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
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.
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.