Cam lift

brxpb · Apr 6, 2014

When calculating valve spring travel should the max lift of the 4s cam be multiplied by 1.13 to find the spring travel, ie, 390 x 1.13 = 440.7(441)?
.441' +.080' clearance = .521' and my free travel is .558' so I should be more than o.k.??

Is that too much free travel. Should I shim some of that out?

Sorry for the inane questions but I'm only learning

Thanks Paul.

pommie john · Apr 6, 2014

According to this page http://atlanticgreen.com/camsurvey.htm ( which has tonnes of useful info) the .390 lift of the 4S is cam lift not valve lift, so you are right that you have to multiply it by the rocker ratio ( 1.13)

brxpb · Apr 7, 2014

Thanks Pommie John, I sort of new but needed to hear it from someone else in case I'd overlooked something.
Do you have an opinion on the free travel figures, when people talk about checking for coil binding I expected the figures to be more touch and go with a 4s, but there's .117' to spare so without having the means to measure it, I wonder if I'm getting enough pressure on the seat? (Granted that this is being measured on the bench and will be verifyed when assembled, turning by hand.)

Matt Spencer · Apr 7, 2014

You really need a D T I ( dial guage ) & a Virnier .

Use the DTI at the top collar to measure actual lift .

Weigh a can of baked beans , then stack cans of required weight on valve spring , with collars . And measure length at that weight . This'll tell you if youre
under or over , or just right .

brxpb · Apr 7, 2014

Do they have to be "geniune norton" baked beans or will Heinz be o.k.??

Bernhard · Apr 7, 2014

brxpb said:
Do they have to be "geniune norton" baked beans or will Heinz be o.k.??

You can get a good fart with either

:shock:

pommie john · Apr 7, 2014

brxpb said:
Thanks Pommie John, I sort of new but needed to hear it from someone else in case I'd overlooked something.
Do you have an opinion on the free travel figures, when people talk about checking for coil binding I expected the figures to be more touch and go with a 4s, but there's .117' to spare so without having the means to measure it, I wonder if I'm getting enough pressure on the seat? (Granted that this is being measured on the bench and will be verifyed when assembled, turning by hand.)

I would be very surprised if you had a problem with that.

People talk about .045" from full lift to coil bound, but unless you're going to race it and hit the red line at every gear shift I think you'll be fine.

acotrel · Apr 8, 2014

I've got a very simple question - which cam gives the best mid-range power in a standard 750 commando motor ?

brxpb · Apr 8, 2014

If it's a standard 750 motor then it would have to have the standard cam in it otherwise it's no longer standard!

Keith1069 · Apr 8, 2014

I've got a very simple question - which cam gives the best mid-range power in a standard 750 commando motor ?

In a 750 or 850 it has to be the std cam but I guess it depends what you mean by mid range? 3500 rpm +/- or more? Look at the attached curves which also make the 850 look like a true stump puller. However, I have never seen these figures replicated which are allegedly from Norton factory tests and attributed to John Favill...........

acotrel · Apr 8, 2014

By 'standard 750' I meant standard motor with only the cam changed. With my own 850, I've advanced the standard cam and use a 2 into 1 exhaust and the torque seems massive right up to 6,000 RPM. Makes gearing a problem. Does the Combat SS cam give more midrange or does it move the power band upwards to get more urge ?