MGA MGA overdrive - that is the question

[tdskip]

So the engine and transmission are out of the MGA and I happen to have an early MGB O/D handy. Hmmmm....

I know there is a bunch of work to do for the conversion to work.

Worth doing? What do you think about impact on the cars value (she is a solid driver, not show)?

 

[Steve_S]

I'm a purist, but I'm doing it to mine. D-Type OD to MGA box. The OD will be activated by using the fog lamp switch on the dash panel.

As far as value, I think it would be like any other modification. Less valuable to some, more to others. The type of person who would worry about losing points in a concourse show probably wouldn't be looking to buy your driver MGA anyway.

Heck, it's ten times better than a 5-speed conversion if you're looking at originality. At least you retain the sound and feel of the original MGA unit, but with a switchable final drive.

 

[tdskip]

Hi Steve! Hope you are doing well.

Do you have a parts list together yet? Are you expecting to notch or bend sheet metal to give the O/D room to fit?

 

[burgundyben]

I looked into doing that, and decided to go for a five speed.

I've just completed the whole job, MGA1500, now an 1800 with 5 speed and a 3.9 diff.

I reckon the biggest hit is the 3.9 diff, I'd do that first and see what you think.

 

[billspohn]

With an OD you don't NEED a 3.9 diff.

You need to have a spacer machined up for the driveshaft - 1 18", so that it fits snugly into the yokes.

You need to bump the tunnel a bit where the solenoid is.

You need to bend the MGB shifter into an 'S' so your knuckles aren't under the heater controls.

You need to reconfigure the top of the tunnel around the shifter a bit.

You need to either hack the ears off the frame and weld on another one, or (the best way IMO) make up a mount that has a straight piece of tube that fits between the ears and is welded to a bracket that bolts to the stock MGB pads on the trans. This raises the rear of the trans a bit, but not into contact with the inside of the tunnel, and the changed angle seems to do no harm.

I must take some pics of the bracket on my race car If I ever take the trans out.

I applaud the use of the fog light switch (F=faster)

 

[Steve_S]

Excellent post Bill, thank you! I would love to see photos of your mounting adapters. Cutting and welding has been my biggest concern because I'm not willing to modify the car in such a manner. All modifications need to be fully reversible.

I have not started this project yet and don't plan to until next year. Too many other things on the plate right now and I need to rebuild the OD unit. I'm hoping to mate the D-Type OD assembly to the stock MGA gearbox.

I already have a 3.9 rear end because I have a 3-main 1800 block and head. M.G. should have put that in the car to begin with, in my opinion.

F=Faster indeed!

 

[ronzet]

Hey Steve, I hope you are well....

Just a quick query re: the 3.9. I have been talking about such a conversion for a little while BUT, even with the original 1500 in my A and 4.3/1 rear I do quite a bit of downshifting as it is....

If I finally get around to converting, how <span style="text-decoration: underline">large a price </span>will the Piper take on the performance at the low end?

 

[Steve_S]

I've never driven a 1500 with a 3.9 rear end but I'd imagine it will struggle a bit. In lower gears it probably won't make much of a difference, but climbing a hill in 3rd may be more difficult, and achieving highway speeds may take a bit longer and require more throttle to sustain. Again, I have no personal experience with this particular setup but I have changed gearing in other cars, including increasing the gearing in my 1250 TC.

 

[ronzet]

Steve,

<div class="ubbcode-block"><div class="ubbcode-header">Quote:]... including increasing the gearing in my 1250 TC [/QUOTE]

Using the TC as a reference ... How much of a difference in the 'climbing a hill in 3rd' did you notice in the TC after the gear ratio change??

Thanks...

 

[Steve_S]

The thing about the TC is that the stock gearing is all very low. So what happened is that 3rd became MORE useful for climbing, while 4th became a bit less useful. But the reasoning for increasing the final drive ratio was to lower RPMs on the highway AND gain top speed, where the MGA only needs to lower RPMS. Driving the TC in the past, I would climb long, gradual grades in top gear. Now when the grade gets a little steeper I find myself having to shift down to maintain speed where before I could give it more gas.

I didn't increase much so the difference isn't that great, and it was a worthwhile trade-off between acceleration / climbing and highway cruising. There are higher ratios that people sometimes use, but 4th gear becomes completely useless without significant engine modifications.

 

[ronzet]

Steve,

Thank you for your well articulated reply.

Although the TD vs MGA comparison may not be a good one, it appears that a reduction in highway RPMs with a loss in grade performance is likely a reasonable exchange since most of my driving is on the SoCal freeways...

The conversion from 4.3 to 3.9 has been moved up a few notches on my list of 'things needed' in the on-going restoration...

 

[Steve_S]

If you're sticking with 1500cc and plan to put an overdrive in, I would consider leaving the 4.3 in as Bill suggests. If you stick with tall tires then the point is even more valid.

By the way, if you want to figure out the percentages, I went from a 5.125 to a 4.875. There is also a 4.625 which is too tall for my stock engine unless all I'm doing is highways. You can probably compare the percentages between what I did and what you're planning to do.

 

[burgundyben]

Last night I took mine for its first proper drive since the conversion.

For reference, I pulled out a tired 1500 with 4 speeder.

I fitted a rebuilt 1800 18V motor with poilished ports and chambers and bulleted guides, currently running the 1500 carbs and MGA manifold, fitted a ford 5 speed and a 3.9 diff.

So its pretty much MGB running gear as 5th on the Ford box is the same ratio as OD on the MG box.

It goes like the wind! I cant believe how big the difference is! Its a totally different car.

 

[Steve_S]

I'm not usually one for engine swaps, but the early 3-main 1800 not only looks identical (unless you're REALLY looking) but it also sounds the same, and has the same driving characteristics. The only change in the car is more power. It's really the same block as late MGA, only bored a little bigger. In fact many people have bored out late MGA engines to 1800. Considering this, I look at the early 1800 as more of a period-correct factory replacement.

 

[billspohn]

The easiest way to go OD is use a 3 main MGB trans and rear engine plate. Bolts in.

You can install an OD and main shaft in an MGA box - more work.

As for diff ratios, here is my view gained from experience over the years.

A 1500 will not like a 3.9 with stock or near stock tires. It will work passably well with replacement 185 x 60 - 15 tires however, which require wider (5 - 5.5") rims

A 1600 is similar but puts up with it a bit better as it has more torque.

A 1622 or 1800 has no problem ulling a 3.9 even with stock tires.

If you fit an OD trans there is no point in changing the diff.

The absolute sweetest box ever is a close ratio with OD and a 4.55 diff. Splits between gears (incl. 4th OD) of only 1100 RPM at 6000.

You can fit a 4.88 or a 5.125 - I have done this for hill climbs. Youi accelerate quickly indeed - up to around 90 MPH at which point you are topped out.

So an 1800 with 3.9 and 15" tires is nice, the same engine with an OD and any tires you like is darn near perfect.

 

[Steve_S]

My MGA gearbox is in perfect order, but the MGB gearbox with the OD unit is in poor condition. I figure it's better to put the OD parts on the good gearbox than rebuild the entire MGB box and have a perfectly good but worthless MGA unit sitting around. With all equal I would probably use the MGB box as you suggest.

This is all in the future anyway, and I'm in no hurry since I have a 3-main 1800 and 3.9 rear end already. The OD would be mainly for extended highway driving.

 

[ronzet]

Bill,

Wow... Thanks for the well thought out dissertation on this topic...

'Me thinks' that I will stay the course on my A... leave it with the newly rebuilt running nicely 1500, tranny, and the 4.3. I still need to finish the interior, buy the chrome spokes, and drive it.

I will begin and finish my '66 B restoration to placate my wife and daughter who want it done so they can use it....

After that, should I still be 'with us', I will consider an 1800 3 main for the A and its ancillary accoutrements....

I think that the proclivity to 'fix it even if it works' should be held in check here....

Thanks again.

 

[billspohn]

Steve, you have to gut the box to change out the gears on the mainshaft anyway, so you get your choice of cases. I agree that sticking with the MGA is probably a good idea. I'd use a diaphragm clutch and that means using the MGB front trans cover, if you haven't already gone that way.

Ron, the only thing a 1500 could use that isn't 'make work' are front disc brakes. Keep an eye out for some 1600 bits.

 

[tdskip]

Well, after taking all of this in after kicking off the thread I think I've settled on the following.

If the original 1500 looks good for another 10K miles then I'm going to leave well enough alone there and swap in the O/D. If the original 1500 looks like it needs any work at all I'm going to pull the 1800 from the idle BGT (really strong engine) and go with that.

The BGT needs so much body work that is isn't economic to even consider fixing it. Besides, I have a spare Rover V8....

 

[Steve_S]

Bill, I don't know what's in there. The PO installed the engine and trans and I haven't had a reason to pull it apart, thankfully.

Tom, the early 3-main 1800 is really the way to go in an MGA, if you can get your hands on one. Alternatively I know of a guy locally with a 1622 bored out to 1800 and ready for final assembly, which he's selling.