Tuesday, August 11, 2015

Cam Lift v's Valve Lift - A Descent Into Madness!

Well after a nice relaxing break in Scotland with virtually no internet connection, I'm back and keen to get on and finish off the engine. Before I went away, I was getting bogged down worrying about the camshaft and how accurate the lobes were. One of the great things about getting away from a project is that it changes your perspective and while I was away I realised that I was going into too much detail and it was holding back the progress. Yep, even for me there's a limit as to how 'into' camshafts one can go and I was going too far!! That's not to say that I haven't learnt a stack of useful stuff in the process though...


First things first, it's important to know what we are talking about when we are referring to cam lift and valve lift and what the difference is. Like me, if you are desperately interested in the subject, the internet will provide you with most of the info you need, for that reason, I'll keep this post as light and fluffy as I can, but it does get a bit heavy in parts!! I did my best.


Cam Lift v's Valve Lift
Cam lift is fairly self explanatory really, it's how far the cam pushes the cam follower (and consequently the pushrod) up and down as it rotates. As the pushrod lifts, it pushes on a rocker that acts like a seesaw. As one end of the rocker goes up, the other goes down. The other end of the rocker, as it goes down, pushes on the top of the valve stem, pushing open the valve creating Valve Lift. Even though the valve is actually moving down to get off the seat, it's still called 'lift'.

I drew some pretty pictures in Sketchup to illustrate...

Cam Lift

Valve Lift

Rockers
Now the critical point about the two different kinds of lift is down to the rockers, as they don't pivot around a central point. If they did, then the valve lift would equal the cam lift, just like an actual seesaw. The standard rockers that I own and that were stuffed into almost every Mini over the years are pivoted off centre towards the cam end so that you end up with more valve lift for your cam lift. The standard ratio for the difference is 1:1.275 - meaning that for every 1mm of cam lift, you get 1.275mm of valve lift. Simple enough so far.




Measuring the Lift
Before it was possible to measure either type of lift, a little ground work had to be done. I decided to take measurements every 5 degrees of cam rotation, but the problem was the camshaft was now hidden away behind the timing cover and I had no intention of taking it to bits again. I knew though that for every one rotation of the camshaft, the crank does two. So if I could measure the crank rotation, I could therefore measure the camshaft rotation.

The best way I could think to do this was to print out a protractor from the internet and stick it to the crank pulley with some blu-tak and use a bit of wire as a pointer.


I found it more accurate to use a phone to 'look' at the wire so that I got the same point of view each and every time I looked at the screen. Plus it zoomed up pretty well.



So that's the rotation covered, now to measure the actual lift on the pushrod. A few months back I bought a dial gauge with a magnetic base and I have to say it was cheap and cheerful. However it has proved itself during this job as it worked very well. To measure the cam lift, I needed to remove the rockers and mount the dial gauge on the head so that the plunger sat in the cup of the pushrod.



For the valve lift, it doesn't matter if the rockers are on or not as long as the base of the gauge can be mounted properly and the plunger sits on the top of the spring retainer.



The very last thing to do was to find the centre of the dwell point on the nose of the cam. The method used here is a lot like setting the timing:
  • Rotate the cam until it's more or less where you think it should be.
  • Set the dial gauge to 0
  • Rotate the crank until the gauge is 0.2mm above 0 and read the angle.
  • Rotate the crank the other way to 0.3mm below 0 then back 0.2mm again. This gets rid of any backlash in the timing gears, then read the second angle.

Now get a calculator and add the two angles together and divide by two, the result is the angle that the crank needs to be set at to ensure that the centre of the dwell point is set. It sounds harder than it actually is.


With all that groundwork complete, it's now the tedious job of rotating the crank 10 degrees on the protractor (which is 5 degrees on the cam), measuring the lift on the gauge and writing it down. I created a spreadsheet to record the readings. I say tedious as there are 72 measurements to take per valve, and for consistency and double checking, I like to go round twice, so that's 72 checks also! - Yeah, it's quite boring and I must admit that after doing the exhaust and inlet on cylinder 1, my initial enthusiasm started to vanish very quickly!!

After a little manipulation of the data I collected on these two valves though, I did manage to plot a nice graph, so it was kind of worth it, I think.


The initial plan was to do this for all the valves and do a graph for each cylinder, but the time it takes to do just one valve made it impractical, plus it was as boring as watching paint fade!!! So I left it at these two valves. For the other valves, I just measured the max lift points.


Clearance Gap
Tappet clearance, valve lash, clearance gap, whatever you call it, has an interesting affect that I had not considered until I started messing about with all this. In the spec for this cam states a gap of 0.015 inches which is 0.38mm. As this gap is on the valve side of the rocker, that means that the cam needs to lift the pushrod, 0.38mm divided by the rocker ratio of 1.275, which is round about 0.3mm before the rocker finger even starts to touch the top of the valve, let alone start to move it.


Okay, I admit that it gets a bit heavy now as this concept really got my head mashed up as I was trying to understand how this gap effects the valve lift value given in the cam spec. For my cam the valve lift was stated as 0.318" which is 8.08mm. What I couldn't work out was; does that 8.08mm of valve lift include the clearance gap or not? That is, does the valve itself actually lift off the seat by 8.08mm, or does it mean the rocker finger on the valve side moves 8.08mm as they are different?

If it's referring to the rocker finger, then surely you need to take into account the 0.38mm gap that it has to travel before it reaches the valve top. This whole thing did my head in so much that I ended up buying the kindle version of a book to try to understand it.


The book wasn't massively thick, but it was quite useful and it did have one crucial point that helped to make sense of this issue. It stated that the valve clearance must be subtracted from the total valve lift to obtain the net (or true) valve lift, so...

Total valve lift, ie what is stated in the spec as 8.08mm, needs the valve clearance of 0.38mm subtracting in order to determine the distance the valve actually lifts off its seat.

In this case 8.08mm - 0.38mm = 7.7mm,

or is it????

I admit that it's getting quite heavy going now, but stick with as after further reading I discovered that cam lift is defined as the difference between the cam's base circle and the top of the lobe. That would be all fine and dandy if the base circle was easy to find, it seems though that there is a clearance cut into the heel of the cam that eats into the base circle. Why is this a problem, well it makes it impossible to measure the true cam lift as I can't find the base circle with the method I'm  using. If I'm not able to locate the base circle, I cant measure the lift.



It's super heavy now as this is where I got really stuck as I suspect that this clearance cut into the cam heel takes account of the gap and cancels it out! If this is the case then the true valve lift will be back to the 8.08mm. At this point I have to say that I'm just not sure and I'm starting to conclude that maybe, just maybe I've overthought the subject a little.

The last thing to note about the size of the gap is that the 0.38mm given in the spec is not the same as the Haynes manuals' 0.3mm. Reading between the lines I suspect that the spec for the cam is for 1275cc engine. As mine is a 998cc, I'll stick with the 0.3mm.

As I've spent so long creating these pictures in Sketchup, I thought I'm milk them a little and stick a final one here as a reward for getting this far. Plus it's a bit of eye candy for the post skimmers!





Conclusions
Since I started to look at this subject, I've realised that it's a bit like an endless rabbit hole and at some point you have to decide to stop before you go insane! Luckily for me, I've been on holiday and the break has helped me to realise that I was getting bogged down with this and it was holding up the project. If you managed to wade though this article them thanks for sticking with it, if you skipped to this bit then I can't really blame you.

So it's time to put the rockers back on, set the gaps to the 0.3mm and leave it at that, before I get stuck any further. You see now why I subtitled it 'A Descent Into Madness!'




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