Adrenaline_Z

I'm going to reply to this thread to get some notifications.

I'd also like to state, that some of you are going to get a surprise about
peak values when you refer to valve overlap in degrees (EVC/IVO), as opposed to LSA.

BAD FNZ

Just for the record, I love this forum. This is the type of stuff I've been waiting to see in the stickies!

I would also like to see a break down with the reverse split cams vs standard split cams Cams are my best subject with these cars and I'm only hoping to become more knowledgable as time progress'. Thanks Tony!

DAPSUPRSLO

iTrader: (14)

I agree with the overlap is what counts idea; however, there are many ways to get the same amount of overlap with very different exhaust opening and intake closing events which will result in very different powerbands. I agree with Vizard in that there is only one lsa that provides optimum results for a motor IN A GIVEN POWER BAND. Why, cause there is only one overlap value and one set of valve events that will provide those optimal results for a combonation within it's intended powerband. Now only if it was easy enough to choose the "one lsa and one set of valve events" that was optimal for a combonation. Start factoring in the different lobes and there shapes and its no wander that few in the world can spec a cam within the last few percent of maximum performance for the given combo and powerband.

Joe Overton

I agree with that statment... I can think of many with lots of knowledge who can get somebody close for a given combo... But any or all of those learned off the coat tails of others ..Or with thier own R&D.. But as mentioned above ,will it or is it optimal? is the question! Unless you have the undieing need for every ounce of power. A person will go through many selective cams to find one close and many more to get it within the vicinity of optimal..Any wizards wanna give a shot on the next cam I should try?????

DAPSUPRSLO

iTrader: (14)

I'll give it a go. My 276/280 cam I have for sale. Oh wait, it's for an ls1, ha ha. What's that tell ya, time for ls1 maybe, ha ha, just fooln' with ya.

Zymosis

iTrader: (7)

This is a great topic, and it really clears up alot of things I have read about cams, specifically LSA. I know that when I was looking for cams, I was def a newbie to it all. I knew I wanted something big that would make good power, but I had to beable to drive it in town (Being in college, its still my only car) For the longest time I was shopping by LSA because thats what people seemed to do. Somehow I ended up talking to Chuck at HPE, and I ended up learning LSA isnt all of it (238/242, .608/612 115 LSA) That cam was exactly what I was looking for. Even on the 115 LSA it has a rough idle which people told me a 115 couldnt, it made great power and torque, and was totally streetable.

How does LSA change tuning? From what I have heard tighter LSA's are harder to tune. Also what makes a higher LSA make power longer after peak as opposed to a 110 or 112 for example?

Adrenaline_Z

This topic is going to become 20 pages long I'm sure. Just think about all of
the other factors that will 'play' on overlap: Pre-load, rocker ratio, etc.

I think it's safe to say, the valve train parts also need to be included in this
discussion, as well as a reference point (0.006" or 0.050") because the ramps
are going to have an effect as well.

I hope we're paying the cam specialists to explain this topic; they're going to deserve it!

DAPSUPRSLO

iTrader: (14)

Yep, there is just about and endless amount of points to discuss on this topic. I'm not sure anyone can explain every last one of them, most, but not absolutely every last detail.

As a side not, I'm definately digging this new advanced tech section too

DaddySS

I am certainly not an expert but I too thought I understood LSA and duration fairly well. This seems to be contradictory though. It would help me if we could explain why it works that way, I always find it easier to undertsand and remember if I undertsand the underlying cause, not just the fact.

So, I thought that the torque and peak torque occured sooner and lower RPMs with wider lsa (less overlap) than with a narrower lsa. The reason (I thought) was that valve overlap and longer duration are intended to help the engine make power at higher RPMs.

FiveSevenEagle

why do we need overlap ? why to have the intake and exhaust valves open at the same time ? ive been asking this question for decades now and NO ANSWER ,

thank you

Adrenaline_Z

^ Here's a hint:

Fill a glass with water

Take a straw and drop it into the glass

Put your thumb over the top of the straw and make an air tight seal

Now pull the straw out of the glass

Did water remain in the straw?

Think about the exhaust gasses trying to escape the chamber with a closed
intake valve.

Ben R

Let's not forget that the pressure inside the cylinder is FAR greater than the pressure in the header tube. (Remember that whole deal about how pressure is always trying to equalize?)

The reason we open the intake valve before the piston gets to TDC is so that by the time the piston has started to move back down the bore to BDC the valve is far enough off the seat to get as much cylinder filling as possible when piston speed is the highest. The reason we keep the intake valve open past BDC is because the air develops a decent amount of inertia through the intake tract as the piston moves down the bore. Even as the piston is at BDC and on it's way back up to TDC more air will be coming into the cylinder. The trick is closing the intake valve as soon as the air looses it's inertia, and before the piston can begin to push fresh air back out of the cylinder and into the intake tract. The exhaust gasses rushing out of the cylinder also create low-pressure area that helps draw the air into the cylinder when the valve first comes off of the seat (Going back to the principle that pressure is always trying to equalize).

If we've decided that the actual amount of overlap is what's important, and not LSA, then it stands to reason that as our camshaft gets larger, we will need more LSA to get us back down to the overlap number we're looking for.

Adrenaline_Z

I'm not forgetting any of that. But at what point in time are you referring to?

There are periods of time when the pressure in the cylinder is less than the pressure in the header tube as well.

Quite simply, there is a period of time when the piston is not altering pressure
in the cylinder very much (IE: about 10 BTDC to about 10 ATDC).

If the intake valve is open under the correct circumstances, a higher pressure
intake wave will move into the chamber as the negative pressure exhaust wave
occurs.

That cannot happen without overlap, and you will be giving up "free" cylinder
filling and chamber scavenging.

Ben R

That's what I said:

'The exhaust gasses rushing out of the cylinder also create low-pressure area that helps draw the air into the cylinder when the valve first comes off of the seat (Going back to the principle that pressure is always trying to equalize).'

Adrenaline_Z

But you also said:

at which point I asked: "At what point in time are you referring to"

Pressure is always changing, so it's not accurate to state that pressure in the
cylinder is FAR greater than the header tube.

I'm not really sure why you highlighted my post, and singled me out to begin with?

Ben R

There is more pressure in the cylinder than the header tube from the second the plug fires and the flame front travels across the piston all the way through the blowdown period as the piston travels to BDC, and even as the piston begins to move back to TDC. How else does the exhaust gas run out of the cylinder when the piston is moving down the bore?

I highlighted your post cause it was the only one I read.

Adrenaline_Z

A few things I can think of:

- Exhaust gas will continue to flow due to kinetic energy

- Higher pressure intake charge can also move exhaust gas out of the chamber

- Low pressure in the exhaust port area which remains lower than the pressure
in the cylinder (*for a period of time) will continue to 'pull' gas out of the cylinder.

Gas/Fluid moves from high pressure to low pressure. If the exhaust port happens
to have a lower region than the cylinder as the piston begins to draw downward,
the flow of motion will continue out the exhaust port until the pressure drops
lower in the cylinder.

SStrokerAce

iTrader: (2)

Well after you two get that all ironed out we can throw tuning pulses into the mix and really screw this all up for everyone.... tuning pulses are the reason we keep the intake valve open past BDC and the exhaust valve open past TDC. The latter pulls into this scavenging discussion.

Bret

DaddySS

This is from our Cam Guide:

Long overlap periods work best for high-rpm power. For the street, a long overlap period combined with long-duration profiles combine to kill low-speed torque

- Reducing overlap on a long-duration cam will often increase midrange torque at the expense of peak power, but if the average torque improves, that’s probably a change worth making.

And this from this thread:

Increasing Lobe Seperation Angle moves your powerband higher in the rpm range, and lowers your low/mid rpm torque output. It also helps with emissions testing and makes the idle smoother. "Increasing LSA" means the same thing as "decreasing overlap". So, a 114 LSA cam will idle better, make less emissions, and show peak HP higher in rpm than a 112 LSA cam.


Aren't these contradictory?

Can someone explain how overlap of varying duration affect teh flow at high versus low rpm, and why more overlap works for high or low RPM power and torque?

Adrenaline_Z

Go for it Bret, I think most of us are ready for some hard core discussion on
valve timing.

As for tight LSA making power down low, I tend to believe otherwise.

You have long durations and overlap contributing to a loss of dynamic compression (DCR)
and cylinder pressure (BMEP).

If the VE curve peaks later due to the valve timing, the effective compression
and cylinder pressure also rise later in the RPM band.