Fuel Pressure Question

[Tom Kelly]

No that is a function of the camshaft I and being naturally aspirated as they have to have different cam timing the old-school carbureted cars which I work on a bunch and have had drag cars that run on carburetion what happens when you put a big cam and a mouse or rat motor the air-fuel mixture gets shoved back up into the intake manifold at idle and lower rpm because the valves are hung open longer due to more duration and more cam overlap which increases the pressure in the manifold hence dropping intake manifold vacuum

Thanks for the explanation. It's all coming back. As to GN's I don't read much about using cams to change power. More talk about electronics and boost. It might be different for the ones running 8's and 9's. I was asking the question because Eric's instructions with the chip say that pressure drop when hose is re-attached should be 3-8 psi and I was wondering why it should vary so much. As I said, mine is 8 which closely matches idle vacuum in psi.

 

[gunzandgearz]

Thanks for the explanation. It's all coming back. As to GN's I don't read much about using cams to change power. More talk about electronics and boost. It might be different for the ones running 8's and 9's. I was asking the question because Eric's instructions with the chip say that pressure drop when hose is re-attached should be 3-8 psi and I was wondering why it should vary so much. As I said, mine is 8 which closely matches idle vacuum in psi.

That's because turbo cams resemble a lot more of a daily driver naturally-aspirated cam and you will see on average an 8 to 10 inch drop in manifold vacuum

 

[Tom Kelly]

After reading all of your excellent points, I scoured youtube for a simplified video bringing it all together. For an excellent and entertaining video clarifying the "pressure differential" cooncept mentioned by Jarryl, check this 8 minute video out. And thanks again. I learned a lot from this thread.

 

[ShaunKris]

It's basically related to flow in other words the injector has a certain size hole or orifice in it just like you have different size drill bits you have different size orifices in the injector the bigger the orifice the more the injector will flow for a certain fuel pressure in the rail so let's say the injector is fully open and flowing the maximum it can flow at 43 PSI if I bump the fuel rail pressure up to 53 it is going to push more fuel through that injector there is a triangle formula f a p which means force is equal to area times pressure that means if you increase any of those in a hydraulic system it will increase the flow so a x p is area X Pressure so you can either increase the hole diameter (area)in the injector or increase the pressure (force)behind the injector and both will have the same effect hope that helps

Great question and answer, thanks guys.