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MG MG Y Type - Brooklands Diff in Y-Type

I have just finished the first 200 miles with a YT fitted with a 4.55:1 ratio "Brooklands" Diff.

Wow, what a transformation. The reduction of revs at 100km/h is only 367, so it makes little difference to the pulling power, but the noise from the engine bay is much calmer.

Around town it has meant that 2nd gear is used for sharp corners where 3rd would have done before, but it is certainly not a burden in this regard.

First gear starts are actually quicker, as you get to 2nd sooner without that extra rev in 1st that is needed with the standard diff.

A standard diff requires more revs for little gain in forward acceration, while the "Brooklands Ratio" of 4.55:1 seems to be ideal in this car.

Can anyone who has also done this ratio change, please add their thoughts to this discussion.
A L SLATTERY

What exactly is a "Brooklands" diff?

I bought a spare rear axle to play with and was planning to mate it with a Morris Minor diff. like some people did with TC's. (http://www.mg-tabc.org/techn-up/diff.htm).

For the heavier Y-saloon 4.55 might be to low, so I'm aiming for a 4.875 ratio.
Willem van der Veer

The offset from the mounting flange of the diff centre and the centre line of the axles is different in the Y-Type and the Morris Minor.

So in order to make the conversion last, you need to machine up a new mounting flange (with the correct bolt pattern) to suit the MM diff, and cut out the old flange, welding in the new one with the correct offset.

Fit the MM centre with the Y-Type sun gears so you can use the standard Y-Type axles.

You will also need to check if the diff housing is bent. I had two housings modified that had no external damage visible, yet one was bent to give 1/8" toe in while the other 1/8" toe out.

I intend to try the other diff in the YA with a 4.55 ratio. A lot of TC's that run 16" wheels also run a 4.55 diff, that's why I settled on that ratio.

A saloon is not so heavy that a 4.55 will struggle. In top gear the rpm difference at 60km/h is only 281, 80km/h only 373, and 467 at 100km/h. A conversion to 4.875 will be hardly worth the effort in my opinion.

I call the conversion a "Brooklands" diff, as it is more than just a MM centre bolted in a Y housing, and I don't want people to carry out dodgy conversions.
A L SLATTERY

The YT and TC's all have twin carburettors and are lighter. I think that for a SINGLE carburettored YA, used as it should be; with a family of four in it, 4.55 might be a struggle especially when it gets hilly.

As I'm still contemplating fitting twin carburettors and hills are quite rare in Holland, maybe 4.55 is a better bet, even with the "high" Dunlops I'm running.

Is the YA-offset that much different from a TC diff? Do you have any pictures of the conversion (in progress?); it would do wonders fot my learning curve.
Willem van der Veer

The mis-aligment is around 5-6mm I believe. Not enough that causes immediate failure of the axles or bearings, but which over time, will certainly cause failure.

I think the mis-aligned straight swaps only survive as they are fitted on low usage cars.

My diffs were modified by a local Y-Typer (hello Dick P), so I do not have any work in progress photos, and once installed in the car there is very little to show of the modification.

It is not a modification for a handyman - you must find a competent tradesman who will properly align all of the components before welding in the new flange, and then make up an alignment jig to get it all straight after the welding.

I will be driving my YT from Brisbane to Melbourne in late March (3 day trip) towing a trailer, so I will be able to give an extended report after then.

BTW - I had 2 spare diff housings modified for the "Brooklands" ratio, so I can refit the original diff at any time in about 2 hours, just for the purists.

I like to drive my Y-Types.
Tony Slattery

The problem with just fitting twin carbs is that it is doubtful that very much will happen to the engine power output. The Achilles heal is the unique to YA/B low lift camshaft. I have made the point before that to increase thermal efficiency by raising the compression ratio is a really effective way to increase power cheaply without any detrimental effects. I have towed a 4 birth caravan with 4 up over the Swiss and French Alpes with just this small mod and covered some 100 thousand miles. I will however say that I have always endeavoured to keep the engine and transmission up to scratch. In passing I would say that in 1947 a small saloon capable of over 70 on pool petrol was considered a pretty good performer, how things have changed!
Bryan

Hi Bryan

How much is your compression ratio now, and did you fit a high-lift cam ? Ever made a rolling road test to see how many bhp and torque values were gained ?
Remo Peter

To raise the CR to 8.6:1 the cylinder head is skimmed by 3/32 inch so that the top-to-bottom head thickness is 74.37 mm [originally 76.75 mm check this first]. I made 4 rocker pillar packing pieces from 1/16 inch mild steal [check the oil way]. 3/32 inch washers are required under the cylinder head nuts, although I found 1/16 inch OK. There has been some talk that this arrangement disturbs the rocker geometry and that shortening the push rods is better. Its up to you to decide, personally the engine has run for some 100 thousand miles with very little wear. You will find the performance much more lively with a top speed near or over 80 mph and with a fuel consumption of 30 to 32 mpg. Adjustments to mixtuure and ignition by trial and error. No other changes were made to the engine or ratios. It is doubtfull if fitting twin carbs will improve the performance furher because the low lift camshaft will restrict the inlet flow.
Towing the caravan the car seemed OK in third at 40 to 45 mph in calm conditions.
Bryan

Bryan is quite right - unless you are going to spend a fair amount of money on an engine strip and rebuild, to simply fit twin carburettors will simply mean you are paying more per gallon (or litre) per mile (or km)of travel than otherwise and be wasting gas (petrol). You will be better off taking the extra you would have spent and donating it to charity.

Paul
Paul Barrow



To pre-empt criticism from the keep it original brigade, in 1963 Brockbank had a word to say!

B Mellem

I have been wondering how I can demonstrate just how good this conversion is ?.

The next time I am out driving with other Y's, I will setup a video camera to record the tacho and view out the windscreen.

Then Paul B will have to find a way to link the video to the website !
Tony Slattery

I am considering converting my YA with a 4.30 MGA rear end. I know this seems like a low gear for a YA with a stock engine, but it has also been converted to 15" (wire) wheels, which should make it nearly equivalent to a 4.55 ratio with 16" wheels. The "Brooklands" rear end approach described is also interesting, but I already have the MGA rear. Also, my original rear end needs some other work as well.

The spring pad spacing seems to be exactly on, and the axle, though slightly shorter, should correct some of the excessive width on the rear wheels resulting from the earlier wire wheel conversion.

Does anyone have any experience with this conversion to an MGA rear end? I know it has been done (I have seen the car) but that was before I thought of doing in on mine, and I didn't investigate it very thououghly.

After reading all of the above, I think just shaving the head and forgetting the twin carbs would be the way to go if the power is disappointing after the conversion. I'd like to stay out of the major engine work until I need to do it.
Bill McReaken

I don't know the details about switching to a MGA unless you change the entire rear axle to MGA. Seems to me that going to a lower ratio rear end depends an the roads where you live. If they are all flat or relativley then it may not be much of a problem,but if you live where I do,with some long and in places very steep hills it is a real problem. My gearbox was thrashed so went with a 5 speed. I think I have the best of both worlds, I can cruise the motorways @ 60-65 at 3800 RPM and still make the mountain passes. Terry
Terry O'Brien

When you are in Australia Terry, you will get the opportunity to drive my car with the "Brooklands Diff" and compare it to your 5-speed option.

I was caught on a very steep hill yesterday, with 4 adults aboard in the YT. Still managed a standing hill start off the handbrake with a little slip of the clutch. I confess that I thought the car would not do it, and we would have to "bale out" the passengers. Yee of little faith !!!

If you send me the rolling diameter of your wheels Bill, I will calculate your RPM for a given speed in top gear for you. Or I can send you the Excel file if you can use it.

Perhaps I should send the Excel file to Paul to include on the website ?.
Tony Slattery

Hi Bill,

I have some pictures of an Y with an MGA backaxle I'll mail them to you, but they are quite large files.

This particular car has centre laced 16" spoked wheels as fitted to many racing TC's. As they don't look to broad I think you may have a problem with the offset of outer laced MGA wheels but I dont know how "deep" they are.
Willem van der Veer

Thanks to everyone for the responses.

Tony, please send the Excel file. I can use that but can not get the rolling diameter data right away. It's 20 degrees (Farenhight) here and snow on the ground. Since the wife (reasonably) insists the car she is driving must be in the garage, I have to store the Y elsewhere and it is difficult to get to it now.

Willem, thanks so much for the pictures. I am not exactly sure what wheels are on my car, but they sit just a little wide on the rear now. Since the MGA rear axle is a bit shorter, I think it might all work out and make the car look better too. If not, I suppose I could always replace the wheels with a different offset.

Bill
Bill McReaken

This thread was discussed between 28/01/2009 and 16/02/2009

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