Posted by: nwracing | September 12, 2012

Camshaft Timing Part 2 – At last I’ve think I’ve cracked it!

In Part One of this blog I showed the method I used for finding an accurate Top Dead Centre which enables us to have an accurate reference point for the ignition. Now we are on to the cam timing proper.

When you buy a cam besides the camshaft itself you get one more thing which is almost as valuable as the cam itself and that is the cam card. This piece of paper is absolutely vital if you are going to install your cam properly. If you go to buy a second-hand cam as a personal view I absolutely would not even entertain looking at it if the spec card is missing. Anyway this piece of paper is covered in numbers and terms which appear confusing, but we need to cut through this to get to the information we need to check we have the cam timing right.

In the first part of this blog I had fitted the timing gears and chain. These were fitted in what is referred to as ‘straight up.’ That is neither advanced or retarded. When you get your performance timing set it usually comes with a crank gear that can be mounted using different keyways. These advance and retard the timing of the camshaft and are usually marked in degrees. Fit it at the 0 degree keyway and use the timing marks on the camshaft gear to time it. This is what we are going to check for accuracy.

Now we need to delve in to the camshaft information. There are a lot of terms on the card but the first one we are interested in is the Intake Centreline spec which is usually shown after the figures on valve timing. The intake centreline is the point at which the intake valve reaches its maximum lift. On my cam card this was 106 degrees. So to check this we use the following steps:

Step1:  Fit your dial gauge and indicator on to the top of number one intake lobe. I have installed the lifter to make this easier.

Make sure that you fit the dial gauge so it is square with the lifter this way you get an accurate reading. Rotate the crank so that the lifter is at the bottom of its bore and then zero the dial gauge. Turn the crankshaft in its normal direction until you get to the point where it looks like it has finished moving.

Step 2:  Now turn the crank back until the dial indicator has moved down by 0.100 inch. (NB. Move it back further to start and then creep up on the 0.100 inch reading so that you take any slack out of the chain.)Note the reading on the crank wheel.

                                                                                                                                                 

This shows 148 degrees. Make a note of this number.

Step3: Now continue to turn the crank in the normal direction of rotation until the dial indicator reads 0.100inch again. Note the reading.

This shows 64 degrees. Make a note of this number.

Step 4: To find the Intake Centreline add these two readings together and divide by two. In my case this 64+148 divided by 2 = 106 degrees. This is your Intake Centreline, which is exactly what was on the card. On your cam card will be the lobe separation angle which in my case is 110 degrees. This means that as standard my cam is 4 degrees advanced so I know that the cam is now timed and installed correctly as the manufacturer wanted. If you want to double check you can use the degree wheel to see when the intake and exhaust open and close as given on the specs on the cam card.

What do you do if the number is wrong? If you have a performance timing set up then either adjust the cam shaft gear or move the crank gear depending on your particular set up. Keep checking until the numbers match the cam card. Modern camshafts are very accurately machined so when I get numbers I don’t expect I assume I’ve made a mistake. If you still can’t get it then I would pack it off to my nearest engine builder as getting this right is really important.

I hope that my article explains a little of the intricacies of cam timing. In my case taking my time and checking and double checking seems to work. Oh and one other thing, I have a black timing wheel you may want to consider buying a white one on which you can make pencil marks as this can make life a bit easier.

 

Advertisements

Responses

  1. Great advice from a Quality Engineer, Excellently documented


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out / Change )

Twitter picture

You are commenting using your Twitter account. Log Out / Change )

Facebook photo

You are commenting using your Facebook account. Log Out / Change )

Google+ photo

You are commenting using your Google+ account. Log Out / Change )

Connecting to %s

Categories

%d bloggers like this: