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Old 24th April 2005, 21:38   #1
ernie
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How does retarding cam timing make more power?

Hey guys,

Just like the title says, I am wondering how changing the cam timing makes more power and how it can can induce detonation with higher intake temps. Thanks

ernie
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Old 24th April 2005, 22:20   #2
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Re: How does retarding cam timing make more power?

This is quite complex and I could ramble for hours.... Basically cam timing starts and stops the inlet and exhaust valve opening and closing - No surprises there !

Now think about the inlet inertia at higher rpm, put your hand anwhere near the plenum and you can feel the airflow, meaning the plenum pressure is working in your favour. It's the tap and water effect. Now look at a single cylinder model (It's easier to contemplate)

Inlet valve opens to let the gas in, if the valve waits until the piston is at TDC (top dead centre) then it's missed a trick - Opening before this will allow the inlet pressure to flow into the cylinder as the piston is still travelling up the bore AND leaving the exhaust open for a little will mean the inlet flow will help push the spent exhaust gases out too. Sounds great ? Yep And as the revs rise you can open the valve earlier and earlier ideally just at the point that the inlet pressure is above the pressure created by the piston moving up the bore.

So why not open bloody early all the time ? Well what happens when the piston is coming up the bore and the revs are low (inlet track pressure is low) ? The gas is pushed by the piston and exhaust pressure the wrong bloody way - Yuk ! Remember 'lumpy' cams at idle ? Well thats whats happening - You can hear it

I know I havent answered your question yet - Wait for it

So now lets look at the inlet closing....... OK, the piston has reached the bottom of the bore 'sucking' the inlet charge in then it starts to travel up the bore on compression so we shut the inlet valve right ? - Wrong... The pistons slow at starting to come up the bore and the air and petrol are still flowing in just fine... More air with the right fuel = more power so lets keep the valve open for a while.. At some point the piston coming up the bore is creating more pressure than the inlet pressure trying to puch air and petrol in so it's time to close the inlet valve or the piston will just push air back out... Yuk again....

No prizes for guessing that as the revs rise, inlet pressure rises and you want to leave the inlet closing later and later...

Hang on why on earth does retarding a cam add more top end power when it's great for later inlet closing but buggers up early opening ? YES it does bugger up ear;y opeining and in an ideal world you would open earlier AND close later - Just like a Race cam with long duration opening earlier and closing later, great for high revs - dreadful for low revs....

So WHY does retarding a cam add more top end ? Well it's simply because inlet closing has more impact on power than inlet opening at higher rpm - it's that simple and visa versa at mid rpm so if the cam is a little advanced more midrange and retard for top end power (interestingly with a single cam engine the most important event for high end power is still the Inlet closing point because you have also mucked around with the exhaust timing- Which I won't go into )

I haven't discussed inlet pulses, hermholts resonance inplenums and inlet charge speed, turbulance etc, etc, life unfortunately gets complex if you want to undestand all of the other things going on and having an influence...

I hope this makes sense ? higher temps will be dependent on power, ignoition timing, cylinder pressure etc.
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Old 24th April 2005, 22:26   #3
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Re: How does retarding cam timing make more power?

wow, thanks for the excellent explanation. it all does make sense, but are you saying that there is no simple explanation for why it retarding cam timing can cause detonation at high temperatures?

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Old 24th April 2005, 22:29   #4
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Re: How does retarding cam timing make more power?

Less Overlap(the time when both the In/Ex valves are open) giving higher cylinder pressure which can cause the detonation.
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Old 24th April 2005, 23:19   #5
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Re: How does retarding cam timing make more power?

remembering from my old engineering classes from way way way back (I'm not a engineer BTW) , retarding the timing does not make more power unless you plan to do some type of force induction (to reduce chance of dentonation). It will actually decrease power. Advancing the timing on a normally aspirated car along with higher octane fuel would be the way to do it.



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Originally Posted by ernie
Hey guys,

Just like the title says, I am wondering how changing the cam timing makes more power and how it can can induce detonation with higher intake temps. Thanks

ernie
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Old 24th April 2005, 23:35   #6
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Re: How does retarding cam timing make more power?

Please read long article, inlet timing of closing is king at higher rpm.....
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Old 24th April 2005, 23:38   #7
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Re: How does retarding cam timing make more power?

The best thing about the the BMW is the Vanos system which can retard the intake and advance the exhaust. Giving more power/torque than a single cam with fixed lobe seperation.
This is why a M62TU or S62 with a Motec system would make a great hot rod engine.
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Old 24th April 2005, 23:48   #8
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Re: How does retarding cam timing make more power?

mmmmm, motec. (picture Homer Simpson saying this)

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Old 25th April 2005, 00:27   #9
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Re: How does retarding cam timing make more power?

I run MoTeC on my RS - Awesome bit of kit, not sure how easy it would be too mod with the M5 loom though....... Easy (relatively) on an older 911 ....
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Old 25th April 2005, 00:34   #10
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Re: How does retarding cam timing make more power?

im sure it would be really, really difficult. with the degree of integration between the factory ECU and other systems (DSC, EMS, list goes on) I think it would be next to impossible unless you were using the engine in a race car without any of these systems.

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