Valve overlap

[digger]

Food for thought.
Twin-cam, 4-Valve engines don't have an LSA

If you define that LSA is how far apart the lobes are on the cam then yes they don't, as there are normally atleast 2 cams. If you look at it or think of it as the average of the ICL and ECL or how far apart the centrelines are In cam degrees then yes they do as the number is defined even if many are variable nowadays

[user-23911]

Was it Nissan YD/ ZD or was it Ford / Mazda.......?

DOHC but each stick does 1 intake and 1 exhaust?

Someone knows?


Otherwise everyone's just making it all up as they go?


Otherwise with normal DOHC, you tweak the cams and you've got a fixed LSA you can refer to?
But it's adjustable depending on what you've got?

[John Wallace]

If you define that LSA is how far apart the lobes are on the cam then yes they don't, as there are normally atleast 2 cams.

Isn't LSA (Lobe Separation Angle) the separation angle of the intake lobe and exhaust lobe?
Doesn't say anything about it having to be on the same camshaft?
Same as the overlap?

Now duration, no can't be changed.

Isn't it why the DOHC so good, you can vary these previously non-changeable events after ground?

If one wants a tighter LSA, change the separation, no need to grind a new camshaft.

If you have your favorite intake cam lobe and your favorite exhaust cam lobe, you can put them at whatever IO and EO you want?
(without needing to have multiple camshafts laying around)

[hoffman900]

LSA is just an average is is relatively meaningless.

Most people when talking DOHC talk cam centerlines, which tells you wayyyy more. Nothing stopping people from doing the same with single cam engines, other than marketing.

[John Wallace]

Most people when talking DOHC talk cam centerlines

About like calling a rose by another name?

[hoffman900]

Most people when talking DOHC talk cam centerlines

About like calling a rose by another name?

It just distinguishes between a LSA of 106* which can mean just about anything centerline wise as long as it averages 106. A cam with centerlines 106/106 is going to perform differently than a 100/112 centerline cam.

Since people think in terms of centerlines and LSA’s in that world, they don’t get bogged down with marketing speak and “rules of thumb” that is so prevalent with the V8 stuff.

[RevTheory]

I think we've all stuck our proverbial flags in the sand so is there really anything left to say here?

[randy331]

What if one started with a SBC combo they were not familiar with and 128 put them at a very close base line on their cam so that they were not just grasping at straws, would it be worth exploring?

128 is grasping at straws.

Think about it Gary. The only thing LSA determines is the deg of separation between peak intake lift and peak ex lift. It defines no valve events/overlap/ or the piston position those 2 peak lift points happen at since the cam can be installed on any IVC. Only the deg of separation between them.

How could that be the most important thing ??

Randy

[CamKing]

LSA is a term that was introduced, for the sole purpose of grinding an OHV camshaft.
The guy grinding the cam, doesn't understand opening and closing points. He just wants to know how many degrees to move the master, when going from grinding the exhaust lobes, to grinding the intake lobes. LSA is a machining term, not a design parameter. Cam companies listed LSA, because it was easier for their customers to understand. When I design a cam, I'll tell the customer it's ground on a 107 LSA, just because it's easier then telling him, its a 102 ICL and 112 ECL cam.

[CamKing]

Could we discuss what is happening in the intake port and combustion chamber etc during the overlap cycle?

Easy. Air is flowing from higher pressure to lower pressure.
As long as the pressure above the exhaust valve, is lower then the pressure below the exhaust valve, air is flowing out of the combustion chamber, into the exhaust port.
As long as the pressure below the intake valve is lower then the pressure above the intake valve, air is flowing out of the intake port, and into the combustion chamber. It doesn't matter where the piston is.

[midnightbluS10]

Was it Nissan YD/ ZD or was it Ford / Mazda.......?

DOHC but each stick does 1 intake and 1 exhaust?

Someone knows?


Otherwise everyone's just making it all up as they go?


Otherwise with normal DOHC, you tweak the cams and you've got a fixed LSA you can refer to?
But it's adjustable depending on what you've got?

Ford/Mazda

I know Mazda had the SkyActiv system that had an eccentric shaft inside the camshaft. I think it was skyactiv, anyway.

[piston guy]

Could we discuss what is happening in the intake port and combustion chamber etc during the overlap cycle?

Easy. Air is flowing from higher pressure to lower pressure.
As long as the pressure above the exhaust valve, is lower then the pressure below the exhaust valve, air is flowing out of the combustion chamber, into the exhaust port.
As long as the pressure below the intake valve is lower then the pressure above the intake valve, air is flowing out of the intake port, and into the combustion chamber. It doesn't matter where the piston is.

Perfect , simple terms explanation Mike. It also shows how a piston in motion alters the dynamic and can cause reversion. The only other thing that "can" hurt flow at overlap is a giant dome on a piston , especially if it is an opposing valve ( hemi) head.
See you next week "to pick up a sticker " LOL

[GARY C]

What if one started with a SBC combo they were not familiar with and 128 put them at a very close base line on their cam so that they were not just grasping at straws, would it be worth exploring?

128 is grasping at straws.

Think about it Gary. The only thing LSA determines is the deg of separation between peak intake lift and peak ex lift. It defines no valve events/overlap/ or the piston position those 2 peak lift points happen at since the cam can be installed on any IVC. Only the deg of separation between them.

How could that be the most important thing ??

Randy

I never said it was most important but if your intake center-line is optimized and your lsa is wrong then your exhaust center-line will not be optimum.
If it did not matter then we would all be running Comp shelf cams on 110 lsa

[408 Nova]

Could we discuss what is happening in the intake port and combustion chamber etc during the overlap cycle?

Easy. Air is flowing from higher pressure to lower pressure.
As long as the pressure above the exhaust valve, is lower then the pressure below the exhaust valve, air is flowing out of the combustion chamber, into the exhaust port.
As long as the pressure below the intake valve is lower then the pressure above the intake valve, air is flowing out of the intake port, and into the combustion chamber. It doesn't matter where the piston is.

This description sounds more like normal operation, without taking into account the affects of overlap on the engine.

[hoffman900]

Could we discuss what is happening in the intake port and combustion chamber etc during the overlap cycle?

Easy. Air is flowing from higher pressure to lower pressure.
As long as the pressure above the exhaust valve, is lower then the pressure below the exhaust valve, air is flowing out of the combustion chamber, into the exhaust port.
As long as the pressure below the intake valve is lower then the pressure above the intake valve, air is flowing out of the intake port, and into the combustion chamber. It doesn't matter where the piston is.

This description sounds more like normal operation, without taking into account the affects of overlap on the engine.

All that applies with overlap as well. An engine works off of pressure differentials, no matter what part of the valve timing curve it’s at. If none of the above exist, you get reversion / charge contamination, overlap or not.