Lots of people on the forums believe that anything bigger than a 20G won’t be fun to drive. They are wrong. Half of them have no idea what they’re talking about, and the other half don’t know how to drive. I suspect they got the “OMG lagz WTF” idea from looking at dyno charts that show big turbos hitting full boost at 4000 RPM or more.
Those charts are very misleading. It’s true that if your turbo hits full boost at 4000 RPM and you floor it at 3000 RPM, you’re basically going to wait for a naturally-aspirated engine to take you to 3500 RPM before things get interesting. That’s a waste of time. It’s not fun at all. There’s a very easy way to fix that problem though: stop doing that.
If you have a big turbo, you just have to get into the habit of keeping RPM high when you want power. You get much less lag that way. If I am trying to accelerate, I’m in the 4000–7000 range. When I start a pull at 4500 RPM, the chart looks like this:
The other nice thing about keeping RPM high is that it means driving in a lower gear that you would otherwise. So, not only does boost come on quickly, you get more acceleration from whatever boost you have, simply because you’re in a lower gear.
Around the time when I was trying to decide on a turbo, I went for a ride in a friend’s Corvette, with a big naturally aspirated V8. That was when it hit me that when you want acceleration, you keep RPM high, even if you don’t have a turbo. High RPM is another way of saying “lower gear” and “lower gear” means more torque at the wheels, and more torque at the wheels means more acceleration. That was the day that I realized that I would be perfectly happy with a turbo that didn’t wake up until about 4000 RPM.
At 2500 RPM my car is boring to drive - it’s almost like a naturally aspirated 2.5L. That’s perfectly fine for getting groceries, so I’m OK with that. Also, that fact has no impact at all on my car’s performance characteristics. When I want performance, I am in the 4000–7000 range. Bam. Problem solved.
With a smaller turbo I’d have less torque from the motor, and since it would run out of breath before redline, I’d have to shift sooner, so I’d be handicapped by a taller ratio as well. Some people will pay those penalties, just to avoid downshifting. I don’t think that’s a good trade.
My turbo hits 20psi at about 3800 RPM in 3rd. If I am cruising in 5th or 6th at 2500 RPM, and I just floor it, nothing happens. It 6th gear I hit 20 psi at about 3200 RPM, which sounds great, until you realize that in 6th gear it takes ten seconds to get from 2500 to 3200. That hardly even counts as acceleration, so I have only done that once, just to see how long it would take.
On the other hand, if I am cruising on the freeway at 65mph in 6th, and I downshift to 3rd, I’m at 5000 RPM when I release the clutch. From there I get full boost after about a half-second. And the car accelerates a lot faster in 3rd than in 6th because not only do I have more boost sooner, I have much better gearing. So, two seconds after releasing the clutch, I would reach the redline, at about 85mph. (But that would be illegal, so of course I never do that either.)
And just for completeness…. When I shift while drag racing, I push the clutch pedal, floor it while shifting gears, and then release the clutch pedal. Boost builds quickly with full throttle at high RPM. So even though I shift as fast as I can, I’ve got 10psi when I release the clutch, and it only takes another quarter-second to get 20psi. But, I know exactly where to have my car towed if I break something while racing, and for DDing I don’t plan ahead that much.
(I have spec B gears, so the numbers above are slightly different than a regular 6MT, but the principle is the same.)