Where dos manifold vacum come from
11-23-2007, 01:28 AM
#1
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Where dos manifold vacum come from If a vehicle is cruising down the road at 2000 rpm(manual trans) and you come to a hill, why does the air pressure change in the intake. I would think that the pistons down travel would cause a certen amount of suction because of intake restriction, but why would that change if the engine stays at a constant rpm. In other words what lets a motor make only 75 hp cruising down the road when it has the potential to make 200hp at that same rpm.
11-23-2007, 07:30 PM
#5
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FWIW:
The engine produces power on demand by controlling the amount of air it eats. The throttle controls the air admitted so it is just a restrictor.
Think of an engine with 10 mm throttle body blades rather than 48 or 52 or 58. With them wide open, power produced would be fairly small.
To hold a constant speed on flat, uphill and downhill roads, either the cruise control or the driver's foot opens and closes the throttle blade(s) to control the air admitted.
The more you throttle (restrict) the air, the higher the vacuum inside the manifold. Put you hand over the suction hose on a running vacuum cleaner and you'll get the highest vacuum.
11-23-2007, 08:54 PM
#6
If a vehicle is cruising down the road at 2000 rpm(manual trans) and you come to a hill, why does the air pressure change in the intake. I would think that the pistons down travel would cause a certen amount of suction because of intake restriction, but why would that change if the engine stays at a constant rpm. In other words what lets a motor make only 75 hp cruising down the road when it has the potential to make 200hp at that same rpm.
Manifold vacuum comes from the pistons moving down on the intake stroke.
Are you asking why engines make less power if they are at higher altitudes?
I believe it's because of the change in atmospheric pressure. At sea level the pressure is greater than 5,000 feet above sea level. My guess is with greater pressure outside, it is trying to equalize (in the cylinders). This might help force more air into the motor. I could be wrong. The MAP sensor is what monitors the pressure so it can adjust the fuel ratio.
11-24-2007, 04:07 PM
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Are you asking why engines make less power if they are at higher altitudes?
I believe it's because of the change in atmospheric pressure. At sea level the pressure is greater than 5,000 feet above sea level. My guess is with greater pressure outside, it is trying to equalize (in the cylinders). This might help force more air into the motor. I could be wrong. The MAP sensor is what monitors the pressure so it can adjust the fuel ratio.
I believe it's because of the change in atmospheric pressure. At sea level the pressure is greater than 5,000 feet above sea level. My guess is with greater pressure outside, it is trying to equalize (in the cylinders). This might help force more air into the motor. I could be wrong. The MAP sensor is what monitors the pressure so it can adjust the fuel ratio.
A MAF sensor also adjusts for altitude, just the newer OBD2 systems use both for accuracy.
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11-24-2007, 04:25 PM
#8
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Kind of. Although this is the source, there needs to be one other event to enable vacuum, and that is a restriction. On a <100% VE engine, you'll see vacuum all of the time, because of the VE it has a restriction, even at WOT. But on a high VE engine (BMW M motors, F1, etc), it is possible to see no vacuum in the intake tract at WOT (This is good). And in fact they can even achieve "boost" into the cylinder without forced induction (Intake harmonic tuning allowing more air into the cylinder than it has volume for = boost).
You are correct, except I believe the air pressure alone isn't the reason for the power loss, though it's the biggest player. The compilation of the air is also different IIRC (Less oxygen by percentage than at sea level).
A MAF sensor also adjusts for altitude, just the newer OBD2 systems use both for accuracy.
You are correct, except I believe the air pressure alone isn't the reason for the power loss, though it's the biggest player. The compilation of the air is also different IIRC (Less oxygen by percentage than at sea level).
A MAF sensor also adjusts for altitude, just the newer OBD2 systems use both for accuracy.
The % of O2 doesn't change, just the total amount because of the less dense atmosphere. It's still about 20% +/- a little, same as sea level, but there is less of all the gasses per cubic foot.
Ever try to jog at the top of Pikes Peak? If you are a smoker, just walking is a chore up there.
11-24-2007, 04:50 PM
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Perhaps you've been in CO too long.
The % of O2 doesn't change, just the total amount because of the less dense atmosphere. It's still about 20% +/- a little, same as sea level, but there is less of all the gasses per cubic foot.
Ever try to jog at the top of Pikes Peak? If you are a smoker, just walking is a chore up there.
The % of O2 doesn't change, just the total amount because of the less dense atmosphere. It's still about 20% +/- a little, same as sea level, but there is less of all the gasses per cubic foot.
Ever try to jog at the top of Pikes Peak? If you are a smoker, just walking is a chore up there.
I climbed Pikes Peak, that was a chore, as were all of the 6 14ers I've done. I also drove up Pikes Peak, in a highly spirited fashion. Nearing the top, I couldn't even get the stock Fiero sideways...ON DIRT!
11-24-2007, 07:59 PM
#10
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I'm old. My first time at the Peak was as a GM engineer. We did lots of cooling tests there. Probably still do. A 421 Bonneville could barely churn up a little gravel @ 14110 ft. MSL. Neither could I!
11-26-2007, 09:49 PM
#11
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Bingo!
FWIW:
The engine produces power on demand by controlling the amount of air it eats. The throttle controls the air admitted so it is just a restrictor.
Think of an engine with 10 mm throttle body blades rather than 48 or 52 or 58. With them wide open, power produced would be fairly small.
To hold a constant speed on flat, uphill and downhill roads, either the cruise control or the driver's foot opens and closes the throttle blade(s) to control the air admitted.
The more you throttle (restrict) the air, the higher the vacuum inside the manifold. Put you hand over the suction hose on a running vacuum cleaner and you'll get the highest vacuum.
FWIW:
The engine produces power on demand by controlling the amount of air it eats. The throttle controls the air admitted so it is just a restrictor.
Think of an engine with 10 mm throttle body blades rather than 48 or 52 or 58. With them wide open, power produced would be fairly small.
To hold a constant speed on flat, uphill and downhill roads, either the cruise control or the driver's foot opens and closes the throttle blade(s) to control the air admitted.
The more you throttle (restrict) the air, the higher the vacuum inside the manifold. Put you hand over the suction hose on a running vacuum cleaner and you'll get the highest vacuum.
So your saying the motor is naturally trying to "rev up" and the throttle has to "Choke it down".
If the intake manifold was removed totally and the intake ports hoged out...........ok you get the picture........would the engine continue to rev to higher ground infanetly?
The next thing is, if the same motor was put in a car, you come to a hill, I'm assumeing(however u spell that) the rpms would drop as the load increased, in the same maner a motor at wot would.
11-27-2007, 08:16 AM
#12
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So your saying the motor is naturally trying to "rev up" and the throttle has to "Choke it down".
If the intake manifold was removed totally and the intake ports hoged out...........ok you get the picture........would the engine continue to rev to higher ground infanetly?
The next thing is, if the same motor was put in a car, you come to a hill, I'm assumeing(however u spell that) the rpms would drop as the load increased, in the same maner a motor at wot would.
If the intake manifold was removed totally and the intake ports hoged out...........ok you get the picture........would the engine continue to rev to higher ground infanetly?
The next thing is, if the same motor was put in a car, you come to a hill, I'm assumeing(however u spell that) the rpms would drop as the load increased, in the same maner a motor at wot would.
At WOT under no load the engine will rev to its mechanical or electronically controlled maximum rpm. Under load it will produce it's maximum at whatever the rpm the load lets it go to.
If you want to maintain the vehicle's speed when climbing a hill, the engine needs to provide more power than it did on level road, so the throttle is opened to allow more air in to produce more power. That's assuming you were not already cruising at WOT (@ 150 or so).
At WOT a vehicle will accelerate to it's maximum speed or rpm cutoff, or pcm limited speed, depending on the load (drag and steepness of the hill).
11-29-2007, 02:47 PM
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turbos dont depend on engine load to spool, turbos depend on exhaust flow.
In neutral u will have no load .therefore it doesnt take much power to rev an engine. u could still spool a turbo in neutral using a rev limiter. people with manual transmissions do it all the time .
In neutral u will have no load .therefore it doesnt take much power to rev an engine. u could still spool a turbo in neutral using a rev limiter. people with manual transmissions do it all the time .
12-20-2007, 03:19 PM
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turbos dont depend on engine load to spool, turbos depend on exhaust flow.
In neutral u will have no load .therefore it doesnt take much power to rev an engine. u could still spool a turbo in neutral using a rev limiter. people with manual transmissions do it all the time .
In neutral u will have no load .therefore it doesnt take much power to rev an engine. u could still spool a turbo in neutral using a rev limiter. people with manual transmissions do it all the time .
If a motor has no load (low manifold vaccum) and wot, the motor will continue to rev up untill there is a load on it (high manifold vaccum or a crackshaft load). Valve train usually starts getting shaeky before the ultimate rpm is reached. I don't know all the tek about turbos but i would think that in a perfect world with intake temp, and valve train ignored, a turbo motor would rev infanatly. Any thoughts
12-21-2007, 09:20 AM
#15
As for the "throttle", the word throttle literally means to choke I believe. If you "throttle" someone, you are grabbing them by the neck and choking them. An engine, if "unthrottled" would increase RPM as per fuel if I understand this correctly.
01-14-2008, 03:52 PM
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Originally Posted by topend
turbos dont depend on engine load to spool, turbos depend on exhaust flow.
In neutral u will have no load .therefore it doesnt take much power to rev an engine. u could still spool a turbo in neutral using a rev limiter. people with manual transmissions do it all the time .
In neutral u will have no load .therefore it doesnt take much power to rev an engine. u could still spool a turbo in neutral using a rev limiter. people with manual transmissions do it all the time .
If a motor has no load (low manifold vaccum) and wot, the motor will continue to rev up untill there is a load on it (high manifold vaccum or a crackshaft load). Valve train usually starts getting shaeky before the ultimate rpm is reached. I don't know all the tek about turbos but i would think that in a perfect world with intake temp, and valve train ignored, a turbo motor would rev infanatly. Any thoughts
01-15-2008, 10:02 AM
#19
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This is a good topic - I have another point / question to ask. How is the maniflod vaccum related to fuel consumption?
Does an engine in neutral at half-throttle use the same ammount of fuel as an engine under load at half-throttle? For the same throttle position, does an engine under load use more fuel than an engine with no load?
Not trying to highjack the thread, as I think the two topics are related.
Does an engine in neutral at half-throttle use the same ammount of fuel as an engine under load at half-throttle? For the same throttle position, does an engine under load use more fuel than an engine with no load?
Not trying to highjack the thread, as I think the two topics are related.
01-15-2008, 12:06 PM
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and it is this airflow that spins the turbines and spools your turbo.
But if I floor a car in neutral, the reason it will not build boost is because it will get to redline so quickly N/A, that the turbo doesn't have enough exhaust volume to overcome the engine's N/A airflow. So say at 4,000rpm, if I had a restriction to slow the engine's acceleration down, then it would spool the turbo. However when there is no load (Restriction), the engine is going to increase to 4500, 5000rpm very very quickly, faster than the turbocharger can overcome it's N/A flow. And if it can't overcome this, it can't boost.
Well once you put it in gear, say 4th, and sit at 4,000rpm WOT, you are flowing the same amount of exhaust, but the engine isn't accelerating very fast compared to neutral revving. So you are spending more time, which means more cubic feet, of exhaust into the turbine, without increasing your air demand. Thus the turbocharger can overcome the N/A flow easily, and pressurize the system.
Furthermore, a load increases exhaust temperatures dramatically, which further increases the energy put into the turbine.