camshaft timing question

[als427]

Still learning about camshafts and understand on a basic level duration , lsa , lift , overlap etc.
So - on the same camshaft when you advance it say 4 degrees what is actually happening to increase low end tq and why?
Duration lift and overlap are the same so its obviously related to do with when the intake/ex opens closes relative to TDC.
does opening intake earlier allow more dynamic compression because of when it fills? pure guess
I see lower rpm rated camshafts have an earlier intake opening so this tallies with the above etc but I cant wrap my head around exactly what this is acheiving , thanks all.

[digger]

Still learning about camshafts and understand on a basic level duration , lsa , lift , overlap etc.
So - on the same camshaft when you advance it say 4 degrees what is actually happening to increase low end tq and why?
Duration lift and overlap are the same so its obviously related to do with when the intake/ex opens closes relative to TDC.
does opening intake earlier allow more dynamic compression because of when it fills? pure guess
I see lower rpm rated camshafts have an earlier intake opening so this tallies with the above etc but I cant wrap my head around exactly what this is acheiving , thanks all.

biggest driver when you do get more bottom end (you dont always depending what the start point is) is generally the inlet valve closing earlier so is better at trapping the mixture before the piston pushes back out so in effect reducing reversion. The reason is the inlet valve closing is normally optimised for trapping at higher rpm and making use of the "ramming" effect which allows the cylinder to fill when the piston is actually on the way up. At lower rpm the inlet valve point is ideally sooner.

The other events do change but the impact is usually less of a driver and often has a counter benefit.

[Tom68]

Early inlet closing, hurts top end because it shuts the valve when the intake has inertia and can overfill the cylinder. At lower speeds the cylinder is full earlier and the piston pushes it back up the inlet tract.
Now if you have long runners.....

4 degrees on top of the 4 degrees ground in may not gain you any more lower rpm power, but it will hurt the top.

[BLSTIC]

It's also worth noting that on vvt engines maximum power is usually a minimum overlap position. Why? Because the IVC is so late and with the fixed cam duration late IVC is more important than overlap at the top end. The range of IVC close options can be as much as 50 degrees...

Of course both is best, which is why the early VTEC engines are only just now being overtaken (in terms of power per litre) by the newest VVT-only engines (also worth noting, those modern power gains come with far superior emissions, so we're talking a lot more about better burning than better airflow and cam timing)

[Geoff2]

There is no hard & fast rule & there are plenty of examples of the opposite occurring to what was expected......

[als427]

ok so on a very basic level , with the intake valve closing earlier it traps more air/fuel in the cylinder before it is pushed out again by the piston going back on the up stroke. So more dynamic compression / low power torque.
The flip side being as rpms increase , inertia increases , this hinders upper power as there is less pull through as the valve is closed before it can take best effect?

[Tom68]

ok so on a very basic level , with the intake valve closing earlier it traps more air/fuel in the cylinder before it is pushed out again by the piston going back on the up stroke. So more dynamic compression / low power torque.
The flip side being as rpms increase , inertia increases , this hinders upper power as there is less pull through as the valve is closed before it can take best effect?

At higher revs you need more time to fill the cylinder by keeping the valve open longer, at high revs when the piston descends there is a vacuum in the cylinder, the inertia rush comes late.

[Dan Timberlake]

There is an engine masters episode of motor home 440 with some thumper cam. They move the cam timing about 8 degrees in 4 degree increments.
moderately interesting.

[als427]

hi Dan - any links to that? or series / episode number? thanks

[Bill Chase]

hi Dan - any links to that? or series / episode number? thanks

Motortrend on demand. The episodes are not free, subscription used to be like $5.50 a month, and not available in the UK, Australia etc. Only available for American consumers.
They have actually done a few different episodes on cam timing, and how it effects power range. Powernation TV did one last year on a 383 sbc with the cam straight up vs 3-4° advanced, their episodes are free on YouTube. But don't expect anything like an indepth technical explanation of what is happening. However, it does give a decent visual representation of the shifting of the power curve.

Along these lines, a cloyes cover and hex a just timing set are not expensive (sbc/BBC), and you can play with cam timing at your leisure and see what works best for your combination.

[als427]

hi Dan - any links to that? or series / episode number? thanks

Motortrend on demand. The episodes are not free, subscription used to be like $5.50 a month, and not available in the UK, Australia etc. Only available for American consumers.
They have actually done a few different episodes on cam timing, and how it effects power range. Powernation TV did one last year on a 383 sbc with the cam straight up vs 3-4° advanced, their episodes are free on YouTube. But don't expect anything like an indepth technical explanation of what is happening. However, it does give a decent visual representation of the shifting of the power curve.

Along these lines, a cloyes cover and hex a just timing set are not expensive (sbc/BBC), and you can play with cam timing at your leisure and see what works best for your combination.

Thats great - we can get it on amazon over here so Ill have a month or two of it!