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I don't get it. Theoretically, applying brakes with the left foot and throttle with the right "keeps the turbo spooled" during turns, presumably by keeping the revs up.

However, upon further thought and experimentation, it doesn't work. I tried it, and the engine revs didn't change with the throttle. If you think about it, unless the clutch is engaged, the engine revs can't vary without varying the speed of the car.

What am I missing?
 

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Correct ECU pin for A/F gauge

if you have a boost gauge watch that, the full load on the engine keeps the turbos spooled. If you close the throttle, it takes a lot of the exhaust pressure off the turbo. This causes lost boost and creates a small lag when you get back on the turbo. By keeping the turbos under full throttle, and therefore full pressure, this lag is eliminated. Think of it like this: if you accelerate at full throttle past 4000, you get more boost than if you held it at 4000 and punched the gas.
 

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maxwolfinger said:
By keeping the turbos under full throttle, and therefore full pressure, this lag is eliminated.
so this is what i don't understand. someone said previously (in another thread) that you only get boost if there's load on the engine; i.e. if mash it when in neutral or clutch in, you don't get boost. is this true? (i don't see how, but then again, this is my first turbo.) if so, then is it even possible to keep the turbo spooled when braking/ cornering?
 

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Because it doesn't take much to keep the engine running at high rpm when in neutral. You don't have the high volume of exhaust pushing the turbo to spool up.

Kinda makes you want a blower doesn't it.... :)
 

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bitabur said:
You don't have the high volume of exhaust pushing the turbo to spool up)
there must be something basic i'm missing then, so bare with me.

isn't the *volume* of exhaust exiting an engine solely a function of rpm, not load? using the rpm and displacement, you can compute the volume of exhaust. i'll keep googling...
 

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doh! just found the answer (emphasis added):

http://www.turbochargedpower.com/Turbocharger Theory.htm

The turbine is powered by hot expanding exhaust gas, a lot of hot expanding exhaust gas, the more and the hotter the expanding exhaust gas the better. I am sure many of you have seen pictures of turbo charged engines with cherry red hot exhaust systems and turbo housings. The captions under most of these types of pictures proclaim outstanding horse power numbers. What most of the articles related to these pictures do not tell you is that the engine was under an extreme load. A load so heavy that the engine was almost at its stall point for a prolonged period of time. A condition that most turbo charged engines will never see.

The real point I am trying to make is that the exhaust turbine will not generate enough power to turn the air compressor fast enough for it to work properly unless the engine is feeding the exhaust turbine a lot of hot expanding exhaust gas, a condition that can only be created when the engine is under a load. There is where the selection of transmission gear ratios and the ring and pinion ratio play a critical part. The fact that the engine must be under a load is the reason why, no matter how high you rev a turbo charged engine with no load on it, you will not see the boost gauge move.

This is also where the term 'turbo lag' came from. Turbo lag is basically the amount of time it takes from the time you place a load on the engine (stomp the gas peddle to the floor and dump the clutch or, get full converter lock up with your automatic trans) until the time the engine develops enough hot expanding exhaust gas to spin the turbine fast enough for the compressor to do its job.
so it's not about volume but the *temperature* of the exhaust gases. that makes sense since under load, you're injecting more fuel than under no load, making a hotter explosion, resulting in hotter exhaust gases.
 

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the way I look at it, LFB allows you to keep the throttle open while off boost and on the brakes. By keeping the throttle open and essentially using the brakes to put the engine under load ( not to mention the brakes!! ) allows you to minimize or sometimes eliminate the turbo lag when launching out of the corner.

It takes alot of practice to do it right. I have tried it will average results. The biggest problem I had with LFB is that I would brake too often and too hard during an autocross. On the track however, you have more time to think about what you need to do, so there the LFB technique was easier to do.
 

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The benefits of left foot braking are much more related to chassis balance and weighting than to turbo spool up. Proper left foot braking (or trail braking) can allow better turn in and mid corner response than cornering. Especially with an AWD or FWD car because the front wheels can actually be allowed to pull the car through the corner while the application of the brakes can help induce more oversteer allowing the car to rotate more effectively and get you pointed through the corner quicker.
 

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Xman said:
The benefits of left foot braking are much more related to chassis balance and weighting than to turbo spool up. Proper left foot braking (or trail braking) can allow better turn in and mid corner response than cornering. Especially with an AWD or FWD car because the front wheels can actually be allowed to pull the car through the corner while the application of the brakes can help induce more oversteer allowing the car to rotate more effectively and get you pointed through the corner quicker.
that's what I thought, the basic purpose of left foot braking is to induce oversteer in the car for better cornering
 

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matadorCE said:
Xman said:
The benefits of left foot braking are much more related to chassis balance and weighting than to turbo spool up. Proper left foot braking (or trail braking) can allow better turn in and mid corner response than cornering. Especially with an AWD or FWD car because the front wheels can actually be allowed to pull the car through the corner while the application of the brakes can help induce more oversteer allowing the car to rotate more effectively and get you pointed through the corner quicker.
that's what I thought, the basic purpose of left foot braking is to induce oversteer in the car for better cornering
this is the basic idea, but on a turbo car it has the added benefit of staying with on boost.
The car can develop boost in neutral, this can be seen in the old F1 turbo cars. If you stomped on the gas at idle, by the time 12k rpm came up the car would be under boost. On the sti you can make some boost in neutral, it is just that you have to lift off very quickly. This is why some turbo cars with tons of traction, and strong drivetrains, you will see a different launch technique. Some of these racers will wait until a split-second(or one amber light, if your a dragracer) before launch. At this time they will floor the car. By the time they release the clutch, the car is at the desired launch rpm, but the turbo already has some load on it. This will increase the chance of tire spin and reduce the chance of bog.
A good way to think of the rpm thing is to think of the throttle as a restrictor. If you are holding 15% throttle, then 15% of the cylinder's volume will fill, this will cause 15% of the total possible exhaust gas to exit the chamber and spin the turbo. Like someone mentioned, this is independent on a blower because 4500rpms is 4500rpms at the blower's pulley. This is why a blower/supercharger has better response, its always making the boost, the bypass valve on the intake pipe just snaps shut under WOT.
Just felt like explaining something.
 

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maxwolfinger said:
A good way to think of the rpm thing...If you are holding 15% throttle, then 15% of the cylinder's volume will fill, this will cause 15% of the total possible exhaust gas to exit the chamber and spin the turbo.
the problem with this example is that it is misleading at best; wrong at worst. it implies that boost is related to exhaust volume, which it is not. this is the same volume-based explanation i'd received many times before which made no sense. as the link and quote above indicate, boost is related to exhaust gas temperature, not exhaust gas volume. you get *some* boost under coasting conditions by mashing the gas because it causes more fuel to be injected, which causes a hotter explosion and hotter exhaust gases. it takes a lot less throttle (and less fuel) to rev to a given rpm (e.g. red line) while coasting than under load, hence the exhaust gas temp is lower and the boost if less. a blower has zero lag because it is driven directly off the crank, and not by the indirect mechanism of exhaust gas temp, as in a turbo.
 

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Well I was intrigued, so I went and looked at my Maximum Boost book, and I quote "Heat and airflow drive the turbine." My thought was that since volume is a function of heat and pressure, it must interelate. I am now confused, by the way.
 

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It is NOT the temperature, it is ONLY VOLUME that causes the spool up to occur.

The REASON that you see this quote mentioning TEMPERATURE is because the VOLUME of a GAS increases PROPORTIONALY to TEMPERATURE.

PV = nRT

This is the ideal gas equation. P = pressure, V = volume, n = the number of moles of gas, R is a constant balancing factor, and T is temperature Kelvin.

Assuming all constants, the volume of a gas is proportional on a 1 to 1 ratio with temperature measured in units Kelvin.

This means that 1kg of gas at 50 degrees kelvin will ocupy half the volume as the same QUANTITY of gas at 100 degrees kelvin (note both 50 and 100 degrees kelvin are well below zero degrees celcius, this is just an example.) If you feed a SET VOLUME of gas through the turbo, it will spool up exactly the same ammount whether the gas is cold or hot. But this volume of cool air will be a much higher ammount. This is also why you want an intercooler. In addition to preventing detonation, it also allows a larger QUANTITY of gas (and therefore oxygen) to enter the cylinder.


So the higher temperature of the gas exiting a cylinder under load will cause the gas to occupy a larger volume as it expands exiting the cylinder.




And just so nobody gets confused, gas = matter in a gas state, if i was talking about gasoline i would say fuel. :)
 

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bitabur said:
It is NOT the temperature, it is ONLY VOLUME that causes the spool up to occur.
okay, here's my chain of reasoning that says this can't be so. i may be wrong so please pick it apart. ;)

a) you have to do *work* (in the physics sense) to spool a turbo.

b) volume is a measure of space; e.g. cubic feet.

c) a "measure of space" *cannot* do work.

d) the ideal gas law says (and as applied in this case): a change in temperature *acting* on a volume, will produce a proprotional change the pressure of the contained gas; when temperature is increase, the pressure increases

e) in contrast to volume, pressure is a form of force, which can do work. (pressure is *force* applied over an area).

f) therefore, the pressure of the exhaust gas is what spools the turbo.

so i think the chain of events is like this:

fuel amount -> exhaust gas temperature -> exhaust gas pressure -> boost

increase the fuel amount (necessary to achieve an given rpm under load) and everything in the chain increases. changing to a no-load condition requires much less fuel to attain the same rpm, causing everything in the chain to decrease.

so the reason i emphasized temperature was because this is what you can directly control. well, you actually control the fuel (which controls temperature).

bitabur said:
If you feed a SET VOLUME of gas through the turbo, it will spool up exactly the same ammount whether the gas is cold or hot.
not according to the derivation above. granted that might be wrong.

bitabur said:
So the higher temperature of the gas exiting a cylinder under load will cause the gas to occupy a larger volume as it expands exiting the cylinder.
sure, the gas is expanding into a larger volume, but it's not the volume that is spooling the turbine. again, volume doesn't do work. it's the pressure of the expanding gas that does the work. in fact, it's the pressure *differential* between the input and output side of the turbine that controls the boost, e.g. high flow cats and exhausts lower the back pressure which increases the pressure differential across the turbine which in turn allows the turbo to produce more (and sometimes too much) boost.

anyway, i think we both have the same general understanding of how the thing work; our only difference seems to be between volume and pressure. convince me it's volume; better yet, convince me that temperature has nothing to do with it, preferably with a step-by-step derivation as above.
 

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actually, looking into the PV=nRT, I meant the nR, the amount of molecules, this and the heat creates the pressure, which drives the turbine. I also believe that it is not only the extra fuel, but the extra air brought in that promotes greated combustion pressure, causing more heat, which dumps into the exhaust and builds pressure with all the extra molecules. This pressure is used to drive the turbine. In terms of the airflow being constant with load or no load, I was talking about a it in a different way. I was saying that an engine expels more exhaust gas under full throttle than under part throttle. The throttle restricts the amount of gas allowed into the cylinders. This is why a engine is under more stress with WOT, the cylinder pressures get up towards peak.
 

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maxwolfinger said:
I also believe that it is not only the extra fuel, but the extra air brought in....
yep. i was just simplifying air/fuel ratio to just fuel. the point is you control something to cause higher exhaust gas temperatures.

maxwolfinger said:
I was saying that an engine expels more exhaust gas under full throttle than under part throttle.
yeah, because the engine is turning faster.

maxwolfinger said:
The throttle restricts the amount of gas allowed into the cylinders.
does it? (by "gas" i assume you mean air, not fuel. :) ) doesn't the cylinder still draw in its full displacement on the intake stroke even when the throttle is shut and the engine is idling? it seems it's more like the throttle controls how fast that air can be drawn in; i.e. how fast the engine can turn.
 

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does it? (by "gas" i assume you mean air, not fuel. ) doesn't the cylinder still draw in its full displacement on the intake stroke even when the throttle is shut and the engine is idling? it seems it's more like the throttle controls how fast that air can be drawn in; i.e. how fast the engine can turn.
Think of gas mileage. If a car injected the same fuel while cruising as when accelerating hard, the car would need to burn the same amount of fuel. It would also create the same horsepower and torque at a given RPM regardless of the amount of throttle. Since a car accelerates when you push on the gas, it is making more power by adding more fuel. Hence it gets better gas mileage with less throttle.
maxwolfinger wrote:
I was saying that an engine expels more exhaust gas under full throttle than under part throttle.

yeah, because the engine is turning faster.
I meant at the same RPM, if it is accelerating past a given rpm, it has to use more gas and air, to make more power, to accelerate. It therefore must be making more exhaust gas. You have no manifold pressure when cruising because of this. The turbo is not spinning fast enough to make boost.
 

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Left-foot braking is used for turning! Especially on FWD; but also on 4WD.

It will reduce understeer and even induce oversteer. Works better the more slippery it is.

Not entirely sure on the physics behind it, but I suppose that because the front brakes are more powerful than the rear, and most of the weight is up front, it will simulate rear torque bias.
 
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