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Which Injectors to get for 305 TPI?

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Old 10-07-2017, 06:45 AM   #1
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Which Injectors to get for 305 TPI?

Hi Guys,

I am rebuilding a '91 LB9 305, bone stock engine. Which Injectors should I go with? Bosch DIII from Southbay? Do they need any retune or should I buy something else for plug & play fitment and function? I can't find AC Delco Injectors anywhere which I have in my old V6 and those work perfect.

Thanks!
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Old 10-07-2017, 09:16 AM   #2
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Re: Which Injectors to get for 305 TPI?

If your car is running fine...

Bosch D3 -700 or -710. You cannot go by the flow ratings on the various websites. They are all over the map.

I have a bone stock LB9, 89 in exceptional condition. I ran the -771 "19 lb" and they were wrong. Went to the -710 "22 lb" and they were near perfect.

I bought an adjustable fuel regulator to fine tune the fuel-air ratio but ended up so close to stock, I figure you may not need it. Apparently there are at least two popular fuel injector places. I went with Southbay and have been very happy- they have excellent customer service and support.

-710/700 Injectors:
https://www.southbayfuelinjectors.co...-l98-chevrolet


Fuel Pressure regulator adjustable housing and new regulator diapram:
https://www.southbayfuelinjectors.co...ator-diaphragm

BLMs
Last bit of advice. After so much discussion on this site (and elsewhere) I have a nagging thought that there is some "witchcraft" with non-stock injectors and the "lb" flowratings. The Multecs are old so we must go to alternatives. I am not 100% certain that putting the -710s in your car will yield the same results as mine. Apparently plenty of -771's have been sold to LB9 owners and no complaints- but who knows what those circumstances are. The only real way to ensure you have appropriate new injectors is to use a scan tool and read your BLMs If your BLMs end up fluctuating while you drive between, say 112 and 140, then the replacement injectors are fine. If your BLMs are stuck at the end of their range 96(?) or 160, then your replacement injectors are not correct or you need either to replace them with different rated ones or add an adjustable fuel pressure regulator to trim the fuel delivery to bring your BLMs into the proper range. All BLMs are, are incremental adjustments to the fuel injector pulse width (essentially in milliseconds.)

By way of example, if the ECM (engine control module) reads a lean condition from the oxygen sensor, it lengthens the pulse width a little (more fuel) to try to reach the perfect mix (called stoic). It will keep increasing the pulse width (BLMs go up) until it reaches a stoic mix reading from the oxygen sensor. The maximum the ECM can adjust to a longer pulse width is 160. BLMs around 128 indicate little or no adjustment needed (the middle of its range). If your BLM readings are at 96 or 160, the ECM cannot adjust anymore and it is still not reaching the correct mix. This is what happened to me when I put in the 19lb injectors. My BLMs pegged at 160 meaning the ECM couldn't increase the pulse width enough- it couldn't add enough fuel, to reach stoic (proper mix). I dropped the -710's in and made small adjustment to fuel pressure (small) and my BLMs are around 128 now.

Last edited by Tootie Pang; 10-07-2017 at 09:44 AM.
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Old 10-11-2017, 11:08 AM   #3
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Re: Which Injectors to get for 305 TPI?

I have been considering replacing my injectors this winter as well but am unsure how much adjustment would be necessary. Lots of posts about needing to make PROM changes with the Bosch Injectors. My car has been modified slightly (also a L98) with a chip, exhaust, and other small performance do-dads already but I don't have scanning equipment. Sounds like to do it correctly I need something to scan with? The car currently has Accel injectors and seems to run fine. Maybe the folks at SouthBay would be able to assist?

I have wondered how a free flowing exhaust has affected things being that I haven't changed my O2 sensor. Thought maybe a heated 3-wire sensor would help.

Maybe I don't want to really know???
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Old 10-11-2017, 11:31 AM   #4
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Re: Which Injectors to get for 305 TPI?

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I have been considering replacing my injectors this winter as well but am unsure how much adjustment would be necessary. Lots of posts about needing to make PROM changes with the Bosch Injectors. My car has been modified slightly (also a L98) with a chip, exhaust, and other small performance do-dads already but I don't have scanning equipment. Sounds like to do it correctly I need something to scan with? The car currently has Accel injectors and seems to run fine. Maybe the folks at SouthBay would be able to assist?

I have wondered how a free flowing exhaust has affected things being that I haven't changed my O2 sensor. Thought maybe a heated 3-wire sensor would help.

Maybe I don't want to really know???
If you're not experiencing problems with the Accels, maybe leave well enough alone?

If you do injectors, recommend scanning the car before hand to establish your current baseline... particularly the BLMs. That way if something changes after the injector swap, you'll know it's due to the injectors.

Heated O2 sensor only necessary if you're falling out of closed loop due to the sensor cooling off. But again you don't know unless you scan the car. The signal it sends is no different than the factory O2.
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Old 10-11-2017, 11:40 AM   #5
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Re: Which Injectors to get for 305 TPI?

Well, if your car has been running fine, then maybe invest in a scan tool instead of new injectors. Injectors last a long time.

Reading your BLMs is the actual output of the fuel-air combustion and will tell you if your car is OK.

I think, but am not certain, that if you are not getting BLM error codes, your ECM is able to adjust to stoic. I think an error code happens if the ECM goes to far to adjust.
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Old 10-11-2017, 11:55 AM   #6
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Re: Which Injectors to get for 305 TPI?

Thanks fellas. So much to learn. To say that I'm intimidated would be an understatement. I have learned a ton from the members here but it seems like trying to learn about the FI system is like drinking from a fire hose. Maybe it is time to contact RBob about the Dynamic EFI system.

Its no wonder folks go back to carbs!
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Old 10-11-2017, 10:04 PM   #7
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Re: Which Injectors to get for 305 TPI?

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Originally Posted by Tootie Pang View Post
Well, if your car has been running fine, then maybe invest in a scan tool instead of new injectors. Injectors last a long time.

Reading your BLMs is the actual output of the fuel-air combustion and will tell you if your car is OK.

I think, but am not certain, that if you are not getting BLM error codes, your ECM is able to adjust to stoic. I think an error code happens if the ECM goes to far to adjust.
I don't believe the SES light will come on if the BLM hits 108 (not sure if that's what you meant). The only way to really know is to get a scanner on it...
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Old 10-11-2017, 10:12 PM   #8
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Re: Which Injectors to get for 305 TPI?

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Thanks fellas. So much to learn. To say that I'm intimidated would be an understatement. I have learned a ton from the members here but it seems like trying to learn about the FI system is like drinking from a fire hose. Maybe it is time to contact RBob about the Dynamic EFI system.

Its no wonder folks go back to carbs!
Fundamentally if you think of the ECM as an electronic carburetor, it's not that difficult to grasp the fundamentals...

I.e., all this talk about BLMs (i.e. the fuel trimming the ECM constantly does while you drive)... think of BLMs as someone named "ECM" riding under your hood constantly changing the jets on your carb... if he sees the engine runs too rich via the O2 sensor because the jets are too large (i.e., VE table entry is too high) he swaps out the jets for smaller ones (i.e., lowers the VE table entry)...only he does it at the speed of light while you're driving!

At the most fundamental level, the VE tables are your best guess at what the engine wants in terms of fuel. You program that into the VE table... but ECM will tell you whether you guessed right by reporting via the BLM. BLMs can swing between 108 and 160, and as long as you're in that range, the ECM can compensate for inaccurate VE table entries. If you're so far off in the VE table, the ECM will peg lean (160) or peg rich (108) depending on which way you're off. At that point it runs out of adjustability (i.e., they don't make jets in that size!). Ideally you want BLMs of 128 everywhere (your VE table entries are 100% perfect).

Any rate, that's how you can "carburetize" the ECM logic...

I swapped out from my carb to EFI in 1995 and never looked back. Yeah, it was difficult at times, but it was worth it. Carbs are dinosaurs... hell even my 30 year old TPI electronics is a dinosaur compared to today's stuff. But it's still lightyears ahead of a carb...
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Old 10-12-2017, 02:30 AM   #9
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Re: Which Injectors to get for 305 TPI?

Wow guys, that's what I call useful infos! Thank you all!
I have the Ostrich 2.0 in my V6 with a modified AZTY .bin. I will swap the LB9 for the LH0 next summer, so editing and tuning the AXXP (?) .bin for the LB9 won't be a problem. I am still new into tuning, just modified EGR and AIR and some other minor things. I never edited the VE tables or something like that.
I think I will go with the Bosch's from Southbay and with their AFPR. Not too expensive at all and they seem to know their stuff.
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Old 10-12-2017, 09:44 AM   #10
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Re: Which Injectors to get for 305 TPI?

Unfortunately, if I am understanding things correctly, just running the scan and finding out things are not kosher is only half the battle. If a correction needs to be made you still have to know how to burn the changes onto a chip. I know there is a ton of info on this site to do that so I guess if I want to keep my TPI setup I needs to get edu-ma-kated.
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Old 10-12-2017, 11:18 AM   #11
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Re: Which Injectors to get for 305 TPI?

Not necessarily. With the mods you mentioned, IMHO, if your BLMs are good your engine is burning the desired stoic and all is well.
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Old 10-12-2017, 11:25 AM   #12
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Re: Which Injectors to get for 305 TPI?

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Not necessarily. With the mods you mentioned, IMHO, if your BLMs are good your engine is burning the desired stoic and all is well.
Agreed
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