Understanding Boost

[JBrady]

Understanding boost is an important step in understanding engines overall.

All engines run on pressure. Naturally aspirated engines run on the natural atmospheric pressure. This pressure varies from the standard 14.7psi at sea level to progressively lower pressure as you gain altitude and in some cases actually higher.

Well developed ram air systems can actually create higher than 14.7 psi pressure as can high pressure areas. This is also called one BAR (one barometric pressure) and is displayed in millibars. Of course 1000 millibars is one BAR. If pressure is less than 1000 you have less natural BOOST to run if it is above 1000 you have more.

Also, one BAR is ZERO boost on a boost gauge but is in fact pressure.

Now to get a little confusing (on the road to more with understanding coming in pieces along the way) on a compressor map you will generally see pressure ratio on the left side of the graph. 1 pressure ratio is 1 atmospheric pressure. 2 pressure ratio is equivilant to 14.7 psi of boost at sea level. 3 pressure ratio = 29.4psi and so on.

Here is a picture of the highly developed and highly successful Corvette C6R race car engine, 7 liters. Notice the very small intake tubes between the large ram air tubes and the carbon fiber intake manifold. These are restrictors required by rules to limit the engines power to "even the field" if you will. By studying this design you can start understanding how to MAXIMIZE a limited amount of pressure. Keep in mind that this engine makes nearly 600hp through these TINY restrictors!

 

[jibbby]

Why would they want to restrict the intake airflow on such a nice performance engine? I don't understand the logic behind that?

 

[Lextreme II]

Race Regulations... my Jibbby.......

 

[jibbby]

Thank you Sir.... Forgot about the race regulations.....

 

[JBrady]

Actually the restrictors (as noted are required by the ACO LeMans racing rules) are the reason I posted the picture. As an interesting side note new for the 2007 season many cars run air conditioning as a driver safety feature in for closed cockpit endurance race cars.

Anyway, those restrictors are a TINY 30.8mm (1.21") each on non-a/c cars and the rules allow 31.3mm (1.23") to accomodate the power required for the a/c compressors. http://www.prattmiller.com/c6rnews.php?NewsID=417

Bottom line... these cars are breathing 295hp through each 1.2" restrictor :shocked1:

So, you could have an engine with ONE 1.2" intake port making 295hp just as this one has TWO 1.2" intake ports making 590hp.

I do not know the pressure generated by the ram air system but obviously this is a fairly well developed and efficienct system.

This is similar to the NASCAR restrictor plate engines that have 4 holes that are less than an inch each ranging from a large of 62/64ths (31/32nds) of an inch to a small of 7/8" inch (56/64ths)
http://www.scenedaily.com/stories/2006/10/02/scene_daily229.html
http://www.scenedaily.com/stories/2007/09/24/scene_daily255.html

These engines are making around 450-500hp with the restrictor vs 850-900hp without.

Interestingly the a change from 57/64ths to 56/64ths looses about 10hp on these engines.

From the above links it looks like going from 7/8ths (56/64ths) to 31/32nds (62/64ths) is good for a gain of 80hp!

This is important to understand because boost works the EXACT same way.

Boost is a measurement of resistance. Let me repeat. Boost is a MEASUREMENT of resistance. If you reduce resistance the measurement (boost) goes DOWN. But, the actual mass flow does not go down and in fact usually goes UP. More mass flow is more power.

Mass flow is NOT volume. Volume is a measurement of space such as cubic feet. Mass is a measurement of atomic mass. How many ATOMS are in a given sample. It is the atomic mass that is important. Increasing volume without increasing mass is a REDUCTION of efficiency.

 

[gloverman]

Many people have problems understanding pressure and atmosphere pressure being higher than internal engine pressure and the flow of airflow between the two. This thread contains some of the best wording to explain it.
Another thing which came to my attention recently was the measurement of map sensors. They generally use the first bar as atmosphere pressure so a 2.5bar MAP sensor will measure 1bar atmosphere and 1.5bar (approx 22psi) of boost. Again you have explained the reasons well.
Cheers

 

[JBrady]

Many people have problems understanding pressure and atmosphere pressure being higher than internal engine pressure and the flow of airflow between the two. This thread contains some of the best wording to explain it.
Another thing which came to my attention recently was the measurement of map sensors. They generally use the first bar as atmosphere pressure so a 2.5bar MAP sensor will measure 1bar atmosphere and 1.5bar (approx 22psi) of boost. Again you have explained the reasons well.
Cheers

Thanks, this is my goal, to share what I have learned and to learn from others.

As you state the MAP sensor (stands for Manifold Absolute Pressure) is in fact as I state above. So, since it is "absolute" pressure the first BAR would be the the same as ZERO on a boost gauge. Obviously to run a very highly boosted engine like a mega power 2JZGTE would require a higher capacity MAP sensor such as a 4 bar or even more as 4 bar capacity is 4-1= 3 times 14.7psi or 44.1psi manifold. So, the boost gauge would read 44.1psi but absolute pressure would be that PLUS one bar for a true total pressure of 58.8psi (4 bar)

 

[JBrady]

Now, understanding the above is prerequisite to my next points. If the above is not clear please re-read and if still needed feel free to ask again (and again) the stupid question is the one not asked.

So, here we go...

A compressor... whether a turbo compressor, a supercharger compressor which among many includes the turbo style centrifugal compressor, the roots or pump style (not a true compressor) and the lysolm or screw type, vane, G-ladder or others...

All can be mapped with a compressor map. A compressor map measures the amount of flow (shown either as volume such as CFM or as mass usually as pounds of air per minute).

Compressor maps also show the adiabatic efficiency at various pressure levels. Adiabatic efficiency is how HOT the air is at a give pressure compared to how HOT it would be in perfect compression.

Perfect compression is the theoretical increase in temperature for the increase in pressure. If you start a X pressure at say 100 degrees farenheit and raise the pressure to Y the "book" may say the actual perfect compression temperature should be 200 degrees. If on the test stand the tested compressor outlet temperature at Y pressure is in fact 220 degrees it is less than 100% efficient. The closer to 200 degrees the closer to 100% compressor efficiency (adiabatic efficiency as there is also volumetric efficiency to be covered later)

Why is the above important? Because the lower the temperature at any given boost (pressure) the more MASS or atomic weight will be contained and therefore more power production potential and less thermal load placed on the system for that power level.

 

[Boosted2.0]

On a side note - I have a Handy excel sheet that has all the turbo sizing formulas embedded in it that I made. If you know VE, intake temp, AF ratio, displacement, engine speed and BSFC you can easily calculate where in the efficiency map your engine will run.

 

[striker]

Hey I saw that engine at 24h-lemans last summer, and heard it.... never thought 2x 32 mm could produce that much noise kept me awake all night long...... which was a good thing offcourse.

anyway a lot more can be put through restrictors that size when presure is added, AER makes over 800hp TT through 2 x 32mm restrictors, unristricted the engine can't be run, it will self destruct which was also the case with the audi R8 TT lemans engines. also the R8 breathes through a slotted tube, the slot is 1cm X 40cm through that slot the air enters the plenum chamber above the velocity stacks, throttle bodys cannot be bigger than 50cm by the looks of them on the outside.

so anyone building a twin entry 1uz plenum i would like to hear what gaines you make theory sugests that gains could be minimal.

Grtz Thomas