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chasinthesun 12-20-2006 12:36 PM

Why do diesels need a vacuum pump?
Does the engine not make enough vacuum?Gas cars usually tie into the intake for the use of vacuum needs ,Why does the diesel engine differ?

janko 12-20-2006 12:38 PM

diesel engine has .........
no throttle. ergo no vacuum.

TheDon 12-20-2006 12:39 PM

plus a mechanical vacuum pump lasts longer than an electric version... reason for vacuum actuated.. everything on the W123... just look at the W116's servo control AC.. the servos fetch big money if OEM replacement parts still in the box

rrgrassi 12-20-2006 12:43 PM

You would have to hook up before the turbo, or onto the air intake for vacuum, but the air intake.

Probably on a non turboed car you could tap into the intake like a gasser.

Now I'm curious to see if I could make a vacuum device work like a siphon type like on a garden hose sprayer.


SD Blue 12-20-2006 12:47 PM

A generic answer without a lot of details........
Gas engines have to generate a vacuum in order to overcome the throttle of the intake. This determines their air/ fuel mixture.

Diesels generate little to no vacuum in their intake, if fact, most of the time there is pressure. They use as much air as you can pump into them. That is what makes them a more efficient design.

vstech 12-20-2006 01:17 PM

Yeah, due to the restriction in the airstream in a gasser, you get vacumme unless you are flooring the pedal. However, high performance motors, like a race car, the cam profile is too large, and the motor makes poor vacumme. also in a gasser, vacume aids in the evaporation of the fuel spray, to get as much fuel atomized for a more efficent burn. a Diesel has it's fuel directly injected into the cylinder at huge pressures, into highly compressed air. this ignites the fuel as it is sprayed in, no mixture or evaporation needed.
basically, unless there is a butterfly flap to restrict air as it is pulled by the pistons, there will be no vacumme to speak of. the intake is open completely to atmosphere.

Shorebilly 12-20-2006 03:23 PM

Boy do times change.....!!!

Originally Posted by tangofox007 (Post 1363500)
Diesels don't have throttles to restrict air flow into the intake manifold. Therefore, the is not much vacuum in said manifold. Even less in a turbocharged diesel.

How about a pressure in a Turbocharged Diesel!!!

My first car had vacuum operated windshield wipers, as does my ol' power wagon.....I wonder how many of y'all have ever had to deal with gettin' off the gas to increase wiper speed.......:D


probear 12-20-2006 03:31 PM

Dunno about the vacuum wipers but my 69 Lincoln had hydraulic wipers that ran off of the power steering pump. You did not want the wiper blades to hit your hand. They did have some real power.. :o

97E300D 12-20-2006 04:00 PM

Hows this for going off on a tangent....
Our cars breath in a sort of reverse aquarium

It's funny how people think of vacuum or "suction"... The pistons don't really pull air in through the intake. It's the weight of the atmosphere *14.7* lbs\sq inch that pushes the air in to fill the non-air bubble or low pressure area. A Diesel will produce a small vacuum and it will build as RPMs increase, but as stated it is not enough to power things.

I always like to think of the benefits of a supercharger or turbo in this way. As the cylinders request more and more air in the same give time the atmosphere can't push it in through the intake plumbing fast enough. By the time the intake valve closes more air is depleted then can rush in. sort of like pushing allot of water through a small pipe or high amps through a thin cable.

Turbos can help to over come this... even if the pressure never exceeds the relative atmosphere, power builds as the cylinders fire twice as fast in the same time, yet can't get as much air as it could without the forced induction of air. Remember the equation work = weight moved over time.

tangofox007 12-20-2006 04:06 PM


Originally Posted by 97E300D (Post 1363691)

It's funny how people think of vacuum or "suction"... The pistons don't really pull air in through the intake. It's the weight of the atmosphere 32lbs\sq inch that pushes the air in to fill the non-air bubble or low pressure area.

Got a reference for that 32 psi figure?

If atmospheric pressure was that high, we would not need turbochargers!!!

But you're right, it's funny how people think. And sometimes even funnier what they think.

97E300D 12-20-2006 04:08 PM

Actaully it 14.7 I think right one atmosphre..Right

97E300D 12-20-2006 04:11 PM

Yep it is 14.7 sorry for the disinformation in my tirade... I thinks it's water presure that build at that rate every 10 meters or something.....I forget

97E300D 12-20-2006 04:24 PM

Actually we would still....need turbos ( I think) It wouldn't matter it's all relative to the surrounding pressure.
That's a good one gota think about that one...

And we would be fighting to over come the increased air resistance. The drag Coefficient increases with the sq of speed. Four time the drag on a car doing 40 as doing 20 not twice as much.

But what the heck are we talking about.... Ha

rg2098 12-20-2006 04:43 PM

Some new diesels are now throttled, but not to the same extent as a gasser. Fuel mixture is now just as crucial as a gasser for emissions. They throttle the intake to create some vac to pull the EGR gasses through.

Mustang_man298 12-20-2006 10:24 PM

You know, its interesting, the 617.912 seems to me to be a 616 with an extra cylinder. No other real differences other than firing order and timing of course to account for the 5th cylinder. Now heres the interesting part, my 80 240 had a throttle plate, the 617 I put in doesnt. They both share the same kind of injector pump operating in the same manner, so whats the point of the throttle on the 616? Or was that some kind of emissions brainstorm?

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