More exh duration

[CamKing]

I like to see an average ratio of 74-76%.

Does it make a difference at what lift, or piston position/speed the average EX flow is achieved at?
If I gained 10cfm of flow at .300" lift, which happens to be at BDC, will that change the cam requirement, the same as if I gained 10cfm of flow at .600" lift, at well past BDC, and at a much higher piston speed?

Yes.
You need to use common sense.
If your exhaust is flowing 110% of the intake at .100" lift, you need to take that into account when looking at the over-all picture.

[jmarkaudio]

Here is my take and I said this in another post, every engine combo has a cam that works the best for a given RPM range. Make any change, the requirements change. As far as more exhaust duration, this is from a recent post by Harold. One of the best explanations I have ever read about cams.

Exhaust events are compromised by the need to extract all the torque possible before opening the exhaust valve, and the need to extract all the burned exhaust gases possible before the intake valve opens. After TDC the exhaust valve closing, if longer than needed for low-speed dynamic control, allows excess exhaust reversion to enter the intake from exhaust gas pulsations. This is why I claim the exhaust valve closing is the least important of the four events; It only affects low-speed power. at higher speeds the other events are much more important.
The exhaust cam gets in a race all its' own, a race between how much torque can be made by delaying exhaust valve opening, and how much exhaust can get out before the intake valve opens. How much exhaust gas is out of the cylinder affects how much intake charge can get in.
Peak Horsepower occurs when the exhaust can no longer effectively clean out the cylinder, and leaves residual gases in the cylinder, hindering intake filling.
Intake opening before TDC is to minimize exhaust gas reversion into the intake port, which aids in cleaning out the port entrance after the piston starts down ATDC. The sooner the intake can start flow into the cylinder, the higher the intake port velocity can be, the more inertia ram you can develop in the intake port, and the longer you can fill the cylinder ABDC.
As long as intake charge is still entering the cylinder, it doesn't matter too much when you shut the intake valve. I have had this proved to me for 30 years, with unsymmetrical cams still providing good bottom-end torque, even with delayed intake closings.


UDHarold

[MadBill]

[quote="CamKing...That's not the case.
On a 2bbl engine that turns 7,200rpm, you don't run any more intake duration then a 4bbl engine that turns 7,200. You do run shorter exhaust duration though.[/quote]

Since there is less cylinder pressure and less burned mass to expel with a 2 bbl. engine, I'd expect the exhaust duration reduction to come mostly off the EDIT: OPENING side as the designer struggles to harness every psi, thus narrowing the LCA and retarding the cam to keep the ICL pretty much unchanged. (If you'll have to kill me if you confirm this, CK: I don't need to know that badly!

[CamKing]

Since there is less cylinder pressure and less burned mass to expel with a 2 bbl. engine, I'd expect the exhaust duration reduction to come mostly off the closing side as the designer struggles to harness every psi, thus narrowing the LCA and retarding the cam to keep the ICL pretty much unchanged.

Not really.
You also have to have the required amount of time for the exhaust to pull on the intake during overlap. You can hurt the power by evacuating the cylinder too soon.

[MadBill]

After carefully reasoning my way through the process, I wrote 'closing' when I meant 'opening".
My remarks about LCA and indexing mesh with the latter scenario. (NOW do you have to kill me? )

[CamKing]

After carefully reasoning my way through the process, I wrote 'closing' when I meant 'opening".
My remarks about LCA and indexing mesh with the latter scenario. (NOW do you have to kill me? )

I spent all weekend driving my son to 4 soccer games.
I'm too darn tired to kill anyone.

[black_z]

Back then, the average exhaust port only flowed about 64% of the intake port, so it took a lot of exhaust duration to empty the cyl.

I'm not an engine builder/cam designer, so this may sound stupid. If the piston reaches TDC on the exhaust stroke, how can it NOT empty the cylinder? Regardless of duration.

[David Redszus]

I'm not an engine builder/cam designer, so this may sound stupid. If the piston reaches TDC on the exhaust stroke, how can it NOT empty the cylinder? Regardless of duration.

Now there we go, thinking again. That's going to get us in trouble with conventional wisdom.

Using Stan's engine, a 4 x 3, V8 Cr 5.09
we find that the volume remaining in the cylinder is represented by the chamber volume which is a function of geometric compression ratio.

for
CR 10, vol = 68cc
CR 11, vol = 62cc
CR 12, vol = 56cc
CR 13, vol = 51.5cc
CR 14, vol = 47.4cc
for an engine with a cylinder displacement of 685.8cc.

Since the unswept volume is much less with a high compression motor, we would expect to see less overlap duration being necessary to evacuate the remaining unburned gas.

But what we very typically see is just the opposite. A high compression motor often has longer duration values including overlap duration. What gives?

Could it be that overlap duration has much less to do with chamber evacuation and much more to do with obtaining an adequate valve lift somewhere else?

[black_z]

Hell, I forgot about residual volume. So, is that what the scavenging is for, to rid the cylinder of the residual volume and allow for more room for the "fresh charge"?

[Ed-vancedEngines]

And then we have cams for nitrous specific engines which is a totally different animal as is blown gas and blown alcohol engines too.

Msot cam designers still like to keep the exhaust lifts down as compared to intake but the exhaust durations up. Recently I have been doing the exhaust lifts to be more near the intakes and it seems like positive results. WE can't tell by dyno becuase all the dyno pulls on these things are all motor with no nitrous. I am going against cam grinder typical wisdom but to me it makes sense.

Opinions?

Ed

[CNC BLOCKS]

[quote="CamKing...That's not the case.
On a 2bbl engine that turns 7,200rpm, you don't run any more intake duration then a 4bbl engine that turns 7,200. You do run shorter exhaust duration though.

Since there is less cylinder pressure and less burned mass to expel with a 2 bbl. engine, I'd expect the exhaust duration reduction to come mostly off the EDIT: OPENING side as the designer struggles to harness every psi, thus narrowing the LCA and retarding the cam to keep the ICL pretty much unchanged. (If you'll have to kill me if you confirm this, CK: I don't need to know that badly! [/quote]

On our 2 barrel engines we run a lot of assymetrical single pattern cams on engines that turn under 7500 and we have been using the BBC cam tunnels and have been using up to 1.8 intake ratio rocker and have found using 1.5 ratio on the exhaust picks the torque up and does not effect the power side.

Plus with the higher rocker ratios we use smaller cams at the seat and .050 and the exhaust system we recomend play a big part in the type of cam we use

[CamKing]

Back then, the average exhaust port only flowed about 64% of the intake port, so it took a lot of exhaust duration to empty the cyl.

I'm not an engine builder/cam designer, so this may sound stupid. If the piston reaches TDC on the exhaust stroke, how can it NOT empty the cylinder? Regardless of duration.

Is the cylinder empty at TDC at the end of the compression stroke?
If the exhaust duration and/or lift isn't enough the rising piston just compresses the air again. The volume of the chamber has very little effect. It's all about the pressure differentials between the exhaust port, the cylinder, and the intake port. High pressure wants to move towards low pressure.

What do you think happens at overlap when the pressure in the cylinder is higher then the intake port when the intake valve starts to open?

[Procision-Auto]

That's not the case. On a 2bbl engine that turns
7,200rpm, you don't run any more intake duration then a 4bbl engine that
turns 7,200. You do run shorter exhaust duration though.

I wondered about this as well. My thought is that a touch less duration
and a little more compression would be the norm when compared to a
4bbl setup.

I'm not an engine builder/cam designer, so this may sound stupid. If the piston reaches TDC on the exhaust stroke, how can it NOT empty the cylinder? Regardless of duration.

The way I was taught to visualize this condition was to think of straw
submerged in water. Place your finger over the open end of the straw
and remove it from the glass.

The water drips out very slowly (let's leave cohesion, and adhesion out of this for sake of the analogy ).

Once your finger is removed from the straw (intake valve opens), the
higher pressure from the intake charge helps to push the exhaust gas
out.

This is especially helpful near TDC when the piston is nearly motionless
and not making a big impact on pressure change.

Of course we can only hope that when the intake valve opens, the pressure
is higher than what exists in the cylinder and exhaust at that point in time.

[panic]

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?

[dacaman12]

You also have to have the required amount of time for the exhaust to pull on the intake during overlap. You can hurt the power by evacuating the cylinder too soon.

Speaking of 60's technology, ever noticed that duration @ the seat keeps shrinking and LSA keeps growing? Seems to be working in the exact opposite direction of the quoted theory, doesn't it?

We keep on gaining and gaining power little by little, and keep discovering and rediscovering that NONE of this power is from the exhaust "pulling" on the intake. Instead we keep making imrpovements by controlling reversion. This is especially true when your intake is restricted.

For an example, lets think about a Nextel cup plate engine. A big port with high vacuum could, in THEORY, certainly use all of the suction power you can offer, in order to start airflow earlier.

However, any one seen a Cup plate manifold? The fact that the "reversion control" baffles in the intake manifold make power prove that the exact opposite is occuring.

Common sense would tell you that, IF the exhaust is "pulling" on the intake, cutting back on overlap with a more aggressive cam would start airflow later, and, as result, show a LOSS in power.

To summarize, if we are gaining power by moving in the opposite direction of the "theory", why are we still using this theory.

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

As far as 2BBL motors go, if your intake limits you to an RPM of 7500, there's really no point in trying to gain power at 8000 with a large exhaust lobe. If you can keep the exhaust valve shut longer and still evacuate the cylinder effectively, you gain power below peak HP, with no loss on the top.