Cvcc and fuel pressure

[Michael]

Im looking at putting an adjustable fuel pressure regulator on my T2 as it leans out up high rpm at high boost. My question relates to the set up of the FPR, would it be best to increase the pressure of both the cvcc injector as well as the 4 primary injectors or just the 4 primary injectors. Would the cvcc combustion chamber be leaning out as well? Perhaps I am wrong in my understanding of the cvcc system but as I understand it the cvcc chamber ignites the main chamber thus in a way acting as part of the ignition system (well sort of). My concern is that if I increase the fuel pressure unnessesarily to the pre chamber and it runs rich the flame will not be as big to ignite the main chamber and thus not give maximum power. Clear as mud?

What do you think?

[BOOSTBOY]

Im not sure but here is my opinion.  The cvcc is not really subjected to combustion chamber conditions and therefore isnt as sensitive to fuel mixture in regard to detonation prevention.  The purpose of the cvcc is to provide a combustion ignition which a sparkplug cannot supply.  You might be right, richening this mixture may not proved the ultimate combustion.  Wether the engine is making 100hp or 200hp shouldnt matter to the cvcc system as far as im concerned...

[Michael]

Thats sort of what I was thinking. What I am looking at doing is running an ajustable FPR and the standard one in line with each other so the cvcc injector gets standard pressure and the 4 primary injectors get increased pressure.  

What do you think Charles?

[JohnT]

As an old physicist .. seems to me that the key reason you need more pressure on all injectors (which is what I would think) is that the air pressure inside the throttle body will be high under open throttle high boost.

Fuel flow through the injector is a function of the pressure drop across the injector, from fuel rail pressure to air pressure in the throttle body.

So at high power / high revs / high boost / open throttle, all of the injectors demand a high flow rate to get a decent fuel / air mixture. Obviously, as you close off the throttle, you need less fuel pressure, which is why the fuel pressure regulator is listening to  the post throttle vaccuum or boost.

We plan to run our beast at 20 lbs or so - working up gradually - and have a sports fuel pump to drive the fuel rail, returning through a variable  pressure FPR hooked into post throttle vacuum. T1/2 hybrid in a Civic body, but should all be the same in a real City.

Regards

(Dr) John

[Michael]

So what your saying is the cvcc chamber will be leaning out as well as the main chamber, right?

[doggystyle]

thats what i wanted to know.. if your increasing boost to say 25psi does the computer increase the output of the cvcc injector to compensate cos surely if the boost pressure goes up and theres more air in the cvcc chamber and it doesnt change the its going to be hard to ignite.. which is the purpose of cvcc to start with.. and if it does is it easy to adjust does it just have a linear pulse width.. blah,blah,blah i could keep going

[JohnT]

Doggystyle, the computer doesn't manage fuel pressure in the CityTurbo engine set up.  There is a continuous high pressure fuel supply that goes pump / fuel line / filter / main injector fuel rail / CVCC injector fuel head / pressure relief valve (which has a manifold vacuum / boost input) and then back to the tank via the fuel return line.

As you open and close the throttle, the fuel pressure regulator opens and shuts at different absolute fuel pressure to keep a constant fuel pressure differential (to the inside of the throttle body & manifold) over the injector.

The computer knows about fuel delivery versus revs / air temp / engine temp & Exhaust O2 and opens the injectors for different amounts of time to let a certain quantity of fuel through. So changing the CVCC injector pressure independent of the other four injectors would probably really screw up the ability of the engine to manage itself. Expect shit performance or short engine life.

John

[QikLude]

why dosent someone hook a multimeter up 2 the ccvc injector and see what its doing?

rung some probes on it back into the car, go for a rip and get a mate to read it.

any changes in voltage (or perhaps pulse frequency) mean that that injector IS being altered.

[JohnT]

Qiklude

Read my response above. The timing is constantly being alterred to the CVCC injector as the computer runs the engine depending on manifold pressure, O2, TA & TW and revs. This ios how the amount of fuel varies.

As to the original idea of different pressure to CVCC and main injectors - you'd need a second fuel pump, second fuel and return lines - or the whole lot would run at the lower pressure - simple hydraulics.

John

[Michael]

You wouldnt neeed to run a second fuel pump and all that. What you can do is run an adjustable FPR for the 4 injectors at a higher presure then run the return line from your ajustable FPR into the fifth injector and from there into the standard FPR that would be running at less pressure than your adjustable one.

Initally I thought it may be possible that the fuel mix for the cvcc could be varied by the flow of the air regulated by the cvcc butterfly, but as it was suggested that the more dense air that occurs with higher boost would cause it to have more oxygen in the mix and cause it to run lean. This suggests to me that the cvcc injector would need to be varied with boost pressure and rpm and all the other sensors that affect the main injectors. The only way to confirm this is to test the injector with a multimeter. If this is proven to be the case then the cvcc injector would require more fuel pressure just as the main injectors would.

Just a thought... As the spark plugs are located in the cvcc chamber which runs at a richer fuel mix than the main chamber to would it be possible to determine if the engine is running lean by analysing the color of the carbon on the plugs as you can on non cvcc engines? What about the mixture readings on a gas analyser - would these be the same as non cvcc engines.  - Food for though.  :)

[Smithy]

My  cats  breath  smells  like  cat's  food.

[Charles]

Run the same fuel pressure  to all injectors! If you have increased airflow (through more boost) then you will have more airflow to both cvcc prechamber and main combustion chamber. Therefore the relationship between cvcc AFR and main chamber AFR needs to remain consistent as well. To do that you need to increase pressure by the same amount to each. If you are worried about cvcc flow you can try swapping cvcc injectors, the T2 flows about 68ccmin and the T1 flows about 45cc/min from memory when I tested them about 10yrs ago. There is definately a difference.

I would be surprised if you are running out of fuel at up to 17-20psi with stock system at the top end unless you are using the stock fuel pump. If fuel is fine everywhere else on full throttle and you have replaced you fuel pump then consider a rising rate FPR that will not affect low speed running. If you increase fuel to bottom end it will be very doughy.

[PGNUTA]

I've got a standard T2 with stock fuel system at the moment...  I've got a 4.1L Falcon fuel pump to replace my stock one.  I'm hoping to hit 25-30psi at some stage, what else is involved (replacement/modifying) to hit this goal?

I realise that i will need a fuel cut defender, i've also got a diesel turbo camry intercooler to front mount.

The other option is plaining the head to get more compression then i wont have to run so much boost to get the front wheels off the ground

Your comments?