TheSamba.com :: View topic - 36 hp cam install--check my thinking here

Fri Aug 26, 2011 11:05 am

I'm working on a 36 for my trike, installing a reground cam for which I don't have any specifications. It is supposed to be a mild performance grind from an old name-brand master.

Here's my setup:

Using .040" checking height with adjustable timing gear here's what I got:

@0deg offset IO=9btdc IC=47.5abdc, EO=42.5bbdc EC=12 atdc
( difference of 3 deg duration between intake and exh--Is this tolerable?)

To me this looked to be 3 degrees retarded offset, so I installed 4 degree eccentric washers in the cam gear and rechecked:

@4 deg offset IO=14btdc IC=45abdc, EO=46 bbdc EC-8 atdc
(note 2 deg discrepancy in intake duration due to sloppy measurement)

This looked 2 degrees advanced, so I installed 2 deg offset washers advanced and:

@2deg offset IO=11 btdc IC= 46 abdc, EO= 44 bbdc EC=9 atdc

given the 3 deg difference in duration between lobes, and a 1 deg fudge factor, this seemed about right to me until I checked lobe centers (max lift) and here's what I got:

Intake lift .335'@ 110 atdc, Exhaust lift .328" @ 103 btdc

If my arithmetic is correct, that puts lobe separation at 106.5 deg, (calculated value would be around 118 deg, right?)

Assuming that this cam is old school, I would think it is a symetrical grind, and should be installed straight up. Questions: 1 Which numbers should I believe, and use to dial in this cam--lobe centers, or opening/closing?

As one can tell, this is not something I do every day, and any expert advice would be much appreciated. Time is short til World of Speed, and I'd really like to get this engine done in time to make it.

//UPDATE// I just checked #2 cylinder cam event and the results are consistently inconsistent:

@2 deg offset, IO=10 btdc IC= 45 abdc, EO=45 bbdc EC=12 atdc
intake lift .331" @ 110 atdc, exhaust lift .332' @ 105 btdc

Looks like I should have just thrown this cam in with zero offset, and forgot about it (just like two of my expert friends told me to...)

Still looking for opinions though...thanks in advance.

Thanks,
Jerry

Fri Aug 26, 2011 3:57 pm

Like you, I usually find open and close numbers to be inconsistent... nothing wrong with the cams, it's just that the ramps lift the lifter slowly at first, not much lift per degree of rotation until you get to about .10" or so lift. Check the same lobe's opening 3 times (with a 36hp radiused lifter) and get 3 answers

Because of that, I personally like the 'split lobe, split lift method' where I write down IO degrees at .15" before full lift, and IC degrees at .15" after full lift, then do the math to split the difference. that number is the lobe center angle in degrees.

So far as weather to advance, run straight up, or to retard... it all depends on the grinder. The trike is light, you could give up a little bottom to maybe shift the torque band a little higher in the rpm range... awe, just set it straight up and forget about it! :D

Fri Aug 26, 2011 6:01 pm

Tom Simon wrote: Like you, I usually find open and close numbers to be inconsistent... nothing wrong with the cams, it's just that the ramps lift the lifter slowly at first, not much lift per degree of rotation until you get to about .10" or so lift. Check the same lobe's opening 3 times (with a 36hp radiused lifter) and get 3 answers

Because of that, I personally like the 'split lobe, split lift method' where I write down IO degrees at .15" before full lift, and IC degrees at .15" after full lift, then do the math to split the difference. that number is the lobe center angle in degrees.

So far as weather to advance, run straight up, or to retard... it all depends on the grinder. The trike is light, you could give up a little bottom to maybe shift the torque band a little higher in the rpm range... awe, just set it straight up and forget about it! :D

Hi Tom,
Thanks for the sage advice. That's exactly what I did. I checked all 4 cylinders, and as you advised, bracketed the lobe centers to reach a consensus of what the separation angle should be, an split it. Ended up within a degree or so of straight up, so there ya' go. Just like I woulda done if I didn't know any better... It will be interesting to see how it works, but then, I guess that's what this is all about :D . Hope to see you at WOS.