I've had the same conversation with several enthusiasts, and have seen posts on many sites about the subject. Even though direct injection is becoming more and more common on cars, many enthusiasts are saying it's "untunable."

The conventional wisdom is, unlike port injection, in which you can simply add higher-flow injectors or an auxiliary upstream injector for forced-induction applications, direct injection inherently limits the maximum fuel flow at high RPMs.

I will preface whatever I say in this post by admitting I'm not an engineer, and I don't purport any of my observations to be technically accurate. However, just a few points I'd like to bring up.

All modern diesels are direct-injected, and the aftermarket tuning scene has taken to them with a vengeance. It is not at all unheard of to get gains of 100 horsepower and more than 200 lb./ft. of torque on the big 3/4-ton diesels with just a little chip tuning and a few bolt-ons. Granted, there is a huge difference in fuel pressure between GDI (gasoline direct injection) and DDI (diesel direct injection). Gas DI systems are around 1,200-2,000 psi, whereas diesel systems are between 24,000 and 29,000 psi.

Also, interestingly, direct injection has some limitations that are not a factor with conventional port injection. The RPM capability of direct-injection engines is directly proportional to how many cylinders they have. The highest-revving DI four-cylinders that I'm currently aware of redline at right around -- or just under -- 7,000 rpm. However, Audi's RS4, with a direct-injected V-8, revs freely to 8,250 rpm. Per crank revolution, there are twice as many combustion events as with a four-cylinder, and therefore half the number of duty cycles per injector.

So in a sense, the power delivery on the new breed of direct-injected, turbocharged 4-bangers will be more diesel-like, in that more of the power will be delivered at a lower RPM. Again, in a segment used to 8,000 rpm or higher redlines, this might be somewhat of a cultural adjustment. As well, with the wider adoption, increased focus, and increased research & development into direct injection, expect the duty cycle limitations to continue to decrease, and the rev limits continue to increase.

Much like fuel injection back in the day when carburetors ruled the streets, I think the declarations of direct injection being untunable are more from unfamiliarity and ignorance. Ignorance...such an ugly word. I mean it in the nicest possible way, in that many tuners simply don't know enough about it to do much with it yet. In time, just like with fuel injection, emissions-controlled engines, The Gen-III GM small-block, and all other new & unfamiliar things, tinkerers will find their way to squeeze more power out of this new-fangled technology.