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How much HP does one psi of boost equal

189K views 56 replies 25 participants last post by  NarrowBody 
#1 ·
Does anyone know this? I can't remeber were I saw this but what I remember is something like for each pis of boost equals 16 hp. Let me know what's up.
 
#5 ·
so how much horses do you think the rex would have without the turbo? just curious
 
#6 ·
The engine in the WRX is made for a turbo. It runs lower compression than say a honda engine. In the WRX it is about 8:1 and a Honda is about 11:1. If you took the turbo out and actually got it running well it would get beat down by a Yugo. This is one heavy little car made to be turboed.
 
#7 ·
I know this is probably stupid to revive this incredibly old thread, but I found this query kind of interesting.

First off, I agree with Tan 14psi = doubled hp over natural aspiration

Baluch on the other hand...thats an overgeneralization and psi/hp is relative to the original performance of an engine.

Here's the theory.

Assuming tuning is done correctly, you get a stoichiometric mix of O2 and fuel igniting in the combustion chamber. We can establish that we have a set ratio of oxygen and fuel and it should never change. So if we dump in twice as much gas then we MUST have twice as much oxygen for proper combustion.

On planet Earth, there is a constant air pressure, 14.7 psi at sea level (this is the base air pressure in the combustion for all non turbo vehicles). So without turbo on a car, people can honestly say they are running their cars slightly over 14psi (relative to the vacuum of space anyways).

We indirectly measure amount of gas particles with pressure, so if we turbo our cars to lets say 14.7 psi (on top of the 14.7 psi the Earth already gives us [totalling 29.4psi]) then we effectively have twice as much oxygen before ignition. Therefore the amt of fuel being pumped in is doubled, then at ignition we should theoretically get doubled the explosive power. So yeah, as Tan said if you do a 14 psi boost then you've nearly doubled the amount of oxygen in your turboed engine for double the power.

Of course you lose potential power the higher you go because you encounter more resistance in the change of inertia of the internal components.

So if your NA car makes about 100 hp
-Boosting it to 14.7 psi will theoretically give you 200hp
-Boosting it to 7.35 psi will theoretically give you 150hp and so on
[ (HP of NA car) x (1 + (boost psi/14.7)) = HP at boost psi ]


As for Baluch, his numbers are close enough but not for all engines. If you want an easy psi to hp conversion just divide your NA hp by 14.7 and that's it.

-If your Suby is 147 hp NATURALLY ASPIRATED stock then, sure, 1psi boost = 10 hp (147/14.7)
-But for a car that is 240 hp NA stock, a turbo would yield an equivalency of 1psi = 16.3hp

To put it more accurately for Baluch, every 1 psi will get you approx 6.8% extra hp over a naturally aspirated setup (again, theoretical).

Cheers
 
#14 ·
LOL, actual responses. Yeah, I did a quick search on the topic and this one was one of the first ones that came up. And since I am a proud new Suby owner, I decided to check this thread out.



I suppose, but remember its only guess. I would ask other members that has played around with turbo. Although there's only a handful that are brave enough to turbo their NA cars, we can calculate backwards what a WRX or STi might have in terms of a NA HP.

It'd be nice to get some real numbers. All that is really needed is just Dynoed HP and corresponding air pressure. Then the rest is basically algebra. In fact if people are willing enough to dump enough numbers in this thread. We can actually calculate the rate of inefficiency turbo gets as the PSI values go higher.

I hope none of you guys are one of those that think having a turbo makes your car run more efficiently.
 
#21 ·
I'm gonna have to agree with TrainRex on this one... it's a well known fact that turbochargers increase the volumetric efficiency of an engine, even in an imperfect world. Turbocharging is a positive feedback loop, allowing a piston engine to overcome the main problem -- using vacuum to draw air into the cylinders -- and therefore able to increase the volumetric efficiency of the engine. Supercharging increases volumetric efficiency as well, but due to the parasitic loss of the blower off the drivetrain, it's efficiency is less than that of the turbocharger. FYI the turbocharger does capture and recycle some of the energy that would otherwise be lost to exhaust, but it does increase backpressure of the engine. Remember that this is necessary for the positive feedback loop to propagate. Also, efficiency of an engine doesn't mean fuel economy, it describes the ability of the engine to produce power with a given amount of Oxygen and a fuel with reduced waste heat.

FYI the Boeing 787 uses a similar technology with the GE GENx jet engine. This engine is a "high bypass" turbofan, which actually uses the giant fanblades to produce the power of the engine as opposed to the exhaust gases. Capturing the exhuast gases increases the effectiveness of the positive feedback loop (this is how a turbofan works) and increases the efficiency of the jet engine, though increasing exhaust back pressure.
 
#34 ·
Also, efficiency of an engine doesn't mean fuel economy, it describes the ability of the engine to produce power with a given amount of Oxygen and a fuel with reduced waste heat.
Oops, I overlooked your post Zax. In response, I think it would appropriate to look at the overall efficiency since any minute changes between combustion and moving the car would be difficult to account for. In fact I think fuel economy should be looked at since it is an actual value we can gather. The only thing making the car go is fuel. The end product we can measure is distance. Now whatever happens in between is left to speculation unless controls and exceptions are agreed upon. So I think fuel economy is a fair assessment in this case.
 
#27 ·
Unfortunately yes, I am very new to the intricacies of turbocharging, so I can't really state much on how complex or dynamic it gets. But that lack of knowledge isn't exactly what I am addressing in terms of efficiency.

I appreciate how much information I am gathering off of this thread but I think we have to address what we mean by efficiency here. As for Trainrex, I have to say that yeah, I am being bound by the physics that encompasses chemical energy to kinetic energy. So to clarify, my definition of efficiency here is ability to convert as much potential chemical energy (fuel) into kinetic energy (movement).

Simply put, the better MPG the better the efficiency. Now if you are talking about the utilizing the maximum potential of the combustion chamber, that's another story and wholly different from what I was referring to as efficiency. I have just simply not found anyone who gets better mpg efficiency from na setup to a turbo setup.

Now I know I only mentioned backpressure being a burden on the potential power that can be garnered from turbocharging, but there are many other factors that keep it from reaching a maximized potential. I am not saying it does not produce more power, but the higher you go in terms of psi the less efficiency you can get out of your car without other modifications.

So if anyone can convince me that a NA vehicle can achieve better fuel efficiency by introducing forced induction, I am definitely open to revamp my opinion. As long as both vehicles retain correct AFR and are identical (with exception to the turbo or supercharger).
 
#29 · (Edited)
Simply put, the better MPG the better the efficiency. Now if you are talking about the utilizing the maximum potential of the combustion chamber, that's another story and wholly different from what I was referring to as efficiency. I have just simply not found anyone who gets better mpg efficiency from na setup to a turbo setup.


So if anyone can convince me that a NA vehicle can achieve better fuel efficiency by introducing forced induction, I am definitely open to revamp my opinion. As long as both vehicles retain correct AFR and are identical (with exception to the turbo or supercharger).



IMO NA (as opposed to any kind of forced induction) engines are more efficient because its a more conservative use of fuel. In fact we see it every day. Driving hard (fast accelerations) consume much for fuel than a steady acceleration. Imagine sprinting 20 meters, then walking 20 meters. I am sure you agree sprinting used up more energy but walking, albeit slow, accomplished the same work utilizing less energy.
The bold part above indicates exactly why that is a completely unfair comparison to use MPG. Also, requiring that a turbo motor runs the same AFR and has the same design (e.g. compression) is completely ridiculous. Turn it around and make the NA motor run low compression and rich and you will have a slug (e.g. take the turbo off a WRX and compare it to a WRX) - not fair is it?

A fair comparison is two motors each optimally tuned and designed making the same total power with similar torque curves driven in the same fashion. This is a properly controlled situation.

You might stop and ask yourself why semi's, where fuel consumption is of huge economic importance, are most commonly turbocharged?

http://www.honeywell.com/sites/portal?smap=turbo&page=turbofactsbenifits_turbobenifits&theme=P1
"More economical: Turbochargers harness and recycle the energy produced by automobile engines, transforming more of the fuel energy consumed into power by creating less parasitic heat and friction. As a result, turbocharged engines deliver significant fuel cost advantages over their naturally-aspirated counterparts.

Greener: Because a turbocharger delivers more air to the engine, fuel combustion is easier, more thorough and therefore cleaner. Today’s turbocharged diesel engines produce 50% less NOx and CO2 emissions than conventional engines. "
 
#32 ·
Hey thanks. I gave it much thought and I see what you're saying. It does make sense. I completely forgot about the compression ratio (essentially rendering using the same engine as NA and w/ forced induction useless) and the fact that the same HP output must be the same as a control.

I am still not fully swayed tho just because I have not really payed any attention to similar hp vehicles and their fuel efficiency b/w turbo and na setups. But at least I understand our single atm of pressure is a limiting factor in how much a NA engine can sustain. So for now, I'll consider that a forced induction engine may be more fuel efficient than a naturally aspirated as long as they have the similar hp. I hope you respect that I still have some reservations on the topic tho.

I tried looking around and there is no real comparison to, let's say, the WRX. Any naturally aspirated vehicle with the same amount of hp will fall into the realm of a V6 instead of a 4-banger. Its hard to find one that is 4cylinder let alone something that is even similar in weight, body style, power curve, drag coeff, etc. If we allowed the comparison of a turbo 4 cylinder with a NA V6, it seems that fuel efficiency seems to favor NA V6s.

For example
Toyota Venza____AWD__V6______268hp__4045lbs___25/18mpg
Subaru WRX*____AWD__4cylinder_265hp__3229lbs___25/18mpg
Subaru Outback__AWD__V6______254hp__3634lbs___25/18mpg
Honda Crosstour__4WD__V6______271hp__4070lbs___27/18mpg
Ford Edge_______AWD__V6______263hp__4098lbs___23/17mpg
MazdaSpeed3*___FWD__4cylinder_263hp__3221lbs___25/18mpg
*turbocharged

Before I get flamed, yes I know I wanted to only compare similar engines but since no NA 4cylinder can make 250hp I decided to look at V6. Unfortunately, it looks like a turbo 4 cylinder might lose out to NA V6s. The Venza has the same mpg as the WRX and Mazda, but it has over 800lbs over the WRX and MazdaSpeed3. The Crosstour is even more impressive with better highway rating, a bit more hp, and still having 800lbs over Mazda and WRX. Within the Subaru family, its a toss up IMO. Really sucks for the MazdaSpeed3 tho, with the few cars I gathered, it is the worst in terms of fuel econ for its lighter weight and FWDrivetrain. Seems that the 2010 model of the MazdaSpeed3 sports a hoodscoop and tmic too...biters. All this info is straight off of their respective websites.

I took it a step further and looked at the STi. At 305hp and 3400lbs, it gets the same mpg (23/17) as the NA 315hp V8 Ford Mustang at about 3500lbs. Getting V8 power out of a turbocharged 4 cylinder is pretty nice, but hell, the mustang uses 87 octane.

I realize that its using the turbo as a way to overcome the standard single atmosphere of pressure. In fact, compress two reactive chemicals together enough and you'll get a spontaneous rxn (combustion in our case) without a spark. That's basically what some diesel engines do, right?

And for those who mentioned the Boeing jet and semi trucks; aren't those mainly used because they have to create a power output great enough without using an incredibly large engine? I mean I understand that turbo allows for more power with a smaller engine; so wouldn't that be a major factor? (Not to mention that a jet suffers even more since air pressure at high elevations is much less).

Thanks for all the input, knowledge and patience guys.
 
#35 ·
In fact, compress two reactive chemicals together enough and you'll get a spontaneous rxn (combustion in our case) without a spark. That's basically what some diesel engines do, right?
guys.
The diesel cycle is a very very different thermodynamic cycle altogether: isentropic compression -> isobarametric event (diesel flashpoint) -> isentropic expansion -> isochoric event (exhaust valve) and repeat (from the P-V diagrams)

The theoretical Otto cycle has two isochoric events and two *adiabatic* events (isentropic in theory but not realistically). In theory, the Otto cycle is more efficient than the diesel cycle, but because diesel has little concern for detonation, it can run MUCH higher compression levels and is therefore more efficient in practice. If by chance, this is also your confusion. turbocharged engines must run lower compression ratios due to detonation concerns. A typical NA engine runs 10:1-11:1 compression ratio whereas the WRX EJ engine runs 8:1-8.5:1. This is what TrainRex attempted to explain earlier. If the turbo is removed from the EJ engine, it will have much lower efficiency compared to the turbo'd engine. The turbocharger makes up for the lack of efficiency of the low compression ratio by increasing the volumetric efficiency of the engine. Volumetric efficiency is how well the engine can transport intake charge from the inlet to the exhaust. A typical NA engine relies on a vacuum to pull charge through the engine (intake to exhaust). This is negative displacement event and loses energy to the natural expansion process. Forced induction aims to change this by creating a positive displacement event, and uses compression to force air through the cylinders by utilizing some of the waste energy from the exhaust. This increases the volumetric efficiency of the engine, but not completely, as you have mentioned, because the turbine itself imparts a backpressure on the system. Also, if we look at the theoretical system, the compression of the intake charge by the turbo is an adiabatic process and therefore greatly increases the temperature of the charge. Luckily, it is not truly adiabatic since the intercooler reduces the temperature of the intake charge. Though this decreases the overall theoretical efficiency of the Otto cycle due to heat-bleed, it does increase the volumetric efficiency of the engine and gives it the ability to more thoroughly burn gasoline fuel.

All in all, if you put in 14.7 parts of O2 and 1 parts of gasoline, you will get perfect combustion. Theoretically, a NA engine will have the same fuel economy as a FI engine given the same specific output. However, this is not a perfect world and a car with higher volumetric efficiency will burn the same amount of fuel with higher torque output, though modern NA engines are further increasing volumetric efficiency through other means (variable valve timing etc.)

I hope you enjoyed my book.
 
#37 · (Edited)
Weight and aerodynamics have to be taken into consideration as well. Not just the engines:thumbup:. Also alot of the newer V8's (GM and Chrysler) pull the old Northstar trick and drop to 4 cylinders when cruising.

Lengthwise32 said:
The turbo doesn't add more power, it's the extra fuel it allows you to add to the engine that creates the extra power.

The turbo does take some energy to run, it doesn't create this out of nowhere.
In bold is the wrong way to look at it and nobody said it doesn't use power to make power but only under certain conditions. Its the simple fact of cylinder over filling and boosting the engines VE. That is what creates the power. And as other have said,there is roughly about a 7% energy loss with Turbocharging compared to a 20-25% parasitic loss with a Supercharger. Either way your making way more power than you are losing or could N/A. And people just generally saying turbos create back pressure are not seeing the whole big picture.

Example: I have a GT30r on my 2.0L. It crosses the boost threshold at 3500 rpm in 3rd gear at 100% throttle and is fully spooled at max boost by 4500rpm and produces say 400whp. I cruise on the highway at 60 mph in 5th at 2500 rpm with around 10% throttle. I am roughly out putting around 30-40whp. There is almost zippy back pressure happening due to the size of the turbine on that particular turbo and the amount of air going in/out of the cylinders.
 
#42 ·
Don't you hate it when engineering types show up here thinking that they know everything?



Oh, wait..



Great thread, actually.
 
#46 ·
Some might even consider the price of the motor in calculating it's efficiency, because it's certainly not efficient to spend $110,000 on a motor when you can get one for $11,000 that only performs a little worse.. efficiency can mean a lot of things. Also how long the motor will run could be factored in, because a great performing motor that will only run for 3,000 miles wouldn't be considered efficient by many.
 
#47 ·
I know I did (but not for this comparison at hand). Before I decided on my suby I was also looking at the Audi A3. It puts out more HP and is far more fuel efficient. Even buying it used, I estimated that I had to drive it for about 20 years before fuel made up for the cost of it. That hatch is way smaller on the A3 as well so that easily cinched it...especially since I bring my 100lb pup everywhere.
 
#51 · (Edited)
:)

Okay, let me rephrase. Most of people who bought a new car, trade their current one in for another new on in 3-5 years.

Even used cars don't usually die because the engine is worn out, but because of various other mechanical problems.

I kept my Toyota Tacoma 4x4 for 9 years. I changed the oil once a year (every 12,000 miles). I traded it in because I had used to off-road / race a lot and it was beaten into the ground. The entire truck was bent, it had been rolled, the grill was held on by zip ties, the battery by a bungee cord, the heater nob didn't work, the AC had a leak.

Was the engine worn out, even w/ 12,000 mile oil changes? Nope, the truck still ran like a champ and had plenty of power to spare! I could have given it 3000 mile oil changes it's entire life and I wouldn't have gotten anything more for it when I traded it in for my WRX.
-Chris
 
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