what does the camshaft figure of 260 degrees mean?

[mrLEE30]

just was wondering if a cam is said to be 260 degrees what does this mean?

i do understand the concept of what a cam does when it opens each valve and the inlet/oulet valve overlap opening times, i also understand the concept of extraction pressures and inlet vaccum..but what does the 260 bit refer to? the overlap? the inlet or exhuast valve open time?

and on which cylinder would it be referenced from or does it not matter?

Cheers peeps!!

mlree

[mrLEE30]

does no one know or is no one bothered!!

mrlee

[Brianmoooore]

The 260 degree bit is the CRANK angle difference between when a valve just begins to open to just before it shuts.

[Turbo-Brown]

It's the angle through which the crank rotates with the valve off it's seat.

So the valve cracks off it's seat, the crank rotates 260 degrees and just as it completes the last bit of rotation, the valve sits down again.

So you see cams for say the M20 quoted as 285/260 (just made that up) which refers to the inlet duration and the exhaust duration.

[Turbo-Brown]

beat me to it Brian

Anyway, all the cylinders should have the same cam profiles / characteristics, unless you've got something like an A-Series engine with a scatter cam, they might be different (just a guess though)

[mrLEE30]

cheers Brian,

ok that does explain the large degree figure, but is it for both inlet and outlet valves if so do they remain open for the same amount of time as each other??

mrlee

[Brianmoooore]

Like waiting for a bus, isn't it. None for hours, then two at once!

[mrLEE30]

and as i typed my next question trubo-brown answered it!! cheers lads

mrlee

[DaveD]

in the same vein what is the effect of the higher lift cams ,,,does it actually increase the bhp ,,if so how?

[mrLEE30]

high lift cams will open the valve more or longer thus more air /fuel into the combustion chamber thus more power... the trade off is the car is then only happy at high RPM where the effects of the exhaust and combustion chamber vaccum help the gasses to flow into or out of the cylinders..this is why poeple with wild cams have lumpy idle, and the reason an F1 car idles at 6-7000rpm!!

mrlee

[Morat]

higher lift means the valves open further to increase gas flow. I'd imagine you need to be very careful that you don't end up smacking the valves with a piston.

[mrLEE30]

was talkin a load of rubbish before for anyone that read my deleted bit!!!


Also worth noting that the more you lift a cam or the more aggressive the cam (i.e. the speed it lifts the valve) call for stiffer springs or else they start bouncing at higher RPM, i think Ducati avoid this with hydraulic/electric cam lifters.... come to think of it i would iamgine F! cars have the same too which kind of sh1ts on my earlier comment!!!

[Brianmoooore]

Long duration cams have to be matched up with inlet and exhaust systems as well, not fitted in isolation, otherwise you'll end up with exhaust gasses from one cylinder ending up in another one.

[Brianmoooore]

the cam can only hit the piston when its down, a high lift cam will take the vlave further away from the piston!!!

Think you're a bit confused here, mrLEE. Both valves are indeed open at TDC and will hit the piston with high lift cams if suitable machining isn't done.

[mrLEE30]

yes you are right Brian i am in upside down mode where in my world the valves open the ohter way, fortunately in real life this is not the case!!!

I am feeling better now!!

[kam-325i]

Ive just googled "Scatter Cams".........

My head hurts now........

[MrDazzle]

higher lift means the valves open further to increase gas flow. I'd imagine you need to be very careful that you don't end up smacking the valves with a piston.

Which is where pocketed pistons come in...

[gooner1]

Can you explain the pocketed piston bit mate, s l o w l y.

[kam-325i]

Basically, the piston has cut outs/pockets to accomodate the valves.....

[Brianmoooore]

was talkin a load of rubbish before for anyone that read my deleted bit!!!

Preserved for eternity in the quote in my reply though!

[Turbo-Brown]

Think Ducati have mechanical valve closers which force the valve to close in a similar way to the rockers in an M20 say, which force the valves to open, in conjunction with a conventional spring.

F1 engines use pneumatic valve closers to prevent float at the stratospheric crank speeds they achieve, although I'd guess at there being some conventional springs in there just to stop the valves falling open when the engine's not running.

Hydraulic lifters are typically used to automatically and dynamically adjust the valve clearances in an engine. However, the higher lift/more agressive the cam, the less suitable hydraulic lifters become as the oil in them starts to froth up so they stop doing that they should as the bubbles are compressable. You'll often see people switching to solid lifters when they make the move to fancy cams.

[mrLEE30]

mrLEE30 wrote:
was talkin a load of rubbish before for anyone that read my deleted bit!!!

Preserved for eternity in the quote in my reply though!

yes i saw that, i was too slow!!!

oh well it was my thread and i know nothing about cams obviously... but i do now!!

cheers all

mrlee

[DaveD]

WELL I know a lot more now ...thank you all!!! but to continue a bit further which cam would you luminaries recommend for the 325

[orangecurry]

http://www.minimania.com/ArticleV.cfm?DisplayID=1272

scatter cams!

I have four 1275cc A-series engines, one of them is a 1332cc overbore, but I think for the racing 'series' I was in the cam had to be standard..... it's still well-pokey

[Chris-W]

Think Ducati have mechanical valve closers which force the valve to close in a similar way to the rockers in an M20 say, which force the valves to open, in conjunction with a conventional spring.

Quite right - called 'Desmodromic'.

[MrDazzle]

Aye, and invented by a british guy IIRC...

They have an ecentric cirlce type cam, and two sets of cam followers. Has the advantage of being easier to turn, but more important does away with valve float and bounce (where the valve doesn't go up and down quick enough to keep up with the cam).