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#2400433 - 11/08/17 09:52 AM Boost and HP
Dr Dave Offline


Registered: 03/12/14
Posts: 228
Loc: Oregon
Is there a general rule of how much HP can be expected from each # of boost in a supercharged engine and is it a linear or exponential increase?

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#2400445 - 11/08/17 10:07 AM Re: Boost and HP [Re: Dr Dave]
polyspheric Offline
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#2400489 - 11/08/17 11:11 AM Re: Boost and HP [Re: polyspheric]
steve660 Offline
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Registered: 04/06/07
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I was told 1# = about 25 hp.
Thats with a Procharger.
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#2400553 - 11/08/17 12:59 PM Re: Boost and HP [Re: Dr Dave]
dizuster Offline
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Registered: 01/12/05
Posts: 3288
Loc: Oakland, MI
My car is pretty predictable. It's be right about 20hp per psi (turbo) all the way from 10psi up to 23psi. Thats based on ET/MPH, not any dyno fabricated numbers.

You have to remember most ways to create boost (good size turbo or supercharger) is REALLY inefficient at low boost. So the efficiency actually gets better the more boost you throw at it.

That's why it doesn't fall of like you'd think after 5psi as long as it's got a good sized unit to work with.

Compressor map at 16psi

http://www.turbos.bwauto.com/aftermarket...rsin=92044&

Compressor map at 5psi

http://www.turbos.bwauto.com/aftermarket...rsin=92044&


Basically ~18hp per PSI with these numbers...
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#2400594 - 11/08/17 02:32 PM Re: Boost and HP [Re: dizuster]
dogdays Offline
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Registered: 01/20/03
Posts: 15814
It should rise proportionally to the weight (mass airflow) of the intake air as it passes the intake valve. This is one area where some usage of the metric system is handy Say the engine makes 240hp at 4000rpm. Assuming we are at or near Mean Sea Level (MSL)and the atmospheric pressure is 14.7 psi or one atmosphere. If you add 14.7psi of boost, the engine horsepower output should double to around 480.

In reality it won't because it takes more power to run the blower and there are increases in friction and cooling system load, among other things. This is a really rough rule of thumb and could be viewed as the maximum power increase possible. Still, one should not equate a pound of boost with a single value of horsepower increase, which is what I am seeing here in some replies.
Put another way, one psi of boost increases power differently for different engines.

There's a whole lot more that I didn't cover, and a whole lot more that I just plain don't know. What I wrote in this post should be correct, as far as it goes.

The latest is the electric turbocharger. I could envision keeping the boost alive with a motor and then at some point the exhaust driven turbine takes over. The problem is it doesn't seem that there would be exhaust driving the impeller at any time.

R.

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#2400618 - 11/08/17 03:13 PM Re: Boost and HP [Re: Dr Dave]
GTX MATT Offline
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Posts: 4744
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It really depends on what you're using to create the boost and where it is on its efficiency map for the amount of airflow needed to generate the boost you are running. Remember that boost just is positive pressure in the intake, it can be generated by putting cramming more air in, or heating the air up.

I.E. You can take a 6-71 and overdrive it until it makes 20 PSI on a 440, but its outside and above the efficiency range of the compressor, so most of the boost is a function of heat and NOT cramming air into the engine.

You might put an 8-71 or 10-71 and it will make more power at the same 20 PSI, and it might make more power at 12 PSI than the 6-71 did at 20 PSI.

As dogdays said above, in my opinion, it is not X HP for each PSI, it is related to the HP the engine makes NA. In theory 14.7 PSI would double your horsepower at sealevel, but it doesn't work that way for a variety of reasons. It MIGHT in a certain application. The tune up changes more as you increase boost, which limits the effect. But you might get 2X hp @ 14.7 PSI running a compressor well within its efficiency map on 118 octane, maybe with alcohol injection too. The efficiency of the combustion chamber would play a role here too.

I usually use this to get an idea and its always pretty close for me, but its HIGHLY SUBJECTIVE: Theoretical NA HP + (Theoretical NA HP * ((Boost/14.7)* estimated efficiency)) = HP

The Estimated efficiency here is highly subjective. On supercharged applications running PUMP GAS usually an efficiency of .6-.65 produces an accurate output if the supercharger is sized correctly for the application. On turbocharged and intercooled applications if the turbo is being pushed towards the upper end of the efficiency range around .7-.75 will work well. If its at the lower end or sweet spot around .8-.85 will work well. Again, this is on pump gas.

But if you got a huge turbo capable of generating 40 PSI efficiently on an engine and you only run 10 PSI you're likely going to be at or slightly over 100% efficiency. So really, to answer your question, no there is not. But the formula above usually works pretty well for me. Its just all subjective to what efficiency you use, but I'm pretty good at guesstimating that. Its all just a guess though.

Also, the fuel makes a BIG effect on the outcome. The bigger the boost the more the fuel can affect it.


Edited by GTX MATT (11/08/17 03:27 PM)
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#2400701 - 11/08/17 06:19 PM Re: Boost and HP [Re: Dr Dave]
GTX MATT Offline
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One more thing worth noting here on the boost vs theoretical NA HP, the efficiency could be inflated by an engine design that favors boost vs NA. I.E. an engine with an intake port thats too big. The engine may lose power NA because of the big port, but that same port thats too big NA will pick up velocity once the intake has positive pressure, and allow more airflow overall now that its not depending on atmospheric pressure alone to fill it. Same for cams, a cam design favoring boost may lose power NA but pick up with boost. It's all about airflow. And its all combination dependent.
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#2400876 - 11/09/17 01:58 AM Re: Boost and HP [Re: Dr Dave]
FastmOp Offline
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#2400950 - 11/09/17 10:02 AM Re: Boost and HP [Re: GTX MATT]
Clanton Offline
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Registered: 05/15/11
Posts: 3219
Loc: northern,Ohio,USA
Originally Posted By GTX MATT
One more thing worth noting here on the boost vs theoretical NA HP, the efficiency could be inflated by an engine design that favors boost vs NA. I.E. an engine with an intake port thats too big. The engine may lose power NA because of the big port, but that same port thats too big NA will pick up velocity once the intake has positive pressure, and allow more airflow overall now that its not depending on atmospheric pressure alone to fill it. Same for cams, a cam design favoring boost may lose power NA but pick up with boost. It's all about airflow. And its all combination dependent.
I played for days at the Wallace calc looking at port speed and fuel dropout at what rpm trying to figure how much bigger a port could be vs a near sonic port.[100 cfm on the big side started fuel fall out but at low rpm idle to 1500rpm]
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#2400955 - 11/09/17 10:09 AM Re: Boost and HP [Re: Dr Dave]
hemi-itis Offline
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#2401112 - 11/09/17 04:31 PM Re: Boost and HP [Re: Dr Dave]
TRENDZ Offline
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Registered: 01/24/05
Posts: 1501
Loc: Milwaukee WI
There is no fixed formula for boost vs hp.
Port velocity never increases with boost.
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#2401240 - 11/09/17 10:42 PM Re: Boost and HP [Re: TRENDZ]
GTX MATT Offline
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Registered: 06/05/06
Posts: 4744
Loc: CT
Originally Posted By TRENDZ

Port velocity never increases with boost.


How do you figure?
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#2401251 - 11/09/17 11:41 PM Re: Boost and HP [Re: GTX MATT]
FastmOp Offline
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Registered: 01/20/03
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Originally Posted By GTX MATT
Originally Posted By TRENDZ

Port velocity never increases with boost.


How do you figure?


It's true. The air Is dense'er but not faster.

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#2401298 - 11/10/17 06:07 AM Re: Boost and HP [Re: GTX MATT]
TRENDZ Offline
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Registered: 01/24/05
Posts: 1501
Loc: Milwaukee WI
Originally Posted By GTX MATT
Originally Posted By TRENDZ

Port velocity never increases with boost.


How do you figure?


My answer to that would be, "How do you figure?" grin
I can only assume that you think that double the air means double the speed. If that's not what you think please explain.
Now, since the air is twice as dense, if speed doubled wouldn't total airflow be mathematically squared?
Lets assume an engine flows 1000 cfm at a specific rpm and load point. If the air density alone is doubled, the engine would flow 2000 cfm.(or would it?)Now, double the speed of that "twice as dense" air. That would come to 4000 cfm. Does anyone think that 15psi boost would increase airflow by a factor of 4? How about 3? How about even 2? The fact is density creates friction. Even a straight port will slow air speed with increased density.
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#2401334 - 11/10/17 09:04 AM Re: Boost and HP [Re: Dr Dave]
GTX MATT Offline
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Registered: 06/05/06
Posts: 4744
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No how do YOU figure laugh2

I wouldn't say that double the air would mean double the air speed. But I would say that the air speed does increase. But maybe that's just the effect of the more pressurized port.

I'm looking at it from the standpoint of the valve is closed, there's some # of boost in the intake manifold behind the valve, now the valve opens and its going to fill the cylinder in less time that at 0 PSI.

It could stand to reason that its simply because the air is more dense. But even assuming a perfect world in which no heat was generated, a turbo or supercharger isn't compressing the air to 2X it's density @ 14.7 PSI of boost, it's creating a pressure differential of 14.7 PSI that acts on the entire engine, cylinder and all. We then measure the level of boost by how much positive pressure is in the intake manifold, which is really kind of a measurement of how restrictive the engine is to the airflow that the turbo generates.

But the intake manifold doesn't contain 2X the amount of air than it would NA like a compressor tank holds air. Rather the turbo or supercharger is creating a pressure differential that acts on the entire engine. This pressure differential acts on the cylinder from the time the valve opens to the time that it closes, filling the cylinder with more air than it would have NA.

When you reduce the restriction of the engine but make no adjustments to the turbo/supercharger system you will see boost change. I.E. if you increase the size of the engine underneath the intake system the turbo/supercharger will need to work harder to create the same boost, and more of the boost will be heat. Or it may no longer be capable of generating the same level of boost.

I won't say its proven fact, but it does seem that ports that are "too big" work very well with boost. But maybe that's just the effect of the more pressurized port. Maybe what we commonly refer to as "port velocity" is really more related to the pressure of the intake port? It could stand to reason that the larger port is less pressurized by atmospheric pressure. But we also know the engine underneath the port has an effect. shruggy

I'm thinking on this now, will probably consume my mind all day. hammer



Edited by GTX MATT (11/10/17 09:07 AM)
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#2401345 - 11/10/17 09:27 AM Re: Boost and HP [Re: Dr Dave]
rickseeman Offline
pro stock

Registered: 01/18/12
Posts: 1335
Loc: Little Rock, Arkansas
Think of the new Dodge Demon. It has a 378 cu in high performance engine. It might make 420 N/A. They give it 14.7 or so pounds of boost (which doubles the manifold pressure) and it makes 840.
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#2401381 - 11/10/17 10:46 AM Re: Boost and HP [Re: Dr Dave]
TRENDZ Offline
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Registered: 01/24/05
Posts: 1501
Loc: Milwaukee WI
This from your last post GTX MATT...

"I'm looking at it from the standpoint of the valve is closed, there's some # of boost in the intake manifold behind the valve, now the valve opens and its going to fill the cylinder in less time that at 0 PSI."

Hard to argue if that's where it ends. But that's not where it ends. What is the pressure on the other side of the intake valve when under boost?
The cylinder has only two points of reference, the intake charge, and the exhaust. As boost increases, so does exhaust pressure. It doesn't matter if it's supercharged or turbocharged... exhaust pressure gets very high under boost. So at best, cylinder fill starts to happen (loosely)at around a 1:1 pressure ratio.
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#2401395 - 11/10/17 11:25 AM Re: Boost and HP [Re: Dr Dave]
1mean340 Offline
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Registered: 10/09/08
Posts: 283
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You guys can throw out all the fancy engineering formulas you'd like, but in the real world there is no way you can figure it out mathematically unless you can take into consideration the efficiency of the compressor at the boost levels you are seeing (i.e; how much heat it will be generating to do what you want it to) , friction losses from the intake plumbing, efficiency of the intercooler, IATs before the blower, AFR etc...

When this question gets posted on most boosted forums, I have just about always seen guys post that they are seeing somewhere in the neighborhood of 18-20rwhp per PSI up until about 15 psi and then you start seeing some diminishing gains.


Edited by 1mean340 (11/10/17 11:26 AM)
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#2401396 - 11/10/17 11:31 AM Re: Boost and HP [Re: Dr Dave]
Blusmbl Offline


Registered: 01/20/03
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John Heywood wrote a book that has equations for this exact discussion in it.

At some point during the intake cycle you will be @ choked flow of the port, at which point the downstream pressure in the cylinder and exhaust pressure doesn't matter. Even prior to choked flow the airflow will increase with boost. Flow will increase as the boost is increased.

If you reduce the orifice size (turbine housing OR intake port) the flow will go down, but for a given restriction the flow will be increasing if you increase boost.
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#2401400 - 11/10/17 11:45 AM Re: Boost and HP [Re: TRENDZ]
GTX MATT Offline
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Registered: 06/05/06
Posts: 4744
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Originally Posted By TRENDZ
This from your last post GTX MATT...

"I'm looking at it from the standpoint of the valve is closed, there's some # of boost in the intake manifold behind the valve, now the valve opens and its going to fill the cylinder in less time that at 0 PSI."

Hard to argue if that's where it ends. But that's not where it ends. What is the pressure on the other side of the intake valve when under boost?
The cylinder has only two points of reference, the intake charge, and the exhaust. As boost increases, so does exhaust pressure. It doesn't matter if it's supercharged or turbocharged... exhaust pressure gets very high under boost. So at best, cylinder fill starts to happen (loosely)at around a 1:1 pressure ratio.


Correct, I would say it absolutely doesn't end at the intake valve, once the intake valve starts to open the pressure wave is now going through the cylinder and into the exhaust path (assuming overlap) until the exhaust valve closes, then its ending at wherever the top of the piston is. Pressure on the other side of the valve changes depending on where the piston is of course, cam specs, etc. But it should be less than the pressure on the intake side of the valve under boost.


Edited by GTX MATT (11/10/17 11:52 AM)
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