Turbo lag can be defined as the time it takes to bring the turbo up to speed for it to build boost and operate efficiently. You can also think of turbo lag as the time it takes for your turbo to start compressing air and the engine to begin producing significant power. Let’s look at the issue of turbo lag and what to do about it.
Stock Subaru WRX / STI cars are fitted with relatively small turbochargers, so the turbo lag is fairly small. Since the exhaust gases operate the turbocharger, a certain amount of exhaust gas is required for turbo to begin spooling up and compress air / build boost. A 4 cylinder engine can only produce so much exhaust gas at low RPMs. A 2.5L Subaru WRX / STI engine needs to rev up to about 2,500 RPMs to provide enough exhaust gas for the stock turbo to begin operating efficiently and build boost. At about 3,000-3,500 RPM the stock 2.5L engine produces enough exhaust gas flow to keep the turbo spooled up to stock 14.7 psi of boost. In fact, at those RPM ranges there is enough exhaust gas flow to spool up a larger turbo than stock, so the boost controller operates the wastegate to keep the boost level at stock 14.7 psi.
If you were to install a bigger turbo, say, GT30R, on a stock 2.5L engine, you will find that it takes more RPMs to spool up the GT30R, since it will require more exhaust gas flow to begin operating efficiently. Thus, the turbo will not build boost until later in the RPM range and you will not produce power until later in the RPM range.
So the difference in time between when you step on the gas at certain RPM and when the turbo ‘kicks in’ is the turbo lag.
Now, if you know at which RPM your turbo becomes efficient, you can downshift to bring the RPMs into that range, and then step on the gas – minimizing the turbo lag. It doesn’t mean that you won’t have the turbo lag if you stepped on the gas at say, 1,500 RPM in 4th gear, but if you drove at 3,500 RPM in same 4th gear, and stepped on the gas, the turbo lag will be minimal. This brings us to the first way of how to combat turbo lag – you should know at which RPM the turbo lag is minimal and compensate with gear shifts and rev matching.
OK, so small turbos have less turbo lag since they need less exhaust gases to spool up. Big turbos need more exhaust gas to spool up, so they inherently have more turbo lag.
What other things can you do to minimize the turbo lag on your car, independently of which turbo you have? There are several things you can do:
- heat-wrap your exhaust manifold – this will keep the exhaust gases hot inside the exhaust manifold, thus they will move faster and spool your turbo up faster;
- run leaner, but still safe AFRs – running leaner produces higher exhaust gas temperatures, contributing to faster turbo spool;
- invest in a good aftermarket equal-length exhaust manifold and uppipe for better exhaust gas flow;
- get a programmable electronic boost controller that will try to spool your turbo faster by keeping the wastegate completely shut until a certain level of boost is achieved;
- do not get a front-mount intercooler – FMICs increase your turbo lag due to the length of piping and have a lot of other significant downsides. Read the article on “Why You Don’t Need A Front-Mount Intercooler” to know more about why you want to stick with the top-mount intercooler on a WRX / STI.
Remember, the bigger the turbo, the more turbo lag it introduces. You need more RPM to spool up the bigger turbo. In extreme cases, when a huge turbo is installed, 2.5L engine will not have enough gas flow even at 7,000 RPM and the turbo will not operate efficiently.
With huge turbos, modifications to heads, valvetrain and engine block are necessary to rev the engine up to 8,000 RPM and higher to produce enough exhaust gas flow for the huge turbo to be efficient and build boost. Huge turbo won’t even spool at lower RPMs, and you will have a huge amount of turbo lag – or wait time before the turbo spools up.
In summary, bigger turbos flow more air, can build more boost, but need more exhaust gas flow to operate. You can have too big of a turbo that will have such a huge turbo lag that the car will be almost useless for daily driving. Turbo lag can be reduced by application of certain measures as described above. In order to
be a comfortable daily driver, yet a monster on the street and track, you should research the turbos before upgrading, have all the information you need for turbo selection.
Finally, you don’t need a front-mount intercooler, unless your turbo is too big for stock location and it absolutely requires the front-mount intercooler.
Here is a Wikipedia article on Turbo Lag