ATC70 guys... Ever use this camshaft?

[Thorpe]

http://www.dratv.com/bez5atcttrxl.html

Any of you 70 guru's ever try this cam? My 70 could stand just a tad more power for dragging around my 210 lbs... (yet wanting to keep it kid friendly) Thoughts/opinions?

[WilliamJ]

I have no figures here for a standard 70 cam to compare but I can compare it to a performance car cam that has a similar head design and valve gear.

By the way - the 50 thou is a standard checking clearance for doing an approximate cam comparison from one maker to another. The seat to seat timing figures are not so good for comparing the real shape of different makers's cams. The car cam I am comparing with has 8 thou running clearance / lash and 10mm lift at the valve. It has 290 / 294 degrees of seat to seat timing and 70 degrees of overlap. Measured at 50 thou it has only got 235 degrees timing and 14 degrees overlap - so you see why I am confused by the quoted numbers.

It looks like the seat to seat duration is what he is quoting but he doesn't make it clear. IF he is quoting at 50 thou lash then this is a very wild cam and likely in excess of 320 degrees seat to seat - which is a full race opening period and not what you are looking for. The 40 degrees overlap is not very much so that's why I think 255 / 265 are seat to seat numbers and at 255 / 265 it isn't a wild cam. In fact its rather ordinary but you need to take into account the shape of the lobe. Modern profiles lift the valve very fast but over a shorter period. This gives a lot better power spread. Long opening always ruins the bottom end torque but lets you rev the engine high. The design of the cam rocker system allows for very fast opening cams. The overlap and valve to valve contact may well be the limiting factor.

He quotes the full lift points of each lobe but the number indicate that the cam is running some 7.5 degrees advanced. (If the quoted numbers were 110 and 110 the cam would be timed straight up - neither advanced nor retarded) You can advance and retard the cam yourself to see what works best for you as long as you are careful with piston to valve clearance. He talks about valve to valve contact but as the lobes are fixed in relation to each other you can do nothing to adjust this out. Grinding the valve OD is not a great idea unless we're talking 10 thou.

If you can get some confirmation of the numbers quoted and compare those to a standard cam that will tell you if it is worth having. I should now go and carefully measure a 200x cam that I have lying around so I know what Honda's standard profiles look like.
Bill