A few nub questions: Fuel pump and compression

[krogoth]

hey guys just need to clarify a few things

my mate with his r32 was getting a new fuel pump put in....and he was telling me how his mechanic was pissed off coz he dropped off his car with a full fuel tank

the mechanic was pissed coz its much harder and more messy to get the fuel pump out of the fuel tank when the tank is full...

First Question:

Do all cars have their fuel pumps inside their fuel tanks?

Second Question:

If a car has the fuel pump in its fuel tank...does that mean its better for the pump to run on a full tank?

coz if u run around on a 1/4 of a tank all the time, that means the fuel pump is running dry, possibly overheating, which may reduce the life of ur fuel pump?

Compression

This is really vague, and im just trying to get a few more facts straight

-NA cars generally have a higher compression ratio than turbo cars do

wat i want to clarify is how does increasing the boost of a turbo in a turbo car affect the engine compression ratio?

also, if u bolt on a turbo on to an NA engine, does the compression ration change?

is it safe to say that the only way to change compression ratios is by playing with the valve timing?

i know im really simplifying a massively complex situation, but i just want to hear ur thoughts

i did search b4 creating this thread, confused more than clarified, lol

PS, sorry about the long thread and sorry bout the crappy grammer and spelling, i was in a rush

[zco]

2nd question.. if the fuel pump is running dry, wouldnt it suck up petrol at all ?
or are you scared that its scuking up half water, half air ? if it does this, you can hear it as its very loud. it happens on my wallbro fuel pump.. too damn loud.

as for compression. you can change it with headgasket. decking your head/block. change pistons inside obviously.

[krogoth]

sorry for not being clear enough

with liquid pumps, they prefer/its better for them to run completley covered in fluid

for example, we deliver big outdoor fountains for work...and i always tell customers to keep the pump covered in water, keep the fountain full of water so the pump is completley covered in water, this way it cant overheat

if the fountain was left for ages without topping up water, the pump would be slightly exposed, like, the bottom half of the pump is covered in water and the top half is dry, possibly causing to overheat and not being able to pump the appropriate amount water

If pump is half covered in water, it may overheat...possibly reducing its life and may cause it to stop working

i thought it might be similar with fuel pumps, i know im really overthinking this, but i like to know how stuff works, bloody uni, lol
__________________________________________________ ___________

as for compression, i agree that compression can be changed/made higher with a thinner gasket, aftermarket pistons, cams etc, but how does turbo affect compression?

say, if u have a d17 with a bolt on turbo, stock internals, would ther be a difference between cylinder compression if the turbo is running 6psi or 10psi?

sorry if its a really nub vague question, but ive heard so much conflicting rumours and crap i want to clarify

[1996ek1]

wat i want to clarify is how does increasing the boost of a turbo in a turbo car affect the engine compression ratio?

Increasing the boost increases the amount of air being forced into the combustion chamber, which is increasing the compression ratio. This also makes the air/fuel mixture leaner, increasing chance of detonation


also, if u bolt on a turbo on to an NA engine, does the compression ration change?

The compression ratio will remain the same, untill the turbo beings to spool, and make boost. Depending on the amount of boost the turbo is making, but the motor can be 10:1 CR, and once it is on boost, can be more like 13:1 etc


is it safe to say that the only way to change compression ratios is by playing with the valve timing?

Chaning to low comp pistons, you can change to a thicker head gasket, sometimes changing rods changes compression, and things like that. Although im not too sure what valve timing does for compression (It might change it, but i know nothing about that)




Hope that helps

[ZeForce]

as for compression, i agree that compression can be changed/made higher with a thinner gasket, aftermarket pistons, cams etc, but how does turbo affect compression?

say, if u have a d17 with a bolt on turbo, stock internals, would ther be a difference between cylinder compression if the turbo is running 6psi or 10psi?

sorry if its a really nub vague question, but ive heard so much conflicting rumours and crap i want to clarify

Boost doesnt change an engines static compression, obviously, but it does have a big impact on the engines dynamic compression or actual cylinder pressures.

So yes, more boost = higher cylinder pressures = more torque = more power

Static compression is pretty much meaningless and only applies if the engine is not running.... Im pretty sure most of us prefer to have our engine running while we are driving.

[ZeForce]

Chaning to low comp pistons, you can change to a thicker head gasket, sometimes changing rods changes compression, and things like that. Although im not too sure what valve timing does for compression (It might change it, but i know nothing about that)

Using a thicker head gasket is just asking for trouble. Valve timing can be used to reduce/increase the cylinder pressures by increasing/reducing the amount of overlap. The pistons cant actually compress the fuel/air mixture until the valves are closed.

[1996ek1]

Think ZeForce said it better lol :p


Edit: Forgot to mention, i personally wouldnt touch a different size head gasket.

[aimre]

Increasing the boost increases the amount of air being forced into the combustion chamber, which is increasing the compression ratio.


No thats WRONG. Turbo doesnt change your compression ratio.

wheter or not it changes how much air gets compressed in your cylinders is a diffrent story. But u cant say its changed your compression ratio.

it's the ratio between the volume of the cylinder, when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke.

nothing to do with turbos

[1996ek1]

So dynamic compression doesn't change...? You sure?

[ZeForce]

He was refering to static compression ratio, he should have been more specific

[krogoth]

^thanks for all the info guys, would give u all rep points, but its not enabled atm

so its not accurate to say stock turbo engines have a lower compression than stock NA engines?

maybe it can be said that before the turbo begins to spool, a turbo engine has lower compression than the NA engine

but after the turbo has started to spool and max boost is reached that the turbo engine is running higher than compression than the NA engine?

[krogoth]

No thats WRONG. Turbo doesnt change your compression ratio.

wheter or not it changes how much air gets compressed in your cylinders is a diffrent story. But u cant say its changed your compression ratio.

it's the ratio between the volume of the cylinder, when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke.

nothing to do with turbos

yep, that make more sense

i thought logically that the compression of a cylinder has nothing to do with turbo....

good thoughts guys...keep it coming...this is helping me do the discussion part in my report, lawl

[1996ek1]

so its not accurate to say stock turbo engines have a lower compression than stock NA engines?

maybe it can be said that before the turbo begins to spool, a turbo engine has lower compression than the NA engine





Refering to static compression, which is most often the F/I engine is lower then N/A. (if its a stock turbo)

[EUR003act]

it's the ratio between the volume of the cylinder, when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke.

nothing to do with turbos

110% true!

turbo doesnt touch the compression ratio... it increases compression when boosting yes... but do nissan state their SR20DET is a 10:1 compression ratio when restinng and 13:1 when boosting? or is it just 10:1??

answer: its just 10:1

[1996ek1]

110% true!

turbo doesnt touch the compression ratio... it increases compression when boosting yes... but do nissan state their SR20DET is a 10:1 compression ratio when restinng and 13:1 when boosting? or is it just 10:1??

answer: its just 10:1

Yea static. But what about dynamic, it changes. I'm answering for both here...

[EUR003act]

he was asking about engine compression ratio.. that always refers to static...

thats how ive always know it anyways...

each to their own :)

[1996ek1]

he was asking about engine compression ratio.. that always refers to static...

thats how ive always know it anyways...

each to their own :)

I thought i said earlier

Turbo generally run lower compression, but as the turbo spools the compression increases to that higher of a N/A (not those words but to that effect)

Anyway the OP has hopefully got his answer :D

[krogoth]

i dont mind, keep discussing, im studying this crap now, so an outside perspective helps

[krogoth]

ok, then wat about octane levels in fuel

is their more chance of spontaneos combustion, or compression combustion in a high compression NA engine or in a FI engine?

[ZeForce]

More chance of detonation in higher compression N/A NORMALLY but if you overboost FI you can force a detonation situation.

If you design the piston and combustion chamber properly, raising compression can actually reduce chances of detonation via mechanical octane

[krogoth]

wot is mechanical octane?

[ZeForce]

wot is mechanical octane?

It is increasing the engine resistance to detonation through mechanical means, such as the shape of the combustion chamber and piston dome as well as the appropriate use of quench area. Ideally you want the combustion space to be quite small and confined to a single isolated volume so the air/fuel mixture can burn quickly, this requires the static compression ratio to be fairly high. A larger combustion space means the air/fuel mixture is spread over a greater area and takes a longer period of time to burn. Having a 'fast' burn means the engine will not require as much ignition advance in order to burn the entire mixture before the piston reaches BDC. Reducing the amount of ignition timing advance will reduce chances of detonation and can be done without sarcificing power. Another disadvantage of having a larger volume combustion space is that even though the overall air/fuel ratio is satisfactory there may be areas within the combustion chamber where the air/fuel mixture is much leaner and may lead to detonation. A smaller combustion volume ensures the air/fuel mixture is more homogenously distributed throughout the combustion chamber.


To apply some of this 'theory' to the real world, the B18c P72 head has quench pads on either side of the combustion chamber which gives it more "mechanical octane" over the B16a/ITR PR3 head.


Disclaimer: This is my understanding of the concept, however I am still studying this stuff atm. If I am wrong, not you THINK I am wrong, then feel free to correct me.

[krogoth]

nice explanation

would rep u if i could

btw, wot u studying?

[ZeForce]

Automotive Engineering

[aaronng]

First Question:

Do all cars have their fuel pumps inside their fuel tanks?
Not all. Some have inline fuel pumps.



Second Question:

If a car has the fuel pump in its fuel tank...does that mean its better for the pump to run on a full tank?
The fuel pickup point is usually at the bottom of the tank. It depends on the design as well. For example, DC5R's are happy to run hard til the end of the tank, while RX-8's have problem picking up that last bit of fuel because of the tank shape.



coz if u run around on a 1/4 of a tank all the time, that means the fuel pump is running dry, possibly overheating, which may reduce the life of ur fuel pump?
The fuel pump is the least of your worries. If your pump is running dry, it means that there are short instances where your engine doesn't get the fuel it needs and then runs too lean, slowly melting your pistons.



Compression

This is really vague, and im just trying to get a few more facts straight

-NA cars generally have a higher compression ratio than turbo cars do

wat i want to clarify is how does increasing the boost of a turbo in a turbo car affect the engine compression ratio?
There is static and dynamic compression. Static is what is measured and showed in car specs. It's just the ratio of the volume at BDC to TDC. An NA car can fill at max 1 atm (14.7 psi) of pressure (this is at 100% efficiency). A turbo'd engine can fill higher than that depending on boost pressure. A turbo'd engine running at 9psi gives 23.7 psi fill, which is 1.61 times the air that the NA engine has in its chamber. Since the combustion mixture heats up when it is compressed, the turbo mixture will heat up more because of the higher fill pressure.

Dynamic compression is the actual compression ratio that is affected by things like valve timing. Since the intake valve closes after BDC, dynamic compression is usually lower than the static compression. If you further delay the closing of the intake valve into the compression cycle, you lower the dynamic compression because some of the intake charge flows out.



also, if u bolt on a turbo on to an NA engine, does the compression ration change?
No, the static compression ratio doesn't change, only your intake charge mixture pressure.



is it safe to say that the only way to change compression ratios is by playing with the valve timing?
You change dynamic compression ratio through valve timing.

[aaronng]

A larger combustion space means the air/fuel mixture is spread over a greater area and takes a longer period of time to burn. Having a 'fast' burn means the engine will not require as much ignition advance in order to burn the entire mixture before the piston reaches BDC. Reducing the amount of ignition timing advance will reduce chances of detonation and can be done without sarcificing power. Another disadvantage of having a larger volume combustion space is that even though the overall air/fuel ratio is satisfactory there may be areas within the combustion chamber where the air/fuel mixture is much leaner and may lead to detonation. A smaller combustion volume ensures the air/fuel mixture is more homogenously distributed throughout the combustion chamber.

Yup, the Hemi engines used in GM range are notorious for detonation even with a moderate compression ratio and high octane fuel because of the large combustion chamber and hemispherical shape.

[aaronng]

GM doesn't have Hemispherical combustion chamber head engines. They have semi hemispherical combustion chambers but only Mopar has real hemispherical combustion chambers, and NO the new Mopar engine badged as a Hemi doesn't have hemispherical combustion chambers.

Oops, I meant Chrysler, not GM. My bad.

[krogoth]

excellent info aaron

rep urself:p