Power band for turbo vs. normally aspirated with the same cams

[vannik]

Does the inlet to exhaust valve and port size ratio change? In my head I can imagine we are putting more in (which we can do by upping the pressure), but are still governed by the same laws (of pressure differential) on the exhaust - IE, we can't really force it out, but we've put more in (the cylinder) therefore we need a bigger exhaust valve and port. Right or wrong?

The cylinder pressure is also higher during the blowdown and exhausting phases, thus higher density so the same holds for the exhaust valve, same ratio as for NA engine.

[ptuomov]

I think of this as a product of three things: volumetric efficiency, combustion efficiency, and mechanical efficiency. As long as we don’t have exhaust blowdown interference, combustion efficiency is probably not a huge factor. So it’s the VE vs ME.

Simpler then that.
What happens to the lbs/hr of air, as the RPM's increase to max HP, then beyond that ?

Good intercooling, well matched turbo, and intake to exhaust manifolds is absolute pressure ratio of one leads me to say the normally aspirated lb/h air consumption rate gets simply multiplied by the intake manifold density ratio when the engine is turbocharged. Holding combustion efficiency constant, that means that gross power should also scale with that density ratio.

In general, I’d guess that the peak power rpm is at the point where the air consumption of the engine grows with the rpm at the same rate as the mechanical inefficiencies grow with rpm. I’d expect the mass air flow rate slightly increase with rpm even after peak power rpm, just not fast enough to offset the increased losses to mechanical inefficiencies.

My explanation for why well matched turbo moves the peak power rpm up by 5% or so is that the mechanical inefficiencies stay the same while the mass air flow rate is multiplied by the intake manifold density ratio.

[CamKing]

Simpler then that.
What happens to the lbs/hr of air, as the RPM's increase to max HP, then beyond that ?

A guess
before max hp point the air will follow the piston and crashes into the piston when it does, increasing density. Over that point the air can't follow the piston and pulls the air molecules apart decreasing density ?

Engine demand outruns supply so power drops.

For a given duration and port, you reach an RPM where the density of the flow starts to decrease. This is where the power starts to fall. Increasing the density in the ports via supercharging, will slightly move this point up in the RPM band.

As for cam'ing a turbo engine, it depends on the application.
For IndyCar, where you're always "spooled up", we design the cam to the same intake duration, as if it was an N/A engine. The exhaust duration and LSA are different.
For drag racing, we normally design the cam to make peak HP about 400-600rpm lower then if it was N/A, and let the turbo hold the power peak up those extra 400-600rpm.

[CamKing]

My explanation for why well matched turbo moves the peak power rpm up by 5% or so is that the mechanical inefficiencies stay the same while the mass air flow rate is multiplied by the intake manifold density ratio.

I'm sure that's part of it.
We see on restricted inlet engines, a RPM point where the air mass into the engine is still increasing, but the power is dropping. You reach a point where the power you gain from the added mass is less then the power required to pull that extra mass.
This would be the opposite on a turbo engine. By increasing the mass via turbo, you would move the point up in the RPM band, where the power required to pull the extra mass becomes greater then the power provided from the extra mass.

[Orr89rocz]

For a given duration and port, you reach an RPM where the density of the flow starts to decrease. This is where the power starts to fall. Increasing the density in the ports via supercharging, will slightly move this point up in the RPM band.

I've seen this on long runner tuned port injection blown stuff and some on other turbo charged deals.

The centri blown tpi car had increasing boost via rpm so increased mass flow kept hp increasing with rpm or holding steady with rpm and not dropping off like na normally would. Tpi 18-22" long runners on a 383 would peak no higher than 4500 most of the time especially with a 224 deg cam lobe hyd roller. But that deal made 4-5 psi more boost at 6000 vs 4500. Power increased and held on to 6000 rpm

If you control the boost curve via electronic controller, the turbo controls the rpm range if its big enough to flow what the engine needs. Extend the power curve if you can increase mass flow with the given cam.

[Orr89rocz]

On a well matched engine with close to a boost pressure to back pressure of 1:

1. The major difference between a NA and a boosted engine is an increase in density of the fluid, so velocities all stay similar, mass flow increases because of the density increase. So porting, valve sizing, camming etc should all be similar,

2. The minor difference is the increase in temperature, leading to an increase in wave speed and the tuned rpm point moving to slightly higher rpms.

Even with a boost to backpressure of 1:1 or even less, its still usually has more exhaust pressure than you'd see in a na car. Na car seems very sensitive to exhaust scavenging by the header system.

1: does a turbo setup scavenge well with the turbine being in the collector?

2: in a supercharged system with nothing in the header, you should beable to get na like scavenging correct? Intake pressure should be higher than exhaust pressure by a good margin, unless during blow down the higher cyl pressure due to increased mass flow and power per cyl now dumps into the header so overall system differential pressure is similar to na deals? Cam timing seems much different on blown cars vs turbo

[4vpc]

Does the inlet to exhaust valve and port size ratio change? In my head I can imagine we are putting more in (which we can do by upping the pressure), but are still governed by the same laws (of pressure differential) on the exhaust - IE, we can't really force it out, but we've put more in (the cylinder) therefore we need a bigger exhaust valve and port. Right or wrong?

The cylinder pressure is also higher during the blowdown and exhausting phases, thus higher density so the same holds for the exhaust valve, same ratio as for NA engine.

Thanks for the clarification. I've been seeing the odd new engine here and there with quite big ex valves and was wondering why, one of them was a Ford Ecoboost engine, maybe 2 ltr or 2.3 perhaps.

[4vpc]

On a well matched engine with close to a boost pressure to back pressure of 1:

1. The major difference between a NA and a boosted engine is an increase in density of the fluid, so velocities all stay similar, mass flow increases because of the density increase. So porting, valve sizing, camming etc should all be similar,

2. The minor difference is the increase in temperature, leading to an increase in wave speed and the tuned rpm point moving to slightly higher rpms.

Even with a boost to backpressure of 1:1 or even less, its still usually has more exhaust pressure than you'd see in a na car. Na car seems very sensitive to exhaust scavenging by the header system.

1: does a turbo setup scavenge well with the turbine being in the collector?

2: in a supercharged system with nothing in the header, you should beable to get na like scavenging correct? Intake pressure should be higher than exhaust pressure by a good margin, unless during blow down the higher cyl pressure due to increased mass flow and power per cyl now dumps into the header so overall system differential pressure is similar to na deals? Cam timing seems much different on blown cars vs turbo

Like Mike said it's down to application, but also turbo size. A decent sized turbo can take a huge amount of overlap and spool early, much more than an N/A and the duration on the ex cam can often be 10 - 15' more than the inlet.

[ptuomov]

On a well matched engine with close to a boost pressure to back pressure of 1:

1. The major difference between a NA and a boosted engine is an increase in density of the fluid, so velocities all stay similar, mass flow increases because of the density increase. So porting, valve sizing, camming etc should all be similar,

2. The minor difference is the increase in temperature, leading to an increase in wave speed and the tuned rpm point moving to slightly higher rpms.

Is the second effect sufficient to explain the shift in the peak power rpm of more than 5%? Or is it the declining relative importance of mechanical inefficiencies that shifts the peak power rpm with boost?

[vannik]

2. The minor difference is the increase in temperature, leading to an increase in wave speed and the tuned rpm point moving to slightly higher rpms.

Is the second effect sufficient to explain the shift in the peak power rpm of more than 5%? Or is it the declining relative importance of mechanical inefficiencies that shifts the peak power rpm with boost?

I am not sure, lack of data!

[ptuomov]

viewtopic.php?f=15&t=57095&start=15#p820841

[modok]

Wow I'm years late, but here is how I see it.
top of the RPM range NA engine, the torque actually drops off faster then the VE, WHY?
Two ideas.....
because friction keeps going up,
and also as VE drops WHILE piston speed is increasing, you don't get as good a burn.

Often higher CR seems to help "both ends" of the powerband, if you get a better burn, right? same for turbo

Higher charge density.....most of the time just burns more efficient.
The more molecules you can pack in a small space, just seems to work better all around.
You can't decrease the chamber size and surface area past practical limits, but if you can increase the charge density.... then in relative terms, you have dome the same thing..
And it is the same way for friction.

BESIDES the termperature VS runner length thing mentioned on page one, which I think is true for sure.

[3V Performance]

With boost during the overlap cycle it did a better job of cleansing the chamber where the n/a combo struggled ( rpm point of peak hp ). To me it's telling me the n/a cam needed work on the exhaust side.

[CamKing]

With an N/A engine, you will see the pressure in the manifold drop, as the RPM's increase past peak HP RPM. This caused by the cam and/or port no longer being efficient past peak HP RPM.
With a turbo, if the turbo is efficient to a higher RPM, you won't see that drop off in manifold pressure as early, and you will see an increase in peak HP rpm.

[ptuomov]

With an N/A engine, you will see the pressure in the manifold drop, as the RPM's increase past peak HP RPM. This caused by the cam and/or port no longer being efficient past peak HP RPM.
With a turbo, if the turbo is efficient to a higher RPM, you won't see that drop off in manifold pressure as early, and you will see an increase in peak HP rpm.

When you write this, where are you measuring the intake manifold pressure?