More exh duration

[CamKing]

I can't talk for Harold, but I think he was just saying, as long as the air fuel mixture is flowing in, your not closing the intake too late.

The closing point of the intake becomes too late when the pressure on the cylinder side of the intake valve gets greater then the pressure on the port side of the intake valve.

[Stan Weiss]

Jeff, Lets add dynamic change to this picture. Take the engine off the dyno and place it it a moving vehicle. Now as the vehicle moves down the 1/4 mile and speed goes up to 200 mph the intake see pressure above BP. So the faster the vehicle moves the pressure differential will keep changing.

[CamKing]

Jeff, Lets add dynamic change to this picture. Take the engine off the dyno and place it it a moving vehicle. Now as the vehicle moves down the 1/4 mile and speed goes up to 200 mph the intake see pressure above BP. So the faster the vehicle moves the pressure differential will keep changing.

I've never seen more then a 1-2" increase in manifold pressure(30.5-31.9") at 225mph.
That would not be on the top of my list for things to be adjusting for.

[dacaman12]

I'm not as smart as I used to be!

Re: Harold's comment "As long as intake charge is still entering the cylinder, it doesn't matter too much when you shut the intake valve"

I take this to mean that where the intake efficiency (port/valve size, carb/manifold flow, and mean curtain area during TDC-BDC) is less than the demand all the way down the intake stroke (Delta P is still < 1 ATM @ BDC), the engine will be relatively insensitive to IVC point, in inverse proportion to the residual vacuum.

However, the reverse condition, where the intake efficiency is enough to fully satisfy demand (cylinder pressure = 1 ATM @ BDC), the intake closing point will be sensitive, since inertia fill stopped when Delta P drops to unity.
If satisfaction decays at higher speed, the torque curve will ramp up at this point.
Also affected by rod ratio - but that's another story.

Am I on track?

I have had this particular discussion in detail with Harold. He basically means this:

It doesn't matter WHEN the intake valve closes.

HOW MUCH AIR IS TRAPPED when it does close is what is important.

This becomes almost common sense when you consider that cylinder volume at TDC is constant. More air squeezed into the same volume will make more power.

In other words, closing the intake valve too soon will not increase compression in the motor, and, as a result, will not produce a gain in torque.

[CamKing]

If the results don't match the theory, adopt a new theory.

You may want to give that a try.
With your theory, the ristrictor plate engines wouldn't need a longer exhaust duration to pull on the intake. The fact is, they run about an 8 degree longer exhaust.
The walls(skirts) in the restictor plate manifolds are made to normalize the plenum pressures under the plate and reduce the effects from cylinder pulses.
We tried closing the exhaust sooner on a plate motor last year, and it hurt the power.

[CamKing]

As for cams getting shorter and lobe centers getting wider,
The first Restrictor plate cams I did when NASCAR first went to restrictor plates was 304/312 seat duration(valve opening point to valve closing point) on a 110LSA.
The best ones last year were 300/308 seat duration on a 110LSA.
in all those years, the restrictor plate has been reduced, and the stroke has been reduced.

Up until NASCAR added there new gear rule, The restrictor cam's seat durations and lobe centers haven't changed much.

[dacaman12]

If the results don't match the theory, adopt a new theory.

You may want to give that a try.
With your theory, the ristrictor plate engines wouldn't need a longer exhaust duration to pull on the intake. The fact is, they run about an 8 degree longer exhaust.
The walls(skirts) in the restictor plate manifolds are made to normalize the plenum pressures under the plate and reduce the effects from cylinder pulses.
We tried closing the exhaust sooner on a plate motor last year, and it hurt the power.

First off, you can't simply "close the ehaust sooner" without affecting the rest of the closing side, from a dynamic stablilty standpoint.

After all, if the valve doesn't stay on the seat, did you really close it sooner???

Otherwise, you lost high lift area trying to change the EVC point.

The loss in high lift area decreased blowdown slightly, causing more reversion around IVO, which caused airflow to start later and at a slower velocity, which decreased cylinder filling, which hurt power.

"knee bone connected to the shinbone" kinda thing

[CamKing]

First off, you can't simply "close the ehaust sooner" without affecting the rest of the closing side, from a dynamic stablilty standpoint.

After all, if the valve doesn't stay on the seat, did you really close it sooner???

Otherwise, you lost high lift area trying to change the EVC point.

The loss in high lift area decreased blowdown slightly, causing more reversion around IVO, which caused airflow to start later and at a slower velocity, which decreased cylinder filling, which hurt power.

"knee bone connected to the shinbone" kinda thing

That would be a good theory if it was true.

The exhaust lobe was a reverse asymmetrical(opening side was normal, with a shorter closing side. opening point, opening curve, and max lift were in the normal places.
There was no increase in seat bounce on the spintron, and we even tried it with a little higher rocker ratio to see if adding area would help.
This was all part of a 3 month development program.

[Windsor377]

First off, you can't simply "close the ehaust sooner" without affecting the rest of the closing side, from a dynamic stablilty standpoint.

After all, if the valve doesn't stay on the seat, did you really close it sooner???

Otherwise, you lost high lift area trying to change the EVC point.

The loss in high lift area decreased blowdown slightly, causing more reversion around IVO, which caused airflow to start later and at a slower velocity, which decreased cylinder filling, which hurt power.

"knee bone connected to the shinbone" kinda thing

That would be a good theory if it was true.

The exhaust lobe was a reverse asymmetrical(opening side was normal, with a shorter closing side. opening point, opening curve, and max lift were in the normal places.
There was no increase in seat bounce on the spintron, and we even tried it with a little higher rocker ratio to see if adding area would help.
This was all part of a 3 month development program.

Hmmmm....let me ask a couple questions of all three cam guys on this site.

1. Is a shorter closing ramp really the same as setting the valve on the seat harder?

2. Or is the critical area here still the transition to the base?

3. Or can we just get away with a whole lot more agressiveness on the exhaust side?

[CamKing]

1. Is a shorter closing ramp really the same as setting the valve on the seat harder?

I actually made the seating velocity the same as the longer cam.
To do that, I had to make it faster somewhere else, but when the valve met the seat, it was with the same amount of force as the symmetrical cam.

[UDHarold]

Windsor377,

When I design a family of cams, all cams in the family, whatever the lift or the duration, have the same opening velocity, and the same seating velocity.
The critical area is getting all the exhaust possible out of the cylinder before the intake valve opens. If you fail here, very little else matters.
On question #3, what do you mean by 'aggressiveness'?

UDHarold

[Windsor377]

1. Is a shorter closing ramp really the same as setting the valve on the seat harder?

I actually made the seating velocity the same as the longer cam.
To do that, I had to make it faster somewhere else, but when the valve met the seat, it was with the same amount of force as the symmetrical cam.

Thank you CamKing. I was hoping that would be your answer.

Very appreciative.

[Windsor377]

Windsor377,

When I design a family of cams, all cams in the family, whatever the lift or the duration, have the same opening velocity, and the same seating velocity.
The critical area is getting all the exhaust possible out of the cylinder before the intake valve opens. If you fail here, very little else matters.
On question #3, what do you mean by 'aggressiveness'?

UDHarold

Thanks again Harold.

Bhy agressiveness, I was wondering if with all the advances in valve train technology could we now begin slamming the exhaust valve down on the seat harder than I am accustomed...me being accustomed to your old UltraDyne designs.

[black_z]

If the exhaust duration and/or lift isn't enough the rising piston just compresses the air again.

I see.

[MadBill]

Speaking of lift, power gains may show diminishing returns beyond a certain intake lift, but AFAIK, rarely if ever does a loss occur, whereas I have seen and heard of numerous case where too much exhaust lift loses power. Howcome?