I have now totally re-worked my HP fuel system to my TBs and can now adjust the regulator to give the desired pressure in the fuel rail. My question is - what pressure should i be looking for?

I know there are a lot of factors to consider so I will describe my new setup....

I am running a standard 2.0l Sierra EFi HP pump (supplied from a swirl pot which is filled by a facet lp pump)
this goes through a standard Sierra HP filter and on to the fuel rail.
The fuel rail has now been re-worked and flows through with a 5mm bore (up from the standard 4.5mm). at the other end it goes into an adjustable fuel pressure regulator with a gauge and then returns to the swirl pot. The regulator is one of the £30 jobbies off ebay which I know others on here have used with good results, (Managed to drop it down the stairs the day it arrived and broke the glass on the gauge...grrrr:mad.

Engine is a Pinto 2.1 with the head skimmed 1 mm and Kent FR34 cam, so long duration cam and quite high compression ration (don't know how high yet!) running of GSXR 750 K2 TBs with the standard GSXR injectors. All controlled by MSnS extra. I am currently running 2 squirts synchronus and Alpha-N - although I also have MAP connected so could try Hybrid as well....

I have initially set the regulator to 3 Bar and, of course, the car is now running a bit rough as I need to re-tune to cope with the different fuel pressure (no idea what it was before...)

I want to get a good idea of what pressure to run before I start tuning or I will end up in a horrible mess...

Also - I have connecte the vacuum pipe to the regulator - but am I better disconnecting this and letting the megasquirt deal with the increaseed fuel needs?

cheers (I just hope all this solves my brake down problems )

[Edited on 2-7-08 by DaveFJ]

The fuel pressure you choose doesn't really matter so long as you can get the necessary max' flow from your injectors and you compensate for it in the Megasquirt Req fuel calculations.

However; most injectors are designed to run around 3 Bar (45psi). If you can stick to close to this value then things will last longer, behave more predictably etc...

Finally; leave the vacuum pipe attached to the regulator and let Megasquirt assume the fuel pressure is constant.

I'd say (assuming you've got reasonbly sized injectors in the first place) go for whatever fuel pressure is required so that your maximum duty cycle is about 80%. If your max duty cycle is 40% cos your pressure is too high you're just causing uncessary granularity. If your duty cycle is above 80% you might want to reduce it to avoid flogging your injectors and possible running into lean full out throttle running....

That's how I'd do it anyway- within reason anyway. If the pressure is too low (sub 2ish) you may not get proper atomisation, if it's too high (3.5+) you might start blowing injectors etc etc...

Cheers for that...

Think I will leave it a 3bar for now and try to get it tuned to something approaching drivable and then check my pulse width to see how I am doing.... (sound reasonable?)

Agree - 3 bar will be fine for your use - I can't remember the flow rate of the 750 injectors (they may even be the same as the 600's).

But assume they are around 230-240cc/min, so should be perfectly sized for your engine.

I have not used the vacume pipe on the regulator as the cam I have in my CVH engine is quite a long duration and high lift, so at tick over and low revs, the manifold vacume can be unstable - It's easy to compensate for the fuel delivery in the VE table to be honest, but certainly won't do any harm.

Stick with Alpha n for now, get a good base set up so you can enjoy the car, then tweak :-)

Sounds like you now have a fairly standard fuel delivery set up, so hope it all works....

Best of luck.

Paul.

they arnt bike throttle bodies your running are they ?

Dave what fuel pressure regulator are you using? The only ones I am aware of with a glass bowl are low pressure for carbs.