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Robbie (Forum Supporter)
Robbie (Forum Supporter) MegaDork
4/30/24 8:12 p.m.
AxeHealey said:

This may be a stupid question but are all speedo drives geared the same? That is, how do you know that the feed coming out of the Miata trans will be the right RPM to spin the MGB speedo?

 

They're not, and I don't. It's probably possible to do some math based on Miata trans rear gear and tire size and then backing out the mgb rear gear and tire size.

But instead when I get there I will see how far off it is, and then seen if I can get a different drive gear in the trans. Mazda offers many sizes based on the Miata rear gear and tire sizes offered on the Miata.

Mr_Asa
Mr_Asa MegaDork
4/30/24 8:20 p.m.
Robbie (Forum Supporter) said:
AxeHealey said:

This may be a stupid question but are all speedo drives geared the same? That is, how do you know that the feed coming out of the Miata trans will be the right RPM to spin the MGB speedo?

 

They're not, and I don't. It's probably possible to do some math based on Miata trans rear gear and tire size and then backing out the mgb rear gear and tire size.

But instead when I get there I will see how far off it is, and then seen if I can get a different drive gear in the trans. Mazda offers many sizes based on the Miata rear gear and tire sizes offered on the Miata.

You might google one or the other.  I know that Mustangs have calculators all over the place that allow you to figure out the transmission side drive-gear if you know the tire size.

Robbie (Forum Supporter)
Robbie (Forum Supporter) MegaDork
5/4/24 11:29 a.m.

In reply to Mr_Asa :

Good point, and I'll do you one better. 

"Ass"uming that the mgb and Miata speedometer gears both measure the output shaft directly (the output shafts have a screw gear on them so there is a ratio that is likely different), then I really just need to make sure that the number of teeth on the speedometer gears in the Miata trans is the same or similar to the tooth count on the mgb speedo gear. 

I counted the mgb one last night at 20 teeth, and the one in my 5speed Miata trans had 23.

https://www.miataturbo.net/suspension-brakes-drivetrain-49/clarification-request-how-many-teeth-vss-gears-respective-diff-ratios-83911/

According to that thread, there should be a 20 tooth gear available, and I haven't checked what is in my 6speed trans yet. 

So I feel like there is a solid chance I get lucky here and it's close, but that's a future robbie problem.

nocones
nocones PowerDork
5/4/24 1:50 p.m.

In reply to Robbie (Forum Supporter) :

I think that everyone would love to see you get to the point where this projects biggest integration challenge left is figuring out how to make the speedometers work correctly..

Robbie (Forum Supporter)
Robbie (Forum Supporter) MegaDork
6/2/24 3:42 p.m.

Ok I've been diligently chipping away at the car but not so diligently updating this thread.

First I finished the shifter panel:

next up was the fuel system. Of course I left gas in the tank years ago so first up was to remove it. Luckily the pump still worked so I just pumped it out. Like all 8 gallons. Doh.

I didn't take many photos of the next part because the oil grime on the bottom of any mgb is so thick and nasty by now i think it is turning back into dinosaurs. It's sorta like sap, just gets everywhere. But, I persevered. What I did was I pulled out the mgb power wire that normally runs through these cable ties under the car (I won't need it because I'll be using the Miata power cable), and I replaced it with a nylon line that will become my return line for the fuel. 

then, I got to work assembling the rest of the fuel system. In no particular order, I cut the steel mgb fuel line at the engine bay and bent it to a spot that would work with the Miata rubber line to the fuel injectors. Gave the end a flare so that rubber line wouldn't be tempted to fly off. I put a hose barb into the nylon return line so it would connect to the rubber return line from the Miata rail.

then, I made this bad dog. Moss sells these really nice plastic boxes that fit perfectly in the mgb battery box locations, and since that is basically where the stock mgb fuel pump sits it would be an ideal spot for my new high pressure fuel pump and filter setup. So I mounted my ford high pressure external pump and matching fuel filter to the box and made up these nylon lines and quick connects to connect it all. Then I dropped it in. Factory mgb fuel pump wiring worked perfectly and I just had to add a ring terminal to the ground wire end. I even had a couple rubber grommets laying around that were perfect for sealing the holes I cut in the box for wire and hoses.

Better view inside, when building I kept an eye on what work it would be to replace these when needed.

then I dropped the whole thing in the car. Boom goes the dynamite.

To finish off the fuel system I used the specific metal to nylon compression fittings, easy peasy.

to finish off the return line, racec4r helped me by installing the cut off end of the metal line from the engine bay into the fuel level sender plate.

he even kept the mgb threaded disconnect in case it is ever needed to remove, and he bent up the steel line to go up and over the rear axle where it should be good and out of the way. We supported it with a stock steel tab.

the last part was simply to integrate the wiring to the Miata harness. Miata of course controls the fuel pump via the ecu and a few signals, including the start signal and the maf, and I want to keep all that. There are also relays to power the pump. So, I simply found the wire from the relay that powers the fuel pump (happens to be blue with a red stripe) and connected it to the mgb wire that powers the fuel pump. I found a wire in the mgb harness that had the right connector for the extension I needed, and I was blue with a red stripe. Couple crimps, and voila!

with everything connected and wired up, it was time for some testing.....

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