HOHN

As usual, Kevin is right

The Mustang guys you see can also rev their engines MUCH higher. Thus, the have much lower boost levels for a given amount of airflow in CFM.

Follow this:

Engine Displacement (CID) * rpm/3456= CFM airflow (assuming 100% VE)

Now, you have a CTD that revs HALF of what the Mustang will (3200 vs 6400rpm, for round numbers).

Knowing the HP formula that was posted above, we can see that the Mustang is already capable of TWICE the hp for the same amount of tq, just because it can rev higher.

The Mustang can run the large turbo because:
1) It can be laggy because the gasser revs much higher (if it doesn't spool until 5K, you still have some RPM to play with)
2) The charger doesn't have to produce as high a PR (because of the RPM advantage, again).

On the Pressure Ratio--
What REALLY matter in terms of HP potential is the POUNDS of air that the engine can flow per minute. So, Cubic Feet only matters when the engine is naturally aspirated and compared with another naturally aspirated engine. This, because we assume constant pressure (and it's atmospheric).

Now you get into a TURBOCHARGED engine, and you have to account for the increased air density due to boost pressure. This is where the MASS FLOW (lb/min) comes in.


So if our Mustang Turbo and our CTD both have 800hp, the Mustang will have a much lower boost pressure to do it. Because it revs higher, it takes less Density to produce a given mass flow rate. The lower density shows up as lower boost pressure.

Because it runs a much lower boost pressure, the single turbo on the Mustang operates at a much lower Pressure Ratio. Because of the lower PR, it can often be within the efficiency of a single turbo.

It's VERY hard to make a single charger efficient at the 6:1 or higher PRs that a big-hp CTD needs.

HOWEVER, if you go with Compounds on the CTD, and each one operates at only 3:1 PR, then you now have a 9:1 PR-- AND IT'S EFFICIENT!! This efficiency shows up as lower discharge temps and more density-- which enhances the MASS FLOW.


Always remember, MASS FLOW is the SINGLE biggest determinant of HP potential. Hp is best though of in terms of lb/min of air and lb/min of fuel.

Aftercoolers work because because they increase density (and therefore, lb/min of air) and greatly enhance the mass flow rate through the engine.

MASS FLOW! MASS FLOW! MASS FLOW! MASS FLOW!

Justin