Limit of 550cc injectors with a single walbro on a rb25?

[SuicidnS13]

Will the walbro and 550's support 450whp? According to DW's fuel calculator they will. But id like to hear from actual experiences. I dont plan on going any higher than that as I am only running a t3/60-1. So whats the most my single walbro 255lph can flow? And should I go with 650cc over the 550cc injectors?

[Shocker]

With 550's you can make about 500 at 100% duty cycle. I wouldn't do it so Id say low 400's with 550's would be a safe bet.

As for the Walbro granted its getting 13.5-14volts you can make 500-550 wheel with a bosta-pump even more. But your really maxing out its flow at that point.

[eh?]

WhatsADSM eh?
Modified by eh? at 7:10 PM 4/2/2009

[WhatsADSM]

Those online calculators don't account for the fuel pressure increase that you will see. You need to take your desired boost and plug that number + 43.5 into the fuel flow calculator. Then you can use the resulting number as a max limit.

Also I believe those calculators use crank HP, not WHP.

This is incorrect. Use your base pressure in the calculators (probably 43.5psi).A 1:1 regulator is keeping the effective flow across the injector at whatever you have your base pressure set at. Lets say you have 43.5 base + 20psi due to 20psi of boost. You have 63.5 psi of fuel pressure, however you have 20 psi of manifold pressure to work against. Hence the pressure across the injector is still 43.5psi (63.5psi - 20psi = 43.5psi).

As for the OPs question:The walbro 255lph and 550s will get you to your 450whp mark. Your IDCs will be very high, and the pump will be getting close to its limits, but you will make it.

Just as shocker stated, and it is especially true of an lph versus a hp walbro, you really want to make sure you have full voltage at the pump. I suggest you rewire the pump with larger gauge wire to make sure you have 13-14v at the pump.

So my suggestion is to at least rewire the pump, and if you have the cash some larger injectors would help bring the IDCs down into a safer region.

[eh?]

Whoops I stand corrected...

[Cjmartz2k]

I agree with the second half of what WhatsADSM said. As for the first part, it's a 1:1 rising rate, so the flow through the injectors stays the same, but the resistance the pump sees changes. If you were running out of pump, this could be a factor.

Bottom line though, I think it'll be close. I'd bet you'll make it seeing close to 100% IDC, but if you throw another walbro or bigger pump on there, you duty cycles might come back down a bit. Still 550cc is pushing it for 450rwhp. Your pretty much maxing both of them out dude.

[SullivanRacing06]

i maxxed a single walbro out on my 26 w 720s at 470 something, so i added another, im sitting around 550 whp now

[eh?]

i maxxed a single walbro out on my 26 w 720s at 470 something, so i added another, im sitting around 550 whp now

How much injector Duty cycle were you hitting at 470?

[SullivanRacing06]

this was on a neo afc, stock afms and stock ecu

[WhatsADSM]

I agree with the second half of what WhatsADSM said. As for the first part, it's a 1:1 rising rate, so the flow through the injectors stays the same, but the resistance the pump sees changes. If you were running out of pump, this could be a factor.

Bottom line though, I think it'll be close. I'd bet you'll make it seeing close to 100% IDC, but if you throw another walbro or bigger pump on there, you duty cycles might come back down a bit. Still 550cc is pushing it for 450rwhp. Your pretty much maxing both of them out dude.

The first part of what I said was just a response to what eh posted that's all. Just wanted to say that when you use the injector calculators you use the base pressure you set it to, not base + boost.

Otherwise I agree, if you are running out of pump all bets are off the table, but in that case you can't use the injector calculator at all because you hit a flow limit on another component.

But I have seen lots of cars make 450whp on a 255lph, again to be safest you will want to rewire it.

[eh?]

this was on a neo afc, stock afms and stock ecu

What were your AFR's at? I'm hoping to hit 500whp with a single 255.

[Shocker]

What were your AFR's at? I'm hoping to hit 500whp with a single 255.

You might make it you might not. With 14volts to the pump it might do it. VW guys have stretched a single walbro to its max and made over 500wheel with it. I would NEVER trust it however. Thats why I opted for an 044 inline as well as my 255 intank and Y'ed them together at the fuel rail.

You can always run dual intank walbros as well. If you need me to throw up some pics I can.

[SullivanRacing06]

at wot at full boost at 21 psi at peak power around 7100rpm i had a constant 10.3-10.6 afr, i still need to get a pfc so i can control timing

[WhatsADSM]

You might make it you might not. With 14volts to the pump it might do it. VW guys have stretched a single walbro to its max and made over 500wheel with it. I would NEVER trust it however. Thats why I opted for an 044 inline as well as my 255 intank and Y'ed them together at the fuel rail.

You can always run dual intank walbros as well. If you need me to throw up some pics I can.

x2, couldn't agree with this more.

I have heard of cars dynoing into the mid 500s on a single walbro255, however you are just asking for it doing that.

Eh, I would say if you got a little extra cash and can sport for another pump it would definately increase the safety margin. However if you are strapped for cash, just make sure you have it rewired and watch your AFRs closely, if you have a FP gauge then watch that as well.

OP (et al):Here is a nice graph I found courtesy of the 3si guys. It shows the flow rates of the various pumps, at various voltages. Also on the right hand side they have injector sizes converted to lt/hr.

In your case OP, assuming you have it rewired, you will be injector limited. The flow line of a 255 at 13.5v and 20 psi of boost is ABOVE the injector line for 6 550cc injectors. So IMO if your going to pay cash to upgrade something I would upgrade the injectors.

[SuicidnS13]

x2, couldn't agree with this more.

I have heard of cars dynoing into the mid 500s on a single walbro255, however you are just asking for it doing that.

Eh, I would say if you got a little extra cash and can sport for another pump it would definately increase the safety margin. However if you are strapped for cash, just make sure you have it rewired and watch your AFRs closely, if you have a FP gauge then watch that as well.

OP (et al):Here is a nice graph I found courtesy of the 3si guys. It shows the flow rates of the various pumps, at various voltages. Also on the right hand side they have injector sizes converted to lt/hr.

In your case OP, assuming you have it rewired, you will be injector limited. The flow line of a 255 at 13.5v and 20 psi of boost is ABOVE the injector line for 6 550cc injectors. So IMO if your going to pay cash to upgrade something I would upgrade the injectors.

Well the good news is I can still completely change my order. Will the bosch 044 pump be a direct drop in? I am definately going with 650's or 740's now so I have room to grow. I am also running Nistune on an rb20 ecu for tuning.