Volumetric Efficiency.

[Trevor]

Originally Posted by shotgunslade

There is a significant different between the behaviour of positive displacement compressors like the Whipple and the centrifugal compressors typically seen with turbochargers. A positive displacement device delivers a fixed volume of gas depending upon its speed of rotation. The actual mass flow will be dependent upon that volumetric flow and the pressure drop on the inlet side of the compressor. Pressure drop on the inlet side will reduce the density of the incoming charge and thus reduce the delivered mass flow. The mass flow is not significantly affected by the pressure drop on the discharge side. The main effect of restriction on the discharge side is to increase the power required to rotate the compressor. This increase in parasitic power is small compared with the increased power generated by the increased mass flow. So, the power increase of a positive displacement device is not very sensitive to pressure drop between the compressor and the engine, and dynamic tuning of that conduit has little effect. Mass flow through a centrifugal compressor on the other hand is dependent upon pressure drop on both sides of the compressor as the compressor generates a pressure differential without creating a fixed volumetric flow. Thus tuning and resonant design of the conduit between the centrifugal compressor and the engine can generate increased mass flow of air into the engine, by reducing the pressure against which the compressor is working. A centrifugal compressor can be deadheaded without destroying anything, because it cannot generate flow at a given speed of rotation against backpressure above a certain level,. A positive displacement compressor, if deadheaded, will either explode something on the discharge side, or will break the compressor drive.

Well spoken and spot on !

Even so and in accordance with the above, the effects of a resonant design originally intended for a naturally aspirated engine will not be at best when matched with a centrifugal type supercharger.

I am fully aware that certainly you with a shotgun and most others, will appreciate this obvious fact, but felt it necessary to re inforce the point in view of previous strange statements.

Cheers, Trevor.

[shotgunslade]

Trevor:

I still think there could be some advantage to tuning the conduit between the centrifugal compressor and the engine. The centrifugal compressor is creating a higher pressure, higher density engine intake resource on its downstream side. Granted that additional pressure and density will automatically translate into higher power output for the engine. However, there is not really a one to one relationship between compressor rotation and mass flow, so dynamic and resonant effects could increase mass flow into the cylinders, allowing the centrifugal compressor to pump more air. A decrease in total pressure across the compressor will simply move the compressor state along its curve, resulting in more mass flow into the engine. The problem is that the velocity of the pressure waves in the intake fluid (air) caused by the sequential opening and closing of intake valves will vary with the density of the fluid. The density will vary with the amount of boost, which will vary according to the state of the engine, so calculating the optimal length of any ram intake manifolds is much more complicated than with naturally aspirated intake. Anway, with the turbocharger, you could overcome the lack of intake ram effect by just increasing the boost control setpoint.

[mbtoloczko]

Welcome back Trevor. You picked a fine thread to make an appearance! :-)

Originally Posted by Phast SVX

I was under the influence that HP will continue to increase in a positive displacement blower. I dont think tq was really ever the issue.

Seems to me that this was the crux of the discussion on the SC thread. I am getting the impression that people think that since the Whipple SC is a fixed displacement blower, the amount of air being forced into the motor is increaing at a rate proportional the the rpm of the SC. This isn't the situation though. We are controlling boost pressure and air flow is the dependent variable. As the boost pressure rises, air flow into the motor will increase. The Whipple SC will make peak boost at very low rpm, and then control it at the setpoint with the bypass valve. The flow is whatever the engine can do with that increase in pressure. Its really not much different than raising the atmospheric pressure by 7 psi.

[Phast SVX]

Originally Posted by mbtoloczko

Welcome back Trevor. You picked a fine thread to make an appearance! :-)



Seems to me that this was the crux of the discussion on the SC thread. I am getting the impression that people think that since the Whipple SC is a fixed displacement blower, the amount of air being forced into the motor is increaing at a rate proportional the the rpm of the SC. This isn't the situation though. We are controlling boost pressure and air flow is the dependent variable. As the boost pressure rises, air flow into the motor will increase. The Whipple SC will make peak boost at very low rpm, and then control it at the setpoint with the bypass valve. The flow is whatever the engine can do with that increase in pressure. Its really not much different than raising the atmospheric pressure by 7 psi.

guys im done arguing with this. we never said it would be a perfect line,, we said that it would be a differnt curve, and nothing you are saying is telling me anything but the fact that, mychailo, you are simply making your best guess at what you think will happen. I dont know that you have any direct experience with boosting cars of any kind. And im not saying that some of you dont( i know some of you do!) but there is no doubt going to be a differet curve, and a positive displacement blower, with their high adbiatic efficiency, tend to create symmetrical power curves.
anyways im unsubscribed.
phil

[longassname]

Actually that is exactly incorrect. The whipple supercharger is a fixed displacement blower. It does put out an exact amount of air with each rotation and the only thing that will prevent that is what trevor said..a vacuum on the inlet side. If there is no restriction on the inlet side preventing the blower from getting the air into it's lobes it will put the same amount of air out the discharge side with each rotation and it's output will be a 45 degree angle line. On a side note, that is why all of the dyno plots of engines with the whipple super charger are a 45 degree angle line.

The bypass valve does not control "boost." It does not open when boost gets to the desired level. It works exactly the oposite of that. With a positive displacement blower boost is always at the desired level. At any rpm, be it 1 rpm or 9000 rpms boost will always be the same..even if you unplug all the coil packs and the engine wasn't actually running but you used something else to turn the motor over boost would be the same because the blower puts out exactly the same amount of air with each rotation which is always the same ratio of air to the volume of the engine.

The bypass valve opens when the throttle blades are closed. The throttle blades being closed creates the one thing that is capable of preventing the supercharger of putting out it's exact amount of air with each rotation...a vacuum on the inlet side. This vacuum actuates the bypass valve which then opens and creates a tunnel between the discharge and inlet sides to equalize the pressure so that the blower isn't trying to gulp air that isn't there.

Trevor and shotgun were also correct in that the only effects on performance of intake tract's characteristics are on efficiencies and to a much smaller scale than the actual increases and characteristics of the blower's discharge. As I had explained in the other thread. The blower DOES put out a fixed amount of air with each rotation and if that air didn't make it past the valves and into the combustion chamber it would still be in the manifold and with the next rotation the manifold pressure would increase and escalate with each rotation. Significant restriction would produce the observable symptom of increased manifold pressure whouch would take more power to pump against. It is at that time that the characteristics of valvetrain would come back into play and you would need larger valves, more lift, or longer duration to overcome the restriction.

I hope that helps clear things up for you.

Originally Posted by mbtoloczko

Welcome back Trevor. You picked a fine thread to make an appearance! :-)



Seems to me that this was the crux of the discussion on the SC thread. I am getting the impression that people think that since the Whipple SC is a fixed displacement blower, the amount of air being forced into the motor is increaing at a rate proportional the the rpm of the SC. This isn't the situation though. We are controlling boost pressure and air flow is the dependent variable. As the boost pressure rises, air flow into the motor will increase. The Whipple SC will make peak boost at very low rpm, and then control it at the setpoint with the bypass valve. The flow is whatever the engine can do with that increase in pressure. Its really not much different than raising the atmospheric pressure by 7 psi.

[longassname]

Phil, I can understand your frustration but don't get too po'd. Everyone looks at things starting from a different perspective based on their past understandings. They don't mean to missunderstand. Their past beliefs are just strongly held and make it difficult to accept, believe, or even pay attention to something that is conflict with them.

Originally Posted by Phast SVX

guys im done arguing with this. we never said it would be a perfect line,, we said that it would be a differnt curve, and nothing you are saying is telling me anything but the fact that, mychailo, you are simply making your best guess at what you think will happen. I dont know that you have any direct experience with boosting cars of any kind. And im not saying that some of you dont( i know some of you do!) but there is no doubt going to be a differet curve, and a positive displacement blower, with their high adbiatic efficiency, tend to create symmetrical power curves.
anyways im unsubscribed.
phil

[oab_au]

Originally Posted by Trevor

Harvey you are creating a smoke screen by twisting your words during a debate and one which you initiated. You have previously stated here, quote - "anything you do to the engine to increase power, HAS to increase the torque." This statement is incorrect.

Alterations such that effective RPM are increased CAN and often do, increase power, even at the expense of torque; i.e. even if less torque is available there can be more power as a result of alterations.

Regards, Trevor.

Trev, you missed the "same rpms" part. Read it again.

HP can't keep increasing, if the torque is dropping off. Over the same rpm range, the torque has to increase, for the HP to increase. HP is just an effect of torque over time. Anything you do to the engine to increase power, has to increase the torque, or there will be no HP difference.

Harvey.

[oab_au]

We have got a bit off track. The type of blower used will have an effect on the shape of the torque curve. A centrifugal/turbo is not a Positive displacement blower, so its affect on the torque is not constant. A PD blowers output is closer to a constant pressure.

It does not matter what PD blower is fitted, or what the boost is. It is what the resultant VE in the cylinder is, that will affect the torque curve. You can have 7 psi in the manifold, but if the late closing inlet valve lets it leak out, at lower rpms, the VE will still be low. As the rpms rise and the time for the inlet decreases the VE will still drop off.

On the plus side, as the as the Inertia system reaches its peak, it will still raise the VE, and as the engine speed reaches the resonate speed, it will still increase the VE. These effects are built in to the engine, their effects are not negated, because the inlet pressure has been raised.

It is these functions that produce the shape of the torque curve. That is why the curve increases to the maximum, then falls off at maximum rpms. If this was not the case, the torque would just be a flat line, instead of a curve. When the inlet pressure is increased, the VE/torque will increase proportionally, unless the extra mass gas flow is restricted by either inlet or exhaust components.

So the Maximum torque and HP will increase, but will still be produced at the same rpms, with only a slight variation due to heat and pressure changes in the sonic waves.

Harvey.

[AvPPoW]

Woah! Trevor's back!

*waves*

[longassname]

I'm afraid that is not correct with a positive displacement blower. While it is possible for the combination of the valve size, lift, and inlet time to become a restriction and not allow the aircharge which has been pumped into the manifold by the blower to enter the cyllinder this is not a condition under which we operate the engine. This condition causes the pressure in the manifold to increase which at first onset is just an efficiency effect because the increased pressure then forces the aircharge into the cyllinder. As the condition gets worse however manifold pressure will become excessive and the situation will have to be addressed.

If we are pumping air into the engine at a ratio of 1.77 (hint, wink wink) and we are seeing 20 lbs of boost instead of something close to 11 or 12 we don't just drive the car and say well the torque curve is being determined by valve tract. I suppose we could but the reasonable thing to do is to rebuild the engine with larger valves and more lift.

Under the conditions we operate an engine with forced induction where the valve train is not a significant restriction the previous tuning of the intake tract does not determine the ve. The blower and the ratio at which you are driving it is the determining factor with only one exception--how much of your intake charge you are loosing through the exhaust during intake and exhaust valve overlap. Again this is why engines with positive displacement blowers have a power plot that is a 45 degree line which matches the blowers aircharge output. Any restrictions in flow at higher rpms simply increase pressure and require a little more power from the belt to drive the supercharger. The air is going into the cyllinder no ifs ands or buts and your vollumetric efficiency is whatever the ratio is between the amount of air you are pumping and 1/2 the displacement of the engine (a 4 stroke engine inhales it's displacement once every two revolutions). So if we have a pulley ratio of 1.826 and a 1.6 liter compressor we a vollumetric efficiency around 177% at any rpm. If it isn't then the problem is severe enough that we need to rebuild the engine.

Originally Posted by oab_au

We have got a bit off track. The type of blower used will have an effect on the shape of the torque curve. A centrifugal/turbo is not a Positive displacement blower, so its affect on the torque is not constant. A PD blowers output is closer to a constant pressure.

It does not matter what PD blower is fitted, or what the boost is. It is what the resultant VE in the cylinder is, that will affect the torque curve. You can have 7 psi in the manifold, but if the late closing inlet valve lets it leak out, at lower rpms, the VE will still be low. As the rpms rise and the time for the inlet decreases the VE will still drop off.

On the plus side, as the as the Inertia system reaches its peak, it will still raise the VE, and as the engine speed reaches the resonate speed, it will still increase the VE. These effects are built in to the engine, their effects are not negated, because the inlet pressure has been raised.

It is these functions that produce the shape of the torque curve. That is why the curve increases to the maximum, then falls off at maximum rpms. If this was not the case, the torque would just be a flat line, instead of a curve. When the inlet pressure is increased, the VE/torque will increase proportionally, unless the extra mass gas flow is restricted by either inlet or exhaust components.

So the Maximum torque and HP will increase, but will still be produced at the same rpms, with only a slight variation due to heat and pressure changes in the sonic waves.

Harvey.

[mbtoloczko]

Originally Posted by longassname

I hope that helps clear things up for you.

Yep, I totally understand now. Thanks for tolerating my lack of understanding. I am very curious though to see what happens at higher rpm. As you are saying, I think that at some point the engine will not be able to efficiently receive the air that the SC is pumping, and manifold pressure will begin to rise. Wondering what rpm where that will occur...

[longassname]

Hey, no problem man. I'm on your side that's why I bother explaining the stuff.

Mike may well find out now that he will have a manual transmission. The reasonably low 6500 rpms we get with the automatic are a a big part of what makes so much of this stuff work for us. It lets stage 2 work for turbos despite the little fuel injectors, keeps our valves under control despite the light valve springs, and lets us pump more air before we have to address the valve train. I didn't even realize how high we went on the "boost" on Mike's car when we did it. I just cut the pulley to the smallest size I could to get hood clearance. We had gone more hours than i care to remember without sleep at that point. For all I know it might be too much already. We didn't get to drive it very far. I can cut the crank pulleys smaller easily enough t hough. I'll figure it out on my car like I had originally planned. I have extensive logging capabilities.

Hopefully the na guys who are digging into the heads will make some significant progress soon. It's allready been done. One of our customer's already has it done. He got bigger valves and springs that allow much more lift allong with a lot of head work and big cams courtesy of pantera.
I'm hoping not to have to dig into the valve train on my high boost engine but who knows?

Originally Posted by mbtoloczko

Yep, I totally understand now. Thanks for tolerating my lack of understanding. I am very curious though to see what happens at higher rpm. As you are saying, I think that at some point the engine will not be able to efficiently receive the air that the SC is pumping, and manifold pressure will begin to rise. Wondering what rpm where that will occur...

[alltrac]

Originally Posted by longassname

One of our customer's already has it done. He got bigger valves and springs that allow much more lift allong with a lot of head work and big cams courtesy of pantera.
I'm hoping not to have to dig into the valve train on my high boost engine but who knows?

That would be Yon.

[svxistentialist]

Originally Posted by Trevor

Harvey you are creating a smoke screen by twisting your words during a debate and one which you initiated. You have previously stated here, quote - "anything you do to the engine to increase power, HAS to increase the torque." This statement is incorrect.

Alterations such that effective RPM are increased CAN and often do, increase power, even at the expense of torque; i.e. even if less torque is available there can be more power as a result of alterations.

Regards, Trevor.

Hi Trevor!

Just heard you were back. Nice to hear from you again, and Happy belated Birthday.

I don't agree with the above, but on the same semantic level you based your argument. When Harvey said "anything you do to the engine to increase power", we both know he meant anything over the normal rev range of the engine.

Since he is already a biker, albeit a desmodromic one, he must surely understand that fine balancing and increasing the rev range can give more HP, the God of bikers, but will yield fecc all torque.

I am firmly in the American and antipodean camp when it comes to cubic displacement. High revving low capacity engines suck, is my considered opinion.

Joe

[oab_au]

Originally Posted by longassname

I'm afraid that is not correct with a positive displacement blower. While it is possible for the combination of the valve size, lift, and inlet time to become a restriction and not allow the aircharge which has been pumped into the manifold by the blower to enter the cyllinder this is not a condition under which we operate the engine. This condition causes the pressure in the manifold to increase which at first onset is just an efficiency effect because the increased pressure then forces the aircharge into the cyllinder. As the condition gets worse however manifold pressure will become excessive and the situation will have to be addressed.

If we are pumping air into the engine at a ratio of 1.77 (hint, wink wink) and we are seeing 20 lbs of boost instead of something close to 11 or 12 we don't just drive the car and say well the torque curve is being determined by valve tract. I suppose we could but the reasonable thing to do is to rebuild the engine with larger valves and more lift.

Well addressing this bit first, we are looking at your blower installation. It does not operate under these conditions, that we know of. So this does not apply.


Under the conditions we operate an engine with forced induction where the valve train is not a significant restriction the previous tuning of the intake tract does not determine the ve. The blower and the ratio at which you are driving it is the determining factor with only one exception--how much of your intake charge you are loosing through the exhaust during intake and exhaust valve overlap.

Now Mike you can't dispel the tuning of the inlet tract, but accept a charge loss due to valve overlap. All the functions of the engines tuning have an effect on the VE, the boost just raises it by the percentage of pressure applied. The actually VE curve is the result of this, not the pressure increase.

Again this is why engines with positive displacement blowers have a power plot that is a 45 degree line which matches the blowers aircharge output. Any restrictions in flow at higher rpms simply increase pressure and require a little more power from the belt to drive the supercharger. The air is going into the cyllinder no ifs ands or buts and your vollumetric efficiency is whatever the ratio is between the amount of air you are pumping and 1/2 the displacement of the engine

Well " no ifs or buts" I guess that does not include the "one exception" that you made for valve overlap??. If you accept this condition then you can't discount the effects of the late closing inlet valve, or the assistance of the Inertia or the resonate waves

(a 4 stroke engine inhales it's displacement once every two revolutions). So if we have a pulley ratio of 1.826 and a 1.6 liter compressor we a vollumetric efficiency around 177% at any rpm.

Finally, you have miss understood the meaning of Volumetric Efficiency. It is not the pressure coming out of the blower, it is the pressure trapped in the cylinder, when the inlet valve closes. As with atmospheric pressure, there is losses and gains made by the mechanics of the engine that vary that pressure. Raising the pressure does not change this.

If it isn't then the problem is severe enough that we need to rebuild the engine.

Harvey. ...