How much RWHP per psi?

[b00stedgt]

Swapping out my 3.5" pulley to the originally supplied 3.25" which would give me about 3psi more. Does anyone know how much rwhp 1 psi equals?

 

[fdjizm]

Some say roughly 10 hp per 1 psi, not sure if that's crank or rwhp though.

 

[JeremyH]

It all depends on the setup, how efficient it is (both the power adder and the motor) and where you are at/going to.

The widely accepted efficient value is about 20rwhp per 1psi of boost.

For example take a 300rwhp car, at 10 psi of boost (10 x 20 = 200 ) it would add around 200rwhp for a total of 500rwhp. This number can go up or down depending on the volumetric efficiencey of the motor and the efficiencey range of the power adder at that boost level.

On my setup at 12psi I made 550rwhp, 14psi I made 604rwhp and 16psi I made 655rwhp. So every psi of boost is getting me about 25rwhp with a turbo still in the top 75% efficiencey range. However as I approach the cfm limits of the turbo 2 more psi of boost (18psi) I doubt would make 700rwhp probably only get closer to 18-20rwhp per psi at that level. It gets to the point where the the motor or power adder cant move anymore air(cfm) after that you can add psi but the you wont make anymore power.

Different kinds of blowers will be a bit less than a turbo setup since they take power to make it.

So if you want a good idea of what your car will make, take what psi and rwhp you are making over stock right now and divide to get a rough figure of rhwp per psi for your setup. So if you made 300rwhp na and at 8psi you made 450 rwhp. Thats 150 extra hp with 8 psi of boost which is about 18-19hp per psi, so adding 2psi for 10psi total will likely make 485rwhp on your setup.

 

[BruceH]

Swapping out my 3.5" pulley to the originally supplied 3.25" which would give me about 3psi more. Does anyone know how much rwhp 1 psi equals?

With a whipple every .25" will give about 2 psi. Figure 16 rwhp per psi until you get near 14-15 psi which is where a whipple starts to get non linear in returns.

If you are at 450 right now figure low to mid 480's. If you are still on a stock maf and whipple supplied injectors (I didn't read your sig before answering) you will be pushing their limits with a custom tune.

 

[b00stedgt]

Thank you! Does the same apply for TQ? Any need to get it retuned? I am assuming the the MAF will read the increased air flow and adjust the fuel mixture so I should be ok. I know that I will have to check the injectors but I am only adding the pulley that the blower came with so I would think they would be ok

 

[cobra mike]

Deffinatly get it rutuned

 

[BruceH]

Thank you! Does the same apply for TQ? Any need to get it retuned? I am assuming the the MAF will read the increased air flow and adjust the fuel mixture so I should be ok. I know that I will have to check the injectors but I am only adding the pulley that the blower came with so I would think they would be ok

It should be good but I'd either get on a dyno or datalog. If you have a wideband and open highway that would work pretty well too.

 

[JeremyH]

+1 defiently have the a/f checked on the dyno after the pulley swap, better safe than sorry.

 

[jmn444]

I put mine on a Dyno to be safe, BUT I had spoke to frpp and whipple and both said going from a 3.5 to a 3.25 would not need a diff tune if using their tune. Considering some kits came with the 3.125, I would guess that pulley is safe with the tune as well.

 

[Riptide]

KB pretty much told me the same thing. I could use their stock tune and go down a pulley size if I wanted w/o any other adjustments. They did say no more than 10psi though lol

 

[Vapour Trails]

Find the pressure ratio of your current setup (and rwhp), find your new pressure ratio and cross multiply and divide to solve for x.

 

[b00stedgt]

I made 452 at 8 psi so doing a ratio formula would make it 600+

 

[JeremyH]

Find the pressure ratio of your current setup (and rwhp), find your new pressure ratio and cross multiply and divide to solve for x.

I made 452 at 8 psi so doing a ratio formula would make it 600+

Its not that simple this formula only works for a small window of efficiencey as you go outside ideal efficiencey and approach the limits of the blower/turbo it goes down considerably.

 

[Vapour Trails]

Its not that simple this formula only works for a small window of efficiencey as you go outside ideal efficiencey and approach the limits of the blower/turbo it goes down considerably.

Yes, you are correct.... and he is not doing that, not even close.

 

[b00stedgt]

Yes, you are correct.... and he is not doing that, not even close.

Not yet lol