I was just woundering if 650s would flow enoufgh fuel for like 23 PSI combined with a walbro 255lph fuel pump of course?? thanks much and if those arnt big enoufgh what size should I buy and how much PSI can you get out of them???

 

thats not a problem. stockers might even get you there unless your heavily modded. if you upgrade injectors atleast go with something in the 700's. might as well be prepared for future mods.

 

You don't need an injector upgrade.

 

The stock 550s with a fuel pump upgrade should be adequate. Based on my observations the Walbro will raise base fuel pressure from the stock 43psi to 50-52psi. This effectively raises the output of the stock injector to approximately 600cc. The fuel pump alone may provide enough injector duty cycle headroom to avoid running above 90% IDC.

I'm often surprised to see blanket statements regarding the adequacy of a given fuel injector size. A great deal of the decision has to do with the volumetric efficiency of the motor and the state of tune. If you've got a lot of "breathing" mods and are running super rich, you'll run out of injector sooner than if you were properly tuned.

 

Yeah but you really should get a FPR then if the BASE Fuel Pressure gets too High. That means the Stock FPR is over run and there for you do not have linear Fuel control such as 1PSI of Fuel per 1PSI of boost.

If what you say is true, 43PSI to 52PSI, you will have Static Fuel Pressure until you hit 10PSI of boost on the Turbo. You need to lower it to 43PSI at Idle and have it raise in a Linear fashion

 

I agree, the stock FPR is probably being overrun. Although I don't know for certain since I don't have a fuel pressure gauge running all the time. My data is based on some troubleshooting I did with a shop gauge.

Regardless, the majority of folks upgrading to the Walbro are operating with this condition and probably have compensated for the non-linear delivery of fuel via flash or some other tuning tool. I'm not saying it's the best situation to be in but it will work if properly tuned for.

Also, there's nothing wrong with running a higher base fuel pressure as long as you have the means to control the rise in a linear rate. In fact raising or lowering fuel pressure using an adjustable FPR can be a means for affecting AFR across the entire load range. If you do this, just don't assume that a mail-in base flash is going to work as expected. It will prolly require a custom tune.

 

I run 30psi w/ stock injectors... too many people talk too much about upgrading injectors on stock turbo applications... it's just not needed as long as you have some sort of engine management (to bypass fuel cut)... hell, over-sized injectors on stock turbo could even hurt you.

 

Actually, this will be the case only temporarily. Why? Because as soon as the ECU goes into closed loop, it will detect the variation in A/F and will compensate accordingly. This correction is global, and affects both closed and open loop operation.

 

Sould would an Aftermarket FPR help then? Set at 43.5PSI? BTW AL tested the Stock FPR and he found it to be at 52PSI at Idle, can't be good

 

I don't see where an aftermarket FPR is necessary unless the stock FPR demonstrated fluctuating pressure, or if it proves to be unable to adequately control pressure at WOT.

If the FP is 52 psi at idle, the ECU will correct the A/F. It can only correct so far, after which it will flash a CEL and flag a code. The upside to increasing pressure is that injector capacity is increased, although the relationship between capacity and pressure is not linear. The downside is that as an injector's pressure deviates from spec, atomization quality is diminished. These reasons are primarily why whether you're making 300hp or 1000hp, it's always best to run injectors at their design pressure - which is the stock FP settings.

 

Absolutely. I've been geared in my head for running a standalone. The approach won't work with the factory ECU.

I didn't realize LT & ST fuel trims affected open loop as well. Interesting tid bit.


I agree. If the characteristics of the stock FPR are predictable and repeatable there's no reason not to simply tune for the increased fuel pressure. It might not be a linear scale across the load range but the fuel map should work fine once perfected. I'd rather spend the $200-$300 on a good custom tune than on an FPR.

 

Isn't it better to Run the Stock Injectors with the Base Fuel Pressure they were Intended to? I mean 43.5 At idle instead of 52+?

 

Yes, it's always better to run at as close to design pressure as possible, especially if you find yourself experiencing irregularities due to abnormally high pressure.

 

It'd be great if you could measure it and find out for sure. Any good shop should be able to check it.

52psi is roughly 20% too high which means 10% too much fuel. That might just be out of bounds for the ECU at idle. If the problem really is that the stock setup can't return enough fuel it's not quite as bad as you describe. As more fuel goes out the injectors the return line becomes less of a bottle-neck.

Dave

 

Back in early December I had a shop check system fuel pressure since I suspected that I might have had a bad fuel pump leading to a run-lean condition. I have a Walbro 255 installed. At that time they discovered that the base fuel pressure was 52psi. I'd be interested to hear if anyone else has any data.