Austin Healey Electronic Fuel Injection - Page 7

The "tri-carb" look I was after required some modifications to the drivers pedal box because of the clearance required for the injector fuel rail and the fact that the SU's are all 1" taller since they sit on top of the injector block.  This means that the rear SU needed some additional clearance to mount an air cleaner; the forward two SU's have sufficient clearance to fit an air cleaner.  The twin carb arrangement on the stock log manifold did not require any modifications to the pedal box but would require somewhat taller injector blocks (or the addition of phenolic blocks) to clear the steering column.

List of custom bits:

  • Injector blocks (long carb. mounting bolts)
  • ECT mount
  • TPS bracket
  • IAC manifold        
  • Fuel rail mounting and fuel rail
  • GM distributor machined to fit
  • OPS mount (included in mine but optional)
  • Speed sensor adapter (inlcuded in mine but optional) 


Addendum:  A few notes on electronic injection systems, this is extensively plagiarized from MegaSquirt information found on the internet.  (MegaSquirt is an aftermarket fuel injection computer which has some nice features but requires more involvement to build and is probably less sophisticated than GM's ECU that Ric's EFI system is based on.)

There are two common sorts of electronic injection:  1)Throttle body injection - usually one or two injectors for the whole engine  and 2) Port injection (aka. Multi-Port) - one injector per cylinder

Then there are three common modes of injection timing:

  1. batch - all injectors fire at once, but not timed to any specific cylinder event,
  2. bank - 1/2 the injectors fire at once, then the other 1/2, and so on, but not timed to any specific cylinder event,
  3. sequential - each injector fires at a specific point in the 4-stroke cycle for each cylinder (i.e., 8 independent timing events)

Throttle body injected cars are usually batch or bank fire. Most port injection set-ups before the mid-1990s were bank fire as well (including GM Tuned Port Injection).
Sequential injection requires:

  • at least as many injectors as you have cylinders, with one dedicated to each cylinder (i.e., not a 4 injector TBI on a 4 cylinder).
  • as many injector drivers as you have cylinders,
  • and also requires a camshaft position sensor (a crank sensor is not adequate for a 4-stroke cycle engine).

The GM ECU Ric is using has two injector drivers (injection events), and no provisions for a cam sensor signal, which would be required to make it into a sequential injection system.
The benefits of sequential injection are that:

  • you may get slightly better mileage and lower emissions at low engine speeds,
  • you can tune each cylinder's fuel amount independently (if you know how).

The effect on maximum horsepower is generally negligible.  However, sequential injection does not necessarily mean you are injecting into an open intake valve all the time. The intake valve is only open less than 30% of the time in a typical 4 stroke engine. Once you are trying to produce more than about 25% of maximum HP your injectors are firing for longer than the intake valves are open. If your maximum HP is correctly calibrated to a safe 80% duty cycle, your injectors are injecting well over 50% of the time on closed valves. With Ric's GM ECU, fuel is injected only on ignition events, these are related to cam events, but trying to 'squirt' through an open valve under all conditions is generally a bit hopeless, because:

  • fuel that is injected when the valve is closed doesn't go anywhere, it just sits near the valve vaporizing until the next time the valve opens (some OEMs deliberately squirt against a closed valve to improve vaporization). So squirting against a closed valve does not generally affect the AFR for that cylinder (though there may be a small effect on the combustion quality, good or bad, depending on the port wall temperatures, etc.)
  • the valve is generally effectively open (0.050") less than 300° of a 720° 4-stroke cycle (and closer to 200 for 'stock' engines). So hitting the open valve requires precise cam related timing,
  • to inject the full fuel charge at high loads/RPMs through an open valve requires very, very larger injectors, about 2.5 to 4 times larger than is usually recommended,
  • as the duty cycle for the injectors rises, the injectors come closer and closer to squirting all the time, and injection timing becomes irrelevant.

Lastly, the following formula is used to calculate the required injector size.  1) Take your target peak horsepower and multiply by .5 for a naturally aspirated engine (.6 for supercharged and .625 turbocharged).  2) Take that figure and divide it by the number of cylinders.  3) Divide the results of #2 by .8 for a 80% duty cycle.  This formula gives you the lbs/hr you need from each injector. 

Steve Thomton
1963 BJ7

This project paper was originally posted in January 2009

Pictures updated in June 2009

Formula for injector sizing added to end of the addendum (in bold talics) on Nov. 24, 2009