View Single Post
Old 09-26-2006, 11:55 PM   #24
1st Gear Newbie
Join Date: Sep 2006
Location: Canada
Posts: 24
Originally Posted by Mystikal
You seem to be a knowledgeable person, therefore I'm sure you must have come across the simple mathematic formulae used to calculate possible power from a fuel injector. Using these, at 80% of the injector's duty cycle (the highest use generally recommended) a stock M20 injector can only make ~140hp. Using that number and calculating further, one will find that BMW is actually running the stock injectors at 96% of their duty cycle, in a bone stock M20B25. A 14.6lb/hr injector combined with the stock 3.0 bar regulator can only physcially make 175hp. This is simple physics, it's impossible to argue with.
Lets say that the stock injectors are good for around 175hp, which is more then the peak hp of the car (and i'm sure most e30s are puttin out well below the original hp numbers at this age). You don't drive around at peak hp, so the injectors are running 80% or less for the majority of the time. Running at 96% for a split second when the engine hits peak hp isn't going to hurt anything and still isn't maxing out.

Also, the bsfc values used by online calculators are approximate. If we use a bsfc of 0.4 the stock M20 injector is good for the 215-220hp @ 3bar that Alpina claims. There are lots of unanswered questions about the stock injectors, but I guarantee that BMW engineers didn't make a mistake when choosing the stock injectors. Also, keep in mind that all the sites with injector calculators are trying to sell you new injectors.

Just FYI, I just used an online bsfc calculator that calculated the bsfc of the 325i engine to be more like 0.42 and not the default 0.5 used by injector calculators. (bsfc = brake specific fuel consumption - How much fuel you are using per horsepower per hour) Try the online calculators/formulas again but this time change the bsfc to 0.42 and see what you get.

19# Ford Mustang injectors are 30% bigger then the stock injectors. At WOT the ECU does not use the O2 sensor to adjust the fuel delivery, and does not know you have bigger injectors, so the car will run extremely rich. Bigger injectors require tuning, they aren't plug and play for optimum results.

Here is something else to think about from another thread:

"Under normal driving conditions the economy should be the same but under slight variations in load the O2 sensor has a harder time getting things right. Say you've driven the car hard at WOT and have back off and will now be cruising on the highway your AFR may have been 12:1 when the car started to cruise. Now your O2 sensor will try to lean if off by changing the injector pulse time (reducing it) but how much does it it reduce it by? It doesn't know you have bigger injectors so when it reduces it, it is by too much and it runs too lean. So then it tries increase the AFR by increasing the pulse time and it runs over rich again. Eventually it will get to 14.7:1 but it takes longer than would be the case with the standard injectors. The O2 sensors help but they don't not work instantaeoulsy but rather in a closed loop. "

Last edited by Curtis; 09-27-2006 at 12:29 AM.
Curtis is offline   Reply With Quote