PSI in relation to HP

[Sergio]

Does anyone know the ratio between the two on a 951? Im running aprox 14psi, thinking about running 16 or so. What increase will this bring in HP and seat of the pants thrills?
Is it tied to displacement also?

Reason im asking is that my Friend has a 3.0 litre 300zx TwinTurbo, stock setting is 10 psi, we installed some boost jets in the hoses and now he is getting 14 psi. trying to figure the horsepower gain


Sergio

[VIPRKLR]

Very hard to do without a dyno...

At 14.7 psi of boost, you are effectively doubling your unboosted displacment. (14.7 PSI being atmosphereic pressure at sea level) I do not think that means you are doubling your power, however.

[monteness]

As stated very hard to do without a dyno. Take a look at the LR page, dyno sheets.

15 psi = 311 rwhp
18 psi = 370 rwhp

difference of 3psi, 19.67 hp/psi increase.
Each car is different on the setup but the Lindsey race car was the same just different boost levels.

[Macabre]

You can make a pretty good guess if you have the flow chart for the turbo and the volumetric efficiency of the engine. You need the flowchart because as you increase boost at a given RPM, you may fall outside the efficiency range of the turbo and then the additional heat will reduce power.

Also not that peak boost has nothing to do with peak HP, only peak torque. A 951 with K26 running 15psi should make about the same peak torque as one with a larger turbo running 15psi, but the one with the larger turbo will make more HP because it will be able to hold boost efficiently at higher RPM..

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'86 951 Graphite Metallic/Tan

[dan10101]

<font face="Verdana, Arial" size="2">Originally posted by monteness:
As stated very hard to do without a dyno. Take a look at the LR page, dyno sheets.

15 psi = 311 rwhp
18 psi = 370 rwhp

difference of 3psi, 19.67 hp/psi increase.
Each car is different on the setup but the Lindsey race car was the same just different boost levels.

</font>

I tend towards the 10hp figure per PSI. Possibly a high HP engine that can make proper use of more airflow will benefit more (20HP) from more boost. These are the HP figures of a 3.5 liter engine running these boost pressures. I'm sorry, I don't have the source. I find these things everwhere. This was at 8.0 compression ratio.

stock 6psi 8psi 10psi 12psi
207 266 285.5 305 324

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borrowed from some unknown website.
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Interesting to note the lower compression with higher boost made more HP than Higer compression with lower boost (same effective compression ratio).

Here's a chart showing relationships between compression ratios and boost levels, along with the corresponding "effective" compression ratio :

Static
C.R.....6psi....8psi....10psi...12psi
7:01....9.9.....10.85...11.8....12.7
8:01....11.3....12.35...13.4....14.5
9:01....12.7....13.9....15.1....16.3


Now here's a chart showing approximate peak HP numbers for a hypothetical 3.5L engine. The only change made to this motor is level of boost and C.R.

Static
C.R..Stock HP 6psi...8psi...10psi...12psi
7:01..200.....257....276....295.....314
8:01..207.....266....285....305.....324
9:01..214.....275....295....315.....335

(hope this is readable) How about now..

There is no exact calculation because of the many variables, but this gives some idea..


Wierd, can't make spaces show up on my chart... Let's see what happens now...

[This message has been edited by dan10101 (edited 11-06-2001).]