Added Turbo to sbc 350, Injector size increase necessary?

[Tyree]

Gentlemen,,

Last month we got the ECM 1227747 hooked up with a good .bin file and everything is running good on the TBI 350. My friend just added a turbo to the motor. My question is: Will the stock injectors accomadate this or is it gonna be necessary to increase injector size? We are not planning to go more than 7psi boost. Can an increase in fuel demand during boost be compensated for by using a different .bin with factory injectors?

Thanks,
Tyree

[Six_Shooter]

Gentlemen,,

Last month we got the ECM 1227747 hooked up with a good .bin file and everything is running good on the TBI 350. My friend just added a turbo to the motor. My question is: Will the stock injectors accomadate this or is it gonna be necessary to increase injector size? We are not planning to go more than 7psi boost. Can an increase in fuel demand during boost be compensated for by using a different .bin with factory injectors?

Thanks,
Tyree


When using an engine that has boost, it is always best to use code that supports boost, in this case that requires an ECM swap and into pretty uncharted or at least not very documented territory. That territory being TBI and boost together. I have theories about how to make this work, but in the end going to MPFI is how it's been done.

A few things to consider, and you're on the right track with worrying about fuel delivery, but there is one aspect that needs to be looked at and addressed, and that is to have the fuel pressure rise in at least a 1:1 ratio with boost over base pressure. Now it may be possible that just putting a hood over the TBI might force the fuel pressure regulator to automatically do this, since it's atmospheric reference would be inside this hat. I wouldn't rely on theory though, and will need to be watch very carefully. The reason this is important is the delta fuel pressure, the difference in pressure between the top side and the bottom side of the injector that is what actually makes the fuel flow through the injector. If this pressure is not maintained under boost, as in the delta pressure goes down, there will be less fuel flowing for the same pulse width as there would be for higher pressure.

I would say yes, you will need larger injectors, as it is the stock injectors are usually on the small side when naturally aspirated, and the engine is made to breath better (improved heads, intake and exhaust), which is essentially what you are doing with the turbocharger, making it breath better.

At the very least I would use an FMU AND a BTM (Boost Timing Master), to make sure the fuel pressure rises, and that there is some spark retard. The 7747 ECM is not capable of dealing with boost, at least not without adding some aftermarket equipment, such as the Dynamic EFI EBL, bit to me this falls under the "ECM swap category" even though it is nearly a plug 'n' play. Otherwise I'd be looking at a different ECM and likely swapping to MPFI, unless I wanted to test some of my theories.

[tra1nwreckage]

You think even with 7psi? Thats a pretty mild amount of boost. I would assume fuel pressure regulater and bigger injectors wouldnt help? I could tune the retard on the moter that should help. Im not doing race application just little more power with the tbi.

[steveo]

you'll need bigger injectors as far as i know

[Tyree]

Thanks for the reply six shooter. So you're saying that at boost, air pressure will now affect the factory fuel pressure regulator in the TBI unit causing it to no longer be accurate, thus starving the mixture of fuel during boost. There needs to be some type of fuel pressure regulator that follows the boost pressure and adjusts the fuel pressure at the same time? I was assuming that the ecm would adjust pulse width properly to retain proper fuel to air mixture and the injectors would operated within their limits. If with no additional fuel regulation and the mixture becomes to lean at boost, that would indicate not enough fuel, due to not enough fuel pressure. right?

[RobertISaar]

7PSI is another 50% mass airflow..... you will likely need injector flowrate increases at least that great.

[Six_Shooter]

Thanks for the reply six shooter. So you're saying that at boost, air pressure will now affect the factory fuel pressure regulator in the TBI unit causing it to no longer be accurate, thus starving the mixture of fuel during boost. There needs to be some type of fuel pressure regulator that follows the boost pressure and adjusts the fuel pressure at the same time? I was assuming that the ecm would adjust pulse width properly to retain proper fuel to air mixture and the injectors would operated within their limits. If with no additional fuel regulation and the mixture becomes to lean at boost, that would indicate not enough fuel, due to not enough fuel pressure. right?

Fuel pressure alone is not the only thing that effects fuel delivery, pressure is just one of the variables.

The ECM will not add fuel (nor trim) under most conditions where boost is going to be a concern, since the ECM reverts to OPEN loop at 100% throttle (actually lower than that, but I don't recall what it is $42), and above a certain intake pressure, close to 100 kpa, but that's not even in boost yet.

The ECM really has to be set up for boost in order for any chance of a reliable and well performing engine under boost conditions.

The factory FPR may cause a rise in pressure under boost, I don't know, I have tested it personally, it's only a theory, based on my knowledge of other FPRs and how the TBI FPR is installed. I'm not willing to state that it will work one way or another without testing it myself first.

While 7 PSI is not much compared to some set-ups, like my own car where I have it currently set to hit 17 PSIG boost (20+ this summer, my base boost pressure is 9 PSIG lol), it is still a lot of additional air in the grand scheme of things. As suggested, assuming a linear relationship between intake pressure and air flow, 7 PSIG would be nearly 50% more air flowing through the engine when compared to the Naturally aspirated flow. In practice it will likely be closer to 40 to 45% on a SBC (assuming stock heads, intake and exhaust), which is still significant.

How are you planning to "tune the retard" into the ignition system?