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All the power is in the head. Part 6 - Camshafts

Posted: March 23rd, 2007, 12:25 pm
by Guy Croft
The astute reader will notice this is a copy of a post from GCQ&A, but I shall be enhancing it substantially in due course.

1. Fitting camshafts of higher lift might only work better than std lift if the head flows enough air at that higher lift to make the change worthwhile. That needs testing on a flowbench. A std TC Fiat head airflow typically peaks at about 10mm lift - around the std lift figure. And I've found that on many heads. And of course it's obvious why, that's one of the factors in how cams are chosen. So putting a cam of higher lift (per se).

Just putting in a cam of lift much over 10mm won't assure more power on its own.

There are other factors:

2. Without changing the full lift characteristic of the cam the valve event of the inlet can be extended so that the valve is opened earlier before TDC and closed later after BDC. The former gives 'overlap' which generates superior cylinder scavenging and ram-charging in the inlet tract and the latter allows air to continue flowing even as the piston is going up the bore. Both enhance the torque.

3. The exhaust valve can be opened earlier potentially allowing a more powerful primary pulse and superior cylinder evacuation. This has the dual benefit of enhancing beneficial pressure waves inlet-exhaust and also reducing the pumping work the piston has to do on the ex stroke. Closing the ex later helps with scavenge by allowing incoming charge to purge the cylinder. Etc etc.

Both those changes will increase the lift at top dead centre of the cams and within limits can be built into a cam of same-peak lift-as-standard and can yield improved torque and power.

It must be understood that two profiles which appear the same in terms of timing and duration can have markedly different behaviour. I'm afraid when it comes to cams people will believe what they want to but a 300 deg cam (eg an often cited 40/80 cam) from the 'Acme cam grinders' and one of proven pedigree could be as different as, well 'chalk and cheese'. I happen to have reams of great dyno feedback on my own TC profiles, so I know exactly what they do and that's how I can sell them and rarely comment on other peoples' TC profiles - except C&B who are good in my book. There are some truly rotten TC profiles in circulation - mainly regrinds with weak lift integrals but I am not naming names.
When it comes to other engines my friends at Piper and Kent in the UK have profiles (Peugeot, Ford Vauxhall eg) whose performance I can only describe as 'proven and stunning'. Those guys really know what they are doing.

Make a careful study of the graph.

Those graphs are cam degrees vs lift in inches with flat-follower and zero clearance. The (so-called and much copied 40/80) Pittatore cam (a good albeit mild StII profile as it happens) seems to have longer duration than the GC cam. From my data I can tell you that if set at 0.016" clearance the true duration of the 77 is 312 deg and the 3A 300 degrees. So in fact, the 77 might look to some like a more powerful profile. It's not, nowhere near.

For argument sake let's suppose the cams are both set with 0.020" running clearance. From start of the blue line to contact of cam with tappet at 105 the cam isn't acting on the valve at all. Then the cam goes into the opening ramp phase that lasts from a full 5 degrees of cam rotation 10 crank degrees. The 3A if set at 20 thou clearance pretty well just goes straight into the lift phase. I don't set them at 20 thou as it happens. So at 16 thou, harsh ramp? Not a bit of it. You cannot have a powerful competition cam with long ramps, there isn't time for that. Some are even more aggresive. 3A is my top selling cam and I've never had a ramp wear issue with it. So you can see that a generous proportion of the 'duration' of the 77 and any cam - come to that - can easily be eaten up by a completely wasted and non-productive phase - the ramp.

From my data I can tell you that if set at 0.016" clearance the true duration of the 77 is 312 deg and the 3A 300 degrees. So in fact, the 77 might look to some like a more powerful profile. it's not, nowhere near.

Another important feature that is rarely discussed is the dwell at full lift. The 77 has 6 cam degrees and the 3A 10 degrees. May not sound much but a valve at full lift can flow a lot of air in 4 degrees of crank rotation.
Take a look at the lift rates too. The 3A from contact to 2mm valve lift takes 13 deg of cam rotation. The 77 takes 23 degrees.

And remember above all that both these cams are billet not regrind. The incremental lift determines how much air can flow thru the valve region under the influence of the net cylinder-to-inlet port pressure differential at any given time and the acceleration from start-lift to full-lift largely governs how effective the cam is going to be on generating those vital cylinder-filling pressure pulses in the inlet tract. The bigger the lift integral (measured in inch-degrees) is a measure of this. For equal full lift the cam with the bigger area under the curve is always going to be more powerful. Here the 3A has true integral of 39.8 in-deg and the 77 34.4 in-deg, but I have integral data on some regrinds where it is as low as 28in-deg even for a grind supposedly of '300 deg' duration and 10.5mm lift. Might sell but only on paper and quite useless in fact!

Re: All the power is in the head. Part 6 - Camshafts

Posted: July 3rd, 2012, 3:57 pm
by Guy Croft
Inspired by something I saw on Facebook here is a short film demonstrating the phases of the cam cycle - base circle, ramp, flank, full lift.

Because the TC cam acts against a flat-follower these positions are only approximate. Even at full lift there is a significant 'dwell' period during which the valve is held stationary and 'dialling-in' the cams using dial gauge and protractor is essential to find the mid-point of that part of the cycle. With a cam that has symetric flanks the other parts of the cam phase (FL and LATDC) are always set off from the center of dwell - in other words the centerline of FL ('true full lift') is the datum point for cam timing.