GM TBI on my 1970 360 duster

Hi Mike, I was over here looking for a 318 stroker thread referenced on Moparts. I stumbled across your question, and I enjoy EFI discussions, so I joined so I could share my .02. Some other guys have responded while I’ve typed this, so some stuff is redundant. Oh, well.

The GM TBI is, IMHO, the best OEM system for low-cost retrofitting. There is a lot of performance tweaking that can be done to it and it is well-documented on the ‘net if you dig around (although some of the CFM ratings on the TBs things don’t always match up). Regardless of what TB units you use, you will most likely need to modify it for more airflow and also play with fuel pressure to get the right flowrate on fuel. These 2 items are done outside of the ECM’s realm of awareness and allow you to tweak a proper AF based on the MAP pressure the ECM sees (the controller only handles pulsewidth, so more pressure = more fuel).

1. ‘Standard’ Mopar fuel line size was 5/16, should be what you have. I have seen some fuel pickups with built-in return fittings, so see if one’s available for you’re a-body. I’m a C-body guy so I don’t know all the A-B-E parts that are out there. Try Paddockparts.com or Year One.
2. For your HP level a 454 TB unit is the minimum, and it still might not be enough. A 305/350 TB will NOT flow enough air or fuel in stock config. There are modified 305/350TBs out there, but I think a 454 is a safer bet (and it may still need to be modified for more airflow).
305/350 TB has 1 11/16” bores, a 454 has 2” bores. According to the calculator on the Megasquirt website, a stock 454 TB is good for just under 300 hp (I think GMs 454s were rated at 270hp net?).
http://www.megamanual.com/v22manual/minj.htm#throttle

Based on crude calculations, you need approx 106 lb/hr injectors for 425 hp. But you have that at the wheels, so you need even larger injectors. Early 454 TBs have 90lb injectors run at 12-13 PSI, and that is the TBI unit you will need to start with. Late 454 TBs have smaller injectors run at 30 PSI to give the same flowrate, and you can’t raise the pressure on these injectors high enough to feed your needs. I would recommend an early 454 TB with the 90 lb injectors, then change the regulator spring to the late-unit 30 PSI spring. This should get those injectors to about 136 lb each, and should be enough for around 450 hp.

If you can’t find a late 454 30-lb return spring, you could put an adjustable regulator in the return line or modify your existing regulator. This is fairly straightforward and is noted in some of the links at the end.

You should get a spreadbore intake to run a 454 TBI, the blades are too wide for a standard square 4-V plenum area. There is a Trans-Dapt plate that nicely installs one in the larger rear section of the plenum. You could make one (as I have done in the past) but it is SO much nicer to buy it and start bolting stuff on (place a value on your time!).

Here’s a guy that modifies TBI units:
eBay Store - Xtremefi GM TBI V6 5.0 5.7L 7.4L BBC, LT1 LT4 52mm 54mm 58mm LT-1


3. I don’t have good info on the distributor, but look for the ‘DUI’ distributor, which uses an HEI module, it might be what you need. Generally-speaking, ECU-controlled timing can be a huge benefit, but if the GM ECU will still operate properly with no feedback on ignition you could simplify things by staying with what you have. However, if the GM strategies monitor ignition activity, and it sees activity different from what it is commanding, it may go into a limp-mode?
4. TBI is undoubtedly the most stock-looking EFI setup to meet your visual goals. Yes, you need to add sensors and wiring, but these can be integrated tastefully if you put the effort in. Trying to throw a GM pickup harness as-is into the Mopar engine bay will surely be ugly. Plan on shortening and extending wires as needed and putting new loom on it at a minimum. No, you can’t completely hide it, but you can make it blend in somewhat and look nice.


Here is one thing that could be an issue:
It sounds like you have a large-size cam, possibly with a lot of reversion. Your engine’s Volumetric Efficiency will surely differ from a mild, small-cam TBI engine, so there may be some ‘holes’ or flatspots when the GM power curve is not parallel to yours. Meaning, if your engine simply made 50% hp more than the GM, your power curves are proportional, so just flow more air and crank the fuel pressure up appropriately and the ECM maps work fine. However, if your engine comes on strong at 3000RPM, very few OEM EFI maps will fuel properly for that. If you have the ability to hack the GM ECU and change tables this is not a big issue. Also, a vacuum-referenced FPR could help out a little (it is explained in one of the links below).


EFI is not really cost-effective for a limited-use car of only a few thousand miles/year, but I truly believe once you get it installed and the bugs worked out you’ll want to drive your car much more often!



http://dynamicefi.com/4-INJUpgrade.php
http://marine-performance-parts.com/gmtbiperformance.aspx
http://dynamicefi.com/EBL_Flash.php
http://members.shaw.ca/betterthanyoutoo/how-to/fpr/fpr.htm
http://www.strokerengine.com/TBIkit.html
http://dynamicefi.com/EBL_Drawings.php
http://dynamicefi.com/TBI_Fueling.php
http://home.earthlink.net/~jcgebhart/msindex.html