by Jay Roeder
I can't believe it is already June! I have been
REAL busy lately building engines for my customers
and while that is a good thing from a business
standpoint, it has made it rather difficult
to get as much done as I want to on MY engine.
Responsibility can be a real pain sometimes,
ha, ha! Anyway, away we go!
This month I will share some of the thought process that goes into determining the specifications of my custom camshaft made by Comp Cams for Muscrate, and briefly cover some of the steps of the mysterious process know as "degreeing in the cam". Finally, the engine will be done!
At my shop, Roeder Performance Machine, I use Comp Cams almost exclusively for my custom camshafts, lifters, valvetrain, etc. One of the best things Comp has in its favor over other companies is the people that work there, in particular, Chris Padgitt. I have been doing business for many years with Chris, formerly at Lunati, and he is a wealth of knowledge on basically everything in a racing engine. I'm telling you this not because I think everyone should call Chris, but because the personal service you receive at Comp Cams is the difference. All of the major cam grinders have basically the same equipment, but Comp has some equipment and lots of database that other companies just don't have.
The first step in ordering a camshaft, either custom or "off the shelf" is
to be realistic about the rest of the combination
and its limitations. In the case of Muscrates
302, it is a 9.5:1 compression, 310 cube, 217
cfm intake flowing, single 4 barrel HEMI EATER!
The point is that it is not going to make gobs
of low end torque due to its size, and it is
probably not going to make gobs of horsepower
above 6500 rpm due to the head flow. So, I am
aiming for a power range of something like 4500-6500
rpm. I am limited by IHRA rules to .498" valve
lift and it is a hydraulic roller-type cam.
Based on previous experience, I know that you
can definitely hurt performance by over-camming
Ahh. The finished piece! 310 cubes of Hemi
eater! This thing is in the need of a RACE
Although Muscrates 302/320hp "crate motor" is not a typical "stocker" engine, a lot of the same thinking applies. For instance, the Performance Trends Engine Analyzer Pro dyno simulation program I used to run through some different ideas shows that this engine will make best peak horsepower with a duration profile of 266^ intake and 272^ exhaust at .050". That would be cool if we raced dynos. We don't. I ran numerous simulations in the program, changing duration, lobe separation, cam centerline advance and retard, etc., and came up with the best average horsepower and torque numbers the computer could muster. Then, I added a little experience and went through the Lobe Specifications section of the Comp Cams catalog and found the lobes that most closely matched. I called Chris and discussed what I wanted and told him what I had come up with. He amazingly said he thought what I found would work great, at least it would be a good starting point, and I had the custom ground cam to my door in three days. That is another service Comp has that no one else can beat.
So, what did I end up with? Well, part of building a "stocker" engine is not sharing ALL of your secrets. I know, you would like to poke me with a sharp stick right now, but that's the way it works. I don't make the rules! I can give you a good idea of the specs. The duration at .050" lobe lift is somewhere between 245^ and 255^ on both lobes, the lobe separation is 108 degrees, and the lobe lift is .312". (.312" x 1.6 rocker ratio equals .499" valve lift.) Hey, until I run the car I won't even know for sure if it will work!
Now for the "black art" of degreeing in the camshaft. Actually, it is a pretty straightforward operation that MUST be performed on any engine you care about making power. Basically, when you degree in a camshaft you are simply making sure the valves open and close when they are supposed to relative to the position of the piston. All cams come with a spec. card that will tell the opening and closing events of that particular cam. Some companies list the specs. differently than Comp but the way I prefer to degree the cam is by using the opening and closing events of the cam at .050" lobe lift. Keep in mind that lobe lift and valve lift are completely different things. Lobe lift is how far the lifter will move from a resting position (base circle) and valve lift is your rocker ratio (mine is 1.6) multiplied by lobe lift. The reason or reasons it is important to degree in the cam is because there are lots of variables that can affect the actual cam timing. For instance, has your block been line bored? If so, you have probably loosened up the timing chain slack because of the now shorter distance between the crankshaft and camshaft centerlines. This may easily be 1 degree of retard.
Next, what quality of timing set did you buy? Some of the cheaper sets can
easily be off in dowel hole placement 2-3 degrees.
How about the crankshaft keyway? And for that
matter the crank key itself? Does the key fit
snugly in the slot or is it kind of rolled over?
And, believe it or not, that shiny new camshaft
may not have the dowel hole in exactly the right
spot. All of those variables and more are why
you must have the cam degreed in. Also, this
is why most companies such as Comp Cams "build
in" approximately 4 degrees of advance. In the
case of someone who just lines up the timing
dots and hopes for the best this is a built
in cushion to hopefully lesson the effects of
the aforementioned variables. A retarded camshaft
will give lazy engine response and take away
bottom end torque.