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MG MGA - Upgrade 1500 gears

Hi guys, which is better to improve the four speed 1500, is it better to fit type 9 conversion, or install an MGB 1800 with overdrive, and which is easiest ?
Comments would be helpful, I just find I'm revving the hell out of the car with this four speed box.

Cheers Jack.
Jack New Forest

As original with 4.3:1 final drive, 75-mph = 4400-rpm

Change it to 3.9:1 differential from early MGB for 0.9 overdrive ratio. Easy and cheap. This is what I drive all over the country. 75-mph = 4000-rpm.

Change to MGB overdrive gearbox for 0.82 overdrive ratio. This requires considerable tunnel bashing and lots of cut and weld and machining for a removable cross member on the frame. If you don't do that, then you have to gut the interior every time you need to R&R the gearbox. Huge work expense, moderate parts expense. 75-mph = 3640-rpm. You can also change final drive from 4.3 to 4.1 or 3.9, to slow the engine another 5% or 10%, but you might not like the net loss of torque at rear wheels for casual driving. How much time do you have?

Change to a Fort T9 5-speed kit, pretty much same result as the MGB overdrive. Difference here is more expensive parts cost for the kit, but almost a bolt-in installation done in a week end. How much money do you have?
Barney Gaylord

That sure is a fantastic answer to the question Barney.

It first gives the simplest and most economic option, then the most time consuming option and finally, the most expensive one.

If you try just fitting the 3.9 diff first Jack, you can see if you are happy with it.
You can always do some engine tuning later to add bhp if you feel it has reduced the acceleration more than you comfortable with.
Colyn

Colyn Firth

Jack

As Colyn says, interesting analysis by Barney.

If you are ever passing my door your are welcome to try my car out - Type 9 plus 3.9, but on an 1800 power unit..

This is what she will give you in 5th:

1000 rpm = 23.5 mph
1275 rpm = 30 mph
1700 rpm = 40 mph
2000 rpm = 47 mph
2125 rpm = 50 mph
2550 rpm = 60 mph
3000 rpm = 70.5 mph
3400 rpm = 80 mph
3800 rpm = 90 mph
4000 rpm = 94 mph
4250 rpm = 100 mph

I find the 3000 RPM at 70 very comfortable in today's traffic, both for me and the engine.

Steve
Steve Gyles

Gosh Steve, that's VERY tall gearing, even for an 1800 engine to pull. You are much higher geared than an MGB with its standard 3.9 differential if you remember that the MGB has 14" wheels, the MGA has 15".
Off the top of my head in fourth (1:1), the MGB turns 17.9 mph per 1000 rpm, an MGA MK II with a 4.1 differential such as mine, 18.2 per 1000. (The figure for the MK II is incorrectly quoted in some books).

Even with my T9, and newly fitted worked 1800 engine, I never get the feeling that I'd want to go for a 3.9 differential.
Id anticipate that an MGA 1500 would feel terribly sluggish trying to pull a 0.82:1 top gear with a 3.9 differential ratio.
As others have implied with a 1500, I'd have thought a 3.9 diff would be a nice improvement. With the 3.65 first gear it would still pull away crisply despite being geared up at the rear end.
T Aczel

Jack, my personal preference would be to fit the 5 speed gearbox and stay with the 4.3 diff.
You would then have pretty much the same gear ratios but with the added bonus of an over driven 5th gear and a very slightly higher ratio 1st gear with synchromesh.

It's quite an advantage to be able to easily select 1st gear whilst the car is moving, especially on the steeper hills we drove on last weekend on the MGA Spring Tour.

Even though I now have a 1950cc engine in my car, the 4.3 diff still seems ideal.
The gearing is approx 20.7mph per 1000 rpm and it feels like it would easily pull 5500rpm in 5th gear. That would give over 114mph which is more than enough for me!
Cheers
Colyn
Colyn Firth

"You would then have pretty much the same gear ratios but with the added bonus of an over driven 5th gear and a very slightly higher ratio 1st gear with synchromesh."

Actually first gear on the MGA/MGB Mk I gearboxes is exactly the same ratio as the standard 3.65:1 first gear in the T9. I always thought this to be too low in both my MGA and my MGB, so I took the option of the higher first gear Peter Gamble offered, 2.98:1. In my opinion, this is a bit too far the other way based on my own experience with it. Certainly that's the way it felt to me with my original 1622 engine. However the new 1800 I've recently installed in my MGA is fine with the tall first though. I suspect the best compromise for first is probably around 3.3:1, which is the ratio in first in the Mazda boxes, such as will I believe be offered by Vitesse for the MGA shortly.

As for the diff ratio, I would tend to agree with you Colyn. With an overdrive Vth, (both the MGB Mk I in overdrive top and the Ford T9 in Vth are 0.82:1) , a 4.3:1 final drive ratio works very well.
T Aczel

Barney has nailed it - besides money also depends where and how you drive.
I have both an1800 3 brg / close ratio gearbox / 3.9 diff roadster (no trouble keeping up with highway traffic) also a 1950 5 brg / 4 syncro overdrive gearbox / 3.9 diff coupe (no trouble passing all highway traffic). My ideal choice would be a 1950 engine / T9 5 speed / 3.9 diff.
Mike
Mike Ellsmore

Tom

Yes it is very tall geared. It was ideal when I lived up north and did a lot of motorway driving to get down south for events and family visits. I have said in previous threads that now living down south in the country lanes I have it in mind to revert to a 4.3. That said, the car does not lack performance, it is great in the lower gears with 3rd and 4th especially giving it the necessary performance. The 5th remains a good cruising gear.

I have had it to a ton on the GPS in this configuration and that was before the sportsscreen.

Steve
Steve Gyles

A standard 1500 with a 3.9 diff is really sluggish. I tried it years ago and swapped back to 4.3 after only a few weeks.

I too would really recommend the five speed - particularly for cruising longer distances. It's a lot less wear and tear on both driver and engine.
dominic clancy

You are correct Tom, 1st gear is the same, it's the 2nd gear that is a little higher in the std Ford T9 box.

I have just bitten the bullet and had my T9 box upgraded whilst it was out of the car being repaired.
It now has a heavy duty-high ratio 1st gear and I have also changed 3nd and third gears to eliminate the large gap that was present in the std Ford box.
The new ratios are:-
1st 2.66
2nd 1.75
3rd 1.267
4th 1.00
5th 0.82

I am delighted with the result, previously when climbing a steep hill, the engine was revving away in 2nd gear but as soon as I changed up to 3rd, the engine struggled and I had to downchange again.

I suppose the extra 35+ bhp I now have at my disposal helps a little ☺.

Colyn
Colyn Firth

List of great info lads, I'm now going to source a type 9 box, eBay here I come, and also price up the conversion kit, there is one for sale on eBay £685, the guy says it fits most MGA with high starter motor back, what does this mean ?

Cheers Jack.
Jack New Forest

The eBay kit won't fit your car. On later MGAs the starter motor was moved higher, which requires a different backplate on the engine and a bulge in the tunnel / toeboard to avoid the end of the starter motor rattling against the toeboard.

Beware, not all T9 five speed boxes are the same, there are important differences at the input end.

If you contact Peter Gamble, you will be able to get the correct kit for your cater, but it will cost a little more than the one on eBay
dominic clancy

Jack

These are Peter Gamble's notes to me way back in 1999 on suitable Type 9s. The modification was in its relative infancy in those days. I wasn't the first to fit one in the MGA but not far off.


Steve


Steve Gyles

Thanks Steve and Dominic, useful info there, that's why I asked, I didn't want to go and buy the wrong item, I'll be carful looking around now.

Cheers boys I'll keep you posted on my search,
And I'm assuming I can do this myself without too much hassle.

Thanks, Jack.
Jack New Forest

Jack
if you look on Dominics website you may find some detailed info about how you go about fitting the 5 speed to an MGA.
It really helped me to understand how to remove my similar gearbox a few months ago and, more importantly, how to refit it.
Thanks Dominic, your website has been a great help to me.
The website is http://www.clancy.ch/MGA.html

Colyn
Colyn Firth

I see Mike has the same rig I have in the race car - 3 synch close ratio box, although he uses a 3.9 and I went the other way for the track and use a 4.55.

If you have more than stock power, that CR box gives a really nice set of ratios, otherwise you'd find the first gear a bit high (remembering that with non-synchro first you can't/shouldn't downshift into it until you are stationary).
Bill Spohn

This thread was discussed between 12/05/2016 and 19/05/2016

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