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MG MGB Technical - 3.3 diff behind a stock B engine ???

I have a 3.3 rear axle out of an automatic MGC that I am planning on using behind a GM V6 conversion sometime in the future. In the mean time I thought that I would stick it in my 72 tourer just to see what the taller gearing felt like in that application.

My first thoughts are that it would eliminate the "stump puller" characteristics of first gear and that I might end up "rowing" the stick a bit more than usual, but I do wonder what the top gear and overdrive top gear would be like.

If I am doing the math correctly there is a possibility that it would work to get a reduced cruising RPM on the Interstate with an occasional change down for steeper hills. I also understand that if I really put the pressure on early I could end up with unnecessarily wide and inefficient throttle openings.

My question here is, has anyone had any direct first hand experience with this swap and if so what was the "feel" and performance like?

Thanks for any input that you might make.

Jack
jack austin

Been there & done that! You will be slowing the motor down 500-600 r p m. If I remember 65 mph in 4th. is 35oo rpm. O/D dropes down 500 rpm = 3000 rpm @65 mph. 3.3 = 24,2500 rpm. I have a 71 moter in my GT & I turn 2500 rpm @ 65 mph with 185/65/15 tires, Datsun Z turbo 5sp. You will need to change the rear diff flange to the B flange, for the driveshaft be carefull!!!!
Glenn Towery

I know you're looking for first hand info and I don't have it. But I'll offer my thoughts anyway because it's late and I'm not ready to sleep yet. :)

If memory serves, top gear with a 4-sync box in overdrive is equal to a 3.2 rear end without OD. That means your regular top gear would be roughly equal to this and overdrive would take the ratio down to around a 2.7 non-OD equivalent. A stock B powerplant will have a tough time pushing a gear that high, so my guess is that you will never use overdrive in top gear again, until the engine swap is completed.

My BGT is basically stock with a 4-sync OD box, and 4th-OD is useful only for flats and mild grades, and only while at high enough speeds to be in a decent part of the powerband... say around 70+MPH. Since this will be your new standard top gear, I bet you spend a lot of time in 3rd and 3-OD. The lower gears will probably be really nice for around town driving, similar to more modern ratios.

Of course my math is based upon a brain which is barely able to function from lack of sleep, so take it for what it's worth. ;)
Steve Simmons

Jack-
I've worked out a table that will show the road speed in 1,000 RPM increments for 3rd, 3rd Overdrive, 4th, and 4th Overdrive. Unfortunately, I can't get it to post here in a comprehensible form. Post your Email address and I'll send it to you.
Steve S.

Jack,
I did just that with my '66 before I installed an overdrive box. I stuffed a 3.3 CWP in a late B axle and ran it for over a year. It was a very pleasant car to drive and yes, it made 1st gear more usable and gave the car a longer legged feel. I didn't suffer from lack of acceleration either. I'll leave it to the others to quote the actual #'s but I liked it a lot.

Cheers,
David
David

Jean-
Let's try it this way:

3rd gear 3.909 Vs. 3.307 in MPH*:

2,000 RPM 26.0 / 30.8
3,000 RPM 39.1 / 46.2
4,000 RPM 52.1 / 61.6
5,000 RPM 65.1 / 77.0
6,000 RPM 78.1 / 92.4

4th gear 3.909 Vs. 3.307 in MPH:

2,000 RPM 35.8 / 42.3
3,000 RPM 53.7 / 63.4
4,000 RPM 71.6 / 84.6
5,000 RPM 89.5 / 105.7
6,000 RPM 107.4 / 126.9

4th gear Overdrive 3.909 Vs. 3.307 in MPH:

2,000 RPM 43.6 / 51.6
3,000 RPM 65.5 / 77.4
4,000 RPM 87.3 / 103.2
5,000 RPM 109.1 / 130.0
6,000 RPM 130.9 / 154.7

*Presumes road wheels that have an Original Equipment rolling radius of 23.5”
Steve S.

From the looks of the data on that table, this 3.301 gearset is NOT going to work for you Jack, and you REALLY need to sell that MGC axle to me!

:)
rick ingram

Jack,
There are online gear ratio / RPM converters you may want to check as well.

In my opinion most British cars including B's were laughably under geared.
Fine for pottering around the back lanes of Abingdon but way to short for serious long distance freeway / highway running.

As a rule of thumb, most engines achieve their best fuel economy at an RPM corresponding to a piston speed of 5 to 6 m/s (16.4 to 19.8 ft/s).
Piston speed (ft/s)= 2*stroke(inches)*rpm/720.

Piston speed = 2 x Stroke in inches x rpm / 720

Cheers , Pete.


Peter Thomas

"In my opinion most British cars including B's were laughably under geared.
Fine for pottering around the back lanes of Abingdon but way to short for serious long distance freeway / highway running."

Without OD, maybe. A very tall gear may be fine for cruising, but will hit acceleration. Vehicle gearing is usually a compromise between the two, within limits. There is no point over-gearing an engine so much that it cannot pull sufficient revs to get a decent top speed. Normally cars axles are said to be geared to achieve top speed in 4th when that gear is 1:1, but it seems the MGB is capable of better than that, which is where OD comes in and is a benefit. How much further one could go is a moot point.
Paul Hunt 2

" A very tall gear may be fine for cruising, but will hit acceleration. "

That's why gear boxes have more than one gear!

Joking aside , Yes I agree with your comments about the necessary compromises.

I have always thought a gearbox should have:
a start off gear to avoid slipping the clutch unduly ;
A cruising gear to keep revs downs and fuel economy up , and ,
the rest in between for hill climbing and acceleration.

I am sure there is no "perfect" gearbox since every drivers wants more here or less there and of course driving environment also plays a part.

Cheers , Pete.
Peter Thomas

Pete

Just ran the numbers as an exercise.

Solving your equation for RPM gives:

RPM =(Piston speed *720)/(2*Stroke in inches)

Using a stroke of 3.5 inches (88.9 mm) and piston speeds of 16.4 and 19.8 gives the maximum MPG range from 1687 to 2016 RPM.

As a reference my 72 BGT with OD turns about 3000 at 65 mph. Does make a 3.3 rear end look attractive from the mpg perspective. Not sure how much of a hill would require down shifting out of OD.

FWIW

Larry
72 BGT daily driver
Larry Hallanger

Larry-
I'm not so sure if your expectations for fuel economy would be realized anywhere outside of flat terrain. At 3,000 RPM a stock engine is at the peak of its torque curve, permiting a fairly narrow throttle opening. Reducing engine speed at 65 MPH might require a wider throttle opening. Not the best way to get fuel economy. Of course, if you live in Kansas.......
Steve S.

Figures originally published in The Motor September 26th 1962 show the MGB delivered maximum torque at 3000 rpm, minimum fuel consumption at 3500 rpm (but pretty flat from 3000 to 4000 pm), and maximum power (FWIW) at 5500 rpm, all at wide-open throttle.
Paul Hunt 2

Steve

I do not live in Kansas, and it is reasonably hilly around San Diego (I live at about 1000 ft altitude). Did not expect that the numbers from the "rule of thumb" would be a realistic answer to maximizing mpg, but they do give an insight into the "rule of thumb". Considering only one parameter in any complex problem with invariably give you a bad result.


Paul

I can understand max torque and max power at wide open throttle, but minimum fuel consumption?

The minimum fuel consumption being relatively flat from 3000 to 4000 rpm really suits freeway/Interstate driving, especially with an OD.


Larry
72BGT
Larry Hallanger

Larry - I probably should have said the fuel consumption figures were being measured in pints per hour and not miles per gallon. All three sets of figures are derived from the rev range of 1000 to 6000 rpm. 'Steady state' figures were erived by using an engine or transmission brake to obtain the required rpm even though the throttle is wide-open.

"The minimum fuel consumption being relatively flat from 3000 to 4000 rpm really suits freeway/Interstate driving, especially with an OD." - probably why I've seen the high 30s in long distance touring through France in the roadster, and mid-30s in the V8!
Paul Hunt 2

This refers to 'specific fuel consumption.'

Piston speed is only one of several important variables. Different variables become important on different engines: valve/head design, cam overlap, exhaust backpressure curve, and on and on and on.

In the MGB's case, the port design takes center stage. At low revs, cylinders 1 and 4 run rich, 2 and 3 run lean. At very high revs the opposite can happen. Hitting an equal mixture in all cylinders on this engine is, well, hitting a moving target.

Though there's no general case for ALL the variables involved, it is generally true that the torque curve is the end result of the combination of all these factors - highest torque will be achieved at a point where all factors are working together well.

So, perhaps a better rule of thumb would be that the best power/consumption tradeoff typically occurs not too far from the peak of the torque curve. Gear it such that your high cruising speed is not far from the peak of the torque curve, perhaps rev a little lower if you've got more power than you need, or a little higher if your motor's wimpy.

The other problem: 'specific fuel consumption' is taken at high load. If you've got, say, twice as much engine as you need, (e.g. big V8) then you aren't cruising at high load, so none of this matters. (i.e. rev as low as you can without having to open the throttle too wide, the Other Rule of Thumb)
Sam Good

This thread was discussed between 23/12/2007 and 06/01/2008

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