Pitot Tube

[Duckndive]

Does a Blown Fi system need a Pitot tube in the Plenum  ? 

[Arttu]

No. It needs just reliable boost pressure sensing and then fuel delivery can be programmed to whatever is needed. Generally speaking.

[Duckndive]

1 hour ago, Arttu said:

No. It needs just reliable boost pressure sensing and then fuel delivery can be programmed to whatever is needed. Generally speaking.

Thank you 

[jonny1bump]

Its there to pressurise the float bowls Clive on carb bucket, why so important to get right on boosted carbs.

[Duckndive]

2 hours ago, jonny1bump said:

Its there to pressurise the float bowls Clive on carb bucket, why so important to get right on boosted carbs.

yes i get that i was just unsure if it was needed for Fool Injection 

[Slabbie11]

You have the option of either running fixed fuel pressure or manifold pressure corrected fuel pressure.
 

If you run manifold corrected then it’s basically the equivalent of a Pitot as you always get xxpsi relative to whatever is within the manifold either pressure or vacuum. This is slightly more involved as you have to plumb a reference hose to the manifold which also drives the need for an external fuel pressure regulator.

The more modern approach is to use a fixed pressure “dead head” system which runs a high enough static pressure to allow for boost (many systems run at 58psi or above) & avoid vaporisation. There are also closed loop systems with pump controllers but we don’t need to go there right now.
 

You can calibrate (map) the fuelling to suit either scenario but switching between once complete will mean a remap.

 

I’ve personally stuck with the old school manifold referenced method as it means I can run lower pressure most of the time (not on boost) which keeps the electrical loading of the fuel pump sensible during normal running & ramps the pressure inline with boost. Probably not so much of an issue on an oil cooled motor as the alternator is decent even at lower speeds but as we know the air cooled ones need all the help they can get!

 

S.

[Gixer1460]

1 hour ago, Slabbie11 said:

The more modern approach is to use a fixed pressure “dead head” system which runs a high enough static pressure to allow for boost (many systems run at 58psi or above) & avoid vaporisation. There are also closed loop systems with pump controllers but we don’t need to go there right now.

You can calibrate (map) the fuelling to suit either scenario but switching between once complete will mean a remap.

That's a new one on me! I'm aware of dead head fuel systems, generally on NA systems and they run at a set regulated pressure (around 3 bar) But running a pump dead headed permanently at, say 5-6 bar pressure, to cover 2-3 bar boost, not only puts huge load on the pump, i'll overheat the fuel and it would likely be almost un-mappable due to pressure in rail pulsing from 5 to whatever pressure, due to injectors variable opening times. This doesn't sound like a sound prospect - all to save on a plenum air pressure tapping and a boost referenced FPR - a tried and tested solution!

[Slabbie11]

C5 onward Corvette is one example which springs to mind & there are many others which run the fuel pressure reg built into the fuel filter (many of the German OEMs) near the tank which operates without any vacuum ref & then has a single line which runs up to the fuel rail. The standard Suzuki FI systems have the reg inside the tank but do admittedly run at a lower pressure than the cars.

Certainly not ideal for fuel heating but then the counter argument on cars at least is that the older style feed & return means that fuel has to cycle through the hot environment of the engine bay, the fuel rail itself before going through the reg & back to the tank. Side effect is that it keeps the fuel system components “cooled”. Jag actually had a small heat exchanger on the AC loop to cool the fuel on some of the Federal V12s…a lot of heat & exposure!

When considering high line pressure it needs to be viewed in context as 4 bar rail with 1 bar boost is only 3 bar across the injector. Although injectors are still rated at lower pressures they are designed to work at the higher pressures in service so controlling fuelling isn’t really a problem. That wasn’t always the case though as the old method of ramping fuel pressure & decapping injectors with old tech injectors ends up with some very non linear results!

 

Stew.

[Arttu]

As you said fixed fuel pressure is widely used on OEM cars and bikes but mainly on NA engines. It can work also on boosted engines but higher boost you use less ideal it gets. Effective fuel pressure drops as th boost goes up and naturally fuel flow of the injector drops too. So to compensate that you need either bigger injectors or higher fuel pressure. This will make idle and low load tuning harder than what it would be with boost compensated fuel pressure.

And to clarify one quite common misbelief, the purpose of a 1:1 boost compensated fuel pressure isn't to provide any extra fuel for boost. It just keeps the effective fuel pressure constant so that the injectors flow the same amount of fuel per time regardless of the manifold air pressure. All boost enrichment must happen by increased injection pulse length commanded by the ECU.

[Slabbie11]

The old bodge of rising rate fuel pressure regs to attempt to fix inherent issues! Or even fix imaginary issues in certain circumstances…that’s marketing for you!

S