Boost vs HP Correllation

[Paolo Castellano]

I have a question about how much horsepower can be had from each additional lb of boost. I realize superchargers have inherent parasitic loss when compared to turbos. Someone gave me this equation to calcuulate the relative power gain as the following: 29.92 in Hg + #lbs boost all divided by 29.92 = factor to multiply current hp/torque. Here is an example: 6lbs of boost + 29.92 = 35.92 ==> 35.92/29.92 = 1.2 ==> 1.2 X 420 RWHP = 502RWHP or 82 RWHP. 12 lbs boost should yield 164 additional RWHP or 588 RWHP in this example. I remember Dan Black's Doug Levin car made 617 RWHP and 676 RWLbs/ft. Does that mean he is running 15 lbs of boost? 15+29.92 divide by 29.92=1.5 ==> 1.5 X 411RWHP = 617 RWHP and 1.5 X 450 RWlbs/ft = 675 RWlbs/ft. These #'s are for a stock car( ie no headers/exhaust/etc...)Can it really be that simple to calculate? Would a turbo application yield more RWHP due to less or no parasitic loss? Doug and Jason chime in any time you like! I would also like to see a dyno sheet for the supercharger applications in addition to the peak #'s, this would be helpful to map out HP & torque gains throughout the entire range of RPM. Also do you guys have an idea of exactly how much boost your systems are making at any given RPM? Paolo

 

[TOOOFST]

Paolo,what you need is a TOOFAST NOS system and a long vacation,my brother from another mother.

 

[MES]

I'm not sure if this is a good calculation or not but it seems to work. At sea level there is 14.7 lb/square inch (I think) so if an engine makes 400 rwhp at sea level it should make 800 rwhp with 14.7 lb of boost with no parasitic loss. Turbos have 5-10% loss and superchargers 20-30% so you take the loss of HP from the calculated gain and get real rwhp. Don't try and do the HP per lb boost thing, it's a percentage.

example 400 rwhp (motor only)
run 7.5 lb boost (half of 14.7)
200 rwhp more (50% power increase)
less 25% supercharger (200x.25=50) = 150 rwhp gain
That's a 37.5% total increase

so a 400 rwhp motor with 7.5 lb boost should make 550 rwhp with a supercharger assuming a 25% parasitic loss

and a 450 rwhp motor with 7.5 lb boost should make 619 rwhp

and a 90 rwhp (Honda Civic) with 7.5 lb boost should make 124 rwhp

It's all relative to the starting point.

It's not quite as easy as that but for low boost levels it seems to be fairly accurate. Disclaimer - what do I know, I've never even owned a turbo or supercharged vehicle (not yet anyway)

 

[treynor]

Oh, you just KNOW I can't resist this topic.

First off, the original equation is all wrong. You're adding pressure measured in pounds per square inch (PSI) to pressure measured in inches of mercury. Pressure at sea level is about 14.7 PSI. Thus the right equation, which is itself a gross oversimplification of reality, is:

HP (normally aspirated) * ((14.7 + boost #s) / 14.7)

Thus a Viper, which makes 410 RWHP normally aspirated, would at 8 PSI ideally make 410 * ((14.7 + 8) / 14.7), or 633 RWHP. Interestingly, this is quite close to the observed HP (617) of one 8PSI DLM car I know of.

Now, let's consider Scott P's car. We know that at 18 PSI on blower it made 851 RWHP, and at ~13 PSI it made 713 RWHP. His car of course is far further from stock, having headers, exhaust, and about 8:1 compression ratio. However, let's fudge some #s:

Stock viper with heads, headers etc -->455 RWHP.
Drop compression from 9.6:1 to 8.0:1 --> 379 RWHP
@ 13 PSI: 379 * (14.7 + 13) / 14.7 = 714 RWHP
@ 18 PSI: 379 * (14.7 + 18) / 14.7 = 843 RWHP

So this agrees remarkably with our predictions. Thus, we can tentatively conclude that:

For a Viper with stock compression ratio, each pound of boost provides approximately (410 * ((14.7 + 1) / 14.7)) = 28 RWHP
Similarly, with an 8:1 compression ratio, each PSI of boost = 23 RWHP.

 

[Doug Supercharged Vipers]

I have some good technical info I can fill you in on this evening! Right now I have to shove off to work.

I'll keep you all posted,
Doug

 

[v8huntr]

it would be very hard to find HP gain per psi of boost. HP gain will different depends on turbos. my supra made 705rwhp[1500egt] at 26psi using T76 but same car made 560rwhp[1500] at 26pi using T66. That is over 140+rwhp gain at same boost pressure. it really depends on how much volume of air turbo or supercharger can flow at per psi.

chris