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Re: what should the turbo/apc gauge be doing?
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Posted by Ari [Email] (#2847) [Profile/Gallery] (more from Ari) on Fri, 7 Jan 2005 06:38:13 Share Post by Email
In Reply to: what should the turbo/apc gauge be doing?, michael [Profile/Gallery] , Thu, 6 Jan 2005 17:44:58
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The boost gauge reads the pressure in the throttle body. What it reads depends on engine speed, throttle position, and how fast the turbo is spinning.

As syracusespg says, the gap between the white and orange is atmospheric pressure. The gauge will sit right around there when the car is off. White is vacuum (less than atmospheric) and boost is yellow/red. The end of yellow (or orange, depending on your view) and red is about the maximum the boost gauge should go on a stock engine.

An engine is a pump - it's sucking air out of the intake manifold and blowing it out the exhaust. The faster it spins, the more it pumps. At idle, the engine is spinning, but the throttle plate is closed - only a little air is getting in, enough to keep the engine running. That creates a vacuum in the throttle body (TB), and you see the gauge deep into the white. If you are driving down the highway, and take your foot off the gas pedal, the engine will be spinning even faster, but with the throttle closed, you'll see the most vacuum. (even deeper into the white).

As you open the throttle, you allow air to flow in. With a non-turbo engine, if you open the throttle all the way (and the engine is running), you'd see just less than atmospheric pressure. It's just less, because if it were at atmospheric pressure, air would have no reason to flow in.

With a turbo engine, the exhaust causes the turbo to spin. The spinning turbo compresses air, so you can see higher than atmospheric pressure in the TB.

Remember, what you see on the boost gauge is just the pressure in the TB - it isn't power. Boost pressure will move around depending on throttle opening, engine speed, and turbo speed. It takes a bit of exhaust flow to get the turbo really spinning fast, and exhaust flow is greater when the engine is spinning fast, AND the throttle is open. A fast spinning engine with a closed throttle isn't moving much air through (remember, it's a pump). It also take a couple of seconds to get the turbo spinning fast. So in general, you'll see the greatest turbo boost on the gauge when you are under acceleration, and have been accelerating with a mostly wide-open throttle for a few seconds. A great way to check max turbo boost is to find a nice steep hill, and charge up it in third gear. First and second gear come and go too fast to really see the gauge - 4th and 5th, and unless you're going REALLY fast, you don't have the engine revs.

So, as a responsible car owner, what should you look for on the boost gauge? Well, not much, really. Frankly, a boost gauge is mostly a waste of dash space. You don't need to drive any different to make the turbo work - just drive the car. The gauge will move around, but there isn't anything you should be doing to change that.

Where does the gauge come in handy? When something is wrong. If the boost gauge buries itself into the red (and you'll know that because the acceleration is fierce!), then you know you've got a problem with the boost limit system - a loose hose to the wastegate actuator, or a stuck wastegate actuator. Time to fix it.

The other case is when you don't get full boost. You don't always get full boost in normal driving, and that's fine. But it's nice to know it's there. Find yourself some place where you can do a nice, long acceleration, preferably up a hill, because that increases the load. With a warm engine, you should be able to get the boost gauge right up near the end of the yellow/orange to the beginning of red. Remember, it's not a precision gauge, so just a hair before or after is OK. If you're really worried, get a calibrated pressure gauge.

If the gauge only goes to half-yellow, that's base boost. The APC system will limit boost if it detects knock, the brakes are pressed, or the cruise control is engaged. It will limit boost to Base. If it doesn't detect any of those problems, it'll let you go all the way to full, which is the end of yellow/beginning of red. There are lots of reasons why you might be in base boost, from running too low a fuel octane up to vacuum leaks, wrong spark plugs, bad APC solenoid, etc., etc. If you can't get over base boost with the full load test, do a search on Base Boost.

But if you can get full boost, the gauge is pretty much there for bragging rights.

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