Fuel Injection for the MGB 1800cc Engine

Links

Of course the Megasquirt team.

Werner’s site is a huge help.

Most of my ideas for building things came from http://www.sdsefi.com/, in particular the tech pages:

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.

Status

1/12/2007 – Experimenting with a GSXR 600 based dual TB setup

3/11/2006 – Made the modifications, megasquirt is now providing spark advance as well!

3/04/2006 – Tuning has progressed to the point that I can go back to using it as my daily driver J.

2/28/2006 – Upgraded to mega-squirt-and-spark-extra (MSNS-Extra) to use map based acceleration enrichment.  This has eliminated most of the hesitation when the throttle is opened, more to come with tuning.

01/22/2006 - Up and running, I'm starting the tuning process. It's still a little reluctant to fire when cold, and I've got a stumble from idle when the throttle is opened slowly. There's a stumble under acceleration I'm still trying to tune out as well.

Description

Megasquirt

MS1 V2.2. Modified for the coolant sensor (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

Relay Board

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 a grommet. The wideband cables go through the large opening, those are temporary and that opening will be sealed when the megasquirt is tuned.

Throttle Body

From eBay I purchased twin throttle bodies from an RC51 (Honda VTR1000 motorcycle). The nice thing about these TBs is they each have a pair of 335cc/min injectors, and come with a MAP sensor (which I didn't use), TPS and fuel pressure regulator.  Only one TB is needed, I used the one with the TPS already mounted.  I removed the throttle linkage and the fuel connection that connected the two throttle bodies.  The injector mount with the fuel pressure regulator I swapped onto the body I used.

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

From http://www.megasquirt.info/manual/minj.htm, fuel requirements: 92hp: 2 injectors 27lbs/hr or 284cc's per minute, 54lbs/hr (568cc/min) total.

RC51 injectors: 2 x 335cc/min (32lbs/hr) from the RC51s gives 670cc/min (64lbs/hr).

Throttle body areas:

·        SU's à 2x1.5"d = 3.53 sq in (but has restrictive pistons)

·        DCOE à 2x1.77” = 4.9”, but are usually choked down to 33mm (2.65sq in) to 38mm (3.53sq in)

·        Single RC51 à 2.125"d = 3.55 sq in

·        Double RC51 à 2 x 2.125 = 7.1 sq in

So one throttle body is the same area to two SUs or a big DCOE, and I have 2 extra injectors!  The other issue is that of idle pulse width.  Because it uses siamesed ports, the ‘B idles best with as many injection pulses as possible.  4 RC51 injectors are a little big so it’s tricky to get a good idle – fewer injections per engine cycle must be used.  I found the ‘B to run rich at idle – 12:1 or so even with the SUs, and it’s certainly necessary with the RC51.

Intake Manifold

Made from 16g exhaust tubing. Not pretty, but quite functional. The manifold flange is 1/4" flat steel. I used the head as a jig to tack the pieces in place before final welding with a MIG. A cut down piece of rubber coupler joins the TB to the manifold. It's 3" diameter is a pretty good fit to the 2 3/8" tubing that matches the OD of the TB. I wish I'd made it 1/4" shorter to fit the air cleaner in the bay with just a little more room...

Surge Tank

Made from some 16g exhaust tubing and the fuel pump from a 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. Luckily enough it's a perfect fit for the RC51 fuel line fitting. The pump is grounded to the body of the tank, with an insulated connector 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.

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.

 The stock MGB fuel pump fills the surge tank through the bottom fitting. The top fitting runs to a "tee" fitting, one side to the fuel pressure regulator on the TB, the other back to the fuel tank. 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.

I pushed a used 0.025 MIG tip into the line near the end attaching to the fuel tank to provide some back pressure so the MGB pump isn't continuously pushing fuel through the tank to the return line; only enough movement to bleed the system of air is needed.  I was surprised to find the MGB fuel pump isn’t needed once the surge tank is primed, the FI pump can keep fuel flowing with its own vacuum.  The MGB pump does help fill the surge tank initially, and to keep it pressurized and from filling with air while running.

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 economy tuning.

Coolant Temperature

What a pain, there's no easy place to put a coolant sensor on the B, and it only has the one for the gauge. That's driven by a mechanical (!) voltage regulator. I worked out this circuit to piggy back on the stock one. There's still a fair bit of ripple, especially when the fuel pump "clicks", but it doesn't seem to cause too much of a problem.

TempSenderModification.txt

Air Temperature

I used a GM part from the junkyard, works well.  I tuck it into the open end of the air filter cone.  Perhaps later I’ll mount it, but it seems good enough for now.

Linkages

Went to a single throttle body and removed the original linkage.  The throw was too short, so I added a longer crank arm for the throttle.  The stock return spring on the TB is strong enough.

The bike uses a mechanical idle bleed for a faster idle speed during warm-up – this helps prevent stalling and provides smooth running.  The stripped down (to one TB) mount and the original linkage allows me to use it with the MGB choke cable.  I threaded the choke cable to 10-32 so that I could connect it to the linkage. It whistles like crazy but works well.  Note that this forces the use of the speed-density algorithm with megasquirt, which is the preferred method.  I did find that the ‘B engine could be tuned more easily with Alpha-N before I switched to MAP based throttle control.

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.

I tried using TBs at once for a dual intake. This is pretty tricky through the siamesed ports, I had trouble getting a consistent idle. I believe this was due to the charge robbing problem of siamesed ports and the size of the injectors, which forces fewer pulses per cycle. A big balance tube between the two separate intake manifolds might have worked, but it was easier and cheaper to switch to a single TB.

To Do List

·        Replace the mechanical voltage regulator with a solid state (LM317 based) version.  I’ll need to recalibrate the coolant sensor.  The advantage is that the coolant signal will be more stable and accurate, especially during starting.  Right now the temperature signal jumps around as the voltage changes and as the regulator heats.

·        Add a PCV valve.  Probably the older stock mushroom valve mounted on a tube brazed to the top of the intake manifold.

·        Replace the wide-band O2 sensor with a narrow band when tuning is complete.

·        Provide a little more room for the air cleaner - shorten or remake intake manifold.

GSXR 600 Based FI

More details soon.

 

My home page is Mike's 250lb Locost Site.