Trying to understand camshafts, got a headache like someone beat me with one...

Ok so as the title states, I've been trying to understand the various aspects of camshaft design, and all I've gotten out of it is a headache like someone's been beating me with a Lunati product. What I'm trying to understand is if you hold constant the actual air delivered to the cylinder, what affect does the cam profile have on engine feel and power? Let me give an example:

Say you have an engine with a totally unrestrictive intake and exhaust, basically at no point throughout the prescribed RPM range will the engine be starved for air or be unable to expel all the spent exhaust gasses. You want to test two distinctly different cam profiles on that engine, and are holding every other controllable variable constant. For the sake of ease, let us assume that the volume of air the two cams deliver into the cylinder is identical over the prescribed RPM range, that both cams have some form of hydraulic lash adjustment (so lash isn't an issue), and that the RPM range in question is ~1700 to ~6000RPM

Cam A specs (identical to a Comp Cams 70-131-6 grind # 280H):
- 110 (at crank) intake lobe centerline
- 220 (at crank) lobe separation
- 280 advertised duration
- 236 duration @ 0.083" of valve lift - 0.46" maximum valve lift

Cam B specs (made up but lobe centerlines and advertised duration loosely based on Cam A):
- 110 (at crank) intake lobe centerline
- 220 (at crank) lobe separation
- 280 advertised duration
- 255 duration @ 0.083" of valve lift - 0.36" maximum valve lift

how will the characteristics of cam A differ from cam B? I know that long duration cams tend to be a bit hotter performance-wise, but generally that's not altering the tappet-to-valve ratios so the valve opens the same distance but stays open longer, letting in more air...

therein lies my question I guess. Are the characteristics of a cam based mostly on the quantity of air they let into/out of the cylinder, or does the profile of the cam affect the characteristics as much as (or more than) the quantity of air that goes into/out of the cylinder? I'm assuming the latter is the case, but I only know enough about camshaft tuning theory to realize that I am completely clueless about it

I am by no means an expert on camshafts, but this is how I understand it. If the only functional difference between two camshafts is the duration, you will get more air/fuel charge in and exhaust out with a longer duration camshaft. Basically one cam is opening the valve longer over the cylinder event to get more CFM/energy into the engine. However, even two cams with identical specs could be different due to the lobe profile.

Think of the valve opening sequence as a sine wave graph. At 0 you have a closed valve, and when the camshaft lobe starts opening the valve it ramps up in an approximately exponential state until it tops out and closes in roughly the same fashion. Older cams have a less-steep graph (opening) as valvetrain technology did not allow for a steep opening due to loss of control. Newer engines/cams have more "meat" under the graph (opening sooner and greater on the way to max lift), allowing them to flow more CFM's.

Confused yet?

Anyway, getting to the point, the actual lobe profile (how fast it opens) has just as much to do with it's characteristics as the actual duration and lift values. That's why seemingly identical cam specs can be both a regular cam and a "high energy" cam.

Okay, coming back around to your original question again, I see your camshafts have identical advertised duration, with a different duration @ 0.083" of lift. First of all, this is odd. Most camshaft manufacturers give the stated durations at 0.050" of lift. In this case, the advertised duration is when the actual valve opening event starts to happen, but any actual CFM flow doesn't come into play until ~0.050" (industry standard). So what you are seeing here is that cam A is a standard lobe profile with a "slow" acting lobe and cam B has a newer "high energy" profile, opening more and sooner for the same lift, which is affecting it's actual duration.

Hopefully all of that is right, and now I have a headache, too.

I can't answer your question, but I'd be willing to bet Comp Cams would be willing to. They aren't scared to answer the phone or email and their tech support used to be pretty good.

At a guess, one of them is going to have a steeper ramp area and might cause issues with valve spring, lifter durability and valve float.

Edit to agree with Javelin. One of them is going to open and close the valve quicker due to the steeper ramp. That's when you have to pay extra attention to the rockers, springs, lifter and such.

Not the whole answer, but think about these things:

1) The rate of acceleration of the piston obviously varies as it travels up and down. There is interplay between the rate of acceleration and the flow of intake and exhaust that the cam is allowing at any particular point in time.

2) Less lift may give more charge velocity which will increase inertia of the charge.

Second cam will idle rougher due to longer timing events creating more overlap, it will make its power at a higher rpmdue to the longer valve opening event, but make less power due to the decreased lift.

Or something like that.

you've opened a big can of worms... because not only is duration different, but peak lift is different.

the more duration @ will usually require more static compression because the longer duration doesn't close the intake valve until later in the compression stroke. So because the intake valve is open longer... less pressure is built up, so more static compression is needed.

now...dealing with the lift differences... what you need to do is calculate the valve curtain with each cam to see how much flow is getting past the valve at the different lifts...

Look at the extremes. At one end, you have the OEM/RV/Towing camshaft, IE- low lift and small duration. The other end is occupied with what is a race cam for NHRA Stock Eliminator, IE- same low lift, but the lobes are "square" for maximum duration. The difference you will see between the extremes are less vacuum, choppier idle, higher idle speeds, increased overlap, higher powerband range, etc with the "stocker" cam.

Slyp_Dawg wrote: Cam A specs (identical to a Comp Cams 70-131-6 grind # 280H): - 110* (at crank) intake lobe centerline - 220* (at crank) lobe separation - 280* advertised duration - 236* duration @ 0.083" of valve lift - 0.46" maximum valve lift Cam B specs (made up but lobe centerlines and advertised duration loosely based on Cam A): - 110* (at crank) intake lobe centerline - 220* (at crank) lobe separation - 280* advertised duration - 255* duration @ 0.083" of valve lift - 0.36" maximum valve lift

What you have basically done here would have the same effect as increasing rocker ratio. I'll explain.

First of all, advertised duration is useless. Some manufacturers use seat-to-seat, some use .005", some use .008", and others use "a nice big number to make you buy the cam."

What you are simulating with the cam above could be simulated with higher ratio rockers, or by increasing the peak lobe lift which increases ramp speed of the lobe.

So, in effect, Cam B should make more power and torque across the board since it is spending more time in the mid and high-lift areas. The only time it couldn't make more power is if the heads start to suffer flow in the high lifts of the valve... which does sometimes happen.

Now, to further confuse things, the other thing you will change slightly is overlap. Since you are making the lobe "taller" and overlap is usually measured at .005" lift, you add to the overlap slightly. Where its noticed more (in either the lifter change or the lobe change) will be at the valve overlap. If the rest of the engine characteristics are identical, it should show up as a slightly higher peak torque RPM... maybe 200 rpm in your example.

Cam B might also return slightly less idle vacuum... maybe 0.5mmHg?

What is the desired rpm of this engine? Also consider stuff like gearing and transmission cupping. Both cams have a place.

Edit after a hot shower.
If the car/truck/engine is set up incorrectly for each cam it's going to "feel " awful. The "consistent" setup can't be just that.

oldeskewltoy wrote: you've opened a big can of worms... because not only is duration different, but peak lift is different.

I totally glossed over that because I thought it was a typo.

ok, so basically two different cam/rocker packages that ultimately meter the same amount of air into/out of the cylinder, but do so with differing amounts of lift and duration wouldn't have similar characteristics, or putting it a lot more basically, it's not as simple as saying that 3x8 is the same as 6x4, just because they both equal 24?

pulling numbers out of my ass/out of thin air (at this time of night, I'm finding it hard to differentiate between the two ), generalizing cam specs, and shifting the discussion to airflow, assuming neither cam will run out of air anywhere in the RPM range, if cam A has relatively high lift but short duration and can deliver 3 cubic feet of air, and cam B has relatively low lift but long duration and can also deliver 3 cubic feet of air, would it be fair to say that cam A will be stronger at low to mid RPM, and cam B will be stronger at mid to high RPM?

the more I think about it, the more I'm wondering where my head's been. if you have a shaft spinning at 1000RPM, it will rotate 270 degrees in 0.045 seconds. if you accelerate that shaft to 2000RPM, it now rotates 270 degrees in 0.0225 seconds, so anything that needed to happen during that 270 degree period of rotation would only have half the time available to do it in at 2000RPM vs. 1000RPM. that literally just occurred to me, and sorta proves that my original idea (lower lift, increase duration, achieve same end result) wouldn't work...

To add to the potential confusion, the engine compression ratio can be a factor as well. Somethign to do with why high RPM race engines can run such high compression ratios (13:1 or so) is the wider lobe overlap effectively lowers the compression.

Do some googling... others can explain it better than I.

Strongly suggest you think more in terms of the complete package and what you want the engine to do rather than just the cam specs.

As mentioned previously many pieces ( Compression, fuel, exhaust, gearing, rev range, reliability, etc ) greatly affect the final result.

What do you want to do and what do you currently have?

Chet

These thoughts.... taking things out of context.... is such a waste of time.....

Tell us a pair of cams... and then saying you have no engine restrictions is ridiculous.... because it doesn't matter.

For anyone to truly help you.... give us the engine, its flow @ all the lift points, tell us if this is a pushrod, SOHC, or DOHC engine... What is its static compression, what are the valve closing events(so we can calculate DCR)

Taking a set of cams without context is just an exercise in futility

You're right. I posted an answer to the question, but deleted it, because you really can't answer the question that was asked. It makes no sense.

Chet wrote: Strongly suggest you think more in terms of the complete package and what you want the engine to do rather than just the cam specs. As mentioned previously many pieces ( Compression, fuel, exhaust, gearing, rev range, reliability, etc ) greatly affect the final result. What do you want to do and what do you currently have? Chet

right now, I want to come marginally closer to understanding camshaft tuning theory (it's a bit of a black art to me at this point, as I'm sure is readily apparent). I currently have an idea in my head for a streetable, high performance (~50-60hp, ~35-40ftlbs torque) single cylinder (or parallel twin) motorcycle engine that does not currently exist in captivity, and likely will never exist, mainly due to monetary constraints, but it's interesting to contemplate none the less. hence the lack of details about the engine. I'm basically trying to relate two theories, rather than nail down what a specific cam grind will do in a specific motor under a specific set of circumstances

Slyp_Dawg wrote:

Chet wrote: Strongly suggest you think more in terms of the complete package and what you want the engine to do rather than just the cam specs. As mentioned previously many pieces ( Compression, fuel, exhaust, gearing, rev range, reliability, etc ) greatly affect the final result. What do you want to do and what do you currently have? Chet

right now, I want to come marginally closer to understanding camshaft tuning theory (it's a bit of a black art to me at this point, as I'm sure is readily apparent). I currently have an idea in my head for a streetable, high performance (~50-60hp, ~35-40ftlbs torque) single cylinder (or parallel twin) motorcycle engine that does not currently exist in captivity, and likely will never exist, mainly due to monetary constraints, but it's interesting to contemplate none the less. hence the lack of details about the engine. I'm basically trying to relate two theories, rather than nail down what a specific cam grind will do in a specific motor under a specific set of circumstances

LOL- I think you may be aiming too low!

I'm working with a well known engine builder to do a short run of Honda GB662s. ( 1989 / 1990 Honda GB500 with 106 big bore ). Dyno over 60 HP and mid 40s torque for a single cylinder street bike.

Chet

Camshafts are my business. It's what I do. If you have questions, I can probably answer them. Your (actually the preamble before the) question above did not appear to make sense to me. Comparing two cams for the same application makes more sense. For what I think you're asking, some of the "blueprinted" repro performance cams compared to a newer performance cam for the same application might be a more meaningful comparison. To answer the question

Are the characteristics of a cam based mostly on the quantity of air they let into/out of the cylinder, or does the profile of the cam affect the characteristics as much as (or more than) the quantity of air that goes into/out of the cylinder?

The simple answer is yes. Quantity is the most important. The rest is fine tuning.