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Premium Member
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Discussion Starter #1
How can ppl say that diesels do not have "vacuum" (negative pressure)???

A turbo diesel "SHOULDN'T" have negative pressure after the turbo in the intake. But there is negative pressure between the air filter and the turbo. There is also another case where i think the intake manifold may have negative pressure.

An NA diesel has negative pressure clear from the air filter to the combustion cylinder just like a gasoline engine. The only difference being, the gasoline engine produces "vacuum" with a throttle and uses this "vacuum" to introduce fuel or meter air. On a diesel, the way I see them using "vacuum" it is at idle, the EGR uses a little bit of negative pressure in the intake to suck exhaust gasses back through the engine with its butterfly valve.

So to say that there is NO negative pressure involved with a diesel would be incorrect. its just that in a diesel, the negative pressure doesn't amount to as much as it does in a gasoline.

RIGHT?
 

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Good questions. There is negative pressure in the vacuum line system and brake booster system. But how much negative pressure is there if there is no restriction? Not much. The newer engines have intake plates that, while not used for controlling engine rpm, still restrict air. The older ones don't. The air filter also has a very small restriction.

I haven't measured it but I would guess that the vacuum is so small that it's under -1 psi. In addition, the vacuum pump (it exhausts into the crank case) and crank case gasses are pumping air into the intake. Here is a picture from a Passat showing the routing from the CCV to the intake. There are no vaccum lines or anything else from the intake to the turbo but there is positive pressure going into that tube from the CCV. As an experitment, you could try blocking off the CCV tube and opening the oil filk cap to see how much air is pumping out there.

 
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