I've been looking at lots of turbo maps and none seem to indicate how much air is required to get a given level of boost. Say for example a builder wants to limit their torque to 300 ft/lbs but he needs 430hp at the wheels. We know this means ~7500rpm on the clock and 36lbs. of boost on his engine but where is it indicated how much exhaust volume will get him to that level of boost?

 

Garrett has turbine maps for some of its turbos, or the BW match bot may be of some use.

 

The power level correlates very closely to airflow. Whatever mass flow goes through the compressor must go through the turbine. Turbo manufacturers match the turbine to suit the compressor. Fine tuning is done by changing the turbine housing size. In rare cases, the installation may work better with a larger or smaller turbine than the one supplied by default.

 

You can also look at the required pressure ratio on the turbo map to get a feel for what is required. You should have a good feel for volume and temperature, PV=nRT will get you the rest of the info.

 

Thanks for the input but unfortunately, none have answered my question in a way I have understood it. Yes, the turbo maps supplied by manufacturers detail the pressure ratios available for a given volume of gas throughput but they don't state how much gas is required to start the wheel at a given pressure point. Is this solely an external gate function? I guess what I'm aiming to understand is exactly when the wheel begins turning and how its speed is controlled.

 

I might be complicating this more than necessary. Does the turbo spin when the engine idles? When does it actually begin to spin, does the map indicate this with the baseline curve? Is the wastegate the way we keep the boost at a given level or is there some other way to keep a ceiling on the boost?

 

Turbo spins at idle. It increases with speed as engine load goes up. So if you are cruising at highway speed (light load) the turbo is spinning enough to feed the engine.
If you where to push the throttle heavy, engine load goes up, increases mass flow enough to increase exhaust volume , forcing the turbine wheel to spin fast , which increases turbo speed. This sets of a chain reaction of creating boost, which increases exhaust volume, making the turbo spin faster and faster. The wastegate vents exhaust gases preturbo to control the exhaust volume on the turbine wheel , thus controlling overall boost.

Now the biggest hurdle on this is determining when boost comes on. Many variables that make it hard to answer. Engine displacement, volumetric efficiency , cam timing, tune up, exhaust design between engine and turbo, car weight , gearing , and turbo specs , are big ones in how a turbo will respond on an engine. Heck even intake design has huge effect on spool up.
Most just use experience and or advice from turbo companies.

In the case of indy car, or racing teams that have a big budget, they can design the whole program to get them close to what they want. In the end, everybody, everybody, has to do real world testing .

 

The wheel is always turning if the engine is on, think of it like a fan. If a breeze blows through (engine pulling air) its going to spin the compressor wheel, the exhaust is pushing air causing the turbine to spin. The wheels turning doesn't really benefit us until they hit a certain speed, the speed at which the airflow being pushed by the compressor exceeds what the engine is able to take: this is when you start to see boost. You see this RPM on a compressor map as a number next to the efficiency islands.

 

Thanks guys, I think this latest spat of information has made the picture clearer for me. Okay, here's a real world example with common parts. Get a T3-60 map to follow along if you like:
Engine - Nissan 3 cylinder HR12DDR
NA Net Flow Rate - this 3 cylinder can move around 160cfm adjusted for all losses at 800rpm
Turbo - T3-60 (common map shows good for 2.8 P.R.)
If I understood you guys this time, I can dial in 28lbs. of boost and when I hit 2750rpm, I'll have 5lbs. of boost and a 37% increase in torque over my NA number. If I stay on the throttle, the boost will keep building to its peak of 28lbs. at 8000rpm, where I'm moving 29lbs. of air adjusted for all losses. These numbers assume I can deliver the fuel required and time the burn right. If I'm moving 29lbs. of air adjusted for all losses, am I making ~290whp (10 x air mass) or ~370whp (29 x atmospheric pressure - friction losses)?

Next question - the turbo is near its max spin and I come off the throttle, where does the pressure go and how do I mange it as the throttle position changes?

Another question - at what level of pressure do I need special (uncommon) coupling for my vias (air tubes)?

Last question - how do I keep the turbo more or less in line with the throttle changes?

 

Excess boost pressure is blown into atmosphere via a blow off valve or recirculated back to the intake tract pre turbo via a bypass valve.

At 30 PSI you will want as few flexible silicon joints as possible, one at the turbo, one at the intercooler inlet/outlet and one at the manifold and you will want your cold side piping to fit as tight as possible. You will want beads on the piping where silicon goes and t bolt clamps, no worm gear nonsense.

The turbo will regulate based off of load and wastegate.

Since its your first build try and get it going at a lower pressure, say 5-6 psi and work your way up. This way you can track fueling, dial in your tune and find problems as they arise. 30 PSI is a lot in a gas engine there are going to be challenges along the way, the car is going to be totally different at 5 psi than it is NA, enjoy the gains, build some confidence, acquire knowledge as you go.

 

@B E N
Thanks for the continuing attention. I'll be running E98 so detonation shouldn't be a problem with the high boost. Others have informed me 28lbs. is about the limit with flexible couplers and they need to be real short even then if I want to avoid blow offs. I don't own a dyno so each and every tune session for a change in boost will be cost prohibitive. How do you test at the different boost levels for meaningful results without a dyno? What's wrong with dialing in the full boost and regulating it with the tach since the map shows an escalating torque to the redline? I suspect an escalating torque curve that long is a thrilling experience but I've never experienced it. If anyone has that experience, how does it effect the loads on the rear? Does the rear tend to break out more easily, is it harder to feather/scrub off speed? The bypass valve seems like a more elegant solution, are there any cons compared to a B.O.V.? Anyone have an idea of what kind of power result to expect given the specs provided?

corrections: For those confused by my prior post, the 160cfm flow referenced was for 8000rpm, not 800rpm and I don't need to "mange" the pressure at the different throttle positions, I need to manage it.

 

The bypass valve verses blow off valve usually boils down to vehicle requirements. If the turbo and BOV are before you air metering your fine but if the turbo is after air metering you have to use a bypass valve because the engine management is accounting for the airflow. Most people like BOV because they make a neat noise.

If you have no choice but to have someone else tune it you will probably be best off minimizing tuning sessions, I took for granted that you would be tuning this thing yourself, if you have an experienced tuner you trust then just crank it up and go crazy.

I think the pressure limit on silicon couplers is pretty high, some are built to handle a lot of pressure. I have assisted in compound turbo diesel applications pushing over 80 PSI with silicon, you just have to set the tubing up properly, check the torque routinely and buy quality components. There are some fabrication techniques that can help in keeping them reliable, but the fewer you can use the better, hard tubing wont loosen with time or slip or wear out like the silicon will.