Re: Boost and HP
[Re: polyspheric]
#2400489
11/08/17 02:11 PM
11/08/17 02:11 PM
|
Joined: Apr 2007
Posts: 289 Lowes
steve660
enthusiast
|
enthusiast
Joined: Apr 2007
Posts: 289
Lowes
|
I was told 1# = about 25 hp. Thats with a Procharger.
PSO headed 632 from MM. Cracked cylinder, loose valve seats, low oil pressure, low cylinder pressure. ..... Now its a door stop....
|
|
|
Re: Boost and HP
[Re: Dr Dave]
#2400553
11/08/17 03:59 PM
11/08/17 03:59 PM
|
Joined: Jan 2005
Posts: 3,635 Oakland, MI
dizuster
master
|
master
Joined: Jan 2005
Posts: 3,635
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...
|
|
|
Re: Boost and HP
[Re: Dr Dave]
#2400618
11/08/17 06:13 PM
11/08/17 06:13 PM
|
Joined: Jun 2006
Posts: 5,162 CT
GTX MATT
master
|
master
Joined: Jun 2006
Posts: 5,162
CT
|
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.
Last edited by GTX MATT; 11/08/17 06:27 PM.
Now I need to pin those needles, got to feel that heat Hear my motor screamin while I'm tearin up the street
|
|
|
Re: Boost and HP
[Re: Dr Dave]
#2400701
11/08/17 09:19 PM
11/08/17 09:19 PM
|
Joined: Jun 2006
Posts: 5,162 CT
GTX MATT
master
|
master
Joined: Jun 2006
Posts: 5,162
CT
|
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.
Now I need to pin those needles, got to feel that heat Hear my motor screamin while I'm tearin up the street
|
|
|
Re: Boost and HP
[Re: GTX MATT]
#2400950
11/09/17 01:02 PM
11/09/17 01:02 PM
|
Joined: May 2011
Posts: 4,489 northern,Ohio,USA
Clanton
master
|
master
Joined: May 2011
Posts: 4,489
northern,Ohio,USA
|
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]
|
|
|
Re: Boost and HP
[Re: Dr Dave]
#2401112
11/09/17 07:31 PM
11/09/17 07:31 PM
|
Joined: Jan 2005
Posts: 2,540 Milwaukee WI
TRENDZ
master
|
master
Joined: Jan 2005
Posts: 2,540
Milwaukee WI
|
There is no fixed formula for boost vs hp. Port velocity never increases with boost.
"use it 'till it breaks, replace as needed"
|
|
|
Re: Boost and HP
[Re: TRENDZ]
#2401240
11/10/17 01:42 AM
11/10/17 01:42 AM
|
Joined: Jun 2006
Posts: 5,162 CT
GTX MATT
master
|
master
Joined: Jun 2006
Posts: 5,162
CT
|
Port velocity never increases with boost.
How do you figure?
Now I need to pin those needles, got to feel that heat Hear my motor screamin while I'm tearin up the street
|
|
|
Re: Boost and HP
[Re: GTX MATT]
#2401251
11/10/17 02:41 AM
11/10/17 02:41 AM
|
Joined: Jan 2003
Posts: 5,762 Hot Rod Ridge
FastmOp
master
|
master
Joined: Jan 2003
Posts: 5,762
Hot Rod Ridge
|
Port velocity never increases with boost.
How do you figure? It's true. The air Is dense'er but not faster.
|
|
|
Re: Boost and HP
[Re: GTX MATT]
#2401298
11/10/17 09:07 AM
11/10/17 09:07 AM
|
Joined: Jan 2005
Posts: 2,540 Milwaukee WI
TRENDZ
master
|
master
Joined: Jan 2005
Posts: 2,540
Milwaukee WI
|
Port velocity never increases with boost.
How do you figure? My answer to that would be, "How do you figure?" 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.
"use it 'till it breaks, replace as needed"
|
|
|
Re: Boost and HP
[Re: Dr Dave]
#2401334
11/10/17 12:04 PM
11/10/17 12:04 PM
|
Joined: Jun 2006
Posts: 5,162 CT
GTX MATT
master
|
master
Joined: Jun 2006
Posts: 5,162
CT
|
No how do YOU figure 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. I'm thinking on this now, will probably consume my mind all day.
Last edited by GTX MATT; 11/10/17 12:07 PM.
Now I need to pin those needles, got to feel that heat Hear my motor screamin while I'm tearin up the street
|
|
|
Re: Boost and HP
[Re: Dr Dave]
#2401345
11/10/17 12:27 PM
11/10/17 12:27 PM
|
Joined: Jan 2012
Posts: 2,652 Stuttgart, Arkansas
rickseeman
master
|
master
Joined: Jan 2012
Posts: 2,652
Stuttgart, 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.
2011 Drag Pak Challenger
|
|
|
Re: Boost and HP
[Re: Dr Dave]
#2401381
11/10/17 01:46 PM
11/10/17 01:46 PM
|
Joined: Jan 2005
Posts: 2,540 Milwaukee WI
TRENDZ
master
|
master
Joined: Jan 2005
Posts: 2,540
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.
"use it 'till it breaks, replace as needed"
|
|
|
Re: Boost and HP
[Re: Dr Dave]
#2401395
11/10/17 02:25 PM
11/10/17 02:25 PM
|
Joined: Oct 2008
Posts: 370 Suffolk County, New York
1mean340
enthusiast
|
enthusiast
Joined: Oct 2008
Posts: 370
Suffolk County, New York
|
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.
Last edited by 1mean340; 11/10/17 02:26 PM.
|
|
|
Re: Boost and HP
[Re: Dr Dave]
#2401396
11/10/17 02:31 PM
11/10/17 02:31 PM
|
Joined: Jan 2003
Posts: 8,165 Plymouth, MI
Blusmbl
master
|
master
Joined: Jan 2003
Posts: 8,165
Plymouth, MI
|
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.
'18 Ford Raptor, random motorcycles, 1968 Plymouth Fury III - 11.37 @ 118
|
|
|
Re: Boost and HP
[Re: TRENDZ]
#2401400
11/10/17 02:45 PM
11/10/17 02:45 PM
|
Joined: Jun 2006
Posts: 5,162 CT
GTX MATT
master
|
master
Joined: Jun 2006
Posts: 5,162
CT
|
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.
Last edited by GTX MATT; 11/10/17 02:52 PM.
Now I need to pin those needles, got to feel that heat Hear my motor screamin while I'm tearin up the street
|
|
|
|
|