Discussion About Pressure Being Resistance to Flow.

[jc052685]

I tried searching for like the last hour lol. I remember there being a discussion about how PSI boost is irrelevant and its about flow and such. I understand the principle of the idea and was discussing it with someone. There was some really good info and I was just looking for a little guidance in the discussion we were having.

 

Thanks.

[Tony D]

It wasn't a 'discussion', it was a 'definition'...

 

By definition pressure is only a manifestation to resistance to flow. If your turbocharger only flows X cfm (or pph), and you put it through an X sized orifice, then you will get X psi. Take that same orifice and try to push X+Y from a larger turbocharger and your resultant X+Y psi will be higher. People mistake the power boost as coming from an increase in the PSI, where in reality that is only an indication that there has been a FLOW increase in the system, and the resistance to it shows an increase. If it was BOOST that was making the horsepower---then when you take the second example (original head/orifice) and open it up on the larger turbo, and the 'boost' is kept at the original X psi, your horsepower would be LESS from the original first test, but it won't be because flow is making more get into the cylinder. And at the X+Y psi level similar things would be assumed: if it was boost that made the HP increase then at that elevated number with the ported head you would expect the same horsepower, but it won't be, it will be more, as more FLOW is generated and as a result more power is produced.

 

This assumes proper fueling in all conditions.

 

People say 'boost is what makes horsepower'---this is wrong thinking. The above example shows why.

 

Boost is only, and CAN ONLY EVER BE a measure of the systems' resistance to flow from a given flow source. Whether it be a roots blower, turbocompressor, or even a reciprocating first stage compressor (yes you can do that!)

 

Really, there isn't any discussion, it's a definition. This is 'Compressors 101' and the only practical way to explain pressure in a flowing system. This discussion applies to any fluid system. The problem is people apply STATIC definitions to a FLUID system (where pressure means more contents in a given confined space) and make assumptions accordngly. Can't do that.

[jc052685]

Beautiful Tony! Thanks!!!

[cygnusx1]

I just want to add that when you follow the "just turn up the boost" mentality, what happens after a certain point, is that the restricted flow path will cause the air to "bunch up"; and at that point you are beginning to make more heat than flow. Heat is the result of trying to flow 10 pounds of air through a 1 pound path...so to speak. The analogy could be trying to push a sponge through a funnel. Pushing harder on the back of the sponge will cause it to bunch up and compress further, in the wrong direction, making it all the more difficult to fit through the bottleneck..."heat".

Edited May 7, 2011 by cygnusx1

[Cannonball89]

I never could understand how Porsche was able to turbocharge air cooled motors for so many years and still make them reliable and not overheat until I understood what Cygnus is talking about. Large turbos that provides little restriction to the exhaust flow and free flowing heads and intake were the only way that they could accomplish that feat.

[Tony D]

I just want to add that when you follow the "just turn up the boost" mentality, what happens after a certain point, is that the restricted flow path will cause the air to "bunch up"; and at that point you are beginning to make more heat than flow. Heat is the result of trying to flow 10 pounds of air through a 1 pound path...so to speak. The analogy could be trying to push a sponge through a funnel. Pushing harder on the back of the sponge will cause it to bunch up and compress further, in the wrong direction, making it all the more difficult to fit through the bottleneck..."heat".

 

This belies the physics of what is really happening and is misleading as to what is really what is going on. PV=nRT, boyle's law says the higher pressure you have the more heat produced. It's nothing about a bottle neck in the downstream it goes directly to the pressure ratio and efficiency map of the turbo.

 

If you walk outside the most efficient area of the compressor map, you make more heat per psi produced simply through inefficient compression and internal recirculation. Add to that higher pressure (meaning higher pressure ratio) means that more heat is also produced.

 

Heat is produced by pressure ratio, NOTHING ELSE. This is the physics of compression. The FLOW portion puts you on the map in an efficient or inefficient portion of the efficiency islands. The PRESSURE component gives the basic heat generation. It is straightforward on a 3:1 Comp ratio that you will make X heat at an 88% efficient compression area of the map.

 

Move to the 78% portion of the map, and you will make X heat + the additional heat generated due to being 78% versus 88% efficient in compression.

 

This can be very significant.

 

This is a quadratic equation with the efficiency of the stage of compression a fractional exponent, but the math is something anybody with high school pre-calc or algebra 2 could work out, really.

 

The same thing goes for any compressor.

 

What you do with the "crank up the boost mentality" is move the point on the efficiency island where the compressor is operating from one of high efficiency, to one of lower efficiency. If you run vertically on the compressor map, you would get more heat at the SAME pressure due to the efficiency loss. Add a vertical and horizontal component to the equation and you end up set for poor efficency.

 

Rarely do you move to the right on the compressor map in terms of efficiency, the 'crank up the boost' people move up, and to the left on the map almost universally. And look at a turbo map for efficiency islands and you can see the dropoff plain as day.

 

Through engineering, they bias the maps wonderfully to compensate for this (compare a turbo map from 1970 and 2010 to see the change of the island shape!)