Pontiac Engine EFI Recommendation

[jonathonar89]

Hey Guys,

New to the forum so hello!

I have a fun/different project I'm working on and would like to get a recommendation. Basically, I'm trying to utilize GM parts to do a fuel injection conversion.

What I'm working with....
-1979 Pontiac 400 (originally carbureted) that's mildly built to around 350-400hp estimated
-Late 80's 454 TBI unit
-'81 Pontiac 301 7-wire HEI
-'81 Pontiac 301 log manifold (has o2 sensor bung from the factory)
-Spectre efi conversion fuel has (has a built in Walbro 255lph pump)



What do you guys recommend for a computer and wire harness on this type of project? I've been leaning towards Megasquirt EFI but I wanted to see what is the recommendation for GM computer/harness if there is one.

I understand these earlier TBI have high output (low pressure) injectors. Can I simply swap to a 94-95 454 fuel pressure regulator to the amount of fuel I need for my engine?


Let me know,

Jon

[jonathonar89]

Found a 2nd hand new Megasquirt computer.

No recommendations needed at this point but I will let this forum know if I have any other questions.

Thank you,

Jon

[1project2many]

Hi Jon,

Welcome to the forum. Glad to see that you found a solution. I was hoping to have time to reply yesterday...

I believe you'll be pushing the limits of TBI at 400hp. In order to increase fuel pressure you can install the regulator from the later BBC trucks or an adjustable marine regulator. You may be able to build a throttle body that can use BBC medium duty truck style injectors if you can locate a Holley TB or a medium duty truck governor type TB and raiding the injector pod. I believe the last time someone here was on the hunt for this part it was challenging to find, but there are more injector options available for the smaller pod. An intake with a large plenum can be more challenging to tune as intake air temperature will affect the amount of fuel required during acceleration enrichment.

In addition to Megasquirt, a commonly recommended GM computer can be found in 93 to 95 trucks and vans with 5.0 / 5.7 / 7.4l engines with automatic transmission. By using an OE computer you can follow OE wiring diagrams and diagnostic tables if there are problems. It is possible to customize the TBI code to use the intake temperature sensor to affect the fuel delivery during acceleration but the OE application is designed around the OE configuration and has limited flexibility. Additionally there is custom code available which allows you to run a MAF sensor. Some folks find the MAF option easier to tune although I don't know how that code handles acceleration enrichment.

It sounds like you may be looking for an OE appearing conversion? But I have to wonder if the 301 manifolds will be appropriate for the engine. I suppose any quality headers for a Poncho engine would be expensive these days... What to do?

[ScottP]

Hi Jon,

As a Pontiac owner who has wrestled with this same question, I'll offer some input based on my
EFI 455. Firstoff, good move starting with the '81 Pontiac 7-wire distributor. I've hoarded a few
of them over the years. If you're going to use just about any '87-95 GM TBI ECM or PCM, this
makes it pretty much plug and play.

Is your application a manual or an automatic? I ask because one of the great benefits of using
the '93-95 truck ECM that Shannen mentions above is electronic transmission control. The 455
in my '70 Bonneville is not only port injected using a '411 PCM, but its also backed by a 4L80E.
I'd argue that the 4L80E made AT LEAST as much improvement to the overall driveability and
enjoyment of the car as the EFI swap did. If you're using an automatic, you owe it to yourself
to at least consider this factor.

The other nice thing about the '93-5 truck PCMs is that you can convert from TBI to port injection
with relatively little difficulty. There are loads of inexpensive intake manifold options out there now
for the traditional Pontiac that are port EFI capable. I'm currently running a Professional Products
knockoff of the Performer RPM, but I have a single plane replacement sitting on the bench awaiting
install that I'm going to try out this spring. This gives you a clean migration path if you don't like
how the TBI performs, or if you just decide you want more power than the TBI system will support.

As a total aside, one project that I regret never trying is to grab one of the Offenhauser dual-quad
intakes for a Pontiac and run a pair of TBIs on it. If you're feeling adventurous, this one would surely
turn some heads at the local cruise nights.

-Scott

[jonathonar89]

I didn't think I was going to get a response here.

'93-'95 TBI computer sounds interesting. This is going in a manual transmission '79 Trans Am 400 4spd.

Yes, I'm going for a factory appearing EFI install. This is the '81 passenger side log manifold I picked up.
Untitled by Jonathon Randolph, on Flickr
This is how they look on 301's.....but it will be going on my 400.
Untitled by Jonathon Randolph, on Flickr


For the distributor, I actually had Davis Unified Ignition send me out everything to rebuild. Cap harness, computer harness, pickup, cap, rotor and performance coil. Although they sent me a "performance" pack, I opted for a plain cap for a factory look instead of their fancy coil cover. Here's a small mockup....
Untitled by Jonathon Randolph, on Flickr

Right now I have the intake off so I can start grinding out the plenum. This should be fun..
Untitled by Jonathon Randolph, on Flickr

[ScottP]

Just curious, what's the thinking behind the '81 log manifold? Is the intent to run a single exhaust
in this application? Or were you just planning on using the O2 sensor port? If you have existing
manifolds that work well for this application, and a Y-pipe (or separate downpipes), it may be
cleaner to just weld an O2 sensor bung into those. Another reason this might be a good option
is that it would allow you to add in an additional O2 bung for a wideband - whether you just want
one temporarily as you're tuning, or if you choose to keep one permanently installed in the car.

Another thing to think about is your IAT. If you end up using a GM ECM, you will want to have
one. Not all ECMs supported IAT, and those that did not depend heavily on a functioning Themac
system to keep the intake air temps stable to avoid lean stumbles on cold days. The underside
of the aircleaner provides lots of options for a clean and hidden IAT install.

-Scott