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MG MGB GT V8 Factory Originals Technical - Plenum welding & Lamda sensor voltage

I am fitting a 3.9 EFI to a '75 MGB
Was thinking of saving money(machining various surfaces is expensive)and getting more bonnet/hood clearance by shortening the upper plenum cover itself ie. cutting the bottom lcm off (putting this carefully to one side) then cutting out what I need to remove (about 3cm) then welding the bottom lip back on(bolt holes will give perfect alighnment) and of course welding a couple of patches on the bottom of the LHS air intake where the "curve" is.(of course shortening trumpets etc.) Question is how well does this Aluminum alloy weld?, any special considerations?, is it easy or particularly difficult, should I use a TIG?, I've never done Aluminium welding before.

Also, the lamda sensors, I don't have them and was thinking of substituting a variable resistor and vary the voltage going back to the computer and thus controlling the mixture, for more CO & power. Does anyone know what voltage range these oxygen sensors normally send back to the computer?, I suspect it is very small.
Peter

The material will weld, but I doubt if you could save any money. After you tig'd the bottom half to the top half (if I understand your post correctly) the housing would warp and you would have to have it machined to have it trued. Aluminum is somewhat difficult to weld for the inexperienced and you would be well advised to have someone who runs a lot of aluminum weld for a first class job. You could fixture it while welding, but that would be time consuming and expensive too.

As a point of reference, my local machine shop took about an hour to follow a modified version of Rogers instructions on lowering the plenum about 1.2 inches and it ran about $65.

Best of luck,
Phil
Phil

As for the oxygen sensors, I don't think you can put a resistor and get the effect you want. The sensor output is voltage and at stoicometric it has a fairly high gain and slews back and forth between about .3 to about .5 VDC. If it is working properly, it should be constantly occilating in that range. In effect the ECU controls the mixture by constantly changing the fuel flow so the average is near stoiciometric.

If you want to get out of the sensors, find a hotwire system with a tune resistor in it (post 1989) and use that instead of your current one. I have a chassis number around if you are interested when they went to tune resistors around somewhere if you are interested.
The tune resistor forces the box to a schedule that doesn't rely on the o2 sensor.

Phil

Thanks Phil,
My motor is a 1994 with a new short block/motor underneath (near new anyway, 25,000km). Thats Good news about the tune resistor, There is a tune resistor in the web (I had wondered what it did), apparently it "tells" the computer "no oxygen sensors", yes?. Do you by chance know what resistance it should have for my situation? ie do I have the right one?

As for the plenum, it looks like I'll be going the tried and true, there is no escape. Your costing is very useful as it gives me a ballpark figure. When Greenhorns such as myself walk into a machine shop we are fair game!
Peter

Here's the details on the tune resistor -

A Green (470 Ohms) resistor will give a non-cat tune, and White (3900 Ohms) a cat tune

Here's a link to Roger's lowering instructions-

http://www2.mgcars.org.uk/cgi-bin/gen5?runprog=mgbbs&access=&mode=archiveth&subject=early/34&subjectar=36&thread=E199904201956150

I didn't do it this way, but his method will work.

A less expensive way is to take all the stock for lowereing out of the intermediate piece that hold the trumpets. You can do this by plugging and welding the holes for the brake booster (and the others) and machining the flange on the top of the piece almost down to the floor. Machine the bottom of the trumpet support only to the point you still have some stock to grip the shortened trumpet. The trumpets will come out easily in an oven set to about 275F. Don't forget to have them mill off the bosses on the bottom side of the trumpet support so you can clear the fuel rail.

Roger's method requries taking a cut off of the plenum cover, and I found it difficult to fixture this to get the cut. Conceptually, though, it gave me a framework of where to start to get the height down.

Phil
Phil

I also found this lurking on my desktop at work. My apologies for the lack of acknowledgment to the author-

Hotwire Tune Resistor

All Efi "Hot wire" systems will incorporate a Tune resistor, Its function is to make sure the vehicle
runs the correct Fuel map in the ECU for Cat or Non cat vehicles. If you cannot locate yours the
following may be relevant.

There were definitely some EFi looms made after 1990 with the tune resistor taped inside. From
1990 onwards (14CUX systems) they all have to have one but you've got to find it first!

If there is no tune resistor then the vehicle will run on the default "Limp Home" map. Where the
resistor is fitted externally then it will be where the relays sprout out of the loom, not by the
ECU.PRC9060 is a normal ECU with the regular set of maps in it.

A Green tune resistor (green referring to the colour of the wires) will always give a non cat tune,
and White is always a cat tune. By the way the default (Limp Home) tune is a cat map, so this
usually makes a non-cat car run very rich.

You need to run the correct tune resistor to get the correct fuelling from your ECU and you need
to re-chip all Hot wire ECU's if you have upgraded your engine spec in any way, especially if
increasing capacity. or changing cam, and even for stock engines the Optimax chip will refine
your efficiency, economy and performance.

For those "Even More" technical A tune resistor should indeed be connected between pins 5
and 27. The wire colour s are Black/White and Black/Grey (where the first colour is the primary
colour, and the second is the trace). The Green tune resistor value is 470 Ohms, and White tune
is 3900 Ohms. Ordinary 1/2 Watt resistors are just fine, although the original is a Metal Oxide 2%
item (which I always use anyway). In this location I sometimes use a 1 Watt resistor, because
the larger physical size makes them easier to handle.
Phil

Another thing you might consider is a wideband o2 sensor. I'm not sure exactly how it might be interfaced into the PCM control, perhaps the Rover aftermarket hasn't progressed that far yet, but it can still give you a valuable tool for monitoring your state of tune. Try the DIY-EFI site for details on the wb-o2.

I'm having very good luck so far with the Ford EFI. The adaptive function is working quite well at this point, and considering the limited run time so far is giving very good tune results. At the V8 meet this weekend I idled the car for about 30 minutes friday evening after we got there. At first it ran rich and was a little rough, but after about 15 minutes closed loop operation had kicked in and leaned out the mixture to the point that the idle had smoothed out. We took a few laps around the parking lot with different riders, rolling on WOT and hitting 2nd on the back straight, then idling around the remainder of the course and throttle response improved with each circuit. The next day several cold starts went the smoothest ever, and the one or two idling warm ups to charge the battery seemed smoother than the night before, although it still runs a little rich in open loop until the o2 sensors warm up.

This is with a bone stock Ford EEC-IV system with stock narrow band sensors and absolutely no external tuning. The system came off a Ford modular 4.6L 2 valve engine (Crown Vic) and as used is controlling an Olds 215 with 8.2:1 cr and an Eaton supercharger putting out 10-12 lbs of boost. I'd say that's quite a test of the controller's adaptive ability. Later this week after I get tags on the car I'll get it out on the street to see how it behaves under sustained steady conditions and longer acceleration runs. My impression so far is that it'll do just fine. Once this baseline is established, we'll plug in the TwEECer and begin some performance tuning, interface the wideband sensor, and eventually begin datalogging to see where improvements can be made.

The parts cost of the Ford system was right around $200, and of course there was a considerable amount of labor involved in adapting it to the MGB-V8. Some Painless harnesses should be available to decrease that to manageable levels. The TwEECer RT costs $561 and allows USB laptop interface and five on-the-fly selectable tuning programs (such as economy, performance, stock, test, and no-start), and allows the stock PCM to interface with the wideband o2 sensor for precise control of tuning. This is a prototype system but if someone is interested in doing it I can help.

More details later,

Jim
Jim Blackwood

Jim,
We can solve your problem real quick, all you gotta do is ask!! And maybe offer some free rides as payment!! :-P

What you need to do to solve the rough start-up is go to your local wrecking yard, and pull a set of PRE-HEATED O2 sensors off of a 4.6L vehicle. You will have to run a ground and 12v+ (ignition on) lead down to them as well, but it should solve your problem. EFI systems are designed to use self heating O2 sensors for the very reason you were talking about. The non heated ones are more for aftermarket A/F ration guages like what I have in my car..
Larry Embrey

Larry,
That's what I've got, 4 wire stock sensors. I guess that's not the reason it takes a few minutes to switch to closed loop then, it must be waiting for the engine to warm up. Anyway, there's a setting in the PCM that can change that time interval to anything you want, and you can force it to run in only closed loop or open loop if you want. The adaptive strategy will also, over time, modify the closed loop maps to some degree, based on certain internal control sets which can also be modified with the TwEECer, but I'm betting that the stock control sets will give better tuning as time goes on. It's already quite good, in fact I really haven't had any driveability problems other than at the first few startups. Which does bring up one limitation of the adaptive strategy: If you disconnect the battery it loses the new mappings. That is something I did both at the meet and then back here at home because of a 1/4 amp current drain I haven't tracked down yet. (The new alternator I think) The unmodified settings don't seem to be off the mark very much at all.

Jim
Jim Blackwood

Jim-
I've been following with interest your adaptation of a Ford EFI system on your car.

I've had good experience adapting the newer hotwire system to my MG and have had good luck transplanting fuel injection engines into other vehicles.

I've got a couple of projects that I'm contemplating (unrelated to MGs) where I would like to convert a carbureted engine in one instance and a factory fuel injection in another instance to one that can be programmed, preferably to a speed density system.

I've looked at the Haltech, Electromotive and Acel programmable computers, but am put off by the cost. I take it that the Ford ECU can be edited with the Tweecer system. Can you provide a little more details of how this system works and its avaiability? What level of understanding is required to manipulate the maps? Is it a simple graph, a table or do you actually have to get into the code? The fact that you can get your supercharged engine to work with boost and avoid detonation is encouraging, but is it something a mechanical engineer can do?

Please advise,
Phil Dooley

Thanks
Phil

Peter What I do on ALL of my V-8s with the Rover hot wire F/I is take the trumpet stack off & cut .700 off the botton, get shorter bolts to bolt it back down & it is done & over, you might have to get a 1 x 1 of wood & a hamer & push the fule rail down were you get the clearance you need. I have these ready to ship it you like to just buy one. Ph. me if you like 10 - 2 E.S.T. at 302-734-1243
Glenn Towery

On my cars I go to pep boys & buy a 3 wire ford festa O2 sen. & wire it up & away I go for around $40. each.
Glenn

Jim good to hear about the adaptability. If you can, drop me an email outlining what Ford stuff you got. I am strongly toying with making my 302 EFI. The KEY for me is putting it under a stock hood. The 94-95 Stangs have a lower plenum set-up which I HOPE I might be able to shorten further. I also know Mass Air is the only way to go as it is the most adaptive and handles radical engine changes the best.
Larry Embrey

Phil,
Try tweecer.com and there's also a yahoo group for tweecer which is quite active. There's a GUI interface with both digital and analog screens for tuning the major elements (too many to list) plus a whole bunch of scaler functions and another screen where you can look at the memory map. It should be something you can handle (autocad is beyond *me*). They should be taking orders for the next production run right about now.

Larry,
I agree about the MAF. It's really made the job easy as far as tuning is concerned. Since you've got the 302 half the job is done already, but check the above sites and make sure you get a processor that is on the supported list. The Crown Vic is not, and as a result I can't use the GUI and data mapping functions until either Mike Glover (TwEECer co-founder) maps out the functions for me, or I switch to a supported processor. Jury is still out on that one.

There is another good alternative, called the Mega-Squirt. It's a DIY kit, costs around $150, and is fully documented so there's no guessing about functions. It can function with as few as 2 inputs or can run MAF. That group has been getting pretty good results and is in the midst of a group buy.

Jim
Jim Blackwood

Gentlemen,
Many thanks to all for your invaluable advice, especially Phil, just what I needed to know. I have a white resistor (ie the wrong one)and will change it.
I tried the trumpet/oven trick last night and it worked a treat. I was expecting rather a struggle with this based on what I read in Roger Williams book, instead, over & done in 10 minutes!.

Glenn, your approach seems nice and simple. If I understand you correctly it sounds like you actually squash the fuel rail to oval shape. However the chief impediment is the pressure regulator which I'm going to have to distance/relocate somehow(it's right under the tray) Getting a rail from you is an attractive proposition but the exhange rates (& a lousy 25% import tax!)seems likely to make this expensive. I've heard favourable comment that your engine brackets mount the engine quite a bit lower than anyone else and the 17.5mm you mention comfirms this.

Phil, looking at the empty tray I believe you might be right about the way to proceed, weld up the holes and machine top and bottom. One thing though, did you put some more weld into those "low spots" between the 6 centre trumpets, before starting machining?. Or perhaps it's not worth worrying about, with the manifold surface underneath. At 17.5mm off the bottom Glen must also cut these low spots open/away. Also I noticed that there were fine holes drilled from the bottom of these low spots, at an angle down to unneath the nearest trumpet. Some sort of vaccum balancing arrangment??
Does anyone know what they are there for and are they really necessary?
Peter

Larry, you might investigate the intake and plenum for a 90s-something Thunderbird V8 - I remember reading years ago that they had a lower intake arrangement for hood clearnance that cost some horsepower, but I think a little porting could bring that back.

Wayne
Wayne Pearson

JIM,
Any more info on that Mega-Squirt?? Right now I am looking at getting a complete 5.0L EFI hjopefully from a truck, will use the lower intake and make my own upper to fit under the hood and mount the TB.
Larry Embrey

Wayne,
The 94-95 Mustang is the same way. They had the new bodystyle with lower hood. I will keep an eye out for the Tbird as well though..

Larry
Larry Embrey

http://www.bgsoflex.com/megasquirt.html
Check the link at bottom left for group buy info.

The DIY-EFI page is:
http://diy-efi.org/

Jim
Jim Blackwood

This thread was discussed between 15/08/2002 and 22/08/2002

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