not necessarily a 928 specific question, applies to all injection systems.

What is stopping a person from using an oversized injector? I know that when you use forced induction you have to increase the flow rate/duration of the injector to match the increased amount of oxygen, and that the computer can regulate the flow to an extent. So my question is why not use a larger injector to start with? If I am replacing all 8 injectors as part of a rebuild would it be better to get a larger injector that will, if nothing else, help to prevent a lean condition that can damage the engine?

Thanks,
Brandon

 

if you get larger injectors you have to then change the fuel injection map to open at a shorter interval or you will run rich.

You can do this and change the duty cycle but you loose resolution when you go too large and the window shrinks so tuning is harder

 

also if you go with too small an injector in a car that needs more fuel you usually run the injector at a longer rate and increase its duty cycle which will wear the injector out much faster and proves the point that you really want the correct sized injector for the right job.

 

Thanks for the quick reply.

 

Bigger injectors in an otherwise stock engine will NOT make more power....the AFR will be too rich and you'll have less power, poor fuel mileage and smog problems.....

Yes the injectors used in 928's are a standard size & they make HUGE ones....stock is 19lb...I've seen over 100lb injectors....

In highly modified 928's often 42lb injectors are used....possibly larger...but these are high boost or 6.5L engines too.....

 

yeah to your point Brian, I have to ask Brendan....

WHAT IN THE SAM HELL ARE YOU DOING WITH 72lb injectors! Making a rocketship or something, DAMN!

Maybe the E85 requires more fuel so it would need to flow more then gasoline, but 72 lbs are more than 3.5 times the size of stock ones.

 

The '85 - '86 cars use 25# injectors IIRC, and the '87-up S4's use 19# units. Many of us running mild boost are using 30# injectors which will also work in N/A cars if you remap the ECU.

Some nut-bar out there is rumored to be running 42# injectors in his huffed S4, but he drives a jet for a living...........

 

Thanks for the replies. The reason that I was asking was mostly curiosity about what it would take to run E85 if necessary. If I start from scratch, new fuel lines, seals, pump... would a car with 30# injectors be able to run premium and E85. It sounds like it would work but the timing would need to be advanced/retarded depending on the fuel and the fuel maps would have to be changed to match the fuel requirements. So possible, but still not easy.

Thanks again.

 

ok, time for an E85 thread

 

Do the early 85-86 cars really use a 25lb injector????

as a side note, its better to raise fuel pressure due to better atomization of the fuel charge when released. most of the mixture issues are determined by the MAF and ECU, and under cruise, the O2 sensor.other than WOT, the o2 sensor trys to make the fuel mixture pretty lean, near 14.7:1. this is a mixture for best fuel economy, and cleanest burn. there is no issue with a lean mixture, until you get in the higher power settings (near WOT). of course, real lean is even better for fuel economy (just before it starts to miss) but it would be very hard to find and keep this mixture constant, hence why modern cars do it the way they do now. Then, at WOT, things go open loop.
mk

 

Yes.

 

are they a drop in replacement for the S4?

I might want to start with them on the holbert car, especially if they are drop in and fit the same connectors. I am at 75psi on the fuel rail now, and who kows what the hp will be with the new 6.5 liter bottomend. If i expect only 400rwhp, then 25 vs 19lb injection at the same fuel pressure should be plenty.

mk

 

They're 24 # and have a barbed fuel inlet fitting, maybe other differencces, so not a swap fit. You can buy 30# Ford types on eBay/web that will fit with only one O ring swap.

 

Are you sure?

http://www.mailordercentral.com/928i...606%20120%2000

 

Here's a link that shows some cross references for injectors. It looks like the 928's are cross referenced with Ford Motorsport injectors and yes the 85-86's are 24 LBS. Looks to be some nice prices.

http://www.fiveomotorsport.com/Injec...sPORSCHEVW.asp