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BMW 3-Series (E90 E92) Forum > E90 / E92 / E93 3-series Powertrain and Drivetrain Discussions > N54 Turbo Engine / Drivetrain / Exhaust Modifications - 335i > How much HP Per PSI?



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      05-30-2013, 11:46 AM   #1
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How much HP Per PSI?

Does anyone know how much horsepower each psi represents? Before I went to ISO I was peaking at 14.5, now Im getting 19psi. So thats 4.5 more psi. Anyone have an idea of how much power that equates too?
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      05-30-2013, 11:49 AM   #2
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Quote:
Originally Posted by stoes View Post
Does anyone know how much horsepower each psi represents? Before I went to ISO I was peaking at 14.5, now Im getting 19psi. So thats 4.5 more psi. Anyone have an idea of how much power that equates too?
It's not that simple to say X PSI increase=Y WHP gains. Are you sure you're boosting upwards of 19psi without an upgraded intercooler? In Houston (hot)? That would surprise me..
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      05-30-2013, 11:49 AM   #3
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It's not a linear equation, there are a lot of factors in play.
Just dyno the car.
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      05-30-2013, 12:08 PM   #4
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Yeah, I figured it wasnt that simple... And yeah, I can hit 19 when its below 85 degrees with about 7 gallons e85. But for day to day its more consistent at 17.5 to 18psi
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      05-30-2013, 01:50 PM   #5
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when i had my mustang i put a vortech supercharger on it, this was awhile ago but i remember the guy i ordered it from saying eash psi was worth about 20 hp. i have no idea if this is pertians to turbos. with the mods i had and pushing 12 psi it added up pretty close to what i dynoed........ 480 rwhp and over 500 hp at the motor.
R
so if you go 8 psi stock to 18 psi tuned u'd be adding 10 psi X 20 = 200 extra..... i Wish
so that cant be right......
R
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      05-30-2013, 01:52 PM   #6
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I remember Terry saying on this platform its approx 12whp per psi.
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      05-30-2013, 02:31 PM   #7
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use Virtual dyno
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      05-30-2013, 02:44 PM   #8
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Quote:
Originally Posted by Beemw335
I remember Terry saying on this platform its approx 12whp per psi.
lol u def did not.
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      05-30-2013, 03:54 PM   #9
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Dont look at at peak psi, look at psi around 5-6k as thats were peak HP occurs for stock turbos. You can add 5psi at 3k rpms, but you're making 200whp that low in the power pand.
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      05-31-2013, 09:45 AM   #10
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IIRC when you're engine is boosting 1 bar or 14.7psi, it's pushing twice as much air than atm pressure theoretically making twice as much power than NA. However turbos are not 100% efficient so you see about 70-77% of that power. So if our engine makes 230hp stock at 0psi, it'll make 11-12hp per psi.
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      05-31-2013, 10:06 AM   #11
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need to dyno.

law of diminishing return comes in effect here. additional psi above a certain threshold may only get you marginal gains in hp.

Quote:
Originally Posted by stoes View Post
Does anyone know how much horsepower each psi represents? Before I went to ISO I was peaking at 14.5, now Im getting 19psi. So thats 4.5 more psi. Anyone have an idea of how much power that equates too?
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      05-31-2013, 03:38 PM   #12
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I wasn't too worried about my peak psi, although I will say I wanted to hit at least 17 psi coming from the G4 board which I never could achieve that much power with the old board. With that being said I believe the car holds strong between 14 and 16 psi throughout the powerband. I raced a camero ss that was cammed and full exhaust last night. I knew I would beat him, but I didnt know I would absolutely destroy him by at least 10 car lengths! It was fun!!
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      05-31-2013, 04:01 PM   #13
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why get a 335 then? I doubt you are making anywhere close to those numbers with the 335, I am just wondering

Quote:
Originally Posted by Randomaccess335 View Post
when i had my mustang i put a vortech supercharger on it, this was awhile ago but i remember the guy i ordered it from saying eash psi was worth about 20 hp. i have no idea if this is pertians to turbos. with the mods i had and pushing 12 psi it added up pretty close to what i dynoed........ 480 rwhp and over 500 hp at the motor.
R
so if you go 8 psi stock to 18 psi tuned u'd be adding 10 psi X 20 = 200 extra..... i Wish
so that cant be right......
R
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      05-31-2013, 04:54 PM   #14
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Quote:
Originally Posted by msp2three35i View Post
IIRC when you're engine is boosting 1 bar or 14.7psi, it's pushing twice as much air than atm pressure theoretically making twice as much power than NA. However turbos are not 100% efficient so you see about 70-77% of that power. So if our engine makes 230hp stock at 0psi, it'll make 11-12hp per psi.
Pressure alone isn't enough information to determine HP. Need to take the ideal gas law into consideration which in it's simplest form is something like [pressure] * [volume] / [temperature].

Better yet, use something like this:

http://www.not2fast.com/turbo/glossary/turbo_calc.shtml
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      05-31-2013, 05:02 PM   #15
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It depends on a few factors such as turbo, intake temps and air volume.

Not ever turbo flows the same amount of air at the same PSI nor is that intake temp going to be the same.
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      05-31-2013, 05:20 PM   #16
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Quote:
Originally Posted by stoes View Post
Does anyone know how much horsepower each psi represents? Before I went to ISO I was peaking at 14.5, now Im getting 19psi. So thats 4.5 more psi. Anyone have an idea of how much power that equates too?
The very general rule is 1 psi= 10whp, or a tenth of a second gain in the 1/4 mile. Works out pretty well, but again, this is very generic.
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      05-31-2013, 05:38 PM   #17
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Quote:
Originally Posted by Killerfish2012 View Post
The very general rule is 1 psi= 10whp, or a tenth of a second gain in the 1/4 mile. Works out pretty well, but again, this is very generic.
It's not a linear gain when we're pushing the stock turbos so far out of the compressors efficiency range. You'll find that while you will gain HP at higher boost pressures, you're gaining less and less and wearing the turbos more and more.
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      06-01-2013, 01:21 AM   #18
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Compressors generally have the same mass flow rate for the same psi, whether its small or large for the same given engine, ignoring efficiency. In terms of the N54, the difference is that the exhaust back pressure is huge on the stock turbos, hence little gain as you go past 1 bar. The exhaust gas does not evacuate completely and some gets trapped in the cylinder and dilutes the incoming charge. If you had a nice large rear turbine, there would be more gains as you go higher. An intercooled supercharged motor responds is a little more linear with power vs psi as the exhaust restriction is the similar to N/A.
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