fuel economy

[petermorrisminor]

I have been driving a BX 14 for the last 10 years, and would like to know if more petrol is used if you descend a long hill in gear (on the overrun) using engine braking,than if the box was in neutral and the engine just idling ?

Also would it be worthwhile to convert my 1990 cars engine to ECM by fitting all the electronic parts in the engine bay of a later other car with that engine so the errors of cent. advance and vac.advance were eliminated?

[RobC]

As I understand it, engine braking only saves fuel in EFi engines, not those with carbs. Others will know more.

Regardless, one should always descend long hills in gear for safety reasons!

[rutter123]

Would it really be worth all the trouble to go "electronic"?, so far as i know the 14's return reasonable mpg somewhere around 35/40, which isn't far off it's modern equivalents. If you want more miles for your £ diesel is the answer. As /\ says downhill in gear is the way as the natural engine braking will give more control.

[Kitch]

You shouldn't be idling downhill in any car, regardless of how it's fed. I would think there is next to no difference on economy either way. Much bigger differences will be made by the condition of the carb, tyre pressures and the health of the drive/powertrain.

As Rob says, it's EFI engines which get more clever. The work to convert yours to MPFI would far exceed the tiny savings you might make.

[Des Smith]

I can give a definitive answer to the economy question. I have a 1.4 TU engine in my St Tropez and it returns 37-45 mpg. I also have a fuel-injected 1.4 TU engine in my Xsara which returns 39-43mpg. Even taking account of slightly different weight and gearing, it's pretty clear there's no mileage (sorry) in making any mods as you have suggested and I can only foresee dramas and unnecessary expense if you go down that route.

[Dragon Man]

very good topic,

i converted my panda 4x4 from carb, to a very basic early Bosch EFI system purely just to have AFC (auto fuel cutoff) i did this on the Fiat FIRE engine which is 999cc with a BHP of 50.

depending on the throttle body, you will get different power. sadly for me the throttle body was a tad smaller than the carb (30mm and the carb 32mm). it made the engine a little more grunty low down but high up, it did not go as fast :/

the system used the coil trigger to inject the fuel so no need for a crank sensor as you can guess it was not the best way to do it.

one thing you need to do is completely change your driving style when swapping from carb to EFI. with EFI you try to use the brakes as little as possible, and use the engine in its place. it does save a noticeable amount of fuel, going down hill or coasting in gear costs you nothing when your foot is off the accelerator. i went so far as to install a small LED hooked up to the single injector just so i could see it in action.

now sadly i know nothing about the petrol citroen engines of BXs but i guess if you can find an early EFI system it could be done. most of the time the cam position sensor bolts to a built in bracket on the oil pan/sump

sadly i eventually converted it back to carb as the car always seems to start faster with the carb.

if you check my youtube channel out, you will see the videos i made of it.

remember though, most ECU's have built in immobilisers. this would make things pretty hard if you did want to go down the EFI route.

[Kitch]

Speaking as someone who does know the TU3 very well, it's pointless. I mean, really pointless. Economy and power both almost identical in healthy engines. Chances of the EFI engine being healthy are probably stronger than the carb version, but still, that's a case of maintenance, not setup.

Only version it would be worth doing it with is a TU3J2, which is the 100bhp unit fitted to the AX GTi and Pug 106 XSi (before 1995). It's an iron block, so suffers a weight penalty of around 25kg, and you'd need the later gearbox to go with it (I'd go for a Saxo VTR gearbox if it's going in a BX, personally - a bit more long legged, and they're cheap as nobody in the AX/106/Saxo worlds want them). No immobilisers to worry about.

Would be cool, but a lot of work. They also lose out on economy by around 5mpg, but then you would have just gained 25bhp/20lb ft torque.