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lobe symmetry vs centerline

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pastry_chef
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lobe symmetry vs centerline

Post by pastry_chef »

Naturally aspirated engine scenario.
Let's say asymmetrical lobes are optimal for most applications. If restricted to only symmetrical lobes, how would the relative optimal centerlines trend for most cases?
In other words can it be said a symmetrical cam would usually prefer more or less advance? (vs asymmetric).

If this has a clear answer, can we say why?
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Re: lobe symmetry vs centerline

Post by CamKing »

It depends on how you measure centerline.
If you use the old, outdated way, and call max lift the centerline, then the asymmetrical intake lobe will want to be more advanced then the symmetrical intake lobe.

If you call the mid-point between the valve opening point, and the valve closing point, then the asymmetrical intake lobe will want to be on the same centerline as the symmetrical intake lobe.

You shouldn't run an asymmetrical lobe on the exhaust.
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Re: lobe symmetry vs centerline

Post by RevTheory »

CamKing wrote: Fri Jan 10, 2020 9:32 am...

You shouldn't run an asymmetrical lobe on the exhaust.
You're not in the "fire off a strong wave down the pipe with an aggressive opening" camp, Mike?
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Re: lobe symmetry vs centerline

Post by CamKing »

RevTheory wrote: Fri Jan 10, 2020 3:48 pm
CamKing wrote: Fri Jan 10, 2020 9:32 am...

You shouldn't run an asymmetrical lobe on the exhaust.
You're not in the "fire off a strong wave down the pipe with an aggressive opening" camp, Mike?
No. I''m in the "Do what's correct" camp.

If you were going to design a correct asymmetrical exhaust lobe(and I designed a few), it would actually open slower, and close faster then a symmetrical design. This makes more power, but it increases the exhaust lift at TDC, so you need more valve relief, and it's very prone to seat bounce.
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Re: lobe symmetry vs centerline

Post by RevTheory »

CamKing wrote: Sat Jan 11, 2020 12:26 pm No. I''m in the "Do what's correct" camp.

If you were going to design a correct asymmetrical exhaust lobe(and I designed a few), it would actually open slower, and close faster then a symmetrical design. This makes more power, but it increases the exhaust lift at TDC, so you need more valve relief, and it's very prone to seat bounce.
Nobody ever agrees on what's "correct." I started a thread last year on the advanced board hoping to find some info regarding TFX-measured waves and EVO velocity and it didn't get any traction at all.

Do you have a link to the exhaust thread you did years ago? I searched but had zero luck and it's worth reviewing. I'll bookmark it this time.
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Re: lobe symmetry vs centerline

Post by Stan Weiss »

Mike,
The asymmetrical lobe I saw, looked like it are almost made up of 2 different lobe designs. You have one part from the an event to max lift which is more aggressive than the symmetrical lobe was and the other part from max lift to an event which is "lazier" than the symmetrical lobe was. Yet both cams seem to have just about the same area under the curve. Was this just what had been done for the one I saw or is this a somewhat accurate description?

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Re: lobe symmetry vs centerline

Post by CamKing »

Stan Weiss wrote: Sun Jan 12, 2020 10:38 am Mike,
The asymmetrical lobe I saw, looked like it are almost made up of 2 different lobe designs. You have one part from the an event to max lift which is more aggressive than the symmetrical lobe was and the other part from max lift to an event which is "lazier" than the symmetrical lobe was. Yet both cams seem to have just about the same area under the curve. Was this just what had been done for the one I saw or is this a somewhat accurate description?

Stan
My asymmetrical profiles have a completely different opening curve, from the closing curve. Back in the 80's I looked at what Harold was doing with his idea of an asymmetrical profile, looked at what made it work, and then used that information to start designing my asymmetrical cams.
What Harold called an asymmetrical profile, was actually a symmetrical profile, without an opening clearance ramp(or a very fast one).
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