CARBURETION and your Large/Small port 16V 4AG

[firehawk]

The easiest way of making ignition to work with carbs is indeed using the standard ECU. It can be done in two different ways;

1) Leave the sensors (MAP, TPS, air temp, idle pot) disconnected. This will cause the ECU to drop into "limp home" -mode, which means a constant 10deg BTDC ignition advance. By turning the distributor, one can get a locked, say, 30deg BTDC advance over the whole RPM range. Not an optimal solution but it gets the engine running. The "check engine" -light will be constantly lit.

2) Create a MAP signal and fake the rest. For this, you'll need to tap into each runner on the intake manifold, draw vacuum from them into a small plenum and connect the MAP sensor there. Since the injector holes on the head need to be plugged when using carbs, it would be easy to make plugs which have a small hose barb on them. By joining the four hoses (with, for example, three Y-connectors), one could use the purple TVIS vacuum canister (which is no longer needed) as the plenum.

The rest of the sensors are not used for ignition, so they can be faked with a few resistors. Air temp sensor can be replaced with a 2Kohm resistor, TPS voltage can be connected to +5V via 1Kohm and to ground via 10Kohm resistor, and idle pot (VAF) to +5V via 1Kohm and to ground via 4.7Kohm resistor. This method allows one to use the full 3D ignition map of the stock ECU and use the standard revlimit.

Both solutions are relatively easy and cheap to do and require only very basic fabrication skills. For customizeable maps and revlimit, one would need to either "chip" the original ECU (which is what I'm working on), use an external piggyback ecu or go for a completely standalone system such as the MJLJ. All of these are more complex solutions and require a lot more technical knowledge, tuning time on a dyno and a variable sum of money depending on which solution is used.

Distributors using mechanical advance methods are technically simple solutions but adapting them for 4A-GE requires quite extensive fabrication skills and equipment (lathe, CNC grinder) unless a commercial solution is used (finding one may be difficult and costly). Additionally, in most cases the standard ECU would perform better thanks to the factory-tuned 3D ignition map.

Edit: Oh yeah, the brake booster. My TRD manifold has a threaded hole tapped to the #4 runner and the long tube from the stock manifold screws into it. Even the original vacuum hose fits after I shortened it a few centimeters from the check valve side.