Boost Walkaround - 2011 Kia Optima SX Turbo Long-Term Road Test

2011 Kia Optima Long-Term Road Test

2011 Kia Optima SX Turbo: Boost Walkaround

May 31, 2011


When you open the hood of our longterm 2011 Kia Optima SX Turbo, it's sort of a jumble of black hoses and tubes. Hard to tell where the boost is coming and going.

Perhaps this brief walkaround will help those that like to keep tabs on the whereabouts of the air in their engine bays.


First thing is to remove the engine cover. More than simply decorative, the cover has dense padding built in to muffle the noise of the direct-injection system's high-pressure fuel pump (red). These tend to make uncouth tickety-tick noises.

Note the intake plumbing. It's fed from the high-pressure nose area, wide and a straight shot, all of which is goodness in providing minimal pressure drop upstream of the compressor. This, in turn, reduces exhaust manifold pressure, since the turbine needn't work as hard. Particularly important when you're trying to make 274 horsepower out of two liters on 87 octane.

As with many modern engines, the ECU (green) lives in the engine bay. Back in the day the electronics were not robust enough to live in the hot, dirty, sometimes wet engine bay. Mounting it in the engine bay simplifies the wiring harness, vehicle assembly (the engine plus ECU and engine harness can be installed as a module) and eliminates a large perforation in the firewall for better NVH.

A plastic intake manifold is a common item on normally aspirated engines, but until recently you didn't see them on boosted engines for durability reasons. They're coming into favor now. Seems they've got those earlier issues licked.

The blowoff valve (pink; aka bypass valve, aka anti-surge valve) is old-school - it's a separate piece. So many modern turbos integrate this component into the compressor housing for reduced cost/complexity.


bov_arrows.jpg If you're a blowoff valve noob, look at the picture above then click the thumbnail here:

Blue arrows indicate fresh air traveling through the inlet duct, into the air filter housing and then down to the turbo's compressor (out of sight in this shot). Pink arrows are the hot air that's just been compressed. Some of it is vented by the blowoff valve (the silver doohickey) back into the fresh-air intake where it mixes with blue arrows.

This keeps the turbo's compressor happy -- they don't like high pressure at low flow. What happens is they'll either be inefficient, surge, or both. Certain part-throttle circumstances induce just such a high-pressure / low-flow situation.

Recirculating some of the flow via the blowoff valve means the compressor sees high pressure and higher flow rate than before, even though the flow rate to the engine is unaffected. Neat, huh? Venting the flow also keeps turbo speed up during brief periods of closed-throttle operation, improving boost recovery during gearchanges.

The thick rubberized coating on all the intercooler charge tubes is weird. I can only guess that it's some kind of NVH attentuation strategy. Any other guesses?


Can't really see the turbo itself, as it's buried under an elaborate array of metallic heat shields. You can barely see the compressor housing, its discharge section sprouting vertically before turning gently towards the blowoff valve in the previous section.


Likewise, the fully electronic wastegate actuator is tucked away but I caught it peeking out here. With this, engineers have complete control over the wastegate. Among other benefits, one feature of electronic actuators is that they can open the wastegate wide during a cold start to light off the catalyst much more quickly.

Expect to see more and more of these actuators on turocharged gasoline engines as the price comes down and the hardware become more robust.


The intercooler itself is a tube-fin arrangement. Hot air enters at the top and cooled air exits at the bottom. The rubberized tube shown here connects the intercooler discharge to the throttle body. That's a boost pressure sensor right there too.

What's notable here is the very high fin density. This will tend to yank a lot of heat out the compressed air inside the tubes, but makes it harder for the cooling air to make its way through the maze of fins. If you have very good airflow management and can ensure low engine bay pressure, then you can get away with a high fin density as shown here.


It also helps if you can ensure that the cooling air on the opposite face has a nice, direct shot. Ducting that air further helps the cooling performance -- you can't see it here but there is a very nice duct behind the grille that funnels air directly to the intercooler.

Your turn -- is there anything under the Kia's hood that you'd like to see, or see better, or have questions about? I'll do what I can do respond to your requests.

Jason Kavanagh, Engineering Editor

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