I've heard before (on piston motors) that every PSI you gain about 10hp. Is this about right on a Rotary motor? Say if you just replace a downpipe and gain say 1 or 2 PSI would that give you 10-20 hp? Or does the rotary respond better per PSI than a piston motor?

Thanks

 

In general it will respond the same. I sure you will get people argueing one way or the other, but different cars will get different gains in different situations.

1psi = 10 hp is still a good rule of thumb.

 

waaaaaaaaaaaa! i just typed out a big long thing! then i went to click on send, but i slipped and added 13bpower to by buddy list!

 

FOUND IT:
it depends on a lot of things, the biggest being the compressor map of the turbo you're using, and how close your boost is to the peak efficiency. for example, on the stock turbo, going from 7-12 psi might give you 50hp, but going from 12-17 would be pushing the turbo too much, and might only give you say, 20 hp, and kill your turbo pretty quick. also, if someone were to run 15psi on a ht18 (stock turbo) compared to 15psi on a 60-1 (pretty big turbo), the power difference would be enormous! maybe from 270hp on the stock one to 450 on the 60-1. the 60-1 is bigger, so every psi give more cfm of air. BUT a 60-1 compared to a t-66 which is even bigger, would have the 60-1 making more power at 15psi, since the t-66 would be at the lower end of its efficiency. but at 25 psi... well thats a different story.

 

Doesn't theory say that one bar (14.6 psi) of boost in a perfectly efficient turbo set-up will double the horsepower? Of course all the theory stuff never works anyway

 

stock s4 n/a: 146hp
stock s4 TII: 182hp

36hp increase with ~5psi, why not 50?

I'm sure there are other factors.. I'm curious

 

well you have to remember that the S4 N/A has higher compression rotors than a S4 TII. so that's the main reason why the hp difference is 36. i don't know what the hp would be on a S4 N/A with TII rotors but it might make up for that 14hp discrepence.

 

There are too many factors playing against one another to compare psi and boost in relation to an NA and TII of the same generation...firstly, crank hp means little and wheel hp is what should be measured...even then, the S4 NA and S4 turbo have different compression ratios, engine management programming, transmissions, etc...taking all this into account the 10 psi rule of thumb is a good overall guide but you can't really compare the two cars for accuracy...

 

correct me if I'm wrong, but 15 psi is 15 psi, regardless of the turbo making it. The HP levels would therefore be the same. How hard the compressor has to work to make that 15 psi is another story entirely...

 

Yes 15 psi is 15 psi, doesn't mean they are moving the same amount of CFM.

 

exactly!! a good turbo upgrade will make more power at lower boost levels bc it moves more air. You also have to think of the route this air has to flow...different intake manifolds, exhaust manifolds etc...

 

compression ratio has a major effect too.

 

yes, a higher compression motor will make more power at a given boost level provided the motor can take it

 

compression ratio Vs. boost is an interesting theory.
The theory is with a lower compression raito, you can put more fuel and air into the 'cylinder', therefor making more power.
The porsche 917/30 can-am racecar of the 70's had somthing like a 6:1 compression raito and made over 1000hp from a air cooled flat 12. i think its displacement was around 4 liters. no idea what the boost was.

 

I know that one more PSI will give you more power on an upgrade turbo because it flows more CFM's, but I forgot to mention that I was talking about the stock turbo, generally. S5 in particular.

 

well, on a crazy motor lower compression is preferred bc you can run loads of boost. however, response before boost (like a 930) will really suffer. Thanks to engine management improvements, todays factory turbo cars run higher compression than cars 15 yrs ago...

 

This is a little silly.

First off, there is NO correllation between PSI and horsepower. You guys have to remember that an engine is a SYSTEM - every change to the system affects everything else.

If I take a car with a stock turbo, do all the bolt-ons to it, and run about 1 bar of boost, I'll maybe make 260-270 to the ground. Take the exact same car, same bolt-ons, and swap out the turbo/manifold with an HKS TO4 manifold and a 60-1 HiFi, and run 1 bar of boost, and you'll be making around 350 to the ground.

Why?

First off, a better turbo is FAR more efficient than the stock turbo - you can compress more air for a given shaft speed. It's like seeing a little weiner dog running and a horse running - the weiner dog's little legs are flapping as fast as they can go, but he doesn't go very fast. A horse has longer, better designed legs, and can REALLY move. Same principle.

This also applies to the exhaust side of the turbo. Turbos actually work on the hot side by the pressure differential between the downpipe and the exhaust manifold. The turbine inlet pressure is what spins up the turbo. Too much pressure, and you create backpressure on the engine, which is inefficient. But, if you have a more efficient turbo that can do more with less turbine inlet pressure, you win out there.

Not to mention going to an external wastegate - you can keep the two exhaust streams separate for less exhaust turbulance at the turbine outlet.

There's the breakdown for ya .

Dale

 

so with a larger turbo, even when the engine is in vac will flow more air than the stock turbo, also approx how fast is the turbo spinning when the engine is under vac?

 

LOL, yes.

Besides all the other variables involved, turbos are NOT 100% efficient. Therefore, although the pressure ratio may be the same (ie same boost level) the density ratio (hotter air is less dense than colder air) will depend on the thermal efficiency of each turbo. Just like you know from the theory behind a cold air induction system or an intercooler, colder air means more air mass, and more air mass means you can add more fuel, and more fuel = more hp potential. More info:
http://www.timskelton.com/valkyrie/t...percharger.htm

I'll list some generalized peak thermal efficiencies below. However, you will need to look at each particular compressor map to find the published efficiency at a given pressure ratio and flow rate. Much like horsepower and torque, the peak numbers don't tell the whole story. Not all boost is created equal.

50-60% Roots Blower (Camden, Eaton, etc.)
70% Lysholm Supercharger (Whipple, Kenne Bell, etc.)
75% Turbocharger or Centrifugal Supercharger (Turbonetics, Garrett, Paxton, etc.)

More intricate maps:
http://www.turboneticsinc.com/comp_maps/fig1.html
http://www.superchargersonline.com/index.asp

The second site has a hp calculator which is really simple, but some of you may like playing around with it.

BTW, 1Bar = 14.5 PSI.

Hehehe, are you confused because the numbers don't add up? That's because you are right. There are only two things that matter about the flow rate of a turbo:
1) Can it support the maximum flow rate required?
2) Can it support the minimum flow rate required (ie not surge)?

 

Okay so I have read about it, and for the most part grasp the idea..........but. Why is there a HP difference(WW/ all other things being equal) in a motor with a CR of 11.5:1 with 2lbs boost, than a motor with 9:1 with 7lbs boost? Its all compressed air. Is it just that the charge gets less heated from a turbo as opposed to cylinder compression? Enlightem me!

 

5 psi on an 8 litre v12 is a shitload more air (thus power) than 5 psi on a single cyl 250... so making up rules like 1psi = 10hp is useless. A t2 running 14psi should supposedly make twice as much power as one running no boost, but because of losses involved I doubt it'd quite work out that much...

Charlie

 

Basically...
Go read up on "VE"!


-Ted

 

917/30 had 5.4 L of displacement, two valves per cylinder, two spark plugs per cylinder, and mechanical timed fuel injection. 7800 RPM redline. 1.3 bar of boost (about 18 psi.)

 

Ressurect! :P


-Ted

 

Oh dear god..