How much vacuum does an intake port really see?

[cboggs]

I haven't time to add my thought to this right now, .. yep SOME
of us actually have to work,

but this is an AWESOME thread guys, .. keep this going, . .

Curtis

[SWR]

But despite the theoretical conversion factors, surely 'choking' etc. would occur at lower lifts with the higher velocity of extreme depression flows?

If it chokes on the bench at say 120"...wouldn't that say that it would also choke on the engine when it pulls 150"? I.e. it's not optimal?

[plovett]

Then next thought I had was that an engine with a properly size induction system usually sees less than 1.5" of Hg in vacuum in the intake tract. So I looked up the conversion from inches of mercury to inches of water. It is 13.6185 so 1.5" of Hg = 20" of H2O.
paulie

Your thinking about the vacuum in the intake is backward. The 1.5"Hg vacuum is differential pressure across the carb, not the port.

If the intake manifold is 1.5"Hg below atmosphere it still has an absolute pressure of 28.41"Hg (sea level, std conditions) remaining to "push" air into the cylinder. The piston is creating lower pressure inside the cylinder. That is the potential differential pressure across the intake port.

How much actual differential pressure between the cylinder and the plenum, (thats what causes the flow and determines the velocity), depends on how well the port keeps up with the piston demand from the downward motion of the piston in the cylinder.

Rick

Thanks Rick. That's something I was not understanding.

paulie

[randy331]

But despite the theoretical conversion factors, surely 'choking' etc. would occur at lower lifts with the higher velocity of extreme depression flows?

So, should we use 200" water depression when flowing at low lift, and 80" depression at high lift?

Should we put the piston in the flow test bore to simulate TDC, open the ex. and in. valve to the height they are at TDC during overlap, then pull 200" water on the header tube, and measure how well the piston top/in port/ex port, all flow together during overlap? I would think how well,(poorly), that combination flows backwards would be important also.

Would a momentary negitive preasure spike during overlap, significantly affect the depression the piston pulls on the port at peak piston speed, some 75* or so, of crank rotation later? And therefore the power required to simulate that depression at high lift, on a flow bench?

Randy

[MadBill]

But despite the theoretical conversion factors, surely 'choking' etc. would occur at lower lifts with the higher velocity of extreme depression flows?

If it chokes on the bench at say 120"...wouldn't that say that it would also choke on the engine when it pulls 150"? I.e. it's not optimal?

I was looking at it the other way around, SWR.
Let's say that in the running engine (IVL= 0.900"), unknown to us, the actual pressure delta reaches 120 "H2O @ 0.700" lift. We test the head at 28" as usual, and find steadily increasing flow all the way to our maximum test point of 1.0". There is no trace of choking; so we declare it to be a great port and move on.
BUT, after reading and thinking about this thread, we rework a BBC to use 4 of the cylinders as a giant air pump, spinning them with the other 4(!) We hook it up to our bench and re-flow the head at a depression of 125 "H2O. Now we discover the flow dies at only 0.550" lift! Uh, oh! Not so hot a port after all! Get out the grinders!

[Cammer]

A running engine does not emulate a flowbench test.

Transposing a flowbench test at a depression of 28 inches of water with a full inlet cycle finds the flowbench providing but brief snapshots of pressure seen on a running engine.

To even closely emulate a running engine you would need to match lifts and depressions to actual operating parameters.

The higher the level of developmental design of cylinder heads the fewer the gains to be found on a flowbench!

I see the day when optimal cylinder head designs will be cast at the foundry!

Flowbenches are best used to test cylinder heads and validate modifications.

To further twist your cerebral cortex I will say that cylinder head artistry such as performed by several forum members defies scientific understanding!

Trying to understand a great cylinder head artist's work will leave you scratching your head!

_____________________________________________________

The internal combustion engine is a study of contradictions!
_____________________________________________________

[MadBill]

But despite the theoretical conversion factors, surely 'choking' etc. would occur at lower lifts with the higher velocity of extreme depression flows?

So, should we use 200" water depression when flowing at low lift, and 80" depression at high lift?

Should we put the piston in the flow test bore to simulate TDC, open the ex. and in. valve to the height they are at TDC during overlap, then pull 200" water on the header tube, and measure how well the piston top/in port/ex port, all flow together during overlap? I would think how well,(poorly), that combination flows backwards would be important also.

Would a momentary negative pressure spike during overlap, significantly affect the depression the piston pulls on the port at peak piston speed, some 75* or so, of crank rotation later? And therefore the power required to simulate that depression at high lift, on a flow bench?

Randy

All great questions, Randy!
Ideally, we would obtain realistic pressure traces via some of Nitro2s wizardry, or perhaps one of the better engine simulation programs, and use the results to determine the correct test depression at each lift point (good luck though with the 100 psi needed to simulate low lift opening of the exhaust valve..)

As far as overlap flow/piston dome effects, one could indeed move the piston in correct relationship to the valves through the IVO to EVC range, while both sucking and then blowing air though the exhaust port with and without either valve in place (to determine their influences separately).

Keep in mind though that some builders say one advantage of seat angles above 45° is that they restrict low lift flow, (perhaps thereby providing higher suction and velocity/inertia later in the cycle? )

Much to ponder...

[randy331]

But despite the theoretical conversion factors, surely 'choking' etc. would occur at lower lifts with the higher velocity of extreme depression flows?

If choking occured at low lift because of extreme depressions during over-lap, wouldn't this have to be a seat angle/size/shape, problem? The air speed in the port would be slow, as compared to the speed of the air through the valve window area. The window area being the limiting factor, due to it being a smaller area at that lift, vs the csa of the rest of the port, would make me think this is where the problem would be.

Randy

[randy331]

(good luck though with the 100 psi needed to simulate low lift opening of the exhaust valve..)

On Patrick Hale's cd he something about this. He talks about a tank of compressed air being used to simulate the ex preasure at EVO, and I took it, then measure the time it takes to empty the tank of air through the ex. port at very low lifts. I'd think you would have to heat the tank of air, to simulate ex cooling from expantion as it travels through the port/header. Then I'd think there's still some air/fuel burning going on at EVO. That may be tough to simulate.

(
Keep in mind though that some builders say one advantage of seat angles above 45° is that they restrict low lift flow, (perhaps thereby providing higher suction and velocity/inertia later in the cycle? )
Much to ponder...

I've heard that too. It makes sence to me. If the increased VE after BDC, helped power more, than the increased pumping loss before BDC, hurt power, it would = a net gain.

Randy

[MadBill]

But despite the theoretical conversion factors, surely 'choking' etc. would occur at lower lifts with the higher velocity of extreme depression flows?

If choking occured at low lift because of extreme depressions during over-lap, wouldn't this have to be a seat angle/size/shape, problem? The air speed in the port would be slow, as compared to the speed of the air through the valve window area. The window area being the limiting factor, due to it being a smaller area at that lift, vs the csa of the rest of the port, would make me think this is where the problem would be.

Randy

I didn't mean low lift as in say .050" or 0.100", I meant choked at a lower point than when tested at 28 in. Hypothetical example: "At 28 in., the port choked at 0.900". At higher depressions it choked at lower lifts, namely 0.800 at 40 in. and 0.650 at 120 in. "

[randy331]

Sorry Bill, I guess I was thinking about your first and third post at the same time, and jumped to the conclusion you were taking about the overlap period in both.

Randy

[Stef]

Some good posts. I've thought of measuring the intake valve lift at BDC and then testing at that lift at a very high depression on the bench. If you can improve the discharge coefficient around that lift point you should improve the intake ramming process. But, how would that effect the rest of the intake process? I guess with intake port and cylinder pressure plots for the engine your working on you could optimise the discharge coefficient in both directions for what you had determined as important lift ranges. However, that's probably nit-picking and you would just be better off (and have more hair) if you just tried to improve the discharge coefficients across the board.

[Ron E]

Some good posts. I've thought of measuring the intake valve lift at BDC and then testing at that lift at a very high depression on the bench. If you can improve the discharge coefficient around that lift point you should improve the intake ramming process. But, how would that effect the rest of the intake process? I guess with intake port and cylinder pressure plots for the engine your working on you could optimise the discharge coefficient in both directions for what you had determined as important lift ranges. However, that's probably nit-picking and you would just be better off (and have more hair) if you just tried to improve the discharge coefficients across the board.

Actually, I wouldn't say your ideas are just nit-picking. Maybe they're not the end-all, (maybe they are, too) but they're proven valid. If you want to continue positive flow past BDC, you have to have good flow at, and just past BDC. Inertia is all you've got at that point (N/A). If your flow "pinches off" some at, or near peak lift, there's nothing to provide a recovery after your valve returns to pre-problem lifts, as there is on a bench. At the very least, it will waste precious inertia-energy in trying to overcome this momentary loss of organization. More people than Smokey have set records testing only at peak lifts. I can't make myself ignore lower lifts, but "results is results". And on high lift applications, I will confess to being much more concerned about the highest 20% of lift than the lower 80%. It's no secret that if you optimize the higher lifts, you'll do so at the expense of low-lift flow. It's one of the very few things where a "side-result" works in our favor.
Good arguments can be made for flow at any lift, but, I've never heard a reasonable argument (on hi-lift N/A) that allows for the compromising of "over-the-nose" flow.

[plovett]

Thanks guys! This is exactly the stuff I wanted to read about. I've been waiting for someone to say that the best way to flow test cylinder heads is to run them on an engine on the dyno.

Seriously, very good info.

paulie

[cboggs]

I've never heard a reasonable argument (on hi-lift N/A) that allows for the compromising of "over-the-nose" flow.

what if some of that high lift flow gets "choked off" in favor of smaller
throats etc, for increased air speed into the chamber?

Curtis