I've been doing quite a bit of research on injectors.

I'm using this calc here:

http://www.smokemup.com/auto_math/fuel_injector.php

My 383 is supposed to put down ~425-445 rwhp, so figure 500-540 flywheel horsepower. Advanced induction heads etc.


1. I see everyone saying "Get the SVO injectors, they're rated at lower PSI, so with an LT1, they will flow higher because the LT1 runs a higher fuel PSI".

However, if I run an adjustable fuel pressure regulater, all benefits of the SVO injectors lower PSI rating are negated no?

I understand that they're still good injectors, made by Bosch. If that is the case, then I should just get some Bosch injectors for a good price?


2. Also, what about pintle vs. disc injectors? Racetronix says that disc injectors are superior to pintle, but of course people say both sides to the story.


3. What about brake specific fuel consumption? I'm running the calc with 80% duty cycle and 520 HP. At .45 BSFC it says a 36.56 lb/hr (36# injector). With a BSFC of .50, it says 40.63 lb/hr (42# injector). I'm also going to be using a custom tune & OBD-I conversion from Madwolf. Will the AFPR and custom tune allow changes to be made to run a 42# fine? If I decide to run nitrous in the future, I'd be SOL I think with a 36#, since a 36# is already 80% DC at my proposed power levels...

4. Finally, is it worth the extra ~$80 or so for flow-matched injectors?

Thanks!

 

I've crunched a bunch of numbers here as well, and made a combo chart in Omniform that allowed me to input the #/hr rating of the injector and figure other variables such as boost, fuel pump output, duty cycle, and BSFC.

To try & answer what you are asking:

No, injectors are what they are. If you have more PSI, they will flow more fuel up to a point. I ran my chart from 43.5-60 PSI. In this example, a 36 #/hr unit @ 43.5 PSI is equal to a 42 #/hr unit when run @ 60 PSI.

I have not done any research on this issue. I would go w/ Racetronix on this. He seems to have a great deal of expreience with fuel components for our cars.

A lot of "Kentucky windage" here . As with most things, you should utilize a range of power estimates/fuel requirements for low and high BSFC. I started out w/ .60 on my calculator, then settled w/ .55. Most blower (SC) formulas used either .50 or .60, and NA was always less than SC. Bottom line here, buy as big an injector as you can afford based on a worst-case scenario. I started w/ 42 #/hr before I did enough research, and ended up selling those and am looking at a set of flow matched 57 #/hr units @ Racetronix.

I think so. I have not been able to find an unmatched set for the high Z 57 #/hr units anyway.

Hope this helps.

 

Thanks for the info!

I see what you're saying. What I meant was people buy the SVO injectors because they're rated at 39.x PSI, so when installed in our cars which run 43.x PSI, it automatically flows a couple more lb/hr...

But, with an AFPR, no need to specifically buy SVO injectors, because I can set the FP to whatever I want...just wanted to make sure.

To go with that, how much fuel pressure is TOO much? As you increase fuel pressure, you're also increasing the duty cycle right?

It looks like a 42# is what I should buy then...

I can get Bosch 42# for $279. The Racetronix Lucas 42# flow matched are $400! Not sure which is more benefit, not something to skimp on, but of course I don't want to spend money that I don't need to spend.

Damn cars.

 

1. This factor is usually limited by the pump & delivery system (i.e. nylon hoses that normally carry 43.5 PSI, do you want to run 75 PSI through them?). Fuel pump output (i.e. Walbro GSS307 & 340 in tanks) really drops off @ higher pressures.

2. Good question- I don't think this is the case, as one does not affect the other. If I am not mistaken, the DC is controlled by the PCM.

On my chart, I used the 36 # units @ 60 PSI. With the duty cycle set at 80% (which is what I used for the 36 #/hr also), they flow 42 #. My chart is not set up to show the #/hr for upping the duty cycle, just the fuel needed. In this example, the 36 # units @ 43.5 PSI need 145 LPH with a .55 BSFC. The same injectors @ 60 PSI need 169 LPH. If I change the DC to 90%, the LPH changes to 163 & 191 respectively.

I agree it would be hard to justify the $400 over the $279 for most of us. I'm looking at $500 for the Siemens units I need, so they are a while off in the budget.

 

I'm going to get the Racetronix kit w/ the Walbro 255 Lph pump in it.

I was reading up on AFPR's on LT1's, and what people are saying is they don't even matter on the 94-97's, because the PCM takes care of it. Is that just on STOCK 24 lb/hr injectors? Or any injector we run?

I'm doing OBD-II -> OBD-I conversion w/ a Madwolf tune. Is FP adjusted in that programming? Or will I need an AFPR?


I'm going to reread what you wrote about 176 more times to see if I can make heads or tails of it.

 

Duty cycle is not "set" by anything... it is a calculated value. It is the pulse width of the injector at any point in time, divided by the number of seconds it takes for the engine to complete two crank revolutions. At 6,000rpm, it takes 0.020-seconds (or 20 milliseconds) to complete 2 crank revs. That's the maximium time available that the injector could spray, assuming it was open all the time. If your PCM is calculating a 16mSec pulse width at 6,000rpm, your duty cycle is calculated at 16/20 = 80%. A larger injector in the same engine will yield a lower duty cycle.

When you go to nitrous, you won't necessarily be "SOL". If you use a wet nitrous system, you size your injectors for the NA power level. The wet system adds its own fuel, not through the injectors. OF course you fuel pump needs to supply all the fuel, but the injeoctrs only supply the "NA" fuel. Even if you go to a dry system, some of them use a sidestream from the nitrous line to apply a pressure to the FPR and boost fuel pressure while you spray (e.g. NOS 5176).... but I wouldn't use that approach on a setup like yours.

How high a pressure you can run depends on the type of injector. The pintle style will handle very high pressure. The stock ball and seat type will not. Not sure about the disc type. Most of the high performance engines I have seen are using the Bosch low impedance injectors at fairly high pressures.

You do NOT "tune" with an AFPR. It is useful to allow an injector to produce a higher flow rate (I run Bosch 64's at 58psi = 78#/HR). When you have your PCM tuned, you have to tell the tuner the injector size you are using, the injector manufacturer, and the fuel pressure you intend to use. The programmer enters the correct flow rate and offsets in the PCM for that specific combination. If you attempt to alter the fuel pressure after that, without changing the constant in the PCM, (and assuming you are are still running a program with closed loop enabled) the PCM will simply alter the pulse widths to compensate for the increased or decreased injector flow capacity.... doesn't matter if you are running stock 24's, or some other brand 42's... once you set the constant in the PCM, you do not want to diddle with the fuel pressure.

And fuel pressure is a FACTOR in arriving at the injector flow constant for the PCM programming, but the pressure is not an actual part of the program, and the PCM can not adjust pressure.

 

Whoa.

I'm going to reread that one a few more times as well and hit it up in the AM.

Thanks for the input Fred!

 

Dave,
there's lots of very useful info in this thread, but let me ask you just one question. Why complicate things? I mean, the SVO injectors are well tested and lots of people use them with great results. Just buy a set of new 36#ers and be done with it.

Just my 2 pieces of copper.

 

I'm not complicating things, just want to make sure I"m getting exactly what I need for my money.

Not saying I NEED or WANT an AFPR, looks like I misinterpreted what it was used for...

So from what you're telling me, get 36#ers, and if they for some reason don't supply enough fuel, or they are near maxing out the duty cycles, then through the PCM you can increase the fuel pressure, and knock the DC back down in the injectors?

I also wasn't trying to get AWAY from SVO injectors, I just want to know why pintle is better than disc or disc better than pintle...

Also, I'm not 'lots of people', I'm Dave **** Lacerda, and I'm talking about ME!

Thanks for the info man,
Dave

 

In response to Fred's post, which was articulate and informative (as usual) I have one minor point of difference. An AFPR can be considered as a "tuning" aid, at least the way I would use the term. I use them that way pretty regularly. It's one of the first things I go to when tuning a new combo when the car is way too lean at WOT. Crank the FP up and then make further adjustments in software when it's close. Obviously, this will require going back to tuning for light load operation as it may now be too rich. I see what Fred means when he posted "You do NOT "tune" with an AFPR. It is useful to allow an injector to produce a higher flow rate" but to me, that IS tuning.

Just semantics I guess. BTW: my base FP is 55psi and I see 73psi under boost. At that FP, there was a lot of "tuning" needed to get things right at low load. The injectors are big enough to back off the FP, but it works fine this way so why mess with success? As mentioned, you need a lot of pump to flow large amounts of fuel at elevated FP. Some people think that the elevated FP improves atomization of the fuel, which is a potential benefit.

I'm not sure if this post adds anything to the original question, so on that note: flow matched injectors don't make sense to me. The injectors I have seen don't vary much (unless you get a "dud"). So, just flowing them to see if they are in the ballpark is good (to weed out duds). But otherwise, a few percent difference is trivial. You are only tuning for an "average" AF ratio anyway on a typical LT1 setup.

I'd say get 42# injectors, that way if you need them you have them. There is really no downside as far as I am concerned. Don't go wild with the calculations. Too many unknowns to do otherwise than use them as a "ballpark".

Rich Krause

 

I didn't think that SVO 36# were still being made

 

I made a BADASS Excel spreadsheet today but I forgot it at work!

I'll post it up tomorrow.

More questions:

What is considered "high" pressure? I thought I've seen not to really take it over 60 PSI?

I understand now about the PCM, thanks! Just have the tuner set it all up in there when setting the tune.

I will either be running a pintle or disc, not sure which, but definitely not a ball style.

Guess it depends what I can get for the price.

I was dorking around with some calcs today, and I think what I saw happening is that as FP went up, DC went down?

Rich, in ref. to your matched set comment, was that based on observations of multiple injector sets being tested, and most coming out to pretty close flow #'s to eachother? Wondering how you came to that...

Not sure if SVO 36#'s are still made, but you sure can buy them still. Bosch makes the SVO injectors, and according to New Era Performance, the Bosch injectors are rated at 39.2 PSI as well...if they are correct. I was told by a distributor though that Bosch is doing a redesign or somethingorother on their injectors, so you maybe right that they could become harder to come by...
http://web.camaross.com/forums/showt...hreadid=243496

I wish I could get to this site from work to pull up the info.

 

I have flowed a lot of injectors and if you buy a "set", assuming they haven't done something stupid like giving you an odd-ball, they are pretty close. The only reason I flow them before use is to be sure there are no odd-***** or lemons. I really don't care about small differences between the injectors for the resons that I mentioned.

Rich Krause

 

OK I see now. Just wanted to make sure it wasn't just based off a couple sets you and a buddy bought or something (No discredit meant).

What's it cost ~$ to get them flowed?

 

So when Datamaster says the injectors are at 104% duty cycle, are they actually just staying at 100%? They can't really stay open 104% of the rotation.