Slant Six Forum
https://slantsix.org/forum/

Injector size vs. fuel pressure relationship?
https://slantsix.org/forum/viewtopic.php?t=25773		 Page 2 of 3
Author:  CARS [ Sat Nov 10, 2007 9:43 am ]
Post subject: 
http://www.thirdgen.org/tbi-afpr

Couldn't find that "quarter mod" article but this little write-up explains how to modify the regulator to make it adjustable.

(of course, if the Chy TBI is different from GM's TBI just ignore this :lol: )
Author:  sandy in BC [ Sat Nov 10, 2007 10:13 am ]
Post subject: 
They are different.

The modified stock FPRs I use are pretty crude...( I did shim with a dime my first time) I used to make them from a stock FPR by drilling out the tamper plug and filing a wrench hold on the shaft.

The store bought ones sub in an Allen screw for my filed shaft and charge you $60. The store bought ones come with a spring set for different pressure ranges I use the 10-30lb spring adjusted to 13.5 psi
That is not the same as the 0-15lb stock spring adjusted to 13.5 lbs.

That 3rd gen site you posted is good stuff. My Rochester 220 TB is basically an over bored "Ãœltimate TBI" setup with the COP Car injectors. This is installed in a high torque, Vortec head, roller cam SBC off road 4x4 towing rig . The entire setup is fuel limited.

I plan on a stock 220 with 55lb injectors on an offy manifold for the Valaints next incarnation.....
Author:  SlantSixDan [ Sat Nov 10, 2007 11:58 am ]
Post subject: 
Quote:
It sounds like Dan may be trying the vacumn regulated FPR.
Correct. That's the type that came stock on the '88-'91 Dodge truck/van TBI setup. Throttle body made by Holley, fuel pressure regulator made by Bosch. And then we get to discuss injectors...these parts were used for only four model years, but there were lots of supersessions. I show at least seven numbers for the 3.9 injector, three for the 5.2, and thirteen for the 5.9. Some of them ball-and-cone, some of them pintle-type. Some made by Bosch, some made by Holley, some made by Lindy-Lou's Rare Orchid and Fuel Injector Boutique, and who knows, perhaps some made by Shanghai Guangdong Injector Traffic Industrial Concern.
Quote:
I never had any luck with mine doing anything other than bouncing my fuel pressure around and causing puter fits
But you're using custom computer calibrations, and I'm still using the stock Chrysler SMEC which is expecting a vacuum-regulated (guess "rising rate") FPR.
Quote:
Keep in mind these factory puters are very slow learners.
And these particular ones I'm working with are "special needs" students. :roll:
Quote:
I always "kill the puter"by disconnecting the battery before doing any tuning.
Ditto. Also helps improve safety since I'm working with open fuel in the engine compartment.

After some puzzling initial difficulty getting the new FPR to seal, the 21.4 psi regulator and 3.9 injectors (Holley-made, ball-and-cone type) are in. I verified no-leak (yes), engine start (yes), run (yes), idle (yes), and rev (yes), then shut it down and pulled a ZFR5N spark plug to take a look: Clean but worn...no sharp edges and a gap of about 0.050". Installed eight good used BPR6EIX-11s (NGK's super ultra dydimium irridium ones) that I had kicking around in the toolbox, gapped at 0.040". Went around the engine bay and hit all the electrical connections with Stabilant-22a. Reinstalled the battery, verified start-run-idle-rev, came in and cleaned up.

So it cold-idles smoothly on all cylinders. Does it run better? Will it pass a smog check? Will it run well in closed-loop mode? Donno, wait and see. I'm about to go grocery shopping, so should have more news soon.
Author:  SlantSixDan [ Sat Nov 10, 2007 2:47 pm ]
Post subject: 
Update: The truck runs and drives fine. The idle's a little smoother overall, and the bizarre, idiopathic shaky idle in Reverse is gone. Part throttle behaviour is a little better than before. Full-throttle acceleration is noticeably stronger. The exhaust smells maybe somewhat cleaner (real scientific, eh!); we'll see what a smog check shows. Before I do that, I would like to replace the "good used" spark plugs with new ones. I can't remember why I took these out of the truck several years ago, but I did, and there was probably a reason; this ain't no slant-6 (changing plugs is something of a nuisance).

Thanks for the suggestions and advice.
Author:  Sam Powell [ Sun Nov 11, 2007 5:34 am ]
Post subject: 
Hi Dan, That was an interesting exercise and very appealing in nature. Would you pardon my laziness in asking you to cut and paste your start and finish specs side by side. I would like to see the "after" specs beside the chart you provided in your first post. Thanks.

Sam
Author:  SlantSixDan [ Sun Nov 11, 2007 6:55 am ]
Post subject: 
Whatever info I have, I will be happy to post. You're after before-and-after specs...emissions-wise?
Author:  Sam Powell [ Mon Nov 12, 2007 8:59 pm ]
Post subject: 
I am not sure how you can actually compare what I was hoping to see. I think I have seen flow charts for various size injectors at various pressures. I was looking for a comparison of the flow rate of the smaller injector with the higher pressure compared to the larger injector at the lower pressure. Part of the figures would be theoretical I suppose. Any subjective feed back would be interesting. Does it idle better? In theory it should. Does it have better power? If it does, you have a win-win thing there. How does the economy compare? Any noticeable changes? It would be nice to know what your duty cycle was before, and is now, but your ecu keeps that information to itself.

Do you know what conditions are subject to feed back from the O2 sensor? Those conditions that are in feed back, or closed loop should , in theory be providing the same fueling as before. It would be interesting to know if the newer injectors are working at a lower percentage of their full capacity than the old ones. As a rule, you want to keep the injector duty cycle at 80% or lower to avoid starvation of the engine under extreme load.

Sam
Author:  SlantSixDan [ Tue Nov 13, 2007 11:04 am ]
Post subject: 
Quote:
I was looking for a comparison of the flow rate of the smaller injector with the higher pressure compared to the larger injector at the lower pressure.
Ah! I got that info on the fly by applying the info I posted in the first post in this thread, to the third calculator down on the page CARS linked to in the third post in this thread, to get:

Stock injectors @ stock 14.5 psi pressure: 63.3 lb/hr
3.9 injectors @ stock pressure: 53.4 lb/hr
3.9 injectors @ 21.4 psi: 64.9 lb/hr
3.9 injectors @ 22.2 psi: 66.1 lb/hr
Quote:
Part of the figures would be theoretical I suppose
A large part, because this system uses a vacuum-regulated fuel pressure regulator, so the above are flow rates at zero manifold vacuum. Any time there's manifold vacuum, it's applied against the FPR's spring, so the effective spring pressure is less than the FPR's nominal rating.
Quote:
Does it idle better? Does it have better power?
See my post a couple above this one. The one that starts with Update: :mrgreen:
Quote:
How does the economy compare?
Unknown as yet. That'll take a fair bit of time to get a meaningful figure.
Quote:
Do you know what conditions are subject to feed back from the O2 sensor?
everything except idle and WFO, whenever the coolant and O2 sensor are above their respective threshhold temperatures.
Quote:
As a rule, you want to keep the injector duty cycle at 80% or lower to avoid starvation of the engine under extreme load.
Yeah...given that the theoretical max flow from the smaller injectors with the higher pressure is greater than that from the stock injectors with the stock pressure, I don't see much risk of starvation. WFO acceleration is noticeably snappier, and there's no pinging, so I think I'm probably OK on that front.
Author:  Twisted Sixer [ Thu Apr 10, 2008 1:04 am ]
Post subject: 
The reason reason older port fuel injected engines use vacuum regulated fuel pressure regulators is to save the slow old dog of a computer and the programmer a little work. The regulators are not rising rate but 1/1 the pressure drop is equal in the manifold and in the fuel rail so they squirt the same amount without the computer having to calculate the difference in flow rate.

If I'm reading this right and the TBI motors are using vacuum ported regulators it would almost have to mean the injectors are too big to idle so they did this to decrease the flow and wrote the extra code into the computer to compensate. If thats the case would the injector flow still match what the computer expects at higher engine vacuum?

(edit) I don't own one to go out and check but from the looks of my 91 FSM the vacuum hose ataches above the throttle plate and wouldn't have vacuum and the manual stated that pressure is regulated to a constant 14.5psi
Author:  SlantSixDan [ Thu Apr 10, 2008 5:34 am ]
Post subject: 
The fuel pressure regulator hose sees manifold vacuum; the nipple is above the throttle plate, but it's ported to below the throttle plate.

I may wind up going back to stock injectors and fuel pressure regulator; I've got a pronounced idle/cracked-throttle surge and a code 52 (O2 sensor indicates rich mixture). Strange, especially since the truck passed an emission test with excellent numbers. I know the O2 sensor isn't dead (I can watch it switching on my scan tool). Seems the bonebrained SMEC can't handle this change in injectors and fuel pressure, no matter how minor it appears on paper. The other possibility is a not-dead-but-flaky O2 sensor that just coincidentally happened to get that way after I did the injector and FPR swap. Not too likely, since it's a good-quality sensor and not very old.

It is most likely this is a case of a turd not responding to polish.
Author:  Twisted Sixer [ Thu Apr 10, 2008 5:15 pm ]
Post subject: 
I punched some arbitrary numbers into the calc on the megamanual page just to show what happens when you change injector sizes on something that the computer has to compensate for the pressure change.

For simplicities sake I just used the difference between 21psi and the stock 14.5psi then subtracting 6.5psi to show the drop a stock injector would have at that vacuum.
(wich I believe would be around 13 inches)

V6 injector 64.3 at 21psi......V8..63.3 at 14.5....v6 flows %101.579

..................53.4 at 14.5.............47.0 at 8.0.......v6 flows %113.617


So at high vacuum the flow would be off by over %13 from what the computer expects. If the mopar computers "learn" the way stock chevy ones do (from what I recall) it would lean the entire curve out in the process of trying to fix this all the way into open loop WOT.


I noticed Sandy mentioning never having any luck with this type of regulator did Mopar use something different at some point or ?

Also if you have trouble with bouncing as Sandy mentioned you can try "filtering" the vacuum signal by putting a restriction in the vacuum line.People with bigger cams do something similar to this to their MAP sensors to get the car to idle.
Author:  SlantSixDan [ Thu Apr 10, 2008 7:47 pm ]
Post subject: 
Quote:
So at high vacuum the flow would be off by over %13 from what the computer expects.
...except that the behaviour doesn't change at all when I remove the vacuum line from the FPR altogether so it's not seeing any vacuum at all.
Quote:
If the mopar computers "learn" the way stock chevy ones do (from what I recall) it would lean the entire curve out in the process of trying to fix this all the way into open loop WOT.
The computer does learn. The strange thing is the idle/cracked-throttle surge only appears _after_ the computer has learnt. If I clear it and then start the engine...no surge.
Quote:
"filtering" the vacuum signal by putting a restriction in the vacuum line.People with bigger cams do something similar to this to their MAP sensors to get the car to idle.
Now this is likely a very good idea. Sudden vacuum changes really confuse this engine management system — drop the truck into Drive and the sudden load is an obvious problem. I wonder what size restriction is called for.

Anyone?
Author:  emsvitil [ Thu Apr 10, 2008 11:50 pm ]
Post subject: 
Quote:
Now this is likely a very good idea. Sudden vacuum changes really confuse this engine management system — drop the truck into Drive and the sudden load is an obvious problem. I wonder what size restriction is called for.

Anyone?
Get an aquarium air valve and you can adjust it...........
Author:  SlantSixDan [ Fri Apr 11, 2008 6:42 am ]
Post subject: 
Mmmm...I can see how that'd work as an adjustable air bleed, but how would it work as an adjustable vacuum orifice?
Author:  Matt Cramer [ Fri Apr 11, 2008 8:27 am ]
Post subject: 
The usual restriction I've seen is to use a combination of a fuel filter and a MIG torch tip.
Page 2 of 3 All times are UTC-08:00
Powered by phpBB® Forum Software © phpBB Limited
https://www.phpbb.com/