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DynoMike
Barcelona


Joined: 25 May 2012
Posts: 1286
Location: The Cotswolds

2003 Porsche 996 Turbo

PostPosted: Sat Jul 15, 2017 4:31 pm    Post subject: Camshaft basics. Warning, long and boring..... Reply with quote

Camshaft basics.

A few members have contacted me recently regarding performance cams for their cars, asking what the process involves. Here is a brief look at what goes into selecting cam profiles for a given application.

A good camshaft profile is one which satisfies many different criteria.
The cam must allow the engine to pull from the desired rpm at the bottom end through to the maximum desired rpm up at the top end of proceedings. This can often be difficult to achieve. A good starting point is to look at the current ability of the stock camshaft with its current timing (the placement of valve events relative to crankshaft position expressed in crankshaft degrees), working forward from this point. Porsche has the clever Variocam and Variocam plus systems, which advance and retard cam timing relative to rpm and load request, along with altering the cam profile entirely on the ‘plus’. All good stuff but the system has implications on profile design as we will see later.

Assuming we are dealing with engines that are currently in a relatively high state of tune, say 90hp/litre and up, the final design will be matched to the total airflow characteristics of the intake and exhaust systems, bearing in mind that the factory will have already run through this process at the design stage. So the system in place will already be a well-balanced one, making the stock cams a good starting point. In the case where engine size is increased, or airflow characteristics are improved, the cams will need to be altered to match the new specifications.

A larger engine can tolerate a larger cam with little or no ill-effect on low end running, but with much better cylinder filling at high rpm, the nett result of which is no drop off in power as the cam runs out of puff, just a climbing power curve. As mentioned in another post, any time cylinder volumes increase underneath a standard head, the mean port velocities (mpv) will raise at any given rpm. There will come a point where the original cam and port flow will reach its maximum, after which the torque/cylinder filling will drop off a cliff. To counter the fact that the ports may now be too small for the new cylinder size, a larger cam profile can be installed, giving the mixture more time to fill the cylinder. This is a good halfway-house arrangement to stop the inevitable drop off in toque/cylinder filling at high rpm, but not the best solution.

The answer is to evaluate the standard ports and intake system as a whole, as opening port runners to optimise the mpv will lead to greater area under the toque curve IF the rest of the intake runners/manifold/plenum/throttle body and air box allow a greater throughput of air. Put simply, it is no good uncorking the heads if there is a restriction upstream, or down for that matter.

Getting back to the cam profile design, there are many steps to producing a reliable and high performing camshaft. Firstly, all of the components in the valvetrain are weighed so that accelerative forces can be calculated. The spring is also weighed, the total number of coils recorded along with ‘active’coils (those that physically move when the valve motion takes place). The springs are evaluated for seat load, the value in lbs or newtons that the spring exerts statically between the spring retainer and cylinder head, along with the ‘over the nose’ forces at full lift, then the rate is calculated, expressed as lbsf/inch or N/mm generally. Springs are complex things that exhibit natural frequencies, along with ‘excitation’ frequencies. The latter is the frequency imparted by the profile design.

Once the cam designer has the necessary data he will begin designing the profile to match the engine builder’s request and also balancing the airflow characteristics. For example, if a head flows huge amounts of air at low valve lifts but brick-walls at 8mm of lift, the designer will optimise the lift profile below that number. Cylinder heads often reverse flow at high lift numbers in standard form, ie, go backwards and flow less than they did at lower lifts. Some do the opposite, usually as a result of shrouding within a chamber.

There are four main areas he will concentrate on; Displacement (lift), velocity (rate of change of displacement with respect to time), acceleration (rate of change of velocity with respect to time) and the wonderfully named ‘jerk’ (rate of change of acceleration with respect to time). There is a fifth term known as ‘quirk’ but nobody utilises this as far as I know.

Of those terms, acceleration and jerk are the ones that have the ability to be destructive. Jerk in particular can put a lot of energy into a spring, causing issues such as coil breakage, whereas high acceleration values can lead to ‘float’ (separation of components) and high stab torques being fed back to the chain drive. There is a way to envisage the effect jerk has; most of us will be familiar with the little ‘kick-back’ we get when braking to a standstill in a road car, this is as close an analogy as I can think of to describe what it is, the sudden rate of change of deceleration gives this feeling. Variocam can complicate this aspect of the design, as it has the ability to accelerate the already accelerating valvetrain, so some leeway will be built into the numbers to allow for this.

Many designers will use Fourier Analysis of the spring package, which looks at the excitation frequencies imparted by the profile to the spring, the idea being to use a profile which has low excitation values to enable the spring to have a long life, as in general, the spring is undamped. If anybody fancies looking at a really well designed valetrain at speed, search BMW S1000RR valvetrain in youtube, it is mind blowing! I have some software by Prof Blair and associates which demonstrates the fourier process but do not have the permission to publish it.

The designer will asses the stiffness of the components before embarking on the design, as weak components or design will require a much less aggressive profile to cater for those inadequacies. Finger followers are the stiffest components, direct acting are next followed by wobbly old pushrod and rocker! Which is exactly why the 991.2 GT3 runs finger followers, made even better than before by eliminating the hydraulic element of the 991.1 version. Racing does improve the breed it seems….

Hopefully I haven’t missed anything, though this was typed in a hurry, if I have, just jump in and let me know.

Mike
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ragpicker
Paul Ricard


Joined: 14 Apr 2013
Posts: 3122
Location: North East England


PostPosted: Sat Jul 15, 2017 7:41 pm    Post subject: Reply with quote

worship

Very interesting as every old boy. The youtube vid of the camshafts at 14krpm is unbelievable!

Is there a point then where a camshaft will rotate so quickly (or will have such a profile) that the spring cannot follow its profile and close the valve in time? Presumably at this point they would use some sort of hydraulic valve?

I'd imagine you'd have to be a very clever chappie indeed to come up with a better camshaft than those already installed in the variocam plus engines, considering you would have to factor in the alterations the solenoids can make whilst running...

Mindbending stuff...
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ragpicker
Paul Ricard


Joined: 14 Apr 2013
Posts: 3122
Location: North East England


PostPosted: Sat Jul 15, 2017 8:10 pm    Post subject: Reply with quote

Bit of info on performance cylinder heads, specifically F1 heads.

Amazing thing the internet, the amount of info out there is incredible... Its just finding it!

http://www.f1technical.net/forum/viewtopic.php?t=15385
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DynoMike
Barcelona


Joined: 25 May 2012
Posts: 1286
Location: The Cotswolds

2003 Porsche 996 Turbo

PostPosted: Sat Jul 15, 2017 9:14 pm    Post subject: Reply with quote

ragpicker wrote:
worship

Very interesting as every old boy. The youtube vid of the camshafts at 14krpm is unbelievable!

Is there a point then where a camshaft will rotate so quickly (or will have such a profile) that the spring cannot follow its profile and close the valve in time? Presumably at this point they would use some sort of hydraulic valve?

I'd imagine you'd have to be a very clever chappie indeed to come up with a better camshaft than those already installed in the variocam plus engines, considering you would have to factor in the alterations the solenoids can make whilst running...

Mindbending stuff...


Cheers Andrew!

I thought you may have been interested in this sort of thing, you being a 'fiddler' Embarassed

Yes, valve springs are the key to performance, hence the pneumatic versions shown in your article. That is quite an old engine design, probably around 2003 ish, poppopbangbang will be better placed to answer that than me, but the bucket valve operation dates it. Everything is finger follower these days as you can run jerk numbers in the region of 60% higher than with buckets, whilst still retaining reliability.

I was told by an F1 cam designer that the V8s were running 18mm lift and only 280 degrees of crank duration -something utterly incomprehensible to me. All the time the engine was pulling 18000 rpm, meaning the valve was opening to from closed-open-closed 300 times a second! Surprised

Yeah, the bike head is pretty amazing to watch, isn't it? At no point does the spring separate from the cap, at least to the naked eye. Contrast that with some of the Nascar valve train videos on youtube, they look as though they are about to fall apart at every cycle Grin
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Chief
Indianapolis


Joined: 16 Sep 2010
Posts: 2490
Location: The Middle lands


PostPosted: Sat Jul 15, 2017 9:42 pm    Post subject: Reply with quote

A bit above my pay grade there Mike, although I kinda followed it.
I was always impressed Ducati's desmo valve gear with no spring at all.
Eliminating the weak point and potential of the valve to bounce.
Very interesting pal, thanks for posting Thumb
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Marky911
Suzuka


Joined: 04 Jun 2009
Posts: 1040



PostPosted: Mon Jul 17, 2017 4:00 pm    Post subject: Reply with quote

I'm with Chief. Anything beyond the basics is a bit over my head but kudos to you guys at the sharp end of things and thanks for taking the time to write that.

Very interesting, especially the varying parameters that high rpm's introduce.
As for Variocam etc well that's just black magic.

Good stuff though. Thumb
 
  
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Jamesx19
Monza


Joined: 10 Jul 2015
Posts: 195
Location: Brighton


PostPosted: Mon Jul 17, 2017 8:41 pm    Post subject: Reply with quote

Interesting read Mike.

Topical for those moving up engine capacity. ie 3.4 to 3.7 and 3.6 to 3.9 litres. Unless changing to a more radical cam, they are going to be moving the (increased?) peak torque down the rpm range possibly?

Though I imagine it will depend as you say, on exhaust headers chosen, flow restrictions in the head, and intake etc......

I was thinking you could "simply" readjust the standard cams inlet and exhaust Lobe Center Angle with a vernier cam wheel and get an improvement? I am assuming tightening up the angle will give more overlap but do you reckon that would help high rev cylinder filling on a bigger engine?

Though then I looked into it a bit further....., not only is the cam wheel on the inside of the engine, and a ***** to get to, but the Variocam largely does that job anyway. Though maybe using a "larger" tensioner pad would advance the inlet cam more?

Lastly, designing your own profile sounds fraught with expense. Development costs I imagine could spiral out of all control! Especially if there is already an "off the shelf" from Schrick or similar?

Good stuff anyway, thanks Mike

Cheers James
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bazhart
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Joined: 20 May 2009
Posts: 815
Location: Bolton Lancashire


PostPosted: Tue Jul 18, 2017 3:45 pm    Post subject: Reply with quote

The information from Dyno mike demonstrates that he knows his stuff when it comes to camshafts and engine breathing (something that is rare on here) and he has encountered a problem I frequently have - how to explain things without it all becoming too long. Well done Mike and respect to you.

I therefore don't want to disagree directly except can I just make the point that increasing the capacity will move the bottom end of the torque curve slightly downwards but will also increase the top end because although the specific time area is lower - the piston is a bigger diameter and because the force that creates torque is a function of the force on the piston and that in turn is a function of the burn pressure acting on the piston area - an oversized engine created with bigger pistons makes up for any breathing deficit at higher revs via the increase in piston area and force.

Although different cams may well improve top end breathing, unfortunately these engines are connected to basically an overgeared gearbox and that means that increasing the revs for peak torque before changing up will increase the rev drop between gear changes and can decrease the area under a rear wheel torque graph - basically reducing the benefit - making little difference.

You are absolutely right that the whole engine is balanced for maximum torque at a specific rev band and it can be costly to try and change enough things to benefit from camshaft alterations.

In contrast simply increasing the capacity both spreads the torque out more evenly and also slightly increases top end output enabling the car to be more flexible, faster (as acceleration is more proportional to torque than BHP) and nicer to drive without the need to drop down gears as often and making throttle response more enjoyable.

For my money camshaft changes are essential if you want more performance when the capacity is not increased but to fully exploit should come with different gear ratios (that are usually out of the question).

Thanks to variable cam timing and lift - we have found that while camshaft changes we have tried shifted the torque curve a bit it did not make this type of car and engine any faster - but we will not be as experienced as you and stand to be corrected.

I have replied to some interested parties privately to explain this in more detail but it has taken a lot of pages and I will await their response before saying any more.

Have you any data on camshaft changes to these models?

Keep up the good work.

Baz
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DynoMike
Barcelona


Joined: 25 May 2012
Posts: 1286
Location: The Cotswolds

2003 Porsche 996 Turbo

PostPosted: Tue Jul 18, 2017 8:00 pm    Post subject: Reply with quote

Thanks very much for the kind words Baz, I do indeed have sympathy with you and your eloquent replies, it is harder than it looks as I have just found out!

I would also like to say that I have huge admiration for both you as a person and Hartech as a business, something I hope that one of the enquirers made you aware of regarding the camshaft subject? I would also like to personally thank you for the enormous benefit your posts make to this forum as a whole.

I agree to a point that the overall power will be lifted, but the force being supplied to the greater piston area is a result of volumetric efficiency and cylinder filling at that point (note that one is not the same as the other, here). Without doubt the torque will increase at low rpm for the reasons that you have alluded to, but also to a little realised phenomena that occurs when a larger bore is utilised with the same sized valves as before, namely a relative lack of flow reversal due to the relative valve area being too small for the larger piston. This is obviously further negated by the use of variocam, as overlap can be cut down at very low rpm.

However, at high rpm, the engine will most likely suffer a rapid drop off of cylinder filling and hence torque due to the small valves and ports; this is the reason why I spoke of cam profile changes to effectively make the peak torque and peak power points coincide with the standard engine. The idea is to return the relative peaks back to the position of the 3.4 or 3.6, through increasing flow and ‘flow-area-velocity’ by intelligent modification of the port runners and camshaft profiles. The idea behind this is to maintain the current engine’s characteristics, just raised by the capacity increase. You are right regarding wheel torque but I feel that by sensible and methodical modifications being made, the enhanced low-end torque will be maintained, negating any torque troughs.


The situation with VE and cylinder filling being different is this; namely through-flow on valve overlap. The VE is still high but actual cylinder filling can be less than the VE suggests, but BMEP calculations will reveal this trait if plotted against rpm. As a rule of thumb, torque follows cylinder filling, less frictional losses, so if the torque curve is plotted for a standard engine vs. a larger capacity one, the torque will drop off largely where the cylinders stop filling, but the whole torque curve will probably ‘droop’ with rpm against the standard engine, all other things being equal.


Ideally, larger intake and exhaust valves should be installed, but there are tricks to enable a smaller valve to continue breathing well past their theoretical ‘curtain area’ of 0.25D (the lift at which the developed area is equal to the valve head diameter area), but it remains to be seen if this can actually work on the 996 engine.

As we agree that the engine is a balanced system, some exhaust work would also be required if these targets are to be met. I also agree that piston area is an oft overlooked part of engine development but it usually goes hand in hand with a pro-rata increase in valve area. Time will tell if the performance targets can be met with standard valves, modified breathing capabilities and revised cam profiles.

As ever, I thoroughly enjoy reading your thoughts and explanations on the forum.

Mike
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