boost and lag
 

Too Much Boost
With air being pumped into the cylinders under pressure by the turbocharger, and then being further compressed by the piston (see How Car Engines Work for a demonstration), there is more danger of knock. Knocking happens because as you compress air, the temperature of the air increases. The temperature may increase enough to ignite the fuel before the spark plug fires. Cars with turbochargers often need to run on higher octane fuel to avoid knock. If the boost pressure is really high, the compression ratio of the engine may have to be reduced to avoid knocking.

Turbo Lag
One of the main problems with turbochargers is that they do not provide an immediate power boost when you step on the gas. It takes a second for the turbine to get up to speed before boost is produced. This results in a feeling of lag when you step on the gas, and then the car lunges ahead when the turbo gets moving.

One way to decrease turbo lag is to reduce the inertia of the rotating parts, mainly by reducing their weight. This allows the turbine and compressor to accelerate quickly, and start providing boost earlier.

Small vs. Large Turbocharger
One sure way to reduce the inertia of the turbine and compressor is to make the turbocharger smaller. A small turbocharger will provide boost more quickly and at lower engine speeds, but may not be able to provide much boost at higher engine speeds when a really large volume of air is going into the engine. It is also in danger of spinning too quickly at higher engine speeds, when lots of exhaust is passing through the turbine.

A large turbocharger can provide lots of boost at high engine speeds, but may have bad turbo lag because of how long it takes to accelerate its heavier turbine and compressor.

In the next section, we'll take a look at some of the tricks used to overcome these challenges.

Re:boost and lag
 

hmmmmmm sweet

Re:boost and lag
 

On a side note. Lots of people seem to get confused on the "turbo lag" issue. Your engine still runs like normal. You don't lose power during the lag. It's just a delay before the extra power is brought on. In essence, think of your car at the strip, without the turbo. Light goes green, you step on the gas, and start taking off. Everything is going good, and then you get rear-ended by a bus.

Picking out a turbo is almost a science. You can get close with junkyard parts, or taking what is available... But, if you want the most power, with the least lag, and don't want it to die out before redline, then you need to really know what you are doing. If you don't know how to read a compressor map, then you need to!

Get 2 books before you start any turbo project, and read them both, cover to cover. Then, stick them in the bathroom so you'll keep reading them.
"Turbochargers" by Hugh McInnes
"Maximum Boost" by Corky Bell
You'll save more money on your project by actually knowing what you are doing then you would waste going off of hear-say. And, probably save your motor at the same time.