Technical for non technical people

[Dick Roberts (Web admin)]

Posted by Terry Collier on April 30, 2003 at 13:45:35:

Hi,

Ok I will share some of the findings from my sleepless and very sad nights in Hotels. Before we go on about cams for ever here are some thoughts you may not have considered.

Main Point -- the purpose of the cam is to open valves to let in fuel air mixture and push out exhaust gases. OK but what does this mean to us and our jolly old 602 special.

The task.

At 7000 rpm 1 revolution equals 60/7000=0.0086 seconds. Not very long to do either job.

If the maximum air flow through your head is 100 cubic feet minute (cfm). Then in 0.0086 seconds the amount of air that could be moved is
100/60 = 1.66 cf
or
1.66*1728 = 2868.48 cubic inches per second.
Which equals 2868.48*0.0086 = 24.669 cubic inches per revolution.

However we cannot flow in or out for an entire revolution as the piston sucks in on the way down and pushes out on the way up
Do we need to move 24.669 cubic inches?

The amount we need to move is the volume of the barrel plus the capacity of the Head. We will not worry about the piston shape as the calculation is quite difficult to write down. Suffice it to say that it reduces the volume we need to move.

The barrel is 7.4 cm by 7.0cm (regs) the head is 57cc capacity.

so the total volume is (3.1416*7.4*7.0)+57
or 219.735 ccs

This 219.735*0.061024 = 13.4 cubic inches

Moving air at our maximum rate it would take
13.4/24.669 = 54.3% of a revolution
This is a duration of 195.54 degrees.

If we look at Illustration C -- the timing diagram -- we

So if one´s cam is close to Illustration C then it can be pretty much optimal in performance. This assumes that your Heads flow at least at 100 cfm. If they do not you will have problems, if they flow faster as mine do you will go quite quick.

Fun this far and no real talk of cam design. So what could we do to make a dodgy cam. Actually not very much. You could try raising the lift. this would result in opening the valve quicker and making the hole bigger. Sometimes works. However, if you try drinking a Coke through a straw or through a garden hose you will soon see that big is not always better. If the increased lift increase your air flow it would make a difference, but I will tell you that it does not. Increasing lift only makes up for head design deficiences. Timing can make a difference but there is not much you can do -- look at the dynamics of the piston -- bring it too far forward and you are pushing out before you suck in -- take it back and you are pushing out before you are compressing.

You can put in a flying cam if your head design is too far advanced for illustration C, but this is generating an air flow which is not achievable with our current regs.

The problem we have -- which Paul stated and I made at the AGM is that there is no "bog standard" cam which meets Illustration C. All of the fixes -- verniers etc -- actually are trying to make a bad cam better. This was my point. I felt it was unfair for people to expect certain things from their investment only to be disappointed for no fault of their own. Your cam at the moment is a random one. Those of us who filter cams to find the best approximation to Illustration C have an advantage -- I thought that this should not be a secret and an accurate Club Cam would be the fairest answer. Considering the effort I made last year on this subject I find it both ironic and hurtful to find myself accused of cheating in this respect. Pissed Off is not strong enough.

Anyway -- Sorry another long message. But I hope that you now realise why the way people discuss dodgy cams makes me laugh. Hopefully this may contain information which you never knew or had never considered.

Good Tuning

Terry Collier

If I wanted to make the 2CV engine go much quicker i would do something about the Inlet Manifold. This is very long and has a big effect upon the net air flow you can achieve. So does the carb by the way -- One tip if you are using a Gauze or Foam air filter you are reducing your air flow by over 20%. This is free which as a Consultant goes against the grain.

To do this means we would have to have seperate carbs which is too big a jump.

So next time any of you want to think of a cheat do not go for the single item. Engines work in balance. Just changing things does not do what you expect.