"on cam" - what does it mean?

[Diablos-666]

I read post talking about when the engine is "on cam" but what does that actually mean?

Sorry for the noob question.

[mrmr96]

Some cars have variable valve timing and lift via a variety of systems. This allows for different engine characteristics at different points in the rev range. Typically more power to be produced at high revs whilst retaining better economy/emissions at lower revs. Coming "on cam" refers to the point when the engine transitions from the low rev profile to the high rev. Basically.

[Diablos-666]

Thanks for that, have a better understanding now.

I read a post on the TVR forum about someone saying that the car drones at a certain rev range (around 2.5k I think) someone replied saying that it's probably the car getting ready to come "on cam".

Does that sound right? My car makes that droney noise too if I'm in a high gear and the revs drop to around 2k

[marshalla]

It's not necessarily about variable valve lift, switching profiles etc.

An engine produces maximum power (& torque) at fixed rpm and the rest of the time is sub-optimal. When it comes "on cam" it's in that part of the power-rpm curve where it accelerates more easily and thus feels more responsive. For example, the Lotus 912 in my Excel feels very sluggish below about 3200 rpm, from 3200-6200 it revs freely and accelerates very well, thus I tend to describe it as coming on cam at around 3200 rpm.

[mrmr96]

Ok, your TVR wont have variable timing and in this instance your high performance engine is bias towards producing top end power. Cams duration and lift are a trade off of emissions/economy and indeed torque at low revs vs power at high revs. A variable setup allows the "bet of both". But in your car the profile will be setup with a bias towards top end, so the car runs better at the higher revs where the cam is bias for.

I have mates with Evos who've fitted high performance cams, and while the top end is great they lose some of the lower end drivability.

So coming on cam could be used to describe reaching the range of revs where the cam works best?

[marshalla]

mrmr96 said:

So coming on cam could be used to describe reaching the range of revs where the cam works best?

Also known as being "in the power band".

It relates to things like valve opening points & duration, valve lift and overlap between inlet and output valves mixed with flow rates for the combustible mixture and exhaust gases. If you really want to understand it/blow your mind - I can thoroughly recommend "Four Stroke Performance Tuning" by A. Graham Bell.

ETA : the goal of fancy variable valve timing, or cam changing systems is usually to increase the width of the power band as much as possible.

Edited by marshalla on Thursday 17th November 22:49

[davepoth]

mrmr96 said:

Some cars have variable valve timing and lift via a variety of systems. This allows for different engine characteristics at different points in the rev range. Typically more power to be produced at high revs whilst retaining better economy/emissions at lower revs. Coming "on cam" refers to the point when the engine transitions from the low rev profile to the high rev. Basically.

Nope, not really. In fact VVT was in many ways designed to try and remove the idea of coming "on cam".

Essentially, a cam can be designed to work well in a certain rev range, and then it won't work so well outside of that. The camshaft works by opening a valve to let gas and air in, and also to let exhaust gases out. At low revs you need this valve to only open a little bit, and not for a very long time, so as to get an efficient vacuum from the lower piston velocities found at low engine revs.

As engine speed increases that becomes much less of a problem, so if you want power at high revs you need the valve to open as far and for as long as possible.

The upshot of doing that is that at low revs the engine can't suck the fuel/air mix in properly, which makes it run badly.

If you have a "hot" cam (one designed for high rev and high power) there will be little power low down, but when you get further up the rev range it gets to the point where the cam profile is right for the engine speed, and the engine works a lot more efficiently. That's "on cam".

With variable timing or lift, the engine will switch from a profile designed for low rev power to one designed for high rev power. Effectively a variable valve engine will come on cam twice (or more, depending on how many profiles there are) through the rev range.

[Mr2Mike]

A cam shaft designed to produce good power at high RPM in a normally aspirated engine tends not to work very well at lower RPM (hence the development of variable timing schemes). At low RPM the engine will tend to feel quite flat, but as the RPM rises there comes a point at which the engine starts operating more efficiently, and torque output quickly increases. This is the point at which the engine comes "on cam".

I tried to find a video of an engine for you to demonstrate this, but searching for "coming on cam" didn't quite produce the results I intended

[Steve_D]

As has been said, with a performance cam the engine is not happy at lower revs. As the revs increase the engine will move into its ideal state which some have described as the 'Power Band'.

However to term 'Coming on cam' is used because in many engines the transition between running rough and 'on cam' can be very dramatic.
As a lad I had a Mini with a hot cam which came 'on cam' at 3225rpm and gave you a real kick in the process. The more I learn about engines the more confused I am over this as it makes no sense to me that the changeover should be so precise.

Steve

[Diablos-666]

Thanks for everyones response' to my question, I have a better understanding now.

I knew the engine had a "sweet spot" as you can tell when it comes aline, mine is around 3.5k. The reason the "on-cam" remark confused me somewhat was because in my mind the cam is always turning so was always on. I didn't realise it actually meant the sweet spot of the engine.

Thanks again

[seagrey]

Mr2Mike said:

I tried to find a video of an engine for you to demonstrate this, but searching for "coming on cam" didn't quite produce the results I intended

[Gwagon111]

The Vtec kicked in yo.........