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

[turbo2256b]

Sizing of the turbo has a lot to do with were the power band ends up.

I agree. One the compressor hits the mass flow limit, the power trails off slowly with rpm because of higher mechanical losses and (in some cases) foolish attempts to overspin the turbo simply adding heat to the charge.

Its very important to find surge maps and interpret them for a for a build.

[ptuomov]

Sizing of the turbo has a lot to do with were the power band ends up.

I agree. One the compressor hits the mass flow limit, the power trails off slowly with rpm because of higher mechanical losses and (in some cases) foolish attempts to overspin the turbo simply adding heat to the charge.

Its very important to find surge maps and interpret them for a for a build.

Surge = too little flow and too much boost pressure, air starts flowing backwards in the compressor
Mass flow limit (or sonic choke) = asking for too much flow, compressor tip speed going near sonic or supersonic

As you say, one has to live between these two limits.

[ptuomov]

It's down to the size of your turbo and the length of time the air takes to get there, put a smaller turbo on and see it drop.

I don't understand.

Sizing of the turbo has a lot to do with were the power band ends up.

I agree. One the compressor hits the mass flow limit, the power trails off slowly with rpm because of higher mechanical losses and (in some cases) foolish attempts to overspin the turbo simply adding heat to the charge.

You don't understand, but you agree?

I don’t understand what you mean by the length of the time the air takes to get there.

[ptuomov]

Here's a question about turbocharging a normally aspirated engine. With the same cams, the peak power rpm seems to move up to a higher rpms. So for example if the engine makes peak power of 508hp at 5800rpm when normally aspirated, with 6 psi of turbo boost it makes 713hp at 6200rpm. Peak torque rpm moves only little, from 505lbft at 4700rpm to 690lbft at 4800rpm.

Why is this?

Let me try and answer your question, with another question.
what happens in an engine, that makes the power increase as the RPM increases, until it hits an RPM, where it no longer increases power ?

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.

[user-23911]

The initial statement is wrong.

[CamKing]

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 ?

[Belgian1979]

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 ?

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 ?

[4vpc]

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 ?

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.

[ptuomov]

The initial statement is wrong.

It’s not, as long as you’re in the good part of the efficiency island and asking for constant boost.

[user-23911]

The initial statement is wrong.

Any cam which works well with N/A will also work well with boost.

Take a look at the torque curve when N/A.
Add the boost curve to it and that's the new torque curve.

What you failed to mention is the sizing of the turbos with respect to the engine size.
That's what makes the difference.

in the case of a small turbo, boost comes on early and drops early. Eg, boost comes on at 1K RPM, max boost 2500, holds to 5K then drops by 33% at 7K
In the case of a big turbo, boost comes on late and stays up. Eg boost comes up at 3K, full on by 4K and stays flat to red line.


In the case of a medium sized turbo running low boost, the boost curve is flat, the torque curve will be the same as the equivalent N/A torque curve apart from at the very bottom where there's no boost.
It's the added variable that you conveniently forgot about.

[MadBill]

The initial statement is wrong.

Any cam which works well with N/A will also work well with boost...

I don't know much about turbos, so just to clarify: if you have a highly-tuned NA engine running a lot of overlap, say 80° and you bolt on an ill-matched turbo system that generates perhaps 2X exhaust backpressure vs. boost psi, it will still run like gangbusters?

[user-23911]

Not with a badly matched turbo.
But with the correctly matched turbo, it'll haul.
A badly matched turbo won't work well with any combo.
A big cam needs a big turbo and a small cam needs a small turbo.

Cams normally sold as "turbo cams" are like "towing cams" giving low RPM torque and running out of breath early.

[user-23911]

A small cam with a big turbo.........the turbo is just winding up as the cam is dropping the torque.
A big cam with a small turbo........the turbine is too restrictive at higher RPM, the change in cross section area causes unwanted wave reflections, EGT gets too high and you get an onset of detonation.
Whatever else is in the engine is pretty much irrelevant apart from the CR which needs to be turbo friendly for the octane used. 8 to 1 is usually safe up to 15 pounds boost.
I've run several combinations of cam and turbo over a number of years. My present (N/A)cams are over 300 deg duration and about 110 deg overlap with big enough turbos for over 2 bar boost. I've also broken plenty of pistons and other parts while learning.

[vannik]

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.

[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.

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?