Fuel Injection for the MGB 1800cc Engine

Here's an inexpensive way to provide fuel injection for your MGB. It's relatively easy, a good learning experience, and not an unpleasant looking change to a pair of SUs. If you've got a working pair of SUs it is probably better to stick with them, they run well and reliably. But if they're worn, or if you have a single stromberg this may be a more economical choice than a pair of new SUs, any of the available FI kits, or a weber replacement.

If you want something prettier and much more expensive, the extrudabody kit or the TWM SU kit (I'm not in the habit of advertising products I've never purchased, so sorry, no links provided. I'm sure google will help you track them down). The ideas here should work just as well with those kits.

Be very careful when tuning - bad ignition advanced settings or too rich or lean fuel mixtures can damage the engine, especially when running under full throttle.

What's Needed

• Some automotive, electrical, and minor handiwork skills. No fear :).
• Any working version of megasquirt, newer is probably better. I used a version 2.2 box.
• A set of the older GSXR 600 throttle bodies. These are the least desirable and so the cheapest versions of the GSXR intakes. Get the older ones because you need to separate them and change the spacing; only 2 or 3 are required. Have a close look at the pictures below to make sure you get the right ones. Ebay is a great source.
• Mounts for the air cleaners. If you can get the airbox for the GSXR it's much easier; the rubber air cleaner mounts work very well. You need only 2.
• A pair of fuel injectors. The GXSR ones do not provide enough fuel so you need to get something bigger. Any standard injector will fit. I used a par of 336cc/min (32lbs/hr) injectors which appear to be the correct size to produce 98HP at an 80% duty cycle. They came from a Honda RC51 (Honda VTR1000 motorcycle) throttle body set (which uses 4 injectors).
• A pair of air filters for SUs, or any go faster air filters you want that fit the GSXRs (if you get custom air filters you may not need the GSXR airbox mounts. I used 4 door (wall)protector plates, some lengths of tube, long 1/4" bolts and butterfly nuts to hold the air filters.
• A 7805 or LM317 voltage regulator to replace the mechanical instrument "voltage stablizer", some miscellaneous resistors (see below).
• A GM air temperature sender, readily available from any parts store or junk yard.
• Various connectors and plugs depending on how you run your wiring.
• A high pressure fuel pump, fuel regulator and fuel lines. Some hardlines and low pressure brake or fuel lines from any automotive store.
• Some odds and ends from the plumbing section of your neighborhood hardware store.
• Bits of exhaust pipe and a small piece of 1/4" plate for the intake manifold. Some bolts and nuts to match. Some lengths of suitable ID radiator pipe cut to length to mount the throttle bodies.
• You really should buy or borrow a wide band O2 sensor.
• A steel header or some other way of mounting the O2 sensor. It may fit in the down pipes (put it on the pipe servicing the middle exhaust port) or later down in the exhaust pipe. I was reluctant to try to braze it to the stock cast iron manifold.
• Your MIG welder or a friend or shop that can weld or braze.
• While you can keep your standard ignition, switching to a triggered ignition, especially if crank based, would be better. See my page on electronic ignition.

MGB Engine Behavior

These are a few of my observations while playing with the tuning of my 'B using a techedge wideband O2 sensor.

The 'B is a siamesed port engine. Charge robbing is much more of an issue when using FI than the carbs because carb's add fuel only when air is being pulled into the engine, while FI sprays fuel with every engine event. Using motor cycle based TB's that inject further upstream helps alleviate this problem.

The 'B engine tends to prefer a richer mixture, near 12.5:1. It stumbles when run leaner, especially at idle. It's very important to run this rich at WOT to avoid detonation and serious stumbles (which can damage the engine). When cruising a leaner setting using the O2 sensor readings may help improve economy, I'll know more when the weather here improves and I can get some accurate, long term measurements.

The 'B transitions through MAP readings are rather abrupt. This makes tuning at the top end a bit of challenge. I may switch to alpah-N (throttle position based) tuning at some point to see if I get better results, but for now things seem to be running very well.

Larger throttle bodies seem to make it difficult to tune out a hesitation in the 'B when the throttle is opened at low engine speeds. I believe this is due to the low air speed and the siamesed ports - the engine is "over carbed". It takes a short time for the ECU to detect the throttle change and inject extra fuel (accelEnrich), and longer still for that fuel to be picked up and burned. This results in a hesitation or in extreme cases a "pop" or backfire into the TBs, which isn't very healthy.

The GSXR TBs have the advantage of being 1 1/2" in diameter (one of the reasons they're not it too much demand) which exactly matches the intake manifold of the MGB, and of course of the HS4s. Peter Burgess, in his book, doesn't see any advantage of going larger (the intake ports of the 'B head are 1 3/8" in diameter) unless you have some serious head and engine work done. I've had the same experience.

Consider also that the SUs have a rather intrusive, sharp edged piston positioned in the air stream. This is completely eliminated by the GSXR TBs, which implies that (especially at their low cost) they should be sufficient.

I found it interesting to see that most fuel injected motorcycle engines overcome the slower air speed by using secondary throttle plates controlled by the ECU positioned in the TBs upstream of the throttle plates and injectors. Contrast that with the SU design of varying the venturi to keep up the airspeed. The smaller GSXR had a similar arrangement on the exhaust as well, I assume for the same reason.

Links

The Megasquirt team.
Werner’s site is a huge help.
Paul's twin body MGB intake is very nicely done.
Most of my ideas for building things came from http://www.sdsefi.com/, in particular the tech pages:

• Tech/FAQ Page
• How to build an intake manifold
• How to build a surge tank

I'm using the 2A0 wideband O2 controller from Tech Edge. A little more expensive than some, it's still a good value and has the advantage of being extensible - there are a number of input signals that can be used and logged. I plan to feed my ADXL 202 based g-meter into it, and along with the RPM signal to use the 2A0 as a road dyno eventually.

Description

Megasquirt and Relay Board

MS1 V2.2. Modified for the stock MGB coolant sensor and spark advance (see below). I added a connector for the 2A0 WB O2 board so that I could plug the linear O2 signal directly into the megasquirt.

I installed the megasquirt directly above the map box on the passenger side (you can see the wiper motor in this shot).

The standard megasquirt relayboard. I use it to trigger the fuel pump and provide power to the megasquirt. The megasquirt cable goes into the passenger compartment through the grommet you can see on the right. The wideband cables go through the large opening (the offside brake opening for RHD cars). Those are temporary and the large grommet will be replace when tuning is complete and I replace the wideband controller with a cheap narrow band O2 sensor.

A small brake line is used to run the vacuum from the throttle bodies back to the vacuum sensor mounted in the megasquirt box inside the passenger compartment.

Throttle Bodies and Linkage

SU's are twin 1.5" (38.1mm) diameter with restrictive pistons to provide a constant air velocity. The weber DCOE45 (often used for racing, but read Peter Burgess's book) is a twin throat 45mm (1.77") diameter but are usually choked down to 33mm to 38mm by the (replacable) venturi.

The GSXR 600 TBs are 1.5" ID on the intake side, and 1 3/4" OD on the air filter side. A perfect match to the MGB stock intake manifold and a nice fit for the MGB stock air filters.

These are the ones you want, notice they come apart.

I used 3 of the four bodies. The two with the mounting flange for the fuel rail are the key ones you need to hold the injectors down properly. The third is used to provide an easy way to mount the linkage to connect them. The fourth, the one on the end with the TPS is not used. You also need the plate that holds the throttle cable linkage.

Use two lengths of 1/4" threaded rod, nuts, washers and lock washers and 4 spacers to connect them together. Choose the spacer lengths to position the TB's in the correct spot to mount on the intake manifold. The spacers are 1/2" long. Black 1/4" plumbing adapter pipes work well, are easy to get and cheap. Aluminum would look better :). Make sure they're all exactly the same length - cut them a bit long then file them flat and to fit.

Strip off the throttle shafts and plates; the front ones are discarded (keep the springs and things). Plug the holes with suitable caps, or simply tape over them with electrical tape like I did - that'll last quite some time. Now comes the tricky part... pick the right throttle shafts. The first (rear one) is easy - it's the one to which the throttle plate and cable mounts. The next is a longer one - it floats in the dummy TB and needs to hold the correct mount on each side. The front one is the longer one from the TB to be discarded. It's used to fit into the TPS (see below).

Notice the throttle linkage plate, that comes in handy later, keep it. It mounts on the rear most throttle body.

Note that the linkages are bent slightly to span the gap left between the throttle bodies - this saves a lot of aggravation.

Don't use this drawing exactly, you should wait till you have a better feel of where you want to run your throttle cable. But something like this should work: Turbocad throttle lever.

The throttle cable needs to be mounted somehow, I made a small bracket to hold it with a manifold/flange bolt, and reused an old adjuster I had lying around. The stock cable is threaded through that bracket and adjuster.

The stock choke cable can be attached (with a little fussing around and the right cable end) to the GXSR fast idle lever. I threaded the cable sleeve to take a nut to lock it all it place. This makes cold starting and running a little easier, no need to feather the throttle. And saves hooking up some kind of fast idle arrangement.

The stock TPS is used by mounting it on a separate plate that's tied into the end of the bolts holding the TBs together. This is necessary because the original TB with the TPS can't be used - it doesn't have a flange for the fuel rail. The TPS mounting plate is cut from any thin metal (I used aluminum). Same as with the manifold adapters, print this diagram, glue it to the plate, drill and then cut it out. Turbocad TPS mount.

Trial fit the TPS to the body, measure the resistance to make sure you've got a good range from both of the sensor wires (1k to 4k is pretty good). It should be pretty close to where it is in my picture.

Air Cleaners and Temperature Sender

Note the hole and rubber sleeve added for the air temperature sender. The sleeve was rescued from the pick-a-part at the same time I liberated the sender. It's a standard GM air temp sensor. Grab the connector as well so you can splice it into your wiring harness.

Injectors

1970 MGB 92hp @ 5400 110ftlbs @ 3000 top speed 104mph

See http://www.megamanual.com/v22manual/minj.htm, fuel requirements: 100hp: 2 injectors at 29lbs/hr for an 85% duty cycle.

From http://www.witchhunter.com/injectorcalc1.php4, fuel requirements: 92hp: 2 injectors at 29lbs/hr or 302cc's per minute, total of 58lbs/hr or 604ccs/min.

I used RC51 injectors: 2 x 336cc/min (32lbs/hr) from the RC51s gives a total of 670cc/min (64lbs/hr). I found someone else's test results for them on the web:

Intake Manifold

Short lengths of 1 1/2" ID tubing is welded or brazed to the flanges shown here. Intake adapter flanges (turbocad)

I made the flanges from some 1/4" x 2" flat stock:

1. Print the drawings full size (1:1).
2. Cut out a pair of the flanges and glue them to the stock (sprayable contact cement works well).
3. Center punch and drill the holes.
4. Cut out the flanges.
5. Weld or braze the intake pipes to the flanges. I put a few small beads around the ends of the tubes to help hold the radiator hose (that attaches the throttle bodies to the tubes) in place.
6. A little hammer work may be necessary to flatten out any heat distortion from the welding.
7. Stick some coarse and later fine sand paper to a piece of flat glass and rub the flanges on the paper till they're flat and smooth. The color should even across the flange. Some engineers blue can help.

Drop by the auto store and buy a length of any radiator tube with a 1 1/2" ID. Cut to fit the adapters and the throttle bodies.

I used tube lengths so that the throttle bodies and air cleaners would be in the same position as they are when using the SUs and stock air cleaner, but feel free to adjust to taste.

Not the best choice, but some standard hose clamps hold both the tube and the throttle bodies in place. The hose clamps do tend to cause the radiator tube to creep over time, so keep an eye on them.

Remove the SUs and the spacers, replace the stock manifold studs with some appropriate bolts to hold the adapters, and you're good to go. Use a pair of the spacer to manifold gaskets for an airtight seal.

Reusing the stock manifold simplifies things, for example the PCV valve can be retained.

O2 Sensing

You can just see the bung on the header pipe in the picture showing the manifold and TB. I used the header to avoid fiddling with the cast iron stock manifold or the downpipes that are so hard to get off on the B. I used the middle branch to get a reasonable approximation of the air/fuel mixture - if I had another sensor ($$$!) I've put one in one of the outside branches too.

Once tuning is complete I'll replace the wideband O2 sensor with a cheap narrow band for use when driving (so I can use my wideband for other projects).

Coolant Temperature

There's no easy place to put a second coolant sensor on the B, and it only has a provision for a non standard one for the gauge. It's driven by a mechanical(!) voltage stablizer. I worked out this circuit to piggy back on the stock one.

TemperatureModification

I don't think easytherm is a good way to figure out the curve with this modification, it gives some odd results. I measured the voltage at various temperatures and hand coded my own (well, I wrote a java app to generate it truth be told).

Fuel Delivery

The GSXR fuel rail can be reused. The two throttle bodies with the hold downs for the fuel rail must be used as the actual intakes to hold the fuel injectors in place. Cut the fuel rail, file it round to accept a high pressure fuel line. Try to leave a little lip on the ends to help secure the fuel line.

Cut off the end, drill and tap it out to hold some way to connect the high pressure hose coming from the fuel pump.

Most any high pressure regulator can be used. I was lucky enough to have a surplus one from a Yamaha R1 throttle body assembly. The part number might be 5PW-13906-00-00, but I'm not sure. You can see it in the picture below. Note that it is a perfect fit for the GSXR fuel rail! There are two other ports, this most visible one going to a "tee" connector to the fuel return line to the fuel tank and back to the surge/fuel pump tank. Note that the line restrictor is on the surge tank side of the return line, upstream of this connector and the fuel tank.

The other port is a vacuum port that goes back into the throttle bodies at the bottom.

The surge tank was made from some 16g exhaust tubing and the fuel pump from a junk yard honda civic. I got the entire pump including the mount for the fuel tank. This let me cut the banjo fitting and braze it to the surge tank lid. The pump is grounded to the body of the tank, with an insulated connector plumbed through the lid.

A 12g lip is welded to the top of the tank, then drilled and tapped for the #6-32 SS screws to hold it in place. A cork gasket seals it nicely even under pressure once it's given time to swell in place.

The stock fuel line from the stock pump feeds the surge tank. Use a fuel filter on this line. A new return line made from brake pipe is used for returning the low pressure "waste" fuel to the tank at the back of the car. A used 0.025 MIG tip in the line leaving the surge tank provides some back pressure so the MGB pump isn't continuously pushing fuel through the return line; only enough movement to bleed the system of air is needed.

Fittings are 3/8" to 1/4" black plumbing bushings that are trimmed and brazed to the tank (followed by a quick pass with a 1/4" NPT tap). I used the same bushing brazed to the MGB fuel filler neck at the rear of the car.

I used a bulk head barbed fitting to run the drain into the MGB trunk, then a short length of hose to join that to a barbed fitting on the fuel filler neck. Don't be tempted to weld a bung into the fuel tank, they can blow up when heated! It's much safer to put it into the short filler neck once it's been removed from the car.

Another option to consider:

I'm told the surge tank and Bosch high pressure fuel pump from a fuel injected mid to late 80's VW Jetta or Golf is a great add-on system. It can be located in one of the battery boxes (assuming you've converted to a single 12v system) if you're willing to splurge for that much high pressure fuel hose. It's designed as an external surge tank and has all of the supply and return fuel line attach points to and from the fuel tank and too and from the fuel rail.

Spark Advance

I’ve added electronic advance control using MSNS-Extra by splicing it into my Crane (Allison) XR700 ignition.   I was using a vacuum can that was originally designed for a ported signal direct from the RC51 throttle bodies which was giving full manifold vacuum.  This gives too much advance at idle and runs a little rough.  By programming the advance curve I duplicated the original ‘B curve and no longer need to be worried about worn out or incorrect advance springs.  The modifications are detailed here.

Misfires

I tried to build the surge tank without a return line, using a fluid level sensor from McMaster-Carr. It worked by shutting off the low pressure pump when the surge tank would get full. Unfortunately the switch wasn't reliable enough under road conditions and would allow pressure to build up, spraying fuel out of the air vent on the tank - not good.

To Do List

• Redo the linkage to get rid of the middle (unused) throttle body. Not very urgent.
• Replace the wide-band O2 sensor with a narrow band when tuning is complete.