FAO Rob

[BobMillar]

Hi Rob,

saw the post on the BCBBS about the FPR losing you 8bhp!

Can you elaborate further cos' as you know I've also got the FPR and virtually the exact same setup mods wise as you!

[Rob Bell]

Hi Bob,

sorry for not replying sooner- just been so busy at work etc etc etc...

The bottom line is that my engine was over-fuelled - and there was no equivocation about it. The odd thing is that the torque curve at engine speeds below 5000rpm looks better than my previous RR run @ G-Force - but over 5000rpm the engine started to loose power hand over fist - a case of fuel displacing air, and therefore oxygen - and thus getting inefficient combustion.

It was interesting looking back at what I wrote after initially fitting the FPR - better torque, but a more noticible flatting off of power at the top end: exactly what was shown on the power curves!

However, some cause for confusion: some of the other engines present were not overfuelled despite using the same FPR.

My advice would be to check the Lambda readings during open-loop running (closed loop, MEMS will compensate for any fuel pressure related problems). If you're not able to do that (or find a local RR with this facility), then change back to your original FPR at least until you've had your cylinder head ported...

[BobMillar]

So was your 136bhp obtained with the original or new FPR?

[Rob Bell]

With the new FPR. I had 144 before.

The figures are exagerated by G-Forces' fly wheel estimation algorhythm that has been optimised to Porsches. A more 'real' figure is 126.5 bhp

The stoichiometric figure, BTW was somewhere in the 10 - 11 bracket - hopelessly rich: it should be, as you know, 13.1

Will revert to the old one, and plan to repeat the run and check the stoichiometry again (the last time I was there, the air:fuel sensor was not operational).

[BobMillar]

Been thinking about this over the weekend as I've done a lot of driving.

Think I can actually live with the 8 bhp drop as the low end responsiveness is fantastic. Foot down in 4th gear at 30 mph and the car really takes off which, lets face it, is more useful for day to day driving.

May stick the original FPR on for track days when I'll need the power higher up.

[Rob Bell]

Unfortunately, it may not be that simple Bob - if it were, I'd do exactly the same.

The problem with over fuelling is many fold, but here, in no particular order are the 3 main issues I can think of off the top of my head:
1. Fuel has volume - and therefore will displace air. The more fuel, the less air. The less air, the more incomplete the burn. The less complete the burn through lack of oxygen, the less efficient the engine. More fuel consumption, and as I've already seen, less power.
2. A rich engine runs cool. The cooler the engine, the less efficient it becomes - comes back to point made in (1) above.
3. MOST important bit this: what happens to the incompletely burnt fuel? Some will be converted to carbon soot and hydrocarbons that at best will poison the cat. At worse, it'll contaminate your engine oil and unless changed, will damage the engine internals. Any unburnt fuel will not be blown out of the exhaust either: some will drip down the cylinder bores (bore wash), removing the oil lubricating the cylinder rings. Means accelerated bore wear.... argh!

I'm not saying that all this is happening in your car - afterall, my FPR might actually be running a higher fuel pressure than yours (depends on how reproducible Mat's rig proved to be) - or your engine might be able to better use the extra fuel than mine, but if yours is running as rich as mine, I'd definitely advise returning to the original FPR to avoid potential engine damage. This is what I did on Thursday evening before Silverstone! I'm going to keep my uprated FPR until I port the cylinder head - as Paul's experience shows, it may be better suited to a 100 cfm head!

[BobMillar]

Whats the best way for a quick check to see if it's running rich?

Pop into a garage and ask them to stick an exhaust gas analyser in the tail pipes?

[Rob Bell]

Yes, that ought to do it Bob. Bear in mind though that at a constant throttle opening, the engine is working in closed loop mode, so any over fuelling due to the FPR will not be seen - but you ought to be able to see the impact of the FPR on fuelling at full throttle (open loop map).