6 replies to this topic

[Steve-O 2014]

Was looking at my 998 cam and mg metro cam and noticed the lobes are more pointed on the stock 998 cam and rounded on the mg cam.

Is this normal / right as I was watching Wheeler dealers this morning and Ed changed the cam in a engine, he said the lobes are more of a point on the uprated cam over stock to give a higher lift and more power

[Alex_B]

Cam characteristics are split into two categories: duration and lift, where lift is dictated by the difference in height between the lobe height and the base circle and duration is how long the cam follower is sitting on the lobe before being back on the base circle. Different cams have different lifts and durations and both can be optimised to suit your engine (within reason as to what's available to buy). It's not quite as simple as that but that's the basics of it anyway.

Lots of lift but small duration will mean the valve opens very quickly but opens a lot then closes quickly where as a longer duration cam with less lift will open less but for longer. Depending on a number of variables such as your intake charge speed will dictate what you want the valve to be doing.

[Shooter63]

If you think about what a camshaft does, a cam with more vertical sides with a flatter top means the valve is opening quicker and staying open ( at or around max lift ) longer, hence a performance cam looks more like the mg one. Cam design includes many factors regarding the above with limits.

Ed China has to try and explain technical things very simply for the masses, sometimes they don't seem to come over very well.

Shooter

[Shooter63]

 4cams.jpg   25.83K   25 downloads

Just to show you what people mean here is a picture showing std to race cams ( left to right)
Not mini cams but believe it or not harley davidson cams, you can see the difference in ramps and the amount of time the valve will stay at or around full lift.

Shooter

[ACDodd]

That is normal as they are both virtually the same lift. Giving more duration to the lobe rounds the top off. It also increases overlap lift, if the LSA remains the same, which it does on these cams.
There is more to cams than duration and lift. You also need to think about timing angles, lift at split overlap, and most critically, the relationship of overlap lift to total lobe lift giving a rate of opening.

Ac

Edited by ACDodd, 10 September 2016 - 11:21 PM.

[Ethel]

Higher lift will only give more power if the ports and valves can make use of it. Opening the valves more quickly means the ports and valves reach their full flow potential sooner and for longer, but you also have to consider where the piston is in the stroke to generate that flow as well as the inertia in the induction and exhaust. The A Series isn't a great breather, the valves are limited by the narrow bore to stroke ratio and then there's the compact 5 port configuration.Lifting a valve by half its radius will give as big an opening around its circumference, or "skirt" as if it was totally removed. Since the valve needs to have a stem, the actual maximum is that bit less, even if you could eliminate all the other flow losses. For a, generous, 36mm A Series inlet valve that gives 350 thou of maximum useful valve lift. The MG cam gives about 300 thou with standard rockers. The chances that an A series inlet tract makes the 86% efficiency for that to be a concern are a tad slim!

 

The cam pays dividends in the same way the big valve's increased circumference does: getting the inlet to flow more air as soon as possible.

[Steve-O 2014]

Thanks just when ed said that I went and looked at mine side by side and thought "this doesn't match what Ed said..."

I get it now, basically the rounder the top the more duration the valve will be open