whats a safe PSI to run?

[UncleBob]

Originally Posted by Carfreak18

So are you saying he should not be on pump gas with a setup like this? I dont fully understand what you're trying to say.

what I'm saying is KABOOM!! Very soon. Its not "impossible" to run 28psi on pump gas, but it takes a very thoroughly engineered engine, and IMO, it REQUIRES at the least, alcohol injection. I personally don't believe its feasible with pump gas (92 octane) by iteself, even if you're running 6:1 compression ratio and 5 degree's total timing.

OK, it would probably be feasible then, but it would drive like utter crap.

With alcohol injection, its a whole nother story, but then, its no longer "pump gas" either.

[Carfreak18]

even if he "remapped" his fuel,he shouldnt be running pump gas?

[SaabJohan]

Originally Posted by KiwiBacon

Ain't it funny how hard they try.
Even at 15psi (pressure ratio = 2) the efficiency is on the way out. I've heard of clowns trying to run these at 20psi.

Some turbos can efficiently boost to well over 30psi, but they aren't very common in passenger cars.[/IMG]

From the turbochargers point of view, high boost pressures aren't that much an efficiency problem like it is an turbo speed problem. Sure, when you run very high boost pressures you lose a bit of efficiency, but lack of efficiency is more an issue related to too high flow rates. Of course, if you keep increase the boost the impeller inducer will eventually go supersonic which is followed by a reduction in efficiency. But the first problem with high boosts tend to be turbocharger speed. In most maps, like the one you posted, the highest rpm line is the maximum safe rpm. Above that point, we usually have the third critical speed for the turbocharger rotor, and if it spend much time at that speed it will break. Another problem is the centrifugal forces generated by the high speed, and together with the higher temperatures the stress will be great on the impeller. Most low boost turbos have impeller wheels made out of a simpler aluminum alloy, and these tend to lose strength at about 150 degC, the temperature you typically reach at about PR 2.5.

Compressor designs for higher boost pressures, tend to be made out of a high strength, heat resistant aluminum alloy, or a titanium alloy, preferbly billet machined from a forging. They also have a different design that makes it possible to build a higher boost at a lower turbocharger speed, however, that usually comes at the cost of map width. Some compressors for high pressure ratios also have a transonic impeller design to overcome the normal losses related to impeller speed going supersonic.

In general a turbocharger should be used at a pressure ratio where it offers the greatest map width; that will result in the greatest usable rpm range for the engine. In the map you posted, that is at about PR 2.1, but a higher pressure ratio can be used at lower speeds for more low end power.

Some turbochargers can run with very high pressure ratios, up to 4 or 5 even. However, most small turbochargers found on cars work best at around 2 or so. Bigger diesel turbos tend to favor ratios around 3, but some use 4 or 5. Big diesel engines, such as ship engines also tend to favor very high pressure ratios.

In many cases tough, the turbocharger isn't the component limiting the boost.

[Carfreak18]

Whats a "pressure ratio"...?

[UncleBob]

atmospheric pressure at sea level is 14.5psi absolute.

"guage" pressure, at sea level is 0psi. If you run 14.5psi on the gauge, you have double the atmospheric pressure, or you have a ratio of 2.

3 pressure ratio would be 14.5 x 2 = 29psi. The psi for each ratio changes as you change altitude, so it can be a bit confusing when you are talking about ratio's, absolute, gauge pressures.

But to keep it simple, a ratio of 2 = roughly 15psi of boost on your boost gauge.

[SaabJohan]

Originally Posted by Carfreak18

Whats a "pressure ratio"...?

For a compressor

pressure after compressor / pressure before compressor = pressure ratio (aka compression ratio)

For a turbine

pressure after turbine / pressure before turbine = pressure ratio (aka expansion ratio)

For a compressor the PR is always above 1, while for a turbine the value is always below 1.

Atmospheric pressure is typically around 1 bar, so a compressor running a pressure ratio of 3 will create a pressure of 1*3 = 3 bar given there are no pressure loses before the compressor. That pressure is usually described as 2 bar boost, since that is given in pressure above atmospheric. More imporantly though, imagine that we use a two stage compressor, each running at a pressure ratio of 3, the pressure in the system would increase to 1*3*3 = 9 bar or 8 bar boost.