bmwz4 wrote:
Fuel pressure tester arrived this morning and gave a constant reading of 51psi regardless of engine revs.
That's good, it means that the jettings will be consistent and there won't be any fear of A/F fluctuations.
Something that is still troubling me a little tho is that a couple of other lads on the forum with the same engine albeit in a 3 series were running 3:1 AF ratios, I think I even read somewhere the Wizard said that the beemers like this ratio, although I would rather err on the side of caution.
That ratio is very close to the optimum for peak power, but the downside is that they have to be more mindful of any increases in temp where the nitrous flow might be too great OR the off chance of a tank of poor quality petrol, resulting in bad things. The richer ratio you are running gives you a better safety margin with slightly lower peak power, but also better low end torque due to the extra richness, which makes street driving all the more fun.
Heres a few thoughts, although my intial ratio of a 50N 20F (2.5:1) gives me a nice plug colour and theorectically thats the ratio I should keep, how true is that ratio when you jet higher, ie might it run richer, weaker or the same,
I haven't heard of that one, and I would say that the opposite would be true where the higher it's jetted, the closer to the fuel pump limit and the leaner it would eventually become.
my thinking is that if it generally runs richer with bigger jets you could up the ratio to 3:1, also could it be down to Nitrous temp, the lads with the other 330I's were up north where its a bit cooler than down here in the south or are no 2 cars alike and it just depends on too many variables?
You shouldn't concern yourself with what others run since it is true that no 2 cars are alike. The richer ratio will give you a greater margin of error with little sacrifice in performance. It's always better to increase the jetting at a richer ratio if you want more acceleration on a street car.
For example, two cars are identical, with 50 bhp of nitrous and always result in a tie. One car jets leaner for 10 more bhp (60 bhp) in peak power and just barely beats the other in the next race. But then the losing car jets for 25 more bhp (for 75 bhp which makes for 15 bhp more peak-to-peak than the competitor) at the same rich ratio from the start and puts a real hurtin' on the lean car. Why?
It's because the 75 bhp rich car made GOBS more power over all rpm giving a faster start and more pull throughout each gear (wider portion of rpm), and it did so while running cooler and safer than the 60 bhp car thanks to the extra fuel absorbing heat. Now the 60 bhp car, although did win the second race, BARELY did so and forced the engine to run hotter which COULD have resulted in some detonation (let's assume the weather was unfavorably hot and both engines were heatsoaked). The 60 bhp car didn't gain much torque at all, making it slightly slower on the start while having a narrower portion of the high rpm power available to in each gear. The 75 bhp car had a safer and more powerful setup with very little risk of detonation.
In summary, it's a better option to simply stay rich and make more power with larger jets.
Just trying to understand the finer points and I also want my car to run the best it can
Unless you're competing in some kind of motorsport, then don't worry about it. Those guys running at peak performance are always adjusting something under the bonnet and taking greater risks, whereas regular street cars rarely even look at the engine for an oil change. Stick with the safe jets and just enjoy driving the car with it's new muscle.
