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

[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? Is it just that the mechanical inefficiencies increase with rpm and with boost they become relatively less important compared to the gross power produced? Or something more subtle than that, like how the intake manifold tuning changes?

[cab0154]

I can tell you my opinion from forever ago when I messed with turbos....but its just that, what i found. First I think it depends on the ratio of cross section of the head over cylinder displacement, so if it is something like an LS you will gain more rpm then with an old efi 302 ford. also, intake runner length and cross section: for a given cross section the shorter the runner the harder you can turn it before going into stall. as to why: i think it is because the turbo will keep pushing air though it until the port cant take anymore. when i first had my turbo kit on my car 20 years ago the powerband at low boost was much wider than it was when i would crank up the wastegate. when i would crank the boost up, power would go up but peak would fall to a lower rpm and fall off quicker. but that was probably more due to the small cross section, long runner intakes available back then. that and according to the guys who built my system I was also close to the end of the compressor map based on the power i was making. so a larger compressor might have helped.

[ptuomov]

I can tell you my opinion from forever ago when I messed with turbos....but its just that, what i found. First I think it depends on the ratio of cross section of the head over cylinder displacement, so if it is something like an LS you will gain more rpm then with an old efi 302 ford. also, intake runner length and cross section: for a given cross section the shorter the runner the harder you can turn it before going into stall. as to why: i think it is because the turbo will keep pushing air though it until the port cant take anymore. when i first had my turbo kit on my car 20 years ago the powerband at low boost was much wider than it was when i would crank up the wastegate. when i would crank the boost up, power would go up but peak would fall to a lower rpm and fall off quicker. but that was probably more due to the small cross section, long runner intakes available back then. that and according to the guys who built my system I was also close to the end of the compressor map based on the power i was making. so a larger compressor might have helped.

My guess is that you hit the "sonic choke" or the mass air flow limit of your compressor.

As long as I'm not close to the mass flow limit of the compressor at peak power rpm, I find that peak power rpm goes up with higher boost pressure with everything else (like cam, intake, and compression ratio) held constant. Then when I hit the mass flow limit of the compressor, the peak power rpm starts moving back down in rpm with even more boost.

[cab0154]

i think it went into sonic with the long runner on it. it made like 460rwhp at 5200 at 12lbs. before with the long runner at 6lbs it make 380 at 5600. so then i put the short runner on it and it made 490rwhp at 5800 with 12lbs, but was done at that point. so i am pretty sure the compressor was done, but i didnt push the boost past 12lbs with the box intake since it was just my street car. that combo was just a stock 93 5 liter mustang shortblock and cam, with a set of gt40 irons with 1.7s, a gt40 intake with the lower matched to a 1250. the short runner was a downs box. so small cross section through an aod trans. the turbo kit was a 60-1 street kit from turbo technology. that was back in 1997. a lot has changed.

[4vpc]

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.
If you want to keep the same set up, but lower the peak then change the cam timing (or even cam profiles) and see it fall too. This will see idle and emissions suffer though.

[ptuomov]

i think it went into sonic with the long runner on it. it made like 460rwhp at 5200 at 12lbs. before with the long runner at 6lbs it make 380 at 5600. so then i put the short runner on it and it made 490rwhp at 5800 with 12lbs, but was done at that point. so i am pretty sure the compressor was done, but i didnt push the boost past 12lbs with the box intake since it was just my street car. that combo was just a stock 93 5 liter mustang shortblock and cam, with a set of gt40 irons with 1.7s, a gt40 intake with the lower matched to a 1250. the short runner was a downs box. so small cross section through an aod trans. the turbo kit was a 60-1 street kit from turbo technology. that was back in 1997. a lot has changed.

If you mean "went sonic" at the compressor, then my guess would be that, too. The slightly lower power level by 30hp with longer runners could be additional pumping losses and worse intake port / exhaust port pressure ratio, if the compressor was literally tapped out.

I'm just generally interested in the movement of the peak power rpm as the requested boost is increased.

[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.

[n2xlr8n]

I've been told I make mental models for all things technical due to my lack of formal education, so here goes:

I can see the added mass (with boost) promoting much better cylinder filling (see posts on pressure diff above/below the intake valve) for a given cylinder head / intake tract.

In other words, when your "same" cam would be done at 5800, the mass flow into the cylinder provided by the turbo is allowing your engine much, much more VE, hence more power at a higher rpm.

In a radiation physics sense, it would be analogous to adjusting the RF energy input into a standing wave accelerator, but keeping the resonant frequency the same; in most cases resulting in higher MV output up to a certain point.

But my mental models don't always align with reality

[ptuomov]

I've been told I make mental models for all things technical due to my lack of formal education, so here goes:

I can see the added mass (with boost) promoting much better cylinder filling (see posts on pressure diff above/below the intake valve) for a given cylinder head / intake tract.

In other words, when your "same" cam would be done at 5800, the mass flow into the cylinder provided by the turbo is allowing your engine much, much more VE, hence more power at a higher rpm.

In a radiation physics sense, it would be analogous to adjusting the RF energy input into a standing wave accelerator, but keeping the resonant frequency the same; in most cases resulting in higher MV output up to a certain point.

But my mental models don't always align with reality

The reason why I'm skeptical of that explanation is that the speed of sound only depends on the temperature, while density and pressure effects cancel out. So the wave tuning part should not be much different at all compared to NA and well-intercooled turbocharged engine. That makes me think that things like runner length and IVC should basically tune for the same rpms in Na and well-intercooled turbo engine. Also, if it were a wave tuning thing, then you'd expect the peak torque rpm to move a lot too with boost, whereas in practice it doesn't seem to move much at all (in the rpm range where the turbo is delivering the requested boost). Right? I'm speculating, I don't really know. Hence the question.

[cab0154]

i think it went into sonic with the long runner on it. it made like 460rwhp at 5200 at 12lbs. before with the long runner at 6lbs it make 380 at 5600. so then i put the short runner on it and it made 490rwhp at 5800 with 12lbs, but was done at that point. so i am pretty sure the compressor was done, but i didnt push the boost past 12lbs with the box intake since it was just my street car. that combo was just a stock 93 5 liter mustang shortblock and cam, with a set of gt40 irons with 1.7s, a gt40 intake with the lower matched to a 1250. the short runner was a downs box. so small cross section through an aod trans. the turbo kit was a 60-1 street kit from turbo technology. that was back in 1997. a lot has changed.

If you mean "went sonic" at the compressor, then my guess would be that, too. The slightly lower power level by 30hp with longer runners could be additional pumping losses and worse intake port / exhaust port pressure ratio, if the compressor was literally tapped out.

I'm just generally interested in the movement of the peak power rpm as the requested boost is increased.

that would make sense. i didnt spend a ton of $ on that kit trying to figure it out. but we did go from a p1 procharger (with a fairly small pulley on it to make boost) to an F1 (with a huge pulley to slow it down and control boost) on stroked 300ci 4.6 2v and i was amazed at how much more power and rpm it had just from a compressor swap. and the heads were tiny on that thing. I figured the F1 would make boost (went from 14lbs with the pulleyed up p1 to 19 with the neutered F1 that couldnt have been in the efficient part of its map yet) but it went from making 440rwhp to 630rwhp.

[turbo2256b]

Sizing of the turbo has a lot to do with were the power band ends up. Poor port and intake runner designs can be overcome once pressurized. Example the dog leg end runners on the factory SEFI 5.0 lowers, The Mod motors that respond so well to boost because there is no valve pocket.
At top end there is less stealing of air by the other cylinders as the time to pull air from the intake NA.
Best of all was the variable geometry turbine housings I designed for Schwitzer back in the mid to late 80s. They would allow boost to be controlled throughout the rpm range even at idle.

[n2xlr8n]

I've been told I make mental models for all things technical due to my lack of formal education, so here goes:

I can see the added mass (with boost) promoting much better cylinder filling (see posts on pressure diff above/below the intake valve) for a given cylinder head / intake tract.

In other words, when your "same" cam would be done at 5800, the mass flow into the cylinder provided by the turbo is allowing your engine much, much more VE, hence more power at a higher rpm.

In a radiation physics sense, it would be analogous to adjusting the RF energy input into a standing wave accelerator, but keeping the resonant frequency the same; in most cases resulting in higher MV output up to a certain point.

But my mental models don't always align with reality

The reason why I'm skeptical of that explanation is that the speed of sound only depends on the temperature, while density and pressure effects cancel out. So the wave tuning part should not be much different at all compared to NA and well-intercooled turbocharged engine. That makes me think that things like runner length and IVC should basically tune for the same rpms in Na and well-intercooled turbo engine. Also, if it were a wave tuning thing, then you'd expect the peak torque rpm to move a lot too with boost, whereas in practice it doesn't seem to move much at all (in the rpm range where the turbo is delivering the requested boost). Right? I'm speculating, I don't really know. Hence the question.

Re wave tuning- ignore that; that's why I stated I kept the frequency the same. Only the energy changed. My analogy (flawed as it may be) used the higher RF energy (and resultant higher particle energy) to describe the additional cylinder filling with an engine.

I think of a HP engine in terms of a resonant cavity with a limited frequency range- anything off-resonance (intake tract, exhaust, cylinder heads, flawed cam timing) produces undesirable results, so one would optimize the frequency at which one wanted resonance (rpm range) to occur first, then design for energy (HP).

Yeah, I know- it if were that simple, everyone would do it.

[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.

[CamKing]

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 ?

[4vpc]

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?