Driving "on boost"...

[thewelsho]

What does that actually mean? I'm guessing that it means driving the car on or above the rev threshhold where the turbo kicks in. Right or wrong?

John

[st3ph3n]

Pretty much.

[~ ~ Cal ~~]

If you are like me  a  mong's guide helps :-

Full boost is determined by your ECU hence remaps can yield higher boost ( wit the right mods)

What is a turbo?

Quite simply, a turbo is merely an exhaust-driven compressor. Imagine a small shaft about the size and length of a new pencil. Now rigidly attach a pinwheel to each end of the pencil. One pinwheel (called the turbine) is placed in the path of the exhaust gases which are exiting the engine. These gasses are 'caught' in the turbine, causing it to spin. This in turn spins the whole shaft, along with the pinwheel on the other end (called the compressor). The compressor is placed in the intake air's path; once it begins spinning, it actually compresses the air on its way into the engine.

Why is this beneficial? Well, normally aspirated engines have to work to draw in their intake air. In other words, as the intake valves open, the piston's downward movement creates a vacuum which 'sucks in' some air through the intake system. Ideally, the piston's movement would suck in 100% of the air that could fill the combustion chamber. In the real world this is not the case; the typical engine will draw in only about 80% of the total volume of the combustion chamber. There are many reasons for this--intake restrictions, valve timing, camshaft design, and much more.

Now imagine that the engine mentioned above has a turbocharger. When the turbo compresses the air it builds up pressure in the intake manifold. Now when the intake valves open, air is actually forced into the combustion chamber. (This is one reason why turbocharged engines are sometimes referred to as 'forced-induction' engines.) As you might imagine, this allows more air to fill the chamber.

Okay, so now we have more air entering the engine. To benefit from this, we need more fuel to match. On computerized vehicles such as these, various sensors will "see" this amount of boost pressure and increase the amount of fuel accordingly. Now that we also have more fuel entering the engine, more power is made. (When you get right down to it, the only way to make more power--on any engine--is to shove more of the proper air/fuel mixture into the engine.)

How do turbochargers and superchargers differ?

While they perform the same function, turbochargers and superchargers go about it in completely different ways. As has already been mentioned, a turbo is driven by the exhaust gasses which are already being expelled from the engine. So, in effect, turbos add 'free' power since their compression is created by what was already discarded.

Superchargers, however, are different: they are belt-driven. They feature a pulley whose belt is directly attached to the crankshaft, this allowing them to spin in direct proportion to the engine itself. The upside is a near absence of lag (see below); at least some boost is typically available the instant you crack the throttle. The primary drawback to a supercharger, however, is that they take power to make power. The overall result is more power than there would be without the supercharger; it's just that they aren't as efficient as a turbocharger from an energy standpoint. Other drawbacks include lower mid-range power than a turbo, lower thermal efficiency than a turbo, (sometimes) much harder to incorporate intercooling, etc.

What is turbo lag (and how do I avoid it)?

The majority of turbochargers feature a wastegate--a valve which allows some of the exhaust gas to be directed around the turbine. This allows the turbo's shaft to spin at a reduced speed, promoting increased turbo life (among other things). Think of it as a 'stand by' mode. Since the turbo isn't needed during relaxed driving anyway, this effect is harmless...

...until you suddenly want to accelerate. Let's say that you are loafing along, engine spinning 1500 rpm or so. You instantly floor the throttle. The exhaust gas flows through the turbo and cause it to spool (spin up to speed and create boost). However, at this engine speed there isn't very much exhaust gas coming out. Worse still, the turbo needs to really get spinning to create a lot of boost. (Some turbos will spin at 150,000 rpm and beyond!) So you, the driver, need to wait for engine revs to raise and create enough exhaust gas flow to spool the turbo. This wait time--the period between hitting the throttle at low engine speed and the creation of appreciable boost--is properly called boost response. Many people incorrectly call it lag, which is really something different. Lag actually refers to how long it takes to spool the turbo when you're already at a sufficient engine speed to create boost. For example, let's say your engine can make 12 psi at 4000 RPM. You're cruising along at a steady road speed, engine spinning 4000 RPM, and now you floor it. How long it takes to achieve your usual 12 psi is your turbo's lag time. Between the two, slow boost response usually causes the most complaints.

There are two aspects to consider when dealing with boost response: engine factors and driver factors. As far as engine factors go, there are many things which affect turbo lag... although most are directly related to the design of the turbo itself. Turbos can be designed to minimize lag but this usually comes at the expense of top-end flow. In other words, you can barter for instant boost response by giving up gobs of horsepower in the upper third of your RPM range. (Behold the catch-22 in designing one turbo for all uses.)

Driver factors are another matter. You basically need to understand how a turbo works and modify your driving style accordingly. To sum it up, don't get caught with your pants down! If you feel that there may soon be a sudden need for serious thrust, downshift until your engine speed is at least 3000 RPM. This way there will be noticable boost almost as soon as you hit WOT. If you are going up a hill at WOT around, say 1800 RPM and your speed is dropping, you'll need to downshift just like any other car in the same situation. Remember: turbos need exhaust gas in order to spin. Let them have some when they need it.

[SEEVERS]

You can always rely on Cal to be some kind of weird info-boy...lol.

 

Del.

[~ ~ Cal ~~]

I am back on the Peroni's the night..............Budapest 4 day stagger the 'mara so pre-empting any hangovers by getting lashed now

Back on topic [:$]

[jimser]

very informative cal[8-|] thanks

[thewelsho]

Cheers Cal. Do you have a Word Doc ready to copy and paste at times such as these? [] You said, "... as you hit WOT" What is WOT?

John

[scoob155]

wide open throttle ?