i was told a diesel does not have a vacuum. i installed a boost gauge and there is no vacuum. why is this?

a diesel engine is unthtrottled, hence no vacuum. In a gas engine the throttle butterfly controls the amount of air entering the engine, it is this throttle butterfly and the suction created by the piston pulling against the small air opening that creates the vaccuum. in a gas engine air/fuel ratio is controlled to about 14:1 at all times, by the amount of air entering the engine. Think of breathing through a straw. On a diesel, there is no throttle butterfly, the engine always draws a full "breath" of air and the air fuel ration varies from very high (200:1?) to <14:1 and the amount of fuel controls the amount of power. The lack of vaccuum is also why diesels have less engine "compression" breaking, (actually a gas engine has "vacuum" breaking)

ktmrfs nailed it - a gasser uses piston vacuum to premix a correct stoichiometric volume of fuel/air in the intake to go into the cylinders (carburator supplies a fuel air volume in the whole manifold, injected engines mix in the intake runners after a "throttle body" that controls the air). In a diesel, the correct amount of fuel is injected directly into the cylinder under compression. Actually, in a turbocharged diesel a supply of pressurized air is in the intake waiting to be drawn into the cylinder. Vacuum is maintained in gassers as long as the engine is running and is tapped off just after the throttle body or carb. If you need vacuum on a diesel, you have to run a driven accessory vacuum pump, like on the GM 6.5 diesels.

piston vacuum to premix a correct stoichiometric volume of fuel/air in the intake to go into the cylinders :confused: :confused: :confused: :confused:

Damn I shoulda paid more attention in school:muahaha:

Thanks for the info.

Main Entry: stoi·chi·om·e·try
Pronunciation: "stoi-kE-'ä-m&-trE
Function: noun
Etymology: Greek stoicheion element (from stoichos row) + English -metry; akin to Greek steichein to walk, go -- more at STAIR
1 : a branch of chemistry that deals with the application of the laws of definite proportions and of the conservation of mass and energy to chemical activity
2 a : the quantitative relationship between constituents in a chemical substance b : the quantitative relationship between two or more substances especially in processes involving physical or chemical change

In this context, the stoichiometric amount of fuel means there's enough fuel to consume all the oxygen, no more, no less.

For typical gasoline, and our atmosphere, this happens with 14.69 lbs of air per lb of fuel.

The fact that a standard atmosphere has 14.696 psia of pressure is purely coincidental.

WOW!!! :hail:you guys are shmart :Whoa:

Turbo also does a good job of countering vacuum.

But-- back to the original post -- if your boost gauge has both pressure and vacuum, you have a gasser gauge. A turbo gasser will indicate vacuum until boost builds up and then the boost ranges will be much less than a diesel (1 to 6 psi?) What's the highest boost on your gauge? A proper diesel boost gauge ought to read 0 to about 30 psi (like my Isspro). With the Edge Juice, I've never seen more than about 18-19 psi peak in my truck, never been able to sustain for more than a few seconds around 12-14 and typically run somewhere between 6 and 10 cruising. Now, the big turbo/twin turbo guys and the diesel drag-racing rocket trucks, that's another matter! I had an article on a guy that drag raced modified Navy all-aluminum boat engines in his truck that ran 80 psi with twin turbos and stretched out the head bolts in two or three runs!

I do believe the turbo diesel has that same 14.7 PSI (at sea level) atmospheric pressure all the time plus the turbo boost. Later! Frank

But-- back to the original post -- if your boost gauge has both pressure and vacuum, you have a gasser gauge. A turbo gasser will indicate vacuum until boost builds up and then the boost ranges will be much less than a diesel (1 to 6 psi?) What's the highest boost on your gauge? A proper diesel boost gauge ought to read 0 to about 30 psi (like my Isspro). With the Edge Juice, I've never seen more than about 18-19 psi peak in my truck, never been able to sustain for more than a few seconds around 12-14 and typically run somewhere between 6 and 10 cruising. Now, the big turbo/twin turbo guys and the diesel drag-racing rocket trucks, that's another matter! I had an article on a guy that drag raced modified Navy all-aluminum boat engines in his truck that ran 80 psi with twin turbos and stretched out the head bolts in two or three runs!
Stock LLY run close to 30psi and as Pat has seen more than that w/ power adders.
Get a 50 or psi guage if u will be playing later.

I do believe the turbo diesel has that same 14.7 PSI (at sea level) atmospheric pressure all the time plus the turbo boost. Later! Frank
When I quote "psi" for gauge readings - I'm assuming it's psig (gauge - zero at 1 atmo) not psia.

Stock LLY run close to 30psi and as Pat has seen more than that w/ power adders.
Get a 50 or psi guage if u will be playing later.
Thanks for pointing that out, Tim. Being a "first generation" LB7 guy, I'm "LLY Challenged!":o:

I knew what you meant Tom. Just though some may not know that there was more pressure than what the gauge read. You definitely have a better handle on this subject than I. Later! Frank