcamprofile.pdf

getrdunn

iTrader: (7)

i find this a little interesting. i must say i am surprised what you came up with. i am not by any means a cam guru however even the cam timing amazes me. as you mentioned not only the late exhaust closing but the whole spread of the intakes and exhaust. they appear to look like that of a comp thumper cam. LOL this would probably be a good one for bob to help us understand the concept behind this.

Alan454

I got to looking carefully at the plots and noticed something that I didn't like. The excel "area" plots don't have a constant X-axis scale. In fact I discovered most of the excel plots don't. I used an excel scatter plot this time which has a constant scaling and the results look more appealing. I also remeasured a few points I didn't like and used different points to estimated the slopes. The specs are essentially the same however.

Alan

aj1101

Alan,

Can you point me to an article or write up on how to determine a cam's specs. I have a 350 and think I may have a problem with either my cam or its bearings. I wold like to get an idea of whats going on before tear down. Any info. would be appreciated.

Alan454

AJ – Just do a search on “how to degree a camshaft” and literally dozens will come up. I must say, I skimmed though a number of them but didn’t follow any of them exactly. For me, a picture is better than a 1000 words and this I found about ½ way down the page at the following link:

http://www.compcams.com/Technical/TimingTutorial/

This figure gives you everything you need to know about the relationships between the all the parameters of interest. The important parameters to measure are the crank angles at 0.006 inches of lift (which is how the SAE defines things) and at 0.050” (which is where builders like to define things I think because this is where you start to get some significant flow) and the intake and exhaust lobe points of maximum lift. I went a little overboard with all of my measurements because I wanted to know the complete profile. But hey, once you have everything setup to make the measurements, why not?

I will stress a couple of points though. Pay attention to getting an accurate measurement of TDC. I didn’t like the methods that ran the piston up to a stop so I used the following approach. Just get as close to TDC as you can by your eye and set the pointer of the degree wheel to TDC. Take a few measurements of the piston at given heights below TDC on both sides of TDC with a dial indicator and average the results and then this is applied (subtracted) to all of your raw measurements. Use exactly the same heights on each side of TDC of course. If your heads are on you are forced to use the stop method however.

Do the same kind of thing at the tops of the intake and exhaust lobes and actually calculate the intake and exhaust centerlines from these measurements. These two points are the only points on my plots that weren’t’ actually measured, but rather calculated by the points surrounding them. Don’t go too far down from the peak of the lobes though as some lobes might have a different slope on the opening flank then on the closing flank. You might ask – what can’t I just measure them? When the slope of the lobe is zero, very small changes in lift are produced by relatively large changes in angle and it’s just too sensitive to get a good measurement. You can prove this to yourself by playing with it a bit. The same kind of problem exists in determining TDC.

Have fun!

Alan

aj1101

Thanks Alan. I appreciate the answer and insight. I have been doing some reading and it is becoming a little clearer. I would almost just replace the cam and hope for the best but want to try and identify the problem so that I dont spend next summer "working" on the boat...