How does the TQ/HP curve affect drivability with a turbo?
 

I'm about to turbo my NC and have been reading a lot of threads and researching the very few kits that we have for those. There's really only 2 companies/tuners at the moment and they seem to have different strategies (or at least the dyno graphs indicate that). I really don't have much experience with FI and the only other turbo I've owned was a Chrysler Lebaron (25+ years ago) where you'd mash the throttle, wait a few seconds while nothing happened, and then it would hit you hard but was totally unpredictable.

I put a 2.5L in my NC last year (along with an OS Giken LSD with 3.73 gears, Ohlins, 17x9 with 245/40R17 RE71Rs) so it probably has 185 hp and drives real good but I want a good bit more power everywhere and I want it to be very predictable and controllable power because I live in WNC and mostly use this car on VERY tight roads and autocross. I'm thinking around 300 hp most of the time and then maybe turn it up a bit if I hit a road course or am driving more open roads but I'm definitely not looking for any more than 350.

When I look at different tuner's dynos for their turbo kits, I notice that they can have very different curves and I'm curious what is the best (subjective I know) curve for someone driving really twisty roads or getting power down in 2nd gear at autocross. Some of my closest drives, you spend most of your time in 2nd gear where you come out of the apex WOT and are just about hitting the rev limiter before braking for the next turn. Obviously with the turbo I'll be adding a 3rd gear shift in that short straight away but still, I'll be coming out of all turns in 2nd and getting on it hard so I want the added power to be as controllable and predictable as can be. I'll constantly hear guys say they have no traction in 1st and 2nd with 300 hp but I don't know if they can't drive, have touring tires or if there is something about their tune that makes it hard to put the power down. I realize any tuner can fine tune the tune but as an example, 1 tuner's dyno graph shows max torque by 3500 rpms with a very flat curve tapering off slowly to redline and the other tuner's dyno shows boost just starting at 3000 and doesn't reach peak torque until 4500-4800 rpms and is more of a hump vs flat. So same max torque numbers but one is a big rounded hump and the other is a flat sloping line from 3500 to redline.

Is one of those curves going to be more predictable and easier to drive hard out of a turn than the other? Especially for autocross, I'd love to have as much torque early for coming out of corners but I don't want it to hit so hard that I blow off the tires in 2nd gear. I don't want it to be like that old Lebaron where you were always guessing when, or how hard the power was going to come on when accelerating hard out of a turn. So what am I looking for in a curve to where my right foot can predict how the power is going to come on and I can work the throttle to accelerate as hard as my grip will allow?

FYI- I'm probably going to go with either an EFR 6258 or 6758 turbo although one kit has the GT2860RS which I'm leaning away from at the moment. Thanks!

EFR 6258 will spool very quickly and a proper boost by TPS table will prevent you from overwhelming your rear tires - it should be a very drivable setup.

For a 2.5 liter engine - I think the 6758 would be a better candidate. You might lose a bit of spool up but your larger displacement should make up for it.

GT2860RS is ancient tech compared to the EFR, I'd steer clear of it in this day and age.

I'll suggest TPS based closed loop boost control. 4 port boost solenoid. On a low boost/low hp system, it's not as big a deal. Starting making more boost (10+psi) and being able to precisely control boost by TPS vs RPM goes a long way towards making it feel more like a big sharp V6 than a peaky, laggy old school turbo. The EFR line is so responsive that PTFB (part throttle-full boost) is easily achieved. Fun the first time it surprises you by kicking the tail out in an intersection, not as much fun when it does it on a slick freeway connector at 70mph.

Thanks guys! So it's really not about a flat vs humped curve? How do you setup and control boost by TPS? Is that with an ECU (which ones) or through the ECU? Yeah, I'm not looking to kill myself so I want to figure out the best way of harnassing the power. Thanks!