How do you measure a cam?

[Gas Man]

Ok boys... how do i measure a cam? I have the cam in one hand... I need what and how do I????

Thanks in advanced!

 

[cavracer21]

i believe that u would have to spin the cam between centers with a indictor on the lobes. the difference between the high number and the low should be your lift?

you could pry use calipers and get close by measuring the lobe diameter and then across the raised part of the lobe. then figure the difference.

 

[lee]

Gas Man - I found this - I think this was to measure a car cam but presumably the principles are the same

I don't have a clue what he is talking about but you might - hope it helps


Mike the cam lobe from top of point to heel (up and down), then mike it 90 degrees from that (sideways). Subtract the small dimension from the large, and multiply that figure by the rocker arm ratio. That will give you gross valve lift. You'll have to set the camshaft up in a block with a degree wheel to determine duration

 

[Coolbreeze]

The size lift should be on the side of the last lobe. If you are looking @ the stock cam (s&s) it will be a 545 or 547 or 525.
What are you trying to acomplish. If your changing your cam out, 05 and later you'll half to change the valve springs.

 

[V]

Remember That the difference between the base circle and the max height on the cam lobe are not the true lift of the cam. You also have to figure in the ratio of the rocker arm.

Example: Lobe lift on cam .350 x rocker arm ratio 1.6 = lift of .560 at the valve.

Somebody please correct me if I be wrong.:bang:


Thanks V

 

[jwoolf]

Don't you also have to determine valve opening and closing angles to take into account of what the cam will do for you? Afterall, lift is just how high the valve goes up and down and doesn't have a thing to do with timing.

I'm not expert on the subject but, I'd like to learn a little something about it.

I haven't changed mine yet but, I'm planning on doing it at some point and, I want to be able to get the right cam do work well in the rpm range I'm looking for it to do it's work.

I'm not busting chops here, I"m actually trying to learn something about cams which seem to be a black art.

 

[jwoolf]

Ahhh, I think I found a good paper discussing cam timing, duration, overlap etc.

Here is what I've found with a google search. I'm sure others here my have a better tutorial but, I found this one pretty fast. Timing Tutorial

 

[V]

I don't know as much as i would like to either. When i put in the .600 cam the popping sucked but the bike runs strong. Several cam suppliers including S&S noted that the long duration was causing the popping on decel and that more compression and baffled pipes would help. :loony:

So yes black art definetly.


V

 

[jwoolf]

This is pretty good stuff:

 

[jwoolf]

I don't know as much as i would like to either. When i put in the .600 cam the popping sucked but the bike runs strong. Several cam suppliers including S&S noted that the long duration was causing the popping on decel and that more compression and baffled pipes would help. :loony:

So yes black art definetly.


V

Here's something I found about the high durations...

Long duration, late closing cam designs are necessary to drag the last bit of power out of an engine. Unfortunately, these same cams can perform poorly under more normal riding conditions. In the quest for maximum power output, many-too-many Harley owners choose a late closing, high-rpm cam for their engine. The problem with such choices is that the engine seldom spends time in the rpm range favored by such cams.

The rpm at which a Big Twin gets "happy" can be predicted by the closing point (angle) of the intake valves. The angle is expressed as the number of degrees After Bottom Dead Center (ABDC) that the valves reach .053" from being fully seated.

The following list predicts the rpm at which the engine gets "on the cam" based on the closing angle of the intake valves. These relationships are approximate but should hold true to within 200 rpm or so. They also assume that all other tuning factors, exhaust, ignition, etc., are operating correctly.

30 degrees = 2400 rpm
35 degrees = 3000 rpm
40 degrees = 3600 rpm
45 degrees = 4000 rpm
50+ degrees = 4500 rpm



Seems like this high duration is causing the exhaust valve to open quicker and stay open longer forcing still burning fuel out of the exhaust pipe. Which is really an ok thing for a performance motor. And, besides, it looks cool at the rally when you're doing a burnout and shooting flames out of the pipes. haha

Seems like to me you really need to think about what y
ou're wanting out of a cam before you bolt one in. I'm going to talk to a lot of gear heads before I figure out what I'm putting in. I'm sure the .600 cam is a great cam but, it may not be doing what I want my motor to do so, I'm going to check all the options.

Does anyone know where a good cam parts list would exist? ( along with all of the cam's specs )

 

[lee]

I think longer duration can also increase reversion unless everything is well tuned to suit - this seems logical - it may already say this on some of these posts but as I am not a NASA scientist bio physisisisisisist I wouldn't know!

 

[jwoolf]

Can anyone place the .600 cam and the .640 cam in the above post?

I haven't researched the timing of these cams and, I'd personally like to know.

 

[jwoolf]

I think longer duration can also increase reversion unless everything is well tuned to suit - this seems logical - it may already say this on some of these posts but as I am not a NASA scientist bio physisisisisisist I wouldn't know!

Seems like the overlap number would determine some of that ( when the engine is almost at TDC and the exhaust valve is still open and the intake is opening )but, like you said already, we're not rocket scientists and would need the help of a test engine and a lab with some expensive measuring gear to figure it out ourselves.

 

[jwoolf]

I'm going to end up doing the valve work at some point, having the heads flowed and change the springs and rockers etc. but, I don't want to throw all the driveability completely away. ( I'm willing to throw a lot of it away though. ) LOL That will be the same time I'll go ahead and change the cam and ignition.

I may wait a few more thousand miles before I do all of that work though.

 

[P8RIOT]

That is a great diagram - and it clarifies a lot of the terminology. It still does appear to be largely "ideal theory," though - and for our purposes that's fine. Here's an example of what I mean. The red and blue curves that represent the valve lift appear to be pure parabolas. In real valves, the rise can be accelerated considerably over what this diagram represents, and, as a second example, the high spot can be flattened to maintain wide-open valves for a longer time. This is a complex function of valve acceleration forces, strength of the lifters and supports, RPM that would cause float, valve spring tension, etc. So this diagram serves well to frame the discussion, but it is just a peek into the potential complexities.

Jwoolf really hit it on the head, though - you gotta have a clear objective of what performance you choose to affect. ("If your target is nothing, you're bound to hit it.") The whole valve topic is extremely sensitive to RPM ranges.

 

[Raywood]

Seems like the overlap number would determine some of that ( when the engine is almost at TDC and the exhaust valve is still open and the intake is opening )but, like you said already, we're not rocket scientists and would need the help of a test engine and a lab with some expensive measuring gear to figure it out ourselves.

Here's a little article on overlap, how to figure it out and how much you need. Not meant for motorcycles but still applies.

These are the valve timing overlap ranges that are most likely to work correctly (thanks to David Vizard for providing this):
trucks/good mileage towing 10-35 degs overlap
daily driven low rpm performance 30-55degs overlap
hot street performance 50-75 degs overlap
bracket/oval track racing 70-95degs overlap
dragster/comp eliminator engines 90-115 degs overlap

Calculating Overlap

 

[jwoolf]

Very cool stuff, Ray!!!!

 

[Raywood]

Does anyone know where a good cam parts list would exist? ( along with all of the cam's specs )

Here's a listing of a bunch of cams and there specs.

Camshaft Technology and Calculations

 

[jwoolf]

You just got repped. LOL

 

[Raywood]

You just got repped. LOL

Well thank you Bro.

Here is what RBRacing says about overlap compared to what I posted above.

Overlap, Close the Barn Door

The amount of time, expressed in crankshaft degrees, that describes the window of time between the the Inlet Cam's opening point BTDC and the Exhaust Cam's closing point ATDC. This figure can vary between zero degrees on some stock cams to as much as 70 to 90 degrees on some race motors. In general most street engines will have 20 to 30 degrees of overlap and most performance cams will have 50 to 60 degrees of overlap. Increasing the degrees of overlap tends to move the powerband up the RPM band. Increasing the overlap can increase peak power, but only if the exhaust system is properly designed to scavenge the cylinder. Decreasing the overlap tends to boost lower rpm performance.