Understanding Cams... A Journey
This is a discussion on Understanding Cams... A Journey within the Internal Engine forums, part of the LSx Technical Help Section category; I've been reading up on camshafts and how they affect the performance of an engine. Here's what I've gathered. Please ...
06-22-2011, 03:37 PM #1
Understanding Cams... A Journey
I've been reading up on camshafts and how they affect the performance of an engine. Here's what I've gathered. Please correct me as necessary.
Essentially, the cam dictates the character of the motor and is the lowest common denominator when determining how the motor will behave.
With an overhead valve train, the cam can be designed to provide it's most significant performance at certain rpm intervals. Better low-end torque or more high end power are the basic trade-offs to be considered.
This choice is represented by the lift, duration, overlap and lobe separation angle (LSA).
A higher lift means that the lobes of the cam are pushing the intake/exhaust valves wider open. Conversely, a lower lift means that they do not open as wide.
A longer (higher) duration means that the lobes stay in contact with the lifters for a greater portion of each cycle, allowing air to flow in/out of a cylinder for a greater amount of time.
Overlap is the amount of time (space?) that both the intake and exhaust valves are open simultaneously.
LSA is the angle between the lobe centerlines (which affects overlap, dependent upon duration).
If more low end torque is desired, a shorter duration is better. If more high end power is the goal, longer duration is better.
My understanding as to why this is, is that at high rpm, the cam is spinning faster, which means that the valves are actually open for less time (but same duration, obviously). Therefore, since the amount of time allowed for intake/exhaust is reduced, the longer duration compensates in order to get enough airflow... right...
Question: If this is true, does that mean that shorter duration helps low-end torque or that it hurts high end power?
To put it another way, if an installed cam were adjustable, would shortening the duration IMPROVE low end torque, or would it just HURT high end power without benefiting the lower end?
A smaller LSA supposedly benefits low end torque. Is this related to the reasons why a shorter duration is good for low-end torque (shorter, faster "breaths")? Incidentally, too small of an LSA will reduce vacuum pressure and can harm braking performance.
And that's all I've got for the moment.
Some more questions:
1) With duration and lift: how does setting the intake duration and lift different from the exhaust duration and lift affect the power band? Does more intake and less exhaust create more low-end power or more high end power or what?
3) How can the torque curve be managed to come on strong off-idle and not fall off too quickly as rpms climb? For example, if I have a stock LS1 and I want a wider power band, what cam specs would I have to change? Let's assume that I want the same amount of peak power at the same rpm (305 @ 5200). Can a cam alone increase my maximum torque, and lower the peak rpm without dropping the high end power? (Hypothetically, of course... I know heads and valvetrain need to be replaced, a tune etc).
3) What would it feel like to drive a car at WOT with a very low torque peak and a very high power peak (for example, 400 lb.ft @ 2500 rpm and 400 horsepower @ 6000 rpm). If torque starts to fall off after 2500, does that take away from SOTP feel, or does the climbing power keep you nailed to the seat?
06-23-2011, 03:31 AM #2
This is the major problem with a fixed lobe cam. The cam is designed for peak output at one set rpm.....then trails off at higher or lower rpms. Honda came out with Vtech (variable valve timing) to compensate for this by using this technology they basic "adjust " the cam profile at different rpms give a wider power band.
A narrower lsa will give you a higher peak torque earlier in the rpm range but trail off earlier. A wider lsa will give you lower peak torque, and come into the power later in the rpm range, but also have a wider torque curve. Look at the new Vettes, they are running a long duration cam with around a 121 lsa to give the motor a good idle and very nice wide high power output.
06-23-2011, 03:50 AM #3
it all really comes down to how much air you can pull into the cylinder and trap in there (VE) Volumetric efficiency changes all the time with different loads, rpms, throttle position.....and this is how you truly make power....with your long duration large overlap cams your VE is very low at and around idle because of the valve events it just can not trap as much air as a small cam, which is why you get a rough idle. now at higher rpms on the same big cam the overlap starts to work the opposite way because the exhaust charge running out of the motor actually pulls more intake charge into the cylinder as its leaving.....and this is where it gets interesting.....you with the right overlap, it can pull the intake charge in so well that you can end up having well over 100% VE and be filling the cylinder with more air than it would normally be able to hold.....this is why the larger cams come on the power higher on the rpm range
06-23-2011, 05:01 AM #4
06-23-2011, 07:13 AM #5
06-23-2011, 07:32 AM #6
The fastest car I've been in (did not drive this one) was a turbo-charged NSX. The driver eased into WOT in 1st (so it was like a much stronger version of the feeling of an airplane accelerating for take off), but stabbed the throttle coming into 2nd. The acceleration from the bottom of 2nd gear was massive. The G's pretty much paralyze you all the way to the redline.
I'm pretty sure both motors have a form of variable valve timing.
My car is quick, for sure, but most of the fun is in the 2500-4500 range. I would like to expand that range at both ends by as many rpm as possible.
Is the sensation of G-forces dictated by how much torque is available at a given rpm or how much horsepower the motor is making at that same rpm? It's hard to know using my car, since the torque peak is right around the middle of the rev-range (not a lot of room to experiment with engine response above the torque peak).
I understand that the LT1's 325 lb.ft. came on as low as 2400 rpm and was rated at something like 285 horsepower at 5200. How different would this car feel accelerating than the LS1?
06-23-2011, 09:51 AM #7
06-23-2011, 10:04 AM #8
Yeah, so you know what I mean: a strong pull as soon as you hit the gas, but there's no "surge" as you contniue toward redline.
06-23-2011, 10:10 AM #9
The feeling of G's or put back in your seat is feeling torque.
06-23-2011, 10:25 AM #10
06-23-2011, 10:29 AM #11
I agree to some extent, but when you think about it, torque is just an ingredient of horsepower, which is what is moving the car. Torque can't be the sole source of the acceleration sensation can it? Consider a Mack truck. Over 2000 lb.ft. of torque, but nowhere near the sensation my "measly" 350-ish lb.ft.
Also, the Z06 (or was it ZR1) makes 300 lb.ft. at idle. No SOTP feeling just idling through a parking lot...
06-23-2011, 10:29 AM #12
A big difference between our cars and the NSX/ Porsche is the power to weight ratio....specially when you get to a car like my goat where I had a 500+ lb weight difference over the F-body to over come
06-23-2011, 10:36 AM #13
06-23-2011, 11:10 AM #14
Something like that....Which is why high HP but low TQ cars dont feel as fast as higher TQ but lower HP cars...or dont plant you in your seat. Usually the high HP # is from revving the engine higher.
Also a main ingredient youre leaving out is gearing!
06-23-2011, 11:29 AM #15
Exactly, which is why I don't attribute all of the "feeling" to the torque spec by itself.
06-23-2011, 11:33 AM #16
06-23-2011, 12:09 PM #17
06-23-2011, 12:13 PM #18
06-25-2011, 02:08 PM #19
I love the sound of a thumping cam and I'm a boulevard cruiser. Red-light to red-light and sunday shifting is most pleasing for me. Friday night test and tune at the local track is quite enjoyable, too.
But, my question is how does the cam specs effect a loping idle. I've selected a Trick Flow cam with 232/238 and a .595/.596 lift to compliment my M6 and 4.56 Moser, Hooker LT's, off-road Y-Pipes and Loudmouth LM1 catback. I selected this cam 'cause I want the cam that gives the most radical idle. How will this cam sound at idle compaired to a comp cam with 220/220 and .595/.595 lift?
06-26-2011, 04:53 AM #20
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