I have heard that as a rule of thumb for every 10psi of boost = 100hp. does this have any merit? I have estimated my hp at around 350hp with my mods. but it isn't worth it to me to take it to a dyno to find out. My boost has seen 37psi. The rule seems plausable in my case.
P.S. I have not updated my sig. in a few years so what you are seeing isn't what I have. One of these day I will have to update it.

 

Or maybe look at it like this. Use absolute pressure.... atmosphere plus gauge boost (the atmosphere gives the first 14.7 PSI of boost but does not show on the boost gauge). So 24 psi gauge would be 24 plus 14.7 or 38.7 absolute pressure for 305 HP (crankshaft) on the 305/555 engine. Total PSI of 38 equals 305 HP with little smoke. Add more fuel and it would definitely put out more power, to a point, and smoke more. If it could get to 380 HP it would be about 10 HP per pound of boost, as you said.


John

 

It is more complicated than a simple formula of what boost equals what horsepower. The thing that is important is the amount of air going into the cylinder. Boost is simply a measure of backpressure of the intake system. Air temperature is a big factor since it is closely related to density. The more boost a turbo produces, the higher the temperature will be also. Yes, more boost means more horsepower but the relationship is kind of complicated since boost and airflow are not proportional.

 

Boost is also one part of the power equation. Air alone will not make power.

FUEL is what makes power. Forced induction, helps to burn more efficiently and also burn more fuel, resulting in more power.

 

And remember that horsepower is a term or calculation that was basically developed for laymens use. HP isn't a "real" number. HP is figured from torque. Can't remember the formula right now, I've got it written down somewhere.... One of the smart guys on this forum probably knows the formula.

 

HP is actually the amount of work an average horse could do in an hours time. I don't remember the exact calculations. It's bin a while since it was on discovery/History channel.

 

The formula for horsepower is

HP= PLAN/33000

HP = PLAN
'''''''''''33000


HP= Horsepower
P= pressure on top of piston
L= length of stroke in feet
A=area of piston in square inches
N=number of single strokes per minute

 

I have books that define hp. I just wondered if that rule of thumb statement has a basis in fact. If not reliable then I will discard it. In many industries there are rule of thumb statements that although are not perfect or even applicable in all cases, none the less do serve a purpose and are good for general uses. I realize that volume at pressure would not always be the same hence the need for larger turbos. That is why I asked the question. If that rule were hard and fast an 800 hp truck would push 80 lbs of boost, very unlikely.
What got me thinking of this after I read that rule of thumb somewhere this week was that with what is in my sig I had estimated the truck to be at about 300hp. at that time it would push about 32psi. I then added some 215hp injectors and 4in. exhaust and slid my fuel plate forward about 1/16 in. from standard location. I figured that that would put me around 350hp. After doing this my boost on full acceleration jumped to 37psi. So it seemed to be plausable to use as a rule of thumb at least with the stock turbo anyway.

 

I used to go by the same rule of thumb, but found it is less than accurate. Boost is nothing more than a measurement of restriction for the most part. I have made 860 HP with as low as 55 psi of boost, and I have seen trucks make 75-80 psi that made 550 HP. It's all part of the game. I think in somewhat stock form it can be close, after that it's all out the window.

 

There it is. The more finely tuned the setup, the closer one might get to proving that equation as true, or beyond, as has been stated. Go in the other direction and my 40+psi daily driver is more than likely under the 300RWHP mark, mainly to do with the anemic 12v head and, in part, to do with the tired old VE pump

 

x2. At a peak of 47psi Im still more than likely under 300 as well, for those same reasons.

 

When I dynoed 450, I was at 43 psi. When Dodgediesel did 570, he was around 55 psi he said. So there is merit, but does not fit all scenarios.

 

I say no, I can push 55psi but know I'm nowhere near 550HP. I'd be lucky to break 400

 

willy91; If you still have your stock turbo, you are way out of the efficency zone with it. I would think that you would see a large difference in the power of your truck by putting one on that would give you more volume. I think you would see your boost drop significantly and your egt's would drop too. I am by no means an expert but that would explain your high boost. I think the hx35 is getting out of its efficiecy zone at over 40psi. That is one reason I don't want to mod my truck anymore, I don't want to get into having to replace everthing to go to the next level.