Hey guys
My auto teacher gave us an extra credit assignmnet. He said to use any resource we have to find out why fuel pressure must increase as engine vacuum increases. I decided to use my best resource, HCF. A few things before you post, i know that the Fuel Pressure Regulator works off of engine vacuum and that isn't the right answer. He wants to know why this has to occur. If it makes a diffrence, we are learning about Engine Management.

 

ahhh so you want us to do your homework...just kidding, i tired to find some info on you but if you go to this link http://auto.howstuffworks.com/fuel-injection.htm there is some info on there about it, or just keep going through the sub links on that page..hope this helps

 

Fuel pressure has to increase as vacuum increases? I think it increases as the manifold pressure increases.

FPR regulate fuel pressure according to vacuum because obviously manifold pressure dictates the amount of air being introduced into the combustion chambers. At WOT your vacuum should be 0 and your injectors will be at full duty cycle and will output just as much as the fuel pump will give them (actually about 80% duty cycle but that's another story). More vacuum (less pressure) will cause the regulator to devrease fuel pressure because less air means you need less fuel.

It basically boils down to a driveability issue. You rcar would run like *** if you had full fuel pressure at idle right?

 

As the pressure in the manifold gets lower, doesn't the speed of the engine get higher? or do i have it backwards? My teacher also said that the FPR only changes the fuel pressure in a car with MPI (multiport injection) and in a TBI (throttle body injection) this does not occur, i dunno if it makes a diffrence.

 

do you know how a throttle body works? think about it like that, if you have the TB plate closed, the engine is sucking all the air it uses through those little holes in the plate, so there is a good amount of vacuum in the IM, but if you opened the plate up, what would happen? (besides the revs going up) the pressure in the manifold will equalize with the atmospheric pressure (not quite, but it will get within a psi or two.) so think again about the throttle plate opening, now, we know the vacuum in the manifold will go down (pressure goes up inversely) as will revs with the newfound air supply. so if the pressure goes up (or vacuum goes down) then fuel pressure needs to increase to supply the faster revving engine with fuel. does that make sense? or did i start rambling?

 

Thx sacs, it makes sense now. I thought that because the engine was moving faster, the pistons goin down creates more vacuum cause they are making more intake strokes. But no it makes sense that the pressure would equalize when the plate is opened. So thx. I first thought that the low vacuum made when the engine revs higher and made the FPR raise the fuel pressure. Now, its correct that the high vacuum produced when the engine revs higher makes the FPR raise the fuel pressure cause the engine is gonna need the extra fuel. Also, we were talking about pulse widths in class so i was wondering if it would be correct that the fuel pressure has to be raised because only so much fuel can be sprayed while the pressure is lower, so as the demands of the engine go up, the pressure must also go up so enough fuel can be sprayed in the limited amount of time that the injector can stay open. Is this correct?

 

right. also, manifold pressure goes up with throttle position as well, not just rpm. so under wide open throttle, pressure jumps to feed the engine while its under load.