When it comes to engine tuning for on-track use - whether it's for turbo performance, running on biofuel or just to get the most out of it - it's not all about maximum power all the time, unless you are planning on making a traction control system do all the work (including making up for some suspension shortcomings, but that's another story).
Subscribe now for unlimited access.
$0/
(min cost $0)
or signup to continue reading
I've covered turbos and lag before, but in a nutshell they have an exhaust housing with a wheel in it, and a shaft that connects that to another wheel in an intake housing which is what blows (pumps) the air into the engine at higher than atmospheric pressure. A conventional turbo therefore, requires a good amount of exhaust gas to be flowing through in order to generate boost pressure.
Most conventional turbos also work best somewhere between 80,000 to 120,000rpm, and the time it takes for the exhaust gas to spin the turbo back up to its working speed (after lifting off) is referred to as turbo lag.
Different engine size and turbo size combinations also affect this lag time. A big turbo on a small engine will have lots of lag. The little 1.5L engines in Formula 1 cars of the mid-1980s had, in most cases, such a large turbocharger on them that they didn't make meaningful boost pressure unless the engine was spinning above 10,000rpm.
Road cars though, especially from the mid-'90s on, had far more sensible combinations with a turbo size that would be most useful in the mid-range of their normal rev band.
Still, that alone didn't necessarily stop the turbo coming on like someone had crashed into the back of you, so some boost management was helpful.
By the early 2000s turbo road cars came standard with electronically-controlled forms of boost control. And of course, that made it very easy for tuning workshops capable of remapping a turbo car's ECU (engine control unit; the car's onboard computer) to also mess with the boost map.
The car I was referring to at the start of this column was a 2003 WRX, and it had been messed with by a tuning workshop.
It had a standard turbo, engine, and intercooler, and a bigger exhaust from the turbo's exit (which increased lag a tiny bit, because of lowered momentum, but we'll save that explanation for another exhaust topic).
To simply impress people riding in the car, or perhaps because they were focusing on drag strips or dyno shootouts, what they did in the boost tables of the engine map were to just crank it up as far as they dared push the hardware, pretty much all of the time.
You probably can't read it, but in the top right table it's asking for 12.84psi with just 24.3 per cent throttle (just under a quarter of the way open) at 2330rpm, and it only peaks at 15.78psi at full throttle near the redline. This is a very aggressive boost map, which felt like getting crashed into from behind when the turbo was suddenly able to produce pressure. But the downside was, when trying to drive this car in hillclimb events (short dashes of no more than 2km, often less than 1km, on a twisty course against the clock), was it also exacerbated the car's natural tendency towards power understeer at corner exit.
So, just like a rear-wheel drive car, the driver had to have it pointing in a straight line before the boost came on, and that was a little tricky to predict. That meant there was always a bit of time lost out of every corner in the process of straigntening up, standing on it, and waiting a moment for the boost before scooting off down the straight.
The bottom left was a safe progressive map I put in after repairing a burned exhaust valve, and then once that had settled in the bottom right is the real progressive boost map.
(Also note other changes had been made from the standard ECU maps by the workshop and then by me, but for this we're just talking about the effect of the boost map.)
At the lower end of the table it's only asking for 7.73psi at 2500rpm and 24.3 per cent throttle. The numbers steadily climb as the revs climb and as the throttle percentage increases, before eventually asking for 16.09psi just before the rev limiter at full throttle.
The result was an engine that felt more like a big normally-aspirated one, which meant the throttle could be squeezed progressively much earlier when coming out of corners, and so the lap times came down.