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Doing the cam timing: Twin-Cam

Posted: July 25th, 2006, 4:06 pm
by Guy Croft
The technique for setting up (dialling-in) the camshafts on a twin-cam engine, 8v or 16v is rather different than a single cam unit.

There is more tuning flexibility when you have two cams but setting up is more involved. The main thing you have to watch out for with 'inclined valves' - ie: not mounted vertically - especially on big valve or 8v twin-cam heads is 'valve clash' - where the valves physically hit each other. All twin-cam heads are done the way I describe below.

Never, ever turn either camshaft without looking to make sure the valves are clear of each other. Again, because of this risk, it is MOST unwise to time up a twin-cam engine on the block - always do it with the head OFF.

Tools needed are a large diameter protractor, magnetic base DTI (dial gauge) with enough range to cope with the cam lift, sharp tipped pointer. I stick the protractor to the pulley with 'Blu Tack', an invaluable substance with many uses in engine building. For big valve units like this 8v Fiat head you must make a rig that allows you to turn the cams over in-phase to check the true proximity valve to valve.

If the spec for the inlet cam is full lift at 100deg, this means that the inlet valves should be fully open at 100 crank degrees after tdc, so we need to find full lift on no1 inlet valve, mark it, and then turn the inlet cam counter direction of engine rotation by 50 cam degrees and mark that position too. This will be the position the cam is in at TDC. Conversely the ex valve - if FL 100 crank deg, will be 50 cam deg after full lift at tdc. I usually mark full lift and tdc positions, so we can do a final check and adjust (if we have adjustable cam pulleys) with the belt fitted). There is more on this at:
http://guy-croft.com/viewtopic.php?t=14

I am asked 'do I need adjustable camwheels?' Yes - always use them. This is a race engine website, if you're building a race engine, they are essential. Everyone who did not use them either got in a bit of a mess themselves or I inherited the problem of trying to do cam timing without them to my usual standard of accuracy. I just don't have the patience to explain any more what may happen if you don't use them!

It is not necessary to shim the head prior to doing the cam timing, but I usually get shimming out of the way first. At least that way - when I am measuring the valve lift fro my records, I know the valve really is closed when is appears to be shut, not held open by a tight shim. And, this does matter, if you have the lift at tdc data for the cams, you can cross check the lift done by cam timing method against this. If you find with your cam timing that the lift is way more than it should be at tdc, you have done something wrong.

This is a Fiat TC big valve head - for a Fiat Abarth 124 Rally 1800. It is fitted with race guides, fully flowed, 45/40 valves, GC 3D inlet cam and an original Abarth 068 ex cam. Same valve setup I used on our 'works' 200bhp NHRA engine in the 90s - and many other units.

Doing these big valve units is definitely NOT a DIY job, don't even think about it - far too difficult unless you have done them many times before. You have got to prep it in such a way that you have a pretty fair idea what will happen vis-a-vis shimming and valve proximity before you start work, otherwise you are going to end up with something you either cannot shim - or cannot turn over.