heinrich

Thanks Mark, great point.

FlyingDog

85-86 US cars use one regulator which is the same as the twin regulators on 80-84 US and 84-86 Euros. The pressures aren't close. According to the WSM, 80-86 is 2.5bar w/o vacuum and approx. 2.0 at idle, S4 is 3.8 and 3.3 respectively. Adjustable regulator? They're all aftermarket.

heinrich

So if I ran that one on the S4 for now, I'd be lean I think ...

FlyingDog

With the 19lb injectors, probably. I doubt the O2 could compensate enough.

heinrich

Dangit.

mark kibort

you are forgetting that you cant be too lean at idle, and part throttle. the 02 sensor can MORE than make up for lack of pressure, if even you would have lack of pressure.

anyway, the main concern will be at WOT. part throttle will not matter that much, as the O2 sensor's job is to regulate fuel mixture , which actually would be easier if the pressure is lower (not really easier, but you get the point)

MK

mark kibort

so, if the pressure is supposed to be 2.5 bar on the 85, and i think its 2.0 bar with the US 84, all with the same regulator, what is the differnce in injectors.

if you get more pressure with the one regulator on the S4, and the US84 regulator seems to have a weaker spring in it, but how does the fuel pump pressure and the injector ratings effect things. IM sure we can measure the differences in the regulators. I have both, I may have to do some tests and tell you what i find.

MK

FlyingDog

85-86 US, 80-84 US and 84-86 Euros are all 24lb injectors. The pressure specs are also the same as far as I saw in the WSM. Unless you are drastically changing the fuel flow, it shouldn't change the regulated pressure.

Since the fuel system is a circular loop. The dual regulators was overkill. It doesn't matter whether you use single or dual regulators. Some S2 owners have gone to a single regulator with a rear damper like the 85-86 US. I would assume some of the boosted L-jet owners have done the same to avoid having to buy two expensive regulators.

FlyingDog

I think the computer could only compensate the mixture a certain percentage based on O2 to prevent a failing O2 sensor from making the car inoperable. I don't know whether running 19 lb injectors in a system meant for 24lb injectors is out of that range. LH gurus John and Rich can probably answer that.

heinrich

I haven't pulled the trigger yet on disassembling everything ...

mark kibort

the o2 sensor is very crude. it senses for full rich and then pulls all the fuel out, to full lean, to average in the stoich range (ie 14.7:1 air to fuel) this is easy to do regardless of the pressure, and injectors. Im running way over stock fuel pressure now, and the 02 sensor does a fine job of economy. im still getting 22mpg to the track, with over stock pressure. proving that the 02 sensor is doing its job. at idle, i have NO vacuum attached to the regulators, and this gives a much higher fuel pressure at idle, yet, the o2 sensor still does its job. sure, its over rich for emmissions , but just fine for everything else.

Mk

Jim bailey - 928 International

The pressure at the fuel pump is limited by the regulator which bypasses the excess pressure and only the resistance to flow inherent in ANY plumbing/filters would cause the pressure measured at the pump to be higher than the pressure in the fuel rails. A relatively minor increase in pressure cause a major increase in fuel volume. Pinch off the return line and the fuel pump will try to generate as much pressure as it can which is exactly what a fuel pressure regulator does; block the return line. The vacuum line to a pressure regulator adjusts the pressure by changing the "spring rate" inside the regulator. Giving the engine more fuel when it needs it most.

mark kibort

agree with what you are explaining here , except for your last sentence.

the vacuum line pulls back the plunger in the fuel reg to let more fuel back to the tank, thus reducing the fuel pressure in the lines. (this is why the vacuum is attached to the manifold section of the throttle body, so its under vacuum and working at idle and low throttle. however, once you are at higher throttle, you have no vacuum and now the fuel regulator is on its own . dont know if by pulling back the regulator diaphram, you actually change it's spring rate, but the point is moot, due to the fact that when you need more pressure, you would be on the gas, vacuum would be removed and the system doesnt care about the idle vacuum attached to the regulator. as it is basically like an on off switch. once you are past part throttle, there is no vaccum on that side of the throttle plate.(at least not enough to do much), the venturi vacuum port does act a little differently, and can be used to drive vacuum advance on the older 928s.

to your first point below, we succesfully pitched off one of the return lines of an 84 928 and got an incease in fuel pressure to help WOT mixtures. I guess the one return line on the 84 then carries all the burden and the spring rate is still stiff enough to not allow the same volume of fuel to return . we then used a RRFR to put on the single fuel reg side to adjust the pressure in the loop.

still wondering why you need 2 of them, as the 85 and 87s only use one!

(the above is how i understand it and is based on some experience. maybe I miss understood your last sentence. I thought it should read, " the vacuum takes away fuel pressure when you need it least, at idle" )

Mark

FlyingDog

Overengineering. If there wasn't a crossover hose between the back of both rails, two regulators would be better. With the crossover hose, two is redundant. There is probably a theoretical benefit, that isn't big enough to do much in practice.

That's just symantics. You're saying the same thing.