What does 'on cam' mean

[frankyknuckles]

I've just been reading Dave Jacksons report in low flying about the V8 (top report). dave keeps refering to the engine coming on-cam at 8000rpm etc and it is a term that I have seen used rather alot wrt to all manner of engines.

 

Now, I come from a VTEC background and although I do not fully undersand how it works, I can appreciate that there is a cam profile change when the VTEC engages, so if I was to use the phrase 'on-cam' for a VTEC engine, it makes seme sense. But, as far as I know, in normal engines, like the K, the cam profile does not change at different rpm's, so how does a car/engine come 'on-cam' ?

 

(sorry, I'm sitting at home with the flu and I have nothing better to do 😬)

 

www.R300.net

[duffym]

Hi , my understanding is that with wilder camshafts , which allow the engine to breathe more deeply,there is a critical gas speed into and out of the engine . Below this speed the exhaust gases can get sucked in on the intake stroke due to the overlap between one valve closing and the other opening. This does nothing for combustion! Above this speed everything happens fast enough to make sure all the gases flow correctly and the engine can burn more fuel/air cleanly so produces a lot more power.

no doubt one of the many engine gurus will know better.

 

new owner of 1989 supersprint in brg

[Alex Wong1697456877]

The torque curve rises at higher revs. Torque x rpm = Power. Coming on cam means within coming into the rev band where the engine is delivering more torque. - I think?

 

My engine's power curve does very odd things at about 3000 rpm - after that, the engine power picks up again - I've always felt this as when the engine is on cam.

 

Edited by - Alex Wong on 6 Jan 2005 15:44:15

[rgrigsby]

Hmm I'm sure someone will explain this technically better than me but basically any particular cam has a range of RPM where it is "on cam" and produces the most power/torque, this various depending on the design of the cam.

 

From my xflow background I started of with a 234cam that came "on cam" at around 2.5k but was out of puff by 5.5k, I then moved up to a 244 that came on cam a bit later say 3-3.5k but went ono 6.5k. I finally ended up with a modified 264 which came on cam at around 4.5k but is still going strong at the 8.5k rev limiter!!

 

In the old days when an engine was of cam it could be quite recalcitrant and lumpy although now adays with electronic ECU's this has been elminated pretty much.

 

The exact technical reasons are to do with vavle overlap and things like that which although I have a vague understanding of I probably can't explain!

 

 

Rob G

www.SpeedySeven.com

 

Edited by - rgrigsby on 6 Jan 2005 15:46:42

[Tony Martyr]

Hmmm Alex! I know engines where the torque curve drops at higher revs, so the imprecise term 'coming on cam' isn't simply speed related. The term 'coming on song' used to be used. With engines that are tuned to give maximum power the cams fitted will produce a narrow maximum power band where gas flow etc is at design optimum. It may be between 5 to 8k revs but it will be quite sudden in onset as the torque curve will be steep. You feel it when you hit it!

It is exactly the charactoristic that many users do not want so the VTEC designs and most modern everyday cars are mapped to give flat power curves across a wide rev range

[Alex Wong1697456877]

I was talking 🙆🏻!! What else is new. 😬

[Sheds Moderator]

It's to do with airflow velocities into and out of an engine. Obviously at WOT the air is going through an engine at twice the speed at 6k rpm as it is at 3k rpm. Air has mass and inertia, so cylinders do not fill and empty immediately. This is why things like VTEC have been developed - valve opening and closing at high revs should ideally be different from that at low speeds. In the absence of VTEC engine designers have to decide where the optimal running speed will be. For a big V8 in a Landy, you want lots of grunt low down and you don't care about power at 7500 rpm. The opposite applies in a race car. Therefore in a highly tuned car like a 7 you design the engine to run well between 3 or 4k and the redline. Below this it will run badly and generate low power. This may be termed a "cammy" engine. It is termed "off cam" when you floor the throttle at 1500 rpm, the thing makes a half hearted farting noise and accelerates with all the elan of a fully loaded Nissan Micra. By the same token if you keep your foot down the thing will reach a point where it comes "on cam" (ie the valve timing is correct for the engine speed and hence the gas flows into and out of the engine) and the thing takes off with a howl. The more highly tuned the engine the more marked is this effect.

[Sootysevener]

BOSS has it right but I prefer the less technical approach - - the engine starts to feel much more powerful when it comes on - most tin tops have had the demarcation between on and off smoothed out so its not noticeable but where high performance cams fitted then there is a noticeable difference in performance response rate and the feeling of availability of power at the throttle pedal.

 

As an example the XF with a 234 cam comes on at 3k but a 244 is between 3.75 and 4k

 

David

 

1989 1700XF SS with upgraditis

 

Edited by - Sootysevener on 6 Jan 2005 16:15:57

[Jason Plato]

I'm sorry

 

What I was trying to describe was the sudden increase in performance when the engine hits its sweet spot / comes on cam / picks up its tails / goes like **** ............

 

hope you enjoyed the read

😬

[se7enmad]

Wait a sec?!

 

Coming on cam at 8000 does not make any sense (unless you've got a race screamer engine revving to around 125000). Being on cam till 8000 does.

 

This posting makes me smile a little bit.... most cars I have owned have had pretty radical cams as that is the way we go down this part of the world. at least 285 degrees, 290 to 300 cams for race purposes. A strange 330 Piper thing (that was subsequently withdrawn from the market after a few months only) for my very hairy shopping trolley metro. The cam came on at around 5500, and it revved sweetly beyond 8000 - not bad for a car to go to work with ...

These thingies teach you how to drive and to grin. In a country like the UK where reliability is an issue and you need the engine to get you back home you cannot take so radical an approach.

Anyway, on cam basically is the rev range where the power band is drivable but this again depends on the profile of the cam such that some cams lose revs very quickly (eg during gearshifts) in which case you always have to keep close to max power rpm.

I will not attempt any description involving the techicalities of lift and duration because I know nowt how to explain such matters even though these have been patiently and repeatedly been explained to me over the years. 😬 😬

Having a good look at power plots teaches you a lot in how to launch correctly, how to keep the car on power (on cam), what your shift light should be set at, how much to over rev if you need it etc etc.

I get the impression people think driving styles do not affect the choice of engine tune and cam use from what I read over here.

As a final note, the way with which the cam comes on depends on the cam profile, engine cc's and factors such as the weight of the car. The smaller the cc's of the engine, the more radical a race cam will feel, with more knocking as the cam comes on, as the cc's increase, a cam of similar profile will not be felt as much.

One also needs to adjust the driving style to complement the cam coming on at later rpm's as the cams may get worn if the engine is driven at low revs ie below the power range for the cam.

 

I think this is the best I can explain getting on cam from a drivers point of view. Hope my different perspective to this is not too confusing

 

Antonella

1998 Caterham Classic

my site heremore photos here

 

Edited by - se7enmad on 6 Jan 2005 17:31:52

[dino ferrana]

Antonella Dave is talking about the RST-V8 which is based (losely) around 2 1000cc bike engines. It does rev to 11,000rpm (where is produces 320bhp)!

[se7enmad]

Dino,

 

Thanks for the correction - the cam is still on then at 8000rpm.

The rest of my banter still holds on though!!! 😬

 

Incidentally, as I have already said before I'd love to see a video of the RST with the tacho and speedo showing clearly. My eyesight is pretty bad so maybe I stand corrected if I say the driver keeps the car off cam (...checks videos to confirm 😬) for most of the time. If you notice you will see taht at a particular point, I'm assuming that's the 8000, the car picks up revs incredibly fast and the engine behaves like a beast.

 

for what I can see the unit is pretty much powerful enough below cam, probably very scary on it

 

Antonella

1998 Caterham Classic

my site heremore photos here

[frankyknuckles]

thanks chaps, that sorts it

 

(WOT's a WOTsit WOT? 😬)

 

www.R300.net

[oilyhands]

Extract from cams article on my website..

 

In is imperative when selecting a cam to first understand its nature. A camshaft in a four stroke engine controls the opening and allows the closing of the valves in the cylinder head which in turn allow the fuel/air mixture into the engine prior to combustion, and allow the spent gases out of the engine following combustion. During combustion both valves must be closed to allow the ignited gases to press down on the piston, producing horsepower. Sounds simple doesn't it?

 

In an ideal world, the inlet valve would open when the piston is at Top Dead Centre, following the exhaust stroke, would remain open until the piston reached Bottom Dead Centre after the induction stroke, and then close for the compression stroke. The exhaust valve would then open at Bottom Dead Centre following the power stroke and remain open until Top Dead Centre of the exhaust stroke, at which time the whole cycle would start again.

 

The period for which the inlet valve is open is measured in degrees of crank revolution and is known as inlet duration. Again in an ideal world this would always be 180 degrees. Similarly the period for which the exhaust valve is open is measured identically and is know as exhaust duration (collectively both are simply 'duration'). Again in an ideal world free from the laws of physics, the duration would be 180 degrees.

 

In practice, because the fuel/air mixture inducted into the engine and the exhaust gases expelled have mass and therefore inertia, and the valves and valve train have mass, things do not happen instantaneously. If the inlet valve were to close at BDC of the inlet stroke the cylinder would be in a position of partial vacuum. In fact keeping the inlet valve open PAST BDC allows more mixture into the cylinder, even though the piston is at this time rising to compress the mixture. If the exhaust valve were to close at TDC on the exhaust stroke, spent gases would still remain in the cylinder, so the exhaust valve is left open PAST TDC even though the piston is now falling to induct new mixture.

 

The faster the engine runs, the more it is sensitive to the inertia of the incoming and outgoing gases. The duration of the camshaft in a normal production engine is usually around 255-265 degrees of crankshaft rotation, that is 80 degrees more than the 180 degrees of our ideal-world engine. If we ignore the effects of lift, it follows that if an engine needs to rev higher, the valves need to be open for longer in order to sustain power output.

 

Cam lift also has an effect on engine power, but less on engine characteristics. It follows that the higher the valve is lifted off its seat the more flow is allowed into/out of the cylinder, up to a point. The major limiting factors for valve lift are mechanical, in order to lift higher for any given duration the valve has to be lifted faster. The weight and inertia of the valve train places severe mechanical limitations on the speed at which this valve acceleration can happen without knocking nine bells out of the cam, the followers and all. If you accelerate the valve too quickly, it will lose contact with the cam follower and lobe, and then crash back down onto it and bounce when the spring exerts its authority. This will very quickly knacker the cam, follower and associated gubbins, it may also cause valve/piston contact, coil binding and general unholy nastiness.

 

Generally speaking 'high-lift' cams give an increase in torque across the rev range rather than moving the power band up or down. They also give good emissions (which high overlap cams do not). In the early days valve lift on pinto cams generally tended to be less (although still more than standard) with duration being longer, more recently, the tendency has been to give more lift and less duration so that emissions are more acceptable. Careful design of the lifting ramp of the cam and improvements in metallurgy have allowed these higher lift cams to operate without breaking followers or producing excessive wear.

 

You can see from the previous paragraphs that there is a time during the change from exhaust stroke to induction stroke when both the inlet valve and exhaust valve are open together, this period is known as 'overlap' and is measured in degrees of crankshaft rotation. Having both valves open at the same time would seem on the face of it to be a recipe for disaster. However the dynamics of the exhaust gases flowing out of the exhaust valve and their considerable momentum coupled with the momentum of the charge in the inlet port actually help to drag in mixture via the inlet valve. The incoming mixture also helps to push out the exhaust gases, this process is known as scavenging. Generally speaking, the longer this period of overlap, the less tractable the cam is at lower RPM and the hotter the cam is at the top end of the RPM range.

 

Cams with lots of overlap generally give good engine power at higher RPM, but give that irksome low RPM intractability known as 'off-cam' where the engine growls, spits and jumps, together with a distinct 'coming-on-cam' (very messy) at a particular RPM. This coming on cam feeling results from the harmonics of the exhaust flow reaching a critical point where the exhaust gases stop trying to exit via the inlet port (reversion), and do their proper job of exiting via the exhaust, and promoting scavenging. At low RPM when the engine is 'off-cam', the exhaust gases cause pulses in the inlet tract which lead to a phenomenon called 'stand-off' where inlet mixture is bounced out of the back of the carburettors and hangs in a mist around the inlet trumpets /filters. This contributes to the 'off-cam' feeling as the mixture then fluctuates between too rich and correct and is mixed with spent exhaust gases.

 

Generally speaking, when the engine is off-cam, light throttle openings minimise stand-off; because the butterflies of the carburettors are nearly closed, they minimise the exhaust pulses affect, also smaller throttle openings produce less cylinder filling and therefore less exhaust gases to give those troublesome pulses. Long exhaust primaries can also help minimise standoff by keeping the exhaust inertia from the previous exhaust stroke, and by preventing interference from other cylinders. Mapped ignition and/ or programmed injection can considerably improve the engines tractability at low RPM, as they minimise stand-off, and injection does not rely on induction gas speed to drag fuel in as it is directly injected.

 

Obviously camshaft choice is not the only thing which determines engine output but is the thing which most significantly changes the nature and characteristics of the engines power delivery. A race cam with STD carb, head and manifolds will give appalling driveability, and the top end power expected will evaporate due to restrictions in induction and exhaust. It is important to have an idea of how you want your engine to behave, whether you can live with the engine dropping 'off-cam' at the wrong moment before choosing a camshaft. Note also that as the power band moves up the rev range, it also narrows considerably and also pees away your precious fuel. In a car the weight of a 7, the loss of tractability experienced with bigger cams is less of a problem, but a 'hot-cam' can make driving in traffic extremely wearing.

 

You can't have it both ways, if you want big power at the top end it will always be at the expense of bottom end power and tractability. The answer in the long term is of course variable valve timing which is currently seen in Hondas redoubtable VTEC screamer and Rovers VVC 'K' series engine. These give short duration low overlap timings at low RPM and increase these as RPM increases. They are the mythical holy-grail of automotive technology, the idea of variable valve timing was first put forward by Lanchester , but he did not have the available technology to make it happen.

 

Oily

 

 

 

Edited by - oilyhands on 7 Jan 2005 05:34:47

[Paul Ranson]

'on cam' means 'wahey'.

 

Paul

[Mike Bees]

And if you're on List 1A tyres it means "Waheyyyyyyywwwwwwwwwhhooooooooooooops!!!"

 

Mike

[Jason Plato]

And if your in the RST V8 it means ...... whoooooooooooo shiiiiiiiiiiiiiiiiiiiiiiiiiiiiiiit hehehehehehehe 😬 😬 😬 😬 😬 😬 😬 😬

[Keith Brown]

It was nice to read -Oilyhands- contribution. Thank you. One question answered raises another-

why do Caterham not use VVC in their racing engines?

[blade_runner]

mine comes on cam at around 8k and screams on till 12.2k

 

😬 😬 😬

 

'Pinky Pics' here

[wag]

I understand that BMW have started fitting higher voltage (50V ?) electrical systems so that they have enough poke to operate valves by solenoid. The possibilities there might be interesting.

[oilyhands]

The VVC system, while good in many respects is flawed in many areas, especially in terms of lift and valve acceleration. The mechs are also on the edge of reliability in a road engine at the lowly RPMs used. Stretching them beyond that envelope generally results in failure. They vcertainly wouldnt cope with 12mm lift and much stronger springing.

 

Oily