A friend and I have a disagreement. It involves a turbo diesel and boost. He says when running the engine without any kind of boost controller that the engine will put out the same power when keeping the engine at 10psi for example (by adjusting the accelerator pedal) as using a boost controller set at 10 psi and full throttle. I believe the latter. Any opinions?

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1987 190DT, clackity clack
90 Mustang track car
95 Lightning

could you clarify your question?

boost does not make much power... it's the FUEL matched to the boost that makes the amazing hp numbers in a diesel.
10psi is not much by the way... most of the diesels here on the forum are running 10-12 psi of boost... 120 ish HP... the alda fuel compensator alters the amount of fuel to limit soot production and this makes the engine smooth as the boost builds. instead of raw fuel dumped in, to build the boost and power faster.

He is working on a VW 1.6TD without a boost controller (it makes 18 psi). He says that if he wants to keep the PSI at 12 psi (for example) he will adjust the throttle accordingly to keep 12 psi. I say install a boost controller and set it to 12 psi, the engine will make more power under full throttle and a controller.

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1987 190DT, clackity clack
90 Mustang track car
95 Lightning

agreed.
fuel makes power.
however, if the turbo is putting out 18, and there is adequate fuel, the motor will make more power.
a boost controller, can allow full fuel, without the air to burn it. I.E. Black smoke
using the throttle to control with a pyrometer to verify nothing is getting too hot is wise.

He does have a egt sensor. My thinking is to get the most power at any psi under 18 he should use a controller set for that psi since he will be putting in maximum fuel. Assuming the egt is ok.

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1987 190DT, clackity clack
90 Mustang track car
95 Lightning

If the wastegate is set to open at 18 psi , it wont open before unless you "lower" the spring tension in the wastegate, some are adjustable , some have to be swapped.

a boost controller will help spool the turbo quicker and could be adjusted to block the signal to the wastegate to achieve higher boost, but the opposite is not the case.

Since we're talking technicals here, we probably shouldn't be using the word "throttle" since diesel engines, by design, are not throttled.

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1983 240D 3.0T 4-speed manual, now sold

1989 Subaru GL Wagon 5-speed Touring Edition

The signal line to the waste gate is not hooked up at all. Full throttle is 18 psi. His thinking is if he wants to keep it at 12 psi, he will adjust the throttle to do so. He is thinking that the engine will make the same power doing that as using a boost controller set to 12 psi and a waste gate @ full throttle.

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1987 190DT, clackity clack
90 Mustang track car
95 Lightning

I was using that term instead of "gas" when talking about the tall skinny pedal position.

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1987 190DT, clackity clack
90 Mustang track car
95 Lightning

Could call it the loud pedal.

Technically, the air is not throttled, but the fuel is.

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Gone to the dark side

- Jeff

Technically you and your mate are both wrong.

A petrol / gasoline engine has a fairly fixed fuel/air ratio, floating somewhere around 1:14.

A diesel engine has an almost totally variable fuel/air ratio, with the sole proviso that there is enough oxygen available in the combustion chamber, power is determined by the amount of fuel injected. Unless she is pumping out black smoke (excess fuel) a diesel is always running lean.

A given diesel engine injecting "x" amount of fuel per power stroke will (ignoring power absorbed by turbo/blower) produce the same amount of power, irrespective of boost.

The point of boost is that if atmospheric pressure is 15 psi, then 15 psi of boost, WITH AN EFFICIENT INTERCOOLER, will double the amount of available oxygen in the cylinder, which doubles the maximum amount of fuel that can be burnt per stroke, which will double the "power".

I put "power" in quotes because world + dog talk about bhp, bhp is really a function of RPM, and is (assuming efficiencies are equal) nothing so much as a good indicator of rate of fuel consumption... a 200 BHP motors burns fuel at twice the rate of a 100 BHP motor.

There is an old saying for cars, "You buy the BHP, your drive the Torque"

Torque is what is king, and that, in a diesel, isn't linear, the same engine naturally aspirated and then with 15 psi boost will not produce double the torque.

Compare a medium weight bus/truck diesel with a japanese motorcycle engine, 100 BHP is normal for both, and indicates the rate of fuel (Btu per gallon) consumption, but the amount of torque each produces is wildly different.

In a diesel, boost is good, because you can get a 10 litre truck motor pulling (hard) from 1,000 RPM running 30 psi of boost, theoretically able to consume fuel and this pulling power at three times the normal rate, eg equivalent to a 30 litre engine... but the boost doesn't produce the power, just the means (oxygen) to burn the (excess, without boost) fuel.

As a follow up to the last para, the 10 litre diesel truck engine pulling hard at 1,000 RPM.

With 30 psi boost it will (this is all theory, not practice) produce the power of a 30 litre engine running at 1,000 RPM, without the weight of a 30 litre engine.

The 10 litre engine could produce the same power at 3,000 rpm, but with engines various things scale up not linearly, but with the square and with the cube, so any engine at 3000 rpm is subject to forces not 3 times greater than at 1000 rpm, but the cube, 3x3 or NINE times the forces, which is again expensive...

so the cheapest and lightest way to do for a truck this is 10 litres with 20 psi plus boost...

Detroit were king at torque/weight, note the blower in detroits is just to scavenge, the turbos (if fitted) provide the boost... cos they are 2 strokes.

And a final quote.

"The difference between theory, and practice, is greater in practice, than in theory."

Never forget this.