kep0ne

why is this such a necessary mod to do along with an S-AFC? im curious.

if you keep the 550s and run them at 85% DC, (which ive read is the max you should can run them reliably), you can still support up to 260bhp.

now if you put in 720s and run them at around 72% DC, its still only around 260bhp, and you're still close to maxing out the injectors correct?

so are 720s just to provide enough fuel effeciently? or is it cause the 550s just cant provide enough fuel at all?

someone explain to me how this whole fuel/duty cyle thing works...maybe im misunderstanding it.

and how does psi/cfm work with the injectors duty cycle? maybe i need some formulas to explain it in detail to me.

dp/mp/catback and cone intake with a ported s5 turbo, fcd, walbro. will get the 720s and S-AFC if i need it...but i need someone to explain to me WHY. not just "your motor will blow up"

ive read some people are running this setup and dont have any problems with fuel (scathcart)...

im confused.

kep0ne

anyone wanna chime in, feel free. but im just gonna get the 720s and tune em...no sense in going half assed.

another question pops up though. can you still run lean in the lower rpm range with 550 primaries?...even if you run them at 85% DC? someone explain duty cycle to me, i have a feeling i have it all wrong.

Ryde _Or_Die

Its reliability. Theres no set amount of fuel that every car of a certain type(88 TII for example) is going to need. Its always better to have too much than not enough(reliability-wise). Also, some cars need them, I believe I will once I push my boost up to like 14# so I already have them. Oh and you don't want to run at too high of a duty cycle or the injectors will not last as long.

No7Yet

The big issues are "because you can" and because of pre-3800 RPM full boost.

First, 720s are a pretty cheap mod that will support all the HP you can make on the stock turbo. You're already hacking the car up, so why not go ahead and install some injectors while you're in there? I can have a secondary injector in my hand in 5 minutes flat, and a primary in not much more time. Once you get the S-AFC in, you're going to be tuning the car anyway, so why not allow yourself to raise your performance ceiling to compensate for...

...full boost below 3800 RPM is potentially very dangerous. The stock ECU will NOT drive the primaries over 60% duty cycle before the 3800 RPM switchover to batch fire, so no amount of AFM signal compensation will drive them any harder. Because of this, you need to be able to flow all the fuel that you need at 60%. Don't ask me why Mazda did this, I don't know... they just did.

Brandon

kep0ne

short of replacing the primaries too, what can be done to prevent leaning out at lower rpms? does the walbro remedy this or will it still be a problem? and at what psi does full boost at a lower rpm become a problem? 8psi? 9psi? there has to be a concrete number.

dr0x

One way of ridding yourself of the 60% cap is to get rid of the stock ecu

cymfc3s

also, make sure if you get 720's that if they are low impedance, you will need injector drivers on your s5 to run them.

Ryde _Or_Die

Nope there aren't any concrete numbers for this, though its been asked over and over again...once upon a time by me. Its better to have too much fuel than not enough as I already said. You'll be fine with the stock 550s and a walbro for under 3800rpms with the stock turbo though.


Not sure if I missed him saying he had an S5 TII, but even if he has a mid 87 to a 91 TII he'll need to run resistors.

Spool Up

if i'm runnin the stock turbo/intercooler around 11psi. Will I be okay running 720 just for secondaries, or should I get 720's all around?

cymfc3s

you should be ok with 720's in the secondaries. 10-11 psi is about right.

Evil Aviator

If you mean the psi vs. cc/min, it applies to the injector flow rate. The ECU tells the fuel injectors to inject for a certain amount of time, which is usually measured in milliseconds. The amount of time that they stay open vs. the time that they rest is the duty cycle. If not given an appropriate rest period, the injector will stick open and/or burn out. Obviously, the more pressure that is in the fuel rail, the more volume of fuel will flow through the injector during that given time. This pressure is usually measured in PSI at the fuel rail, and this volume is usually measured in cubic centimeters per minute (cc/min), and at 100% duty cycle. For example, as per the Factory Service Manual, the stock TII "550cc" injectors are actually rated for 532cc/min @ 34.1 PSI, and 568cc/min @ 39.8 PSI at 100% duty cycle. If you average these points, you get 550cc/min @ 36.95 PSI. Therefore, you can see that the fuel pressure makes a difference in the actual fuel volume which is released by the injector. It is important to understand this, because not all injectors are rated at the same PSI. For example, RC Engineering and Bosch injectors are rated at 43.5 PSI (3 Bar), while Python injectors are rated at 40 PSI. Confused yet?

Here is a nice worksheet that you may like to play with:
http://www.rceng.com/technical.htm#WORKSHEET

If I didn't answer your question, and you really did mean "cfm", then these links will help with various formulas:
http://www.grapeaperacing.com/GrapeA...linjectors.cfm
http://www.2gnt.com/www/corbin/chooseinj.html

BTW, rotaries suck gas, so you may want to use about a .64 BSFC or so. Pump gas is about 6 lbs/gallon, depending on the temperature. Also, the rule of thumb to convert from cc/min to lbs/hr is to divide by 10.5. Note that formulas are great and all, but if you could build a perfect machine with just a slide rule, we wouldn't have such things as test drivers, test pilots, and beta testers.

kep0ne

sweet, thanks