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Part 1

Jebu said:
hi guys

can somebody pls explain to me how exactly does fitting cams increase torque?
and what kind of cams are good for RB engines (as in manufacturers etc) .. thinking of putting one in!
would it be advisable?

thx!
I asked a similar question on the Supra Forum and got these posts back...so I hope this helps

Camshaft Science

From it the two post of most interest were from:

Chris Wilson -

Any attempt to even skim the surface here is doomed to failure, whilst i have a decent grasp of cam design and effect you really need a book on the subject, of which there are plenty. I know you won't take this the wrong way when I say your obvious desire to understand your engine and how mods work needs addressing by spending some cash on a few decent books, Ricardo wrote some excellent stuff covering cam design, I'll try and dig up the title.

Basically with a twin cam engine altering cam timing within piston to valve clearnace and "sensible" limits will move the power up and down the rev band, with certain "sweet spots" where torque or BHP is maximized. Changing cam lift and duration will change the whole characteristics of the engine. Being turbo'ed with small exhaust housings and the need to be road usable severely constrains what cams will work in a desirable manner. Without a fully programmable ECU it will be very hard / impossible to get the full benefits of cam or cam timing changes though.

and.....
 

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Part 2

And from Darren Blake - a powertrain engineer at Lotus Cars.

I'm not too sure whether you can put a cam's characteristics just down to its profile alone. How the engine performs will be down to the profile of both cams, and their timing.

Typically, you want as much air in the cylinder as possible on any engine, let alone a sporty one. Assuming that the valve diameter remains constant, the only way to do this is to either make it open further (increased lift) or make it open longer (increased duration). If we make another assumption: that we won't change the valve springs, then the maximum valve acceleration is also fixed. You can still just make the valve open longer by having the same opening and closing ramps and a more blunt profile, but if you want more lift then you are also forced into making the duration longer so that the lift takes place over a longer timespan.

The choice of increased lift or duration (or both) will depend on how well your valve flows air at a given lift. If we imagine for a second that you could freely increase the valve lift you would find that beyond a certain point it ceases to make much of a difference to the overall flow. This is a mathematical function of the lift and the diameter of the valve.

So if increased lift has a top limit which is governed by the valve diameter (which we've assume we want to keep fixed) then the only way to get more air into the engine is to increase the duration.

So why not have as wide a profile as possible and be done with it? Well you could, but there's all sorts of ther things to take into account like the other valve, the motion of the piston, the valve dynamics, intake and exhaust pipe tuning, and (in a production engine) stuff like fuel economy and emissions.

For one thing a long cam duration means lots of valve overlap (when both intake and exhaust cams are open at once). This is good for "blow down" - evacuating all the inert exhaust gases from the cylinder by chasing them out with a fresh intake charge. This happens at the end of the exhaust stroke, which is also the start of the induction stroke. The piston is at TDC here, and the exhaust valve is being held open for blow down and the intake valve is starting to admit the fresh charge. Since there is a mechanical link between the valves and the pistons, the extent to which you can do this effectively is limited by the minimum clearance between valve and piston: i.e you can't hold the exhaust valve open very much while the piston passes TDC (you can with valve pockets but these have their own foibles and I'm going to ignore them). Additionally, in an NA engine blow down only really works at WOT. Before WOT the low intake manifold pressure may cause the exhaust gas to be sucked backwards, causing poor performance at part throttle and idle (which is why race engines don't idle worth a damn - and you thought all that throttle blipping was for show! )

The other penalty is that when both valves are open at WOT, some unburnt fuel will pass right through and out into the exhaust, which is bad for fuel economy.

In an NA engine all of this will be interacting with pressure pulses in the intake and exhaust pipes which as Chris says will create "sweet spots" in the torque curve.

Finally, something which I don't really understand is that by precise timing of the valve events you can reduce the "pumping losses" - the dead work that the engine has to do to get the air into and the exhaust out of the engine.

You might be able to get more torque by opening up wide and then slamming the intake valve shut at BDC, but it can be shown that early trapping of the intake charge does not have as much effect on overall efficiency as other factors, plus it will decrease the overall valve open time (hence limit the airflow) plus it will be beyond the capability of the valve spring. In actual fact, late intake valve closing can just as beneficial for the reasons stated above.

This pretty much explains why variable valve timing (VVT) has caught on in such a big way. Typically, you have a fixed "cooking" profile, but retard it for minimal overlap and late inlet valve closing (pumping losses - don't ask) at low speed / load (and coincidentally during the emissions cycles), but at a certain speed / load point you advance it. You dont get any more flow, but maybe the intake tuning works better with that particular timing above that speed, plus you get more overlap.

First generation cam timing "phasers" were two position with nothing in between, so instead of a single big compromise on valve timing you now have two compromises to choose between depending on what the engine is doing. The current generation of phasers are infinitley variable with engine speed.

Honda take a different approach with their VTEC system, which has two completely different cam profiles to choose between - a "cooking" profile for pootling around town and effectively a race profile with a longer duration and higher lift for WOT stuff.

Even this technology is reaching its limits, which is why so much effort is going into electro-mechanical valvetrains. Once freed from the confines of a direct mechanical link between crank and valve (however "variable" phasers and VTEC attempt to make it) the door is open to maximum valve opening, and infinitely variable profiles and timings.

You may have noticed that a lot of this applies to NA engines. I would have thought that life on a turbo was much simpler because the sensitivty to intake and exhaust tuning was removed when on boost (part throttle and idle performance would need to be retained in the stock engine setup though).

Not really sure if I've answered Alex's question or just blathered on for while, and it didn't all come out of my head. I have a reference book right here I agree that a deep meaningful discussion about it here will be very difficult (I've just paraphrased virtually a whole chapter from one book plus a chunk of what I can remember from another).
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Cheers

Alex H
 
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