SBC 400 TPI $8D with 50lb Injectors

[dave w]

I'm wondering if the '7730 $8D system will function correctly with the injector flow set to 50 lb/hr? Maybe there is a practical limit on the injector flow rate for the $8D system?

I'm hoping to hear back from someone who is willing to share information about using 50 lb/hr injectors with a TPI system.

The SBC 400 TPI system has an aftermarket High Flow Throttle Body, and a set of aftermarket High Flow Runners. The cam is a Competition Cams 292 grind. The vehicle is classic 56 Chevy Belair.

Maybe I should plan on using 24 lb injectors?

Thanks in advance,
dave w

[Six_Shooter]

I can't see why it won't work, people have used as large and larger injectors with other GM codes for sure. I can't specifically recall anyone that is using $8D with 50 lbs/hr+ injectors, but I'm sure there are people.

[Fast355]

I can't see why it won't work, people have used as large and larger injectors with other GM codes for sure. I can't specifically recall anyone that is using $8D with 50 lbs/hr+ injectors, but I'm sure there are people.

I have run 42s on 8D and it ran very well but it was on a 500 cid engine.

[dave w]

I once tried to remote tune a "Fire Breathing" TPI SBC, regardless of the injector flow I set, 40#, 50#, even 60# the "Fire Breathing" SBC was WBO2 "Pig Rich". It's very challenging to argue with WBO2 data! I was thinking like the GM design team, what would be the realistic injector flow expatiation (BPW) for a production '7730 engine (I-4, V6, & V8)? I pondered on the injector flow algorithms of the '7730? I'm inclined to think the displacement has more effect on the injector flow algorithm than the actual injector flow? I once setup a bone stock Vortec 454 with a '7730 with outstanding results! The bone stock Vortec 454 uses 19# injectors, well within the realistic injector flow expectations of a production '7730 engine. I'm thinking, there is some sort of BPW in the '7730 injector flow / displacement algorithm that might not be fully "HACKED"?

Based on feedback from Fast355, I might find the "Sweet Spot" BPW of a 400 CID $8D '7730 with 30ish# injectors????

dave w

[1project2many]

Dave, there are some issues in 8D with injector size. I cannot say exactly what without research, but a friend of mine built a 454 with a fairly stout cam and some larger injectors and although we could make it run reasonably well on the street, it needed some patches in code to help ensure sufficient fuel was delivered.

What you may have experienced on the engine that couldn't be leaned out is the minimum allowable pulsewidth preventing the ecm from calculating a small enough injector on-time. In order to use 42 lb/hr TBI injectors on my turbo 2.2 I had to reduce static fuel pressure from 30 psi to 16, then reduce minimum PW. As min PW decreased I found the minimum opening time commanded by the ecm was so small that idle speed became erratic. Checking injector behavior with a scope showed the injector was not being allowed to open. The pintle would rise some amount then close before the peak current was reached. The solution was to work over the parameters for "Quasi-Asynchronous Fuel Enable" which is essentially (BPW * 2) delivered every other reference pulse. The ecm would switch to QA fueling as rpm and load dropped and switch back to synchronous fuel delivery as the calculated BPW increased. Ultimately the engine would idle in closed loop at stock speed although it took many hours of hands-on work to get it there.

I would opt for smaller injectors if they are available.

[dave w]

The engine starts and runs (poorly) on 24lb injectors. Actually, the engine runs about the same as it did with the 50lb injectors. So I'm thinking I might have a fuel delivery problem, the fuel pressure very slowly bleeds off. The fuel pressure bleed off time is almost an hour after the fuel pump is shut off. The fuel pressure is above 20 PSI for almost 30 minutes. I've been thinking the fuel pressure should bleed off to almost 0 psi fairly quick, like within a minute or less? Am I thinking correctly?

I've been thinking maybe I should try bypassing the factory TPI pressure regulator and convert to the OBD2 "Corvette" return-less fuel filter / pressure regulator set up. Thoughts about the retrun-less system are welcomed.

The 1956 Belair has an aftermarket off the shelf ready "EFI Fuel Tank".

dave w

[dave w]

I've been thinking maybe I should try bypassing the factory TPI pressure regulator and convert to the OBD2 "Corvette" return-less fuel filter / pressure regulator set up. Thoughts about the retrun-less system are welcomed.
dave w

Maybe the Corvette return-less system is not a good plan, I'm thinking $8D would require an injector flow table like the ones pictured (return-less vs. manifold vacuum referenced)

dave w

[1project2many]

Returnless systems should have a very effective check valve to prevent fuel vapor formation in the lines and / or fuel bleed-off during extended shut down. I would expect the behavior of the fuel in the rail during some conditions will make for difficult cranking or starting. My safety net has always been to recommend calibration and fuel system stay together.

A good fuel system will hold pressure for a considerable time after shutdown. Maybe we're used to the GM pumps which are not so good? Bosch built, GM installed pumps in early Vettes would hold pressure for a looong time compared to their GM counterparts. Ford and Chrysler systems are also known to hold pressure. In fact, early Vettes with Bosch pumps would often suffer hard starts due to injector leaking. Deposits would prevent the pintles from closing and fuel would soak the intake. If the pumps leaked down quickly it might not have been an issue.

Oh, and I switched my Sunbird to a Ford pump and the difference in startup time is remarkable.

How does the fuel pressure map out when the engine is running? Does it change with vacuum change? What quantity of fuel is delivered over an interval of time if the supply line is simply dumped into a container?

[dave w]

I did not run the engine for any length of time using the 24 lb injectors. Very long story cut short, the engine will be running a carburetor for next few months. The TPI system is going to be set up on "Mule" engine to get things figured out.

I agree, keeping the TPI fuel system stock (or as stock as possible for a 56 Belair) is a good plan.

dave w