*UPDate*So...lets talk about precision 96# injectors...

[forcefed3.8's]

I ran 8.94@152 with the same injector. Base pressure was 45psi, and boost was at 26psi. Idc was at 80+%

 

[fastblackracing]

id agree with you chuck but id be wrong
on his test at 80 psi rail pressure in atmosphere the injector was unable to lift and flow
i agree in boosted condition when open the injector is seeing 80psi FP less the boost pressure trying to come in so flow stays constant at base 43psi fuel pressure
when the injector is closed the pressure in the rail is what it is, the boost pressure cant help unseat the needle
if the injector wont open with 80 psi on the bench it wont open in a boosted enviornment

does the bench cleaner test at 13.5volts

The differential pressure is all that matters.
AG.

A while back I was not pleased with my 120 injectors after Chuck flowed them and ordered a set from Precision and was pissed when they arrived without a flow sheet as I paid a premium price and expected that info?

I was not happy with how these 120's acted after a few runs and replaced them with with 140's from a quality source and received lots of documentation with them.

Even before a retune, the 140's idled and ran better!

Had same thing happen at lower boost with msd 50s which are similar in style...

The same problem occurs with a lot of injectors when the inlet side is subjected to higher pressures. Some as low as 70psi. Pintle style genuine Bosch injectors or Siemens Deka 3105 or the 3145 have been fine in my car up to 85psi. I've seen pintle style injectors work with 100psi of pressure.


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Excuse my relative ignorance on the subject.

I see three seperate issues.
Size, pressure, and impedance.

Size:
If the injector works at the required pressure, and supplies the required fuel, there are no issues.

Pressure:
If the injector locks up below the required pressure, the injector size is less relevant.

Impedance:
Lower impedance injectors may be less suseptable to lockup to some extend because the supplied amperage for initial opening is higher. (???)

So . . The fact that some can run xxx injector to support higher mass flow could imply that they are doing it at a lower boost level . . Or lower fuel pressure. So a bigger injector may not be the aswer as the injector inlet pressure will be the same. The only difference will be a lower DC for a bigger injector . . . Unless of course, the base pressure is lowered as indicated by fastblackracing.

Increased voltage can also assist "stuck injectors" in some situations.

I am here to learn but believe that the variables need to be reviewed somewhat independently.

This is very interesting and made me take a look at whats going on. While I agree once the injector is opened up then the pressure differential makes an impact with the fuel flow, the thing that needs to be looked at is differential pressures on the valve that opens the injector.

The 95lb Precision injectors are ball and seat design. The MSD 50s are too. Easy to find multiple references on the web where these designs don't like very high inlet pressures, like the 80psi limit the OP found. The pintle design doesn't seem to be affected as much. Looking at the designs I don't think the boost pressure on the outlet side has much to do with it and its probably a function of the valve design and the inlet pressure.

Why? Take a look at the two injector designs:

View attachment 260367

View attachment 260368


On the outlet sides of both injectors you only have a very small surface area seeing the boost coming from the manifold side. The size of the pintle nozzle is very small, and the holes on the director plate of the ball and seat injector are also very small. I don't think its going to add much force to the valve even if the boost was 50psi. Now on the inlet side coming from the fuel pump you not only have a larger surface area, its fluid. 80psi of fluid pressure on the inlet side of the injector on a larger surface area vs 30psi of air pressure on the outlet side over a very small area. I think the boost pressure has a minimal affect on the valve action.

Now when you look at the different designs, for the ball and seat style you got all this fluid surrounding the surface area of the ball working against it while the pintle nozzle being a cylinder the pressure is mainly coming from the sides and not working against it opening. At high inlet pressures it makes sense that the ball would be harder to unseat compared to the pintle nozzle.

Also coming into play would be the coil strength to lift the valve up. High Z injectors usually are weaker due to the lower current. So running high impedence ball and seat injectors at high boost would seem to be of more concern, for example the MSD 50s. Other high z injectors with bad designs or weak coils at high boost would be of concern too. The 96lb injectors are low z and should be better but as the OP found out they can fail too.

Obviously I'm just trying to figure out whats going on and there may be more to it, but its something I never looked at before but definitely has me thinking now. Just put a set of MSD 50s in my truck too darn it.

The injector machine does not take boost into consideration. Therefore, the nozzle end of the inj does not see 32psi.
43 base, + 32 on the reg = 75# fp. Boost working on the nozzle end "counters" the pressure rise, {32}, created by boost on the reg. Therefore: 43+32-32 = 43. Now the injector is working in a 43psi environment.
BTW, I've seen this frequently on the Delphi inj designs....

........OK guys here what we have now, we put the 120# injectors in last and and did some re-tuning last night and this morning. we set the base fuel pressure at 35 vac line off and worked from there. First let me say that these 120# injectors have a little bit but noticeable sharper throttle response and better low speed drive-ability than the 95's........This is with only 2 times out tweaking them....... Now on to the good stuff With the boost at 34# it pulls all the thru completely clean with a very steady 11.5 -11.6 AFR. No missing or cutting out very nice clean pull. So as we were expecting the 96's are not the right injector for me or most likely any one who is going to run over 75-76 pounds rail pressure. And these are the 120# injectors from summit under the Trick flow banner......cost was 300$ for 8 quite a bargain if they work out as good as the initial tests have shown. Final settings were 34# boost 17.5* timing 69-71% duty cycle pulls were done in 3rd gear from 5300 to 6050 rpm.

 

[Jerryl]

Congrats and thanks for sharing.

 

[ttypewhite]

Now on to the good stuff With the boost at 34# it pulls all the thru completely clean with a very steady 11.5 -11.6 AFR. No missing or cutting out very nice clean pull.

I am reading this correctly? Your at 34 pounds of boost and only 11.5 to 11.6 air fuel??? That is waaaaaaaaaaaaaaay off and running super lean as hell. Your air fuel should be north of 10.8.