ModMotors bring a variety of new challenges to the engine builder. One of the more painful is cam phasing. I have followed the tried and true path most of us have or are following in our engine builds. During this death by a thousand cuts it occurred to me there was an easier route available to us, that I have never heard anyone suggest — so I am going to.
This approach uses a homemade fixture and an Excel Spreadsheet to dramatically shortcut the entire process. To start out you will need to fabricate a fixture to hold two dial indicators simultaneously, and a TDC tool to make finding TDC easy and accurate and that’s about it.
This is what the homemade fixture for the dial indicators looks like. This is the front view;
And this is the back view;
The split collet gizmos for locating the dial indicators are Starrett items and as you might guess come at a relatively dear price of $25 to $30 each depending on where you buy them.
The fixture itself is very simple. It bolts to the head at the cam cover attaching screw holes. I used two chunks of aluminum for the indicator mounts and sized them to place the indicator probes just above the valve stem on the tip of the cam follower. To make angle selection easy I used a small stud and wing nut to snug the fixture down so it would not move around during cam phasing.
The next homemade tool you will need is a ModMotor specific TDC tool that is long enough to stand above the sparkplug well when it is installed in the head. I took a spark plug, removed the porcelain and machined an aluminum shaft to push in and protrude into the chamber an inch or so to stop the piston before it reaches TDC. This is what it looks like;
When you install it in the head, this is what It looks like;
This makes it very easy to install and remove the TDC tool any time you feel a need to check your TDC mark on the degree wheel.
I like the Jomar style degree wheels. Mine is a 40 year old version. The Jomar approach to degree wheels uses a collet held to the snout of the crank by the crank stud. The collet has a jam nut to lock the degree wheel wherever you wish. I built a homemade TDC pointer gizmo out of a dial indicator swivel clamp, two lengths of mild steel rod and a $1.50 block of optically clear plastic from Tap Plastics. This is what the pieces look like;
And this is what it looks like on the engine block;
OK now to the fun part. There is a downloadable XL spreadsheet at the end of this post that I built to take the burden of doing all the math, in real time as you are trying to phase the cams, off the engine builder's shoulders. The spreadsheet has one sheet for the Passenger Side cams and one sheet for the Driver's Side cams. This is the Driver's Side Cam Phasing Worksheet;
The only numbers you need to enter are the 0.050” before max lift crank position number and the 0.050” after max lift crank position number. The spreadsheet does the rest for you. The spreadsheet will also accommodate the now unavailable Cloyes 9-Way secondary sprockets, if you are fortunate enough to have them.
Lets put some numbers in. You only enter data into the yellow cells, all the green cells are calculated for you. First thing we want to do is put the cams in straight up with all the 9 ways and the hex adjust set to 0. When everything is set to zero and the pre and post max lift points are entered in crank degrees, this is what we get;
Remember you only enter the data into the four yellow cells. The spreadsheet will calculate the individual lobe centerlines for both the intake and the exhaust, along with the lobe separation angle (LSA) and how much the cams are advanced or retarded, in this case 0.75˚ retarded.
Lets say that I wanted the engine for a street use vehicle so I am going to look for somewhere between zero and -10˚ of overlap so my idle bypass still works reasonably well. Using Mark Olson’s great CamCharting tool we find the following phasing will provide a -3.5˚ overlap target.
Marks CamChart tool confirms an LSA of 114˚ advanced 2˚ will provide -3.5˚ of overlap and open the intake valve at exactly TDC.
Soooo, what do we do now?
Time for the Cam Phasing Worksheet. We already know the cams are at a 109.25˚ LSA with everything set at 0. We want to be at an LSA of 114˚ so lets do this,
By installing the Cloyes 9-Way intake sprocket in the 4˚ retard position we can open up the LSA from 109.25˚ to 111.25˚. Cloyes only provides +/- 4˚ of freedom with their sprockets which means we are still 2.75˚ from our 114˚ LSA target.
If you use another Cloyes 9-Way adjustable sprocket on the exhaust cam it will not change the exhaust phasing but it will change the secondary drive chain phasing with respect to the intake cam. This time though the changes are counter intuitive. Installing the sprocket in a retarded position on the exhaust actually advances the intake cam and vice versa if you use the advance settings.
Lets take the Exhaust 9-Way and advance it 4˚ which is the equivalent of retarding the intake 4˚ and increasing the LSA by 2˚. This is what it will look like;
Now we are at a 113.25˚ LSA. Because of the indexing limits on the 9-Ways of +/- 4˚ this is as close as we are going to get to a 114˚ LSA. We are still confronted with the fact the cams are retarded 4.75˚. To fix this we need to advance both cams 4.75˚. The Cloyes Hex-Adjust range of adjustment as specified by Cloyes is +/- 4˚ however most Cloyes timing sets can sneak up on a +/- 5˚ adjustment range. Let’s set our Hex-Adjust all the way towards it advance limit of 5˚. When we do that the Cam Phasing Worksheet will look like this;
Now we have the cams at a 113.25˚ centerline with a 0.25˚ advance. We wanted a 2˚ advance but that was not in the offing because of the Hex-Adjust design. Cloyes elected to make the range of adjustment on the Hex-Adjust +/- 4˚, to reach our 2˚ advance target we would need an additional 4˚ increase in the range of advance from +/- 5˚ to +/- 7˚.
A crank sprocket has 21 teeth on it which means each tooth is the equivalent of 360/21 or 17.14˚ of crank rotation. To avoid getting caught without enough adjustment range the hex-adjust would need to have a range of +/- 8.57˚ or one half of the angular displacement in each direction of a single tooth on the crank sprocket. Because 8.57˚ is a clumsy number to work with, the range ought to be set at +/- 9˚ or a little more than twice what Cloyes has provided.
Currently there is no clean fix for the reduced range of adjustment with the Cloyes hardware and like the example above, the enthusiast would have to settle for close to but not precisely what was he was shooting for. To be fair this shortcoming does not usually manifest itself but can, as it did here. Whether or not it will affect your install depends on how the cams were ground and where you elect to install them. All things being equal, the Cloyes package is still the best available for our engines today — even if some parts are hard to come by.
Most significantly from an effort on the builders part perspective, the entire cam phasing process has required only two measurements. The first was the initial look see with the cams installed at zero on all the hardware to get your starting point. The second and last install was to set the cams as close as physically possible to your target phasing and then confirm their phasing with one last measurement. You follow this process once for each bank and you are done. When you compare this to the hours of repeatedly installing, measuring, making calculation mistakes on paper, reinstalling and remeasuring to achieve the same result — this is clearly the much easier way to get the job done.
Ed
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