I wonder why Harley uses so much fuel pressure?

That link to the fuel injector calculator includes fuel pressure so you can see what pressure does to fuel flow thru an injector. Is the Harley regulator referanced back to intake MAP or is it a fixed fuel pressure type system. That pressure in the rails compared to danr near 0 psi in the intake on a down shift can lead to some reall short idle pulse widths. That is like 4.4 atmospheres of pressure drop across the injector on a down shift. Most systems are a fixed 3 atm. The wobble fire motor, potato-potato and bouncing idle pulse widths may all be tied into this one. The people that know about the dirty underware don't tend to share.

AW

 

Kent More makes a fuel pressure guage that simply snaps into the same quick disconnect fitting in the tank as the fuel line.

 

hernan, the best way to find out what the true injector pulse width is is with an osciliscope attached to an injector circuit then comparing it to your scanner/logger.

whittlebeast, the Harley EFI, both Magnetti Marelli and Delphi fuel pressures are non vacuum referenced. This may be one reason that actualair/fuel calculations do not always work out. On GM Delphi non referenced systems there is another table called an IFR (injector flow rate) thatindicates the actual injector flow rate Vs MAP sensor signal for proper A/F calculations. Harley Delphi probably has this too hidden somewhere where we can't get to it.

To my knowledge, all injector manufacturers flowrate their injectors at 100% duty cycle but sometimes use different pressures to rate them. The most common pressure and industry standardis 3 bar (approx 43.5 psi). Several compromises have to be taken into account properly sizing injectors, too large and the idle pulse width is too short not allowing them to fully open and close causing a rough idle. Too small and we have the same problem on the full throttle high RPM range with too long of a pulse width not allowing full opening and closing causing erratic fuel flow. Do not want them held open too long due to potential overheating of their electrical coils causing shorter life expectancy. Trying to achieve a lowerduty cycle to coincede with the intake valve open/close is risking a too large of an injector causing poor idle and potential driveability problems due to too short of a pulse width. Many tests that I have studied has concluded that their is negligible benifit at higher RPMs, WOT injecting fuel at the closed intake valve such as 80% duty cycle. Then there is so much air turbulence in the portsthat wetting is the least of the problems not to mention the latent heat principle being given a chance to work to cool and condense the air charge some, one benefit of a carberator.

Doc 1,one of the most valuable features we added to our dyno wasthe ability tolog and graph fuel pressure.We pin pointed more problems with it, it is amazing what goes onsometimes at the last few hundred RPM of aWOT pull when we are least likely to be looking at fuel pressure.

 

Fuel pressure @ 43 psi was done long ago but most manufactures have stepped it up to the 55 psi range back in 1995. It was found running the higher pressure allowed better fuel control for emissions but not enough more that hurt the injector life. With standard injectors you can get the pressure high enough that they will not work correctly so you need to watch how high you go. In an ideal world you would like the injectors to be on slightly longer than the valve is open as there is a volume of air sitting in the manifold between the injectors and valve. Once that air becomes saturated with fuel your in trouble as far as making power and emissions goes.

 

Steve, note that I stated "injector manufacturers" that are often ratedusing 43.5 psi pressure. I'm aware that vehicle manufacturers then take these injectors and operate them at higher or lower (Ford at 39psi) pressures giving them a differentflow rating. What this all has to do with injector timing I'm not sure.

 

My answers were directed at whittlebeast. He seems to think HD uses a lot of fuel pressure when its been the norm for going on 10 years in the industry.

The opening rate from when the injector is told to start Vs when it starts to flow fuel changes as fuel pressure is raised and lowered as do the closing rates. In low volume times it's a major effect on the injection timing but once the load comes up its just an increase in flow and atomization.

 

REF post 7:
"So there is a lot more to it than just duty cycle to worry about when properly sizing an injector".

Steve et al:

I posted a question about injector sizing and valve timing over on HTT. If you would care to elaborate it would be appreciated.

http://groups.msn.com/HarleyTechTalk...=0&CDir=-2


Injector Sizing: For Specific Intake Valve Position Targeting

Doing some research in this area.
Been reading DTT, Marren and Russ Collins' sites etc.
The Marren and RC sits don't specifically discuss what I'me looking for. DTT...good info on siamesed runners but...

Looking for additional knowledge and experiences.

While many of us know, the rule of thumb on injector sizing is to not go higher than 80% duty cycle etc. This of course being in reference to the injector having the fuel flow capacity to meet the engines needs while keeping the at or below 80/20 (open/closed) maximum.

What I'm inquiring about here...is to those that have knowledge of and/or done experiments with sizing the injectors to supply an X quantity of fuel in a narrow pulse width (shorter time) versus wide (longer time) for specific intake valve position targeting.

I.e. a narrow PW to inject the same X quantity of fuel only at the dynamic valve curtain...in lieu of a wider PW that would have be to long enough whereby the fuel spray would begin while the valve was still closed and pool there until the valve opened.

I've read too that's it's good to spray some of the fuel charge on a hot closed intake valve to help atomize the fuel. Spraying the fuel at the valve curtain would mean airflow into the cylinder would have already begun.

Thoughts?...experiences?

Also in the siamesed runner on the 315/405 firing deg HD engine...injector size can increase or reduce the amount of borrowing that takes place of the fuel charge from one cylinder to another.

Thoughts?...experiences here too?

I assume spray patterns would have effect on the situations above as well??

I understand too that too large of an injector can be detrimental in the idle and low speed areas of operation.

TIA

 

Well, just to give some feedback on this subject.

I've found out when I'm at WOT, the injector pulse goes to 100% (or 25ms) because probably I'm having some pressure issue.

I was riding 90mph when I tried to accelerate and the engine didn't follow the throtlle. Checked the AFR and it was at 19.5~20. The only problem is that this is intermitent, only happens sometimes, not always.

I guess is a fuel pump problem or bad lambdas...