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MG MGA - MGA 3.3 Diff

For anyone wanting a really high geared car have a look at this on Ebay:http://cgi.ebay.co.uk/MGA-DIFF_W0QQitemZ260057739978QQihZ016QQcategoryZ27380QQrdZ1QQcmdZViewItem

I can't help thinking that this would take all the performance out of the car. Just seems too high. I have the 1800 with 5-speed and a 3.9. I would not want to go any higher.

Steve
Steve Gyles

Shhhhhhuuuuuussshhhhh
Bob (robert) yes I own an MG Dwarf!

A quick guess would be 90 MPH at about 3500 RPM but it might take a half hour to get up to speed.
Gordon
Gordon Harrison

Assuming a 165/80x15" tire, a 3.3 would let you cruise at 91.57mph @ 4000rpm, according to:-

http://pw1.netcom.com/~sgalaba/mph.htm#GearSpeed

Bob, does this mean that MG will be making a return to Le Mans? 6000rpm in direct top (1:1) will give you 137mph. And a 0.86 O/D will give you 160mph. Better forget Le Mans and head straight for the Bonneville Salt Flats.

Derek Nicholson

Damn I was the high bidder at 200 quid with 7 secs to go but lost out. Never mind save me a job changing my diff.
Bob (robert) yes I own an MG Dwarf!

Interestingly, it is an unused original and it is painted black. I always thought they were, but a recent thread claimed they were bare aluminum.
John DeWolf

FYI,
A company in UK now makes 3.7 banjo ratio's for £175 when I purchased one about a year ago. Not had chance to fit it yet though.

Can't remember their name, but if you search the bbs archives for 3.7 you should find it.

Cheers
Mark.
Mark Hester

On the 1600 I helped disassemble years ago, the center section was bare aluminum. I haven't gotten deep enough into my 1500 yet to say if its diff housing was painted or not.
Del Rawlins

Parts in the inventory stream were not necessarily treated the same as parts in the assembly stream. One could have been painted and the other plain particularly if it was a competition or development part and wasn't part of the regular contract with the supplier.
Rich McKIe

An example of this is that generators on cars were maroon and spare part generators were black.

Mick
Mick Anderson

Forget a 3.3 final drive.

The early 1500 with 68 bhp would propel the MGA to about 95 mph. So, 90 mph would require about 65 bhp to overcome all drag regardless of the gearing used.

Does any MGA engine (stock or even mildly modified and street usable) put out 65 bhp at 3500 RPM? The twin cam doesn't and it's the most powerful of the lot. A chart supplied with the twin cam brochure shows the then current 1500 with 72 BHP puts out only 50 BHP at 3500 RPM and the twin cam 63 BHP.

AS the MGA's areodynamics requires about 65-70 bhp to get it to 90 MPH, that gearing is useless if you want to use all four gears. What you would wind up with is a car in which 4th would only be usable downhill with the wind!

When working with gearing, you must consider the BHP available at a given car's speed. If the BHP available is less than the BHP REQUIRED at that speed, the car will just slow down.

Anyone know of a chart showing the BHP requirements of the MGA body at differen speeds? I've seen some for the speed record cars.

Blake




Bullwinkle

I seem to recall a guy named Chris Caton who raced an MGA in the late 70's at one of the Le Mans sports car races. These were held on the Friday or Saturday before the actual 24 hour endurance race took place.

He had the 3:3 diff fitted and claimed to obtain almost 120 mph on the old Mulsanne straight.

I'm not sure what derivative of the 'B series' engine he had fitted, and of course the car was run without the windscreen to assist in the aerodynamics.

I suspect the biggest problem was getting the car moving in the first place. My car has a close ratio gearbox and coupled with a 3:9 diff hill starts can be quite a difficult.

JB
J Bray

Blake,
I'd be interested in a copy of that if you turn one up. Ideal thing to consider when racing.
Barney turned up the drag coeficiency numbers in a similar thread a few weeks back.

http://mgaguru.com/uta_mga/performance1.htm

My question then became, how much better would it become without the windscreen or with a cut down windscreen. Any idea?

3.3 seems useless to me, but 3.7 I though would be worth a shot for driving across australia, where lower reves would mean less fuel burnt.... well unless the engine was labouring.

Reading further,
Is this the sort of thing you after blake?

http://mgaguru.com/uta_mga/performance2.htm

This is for an electric powered MGA though, but perhaps the power source does not matter so much.

I'm not sure if I reading it right,
but in fourth gear :
MPH BHP
101.7 82.9
110.2 103.4
118.7 127.0
127.1 154.2

Does that sound right ???

Cheers
Mark.
Mark Hester

Yes Mark I know the company who makes these non standard ratios for banjos so hence my top bid of 200 Quid!
A 3.7 gets me near the power band that I want so I may have to go with that.
Bob (robert) yes I own an MG Dwarf!

Bob, - If you can tell me who makes the 3.3 gear set I will add it to my list of available ratios for the MGA/MGB banjo axle here: http://mgaguru.com/mgtech/rearaxle/ra201.htm
This will certainly have some application for a higher power race car with no windscreen, given a long enough straight away of course.
Barney Gaylord

Mark:

There is a graph in John Thornly's book "Maintaining the Breed" (page 155) which gives the "total tractive Resistance" of the TF, MGA, and EX179. This tractive efort is listed as B.H.P.

For 100 mph, the total tractive effort is shown as 70 BHP. BUT that would be the B.H.P needed at the wheels?

This MGA tractive effort could have been derived from EX182s which were the LeMans prototype MGAs with cockpit covers, full undershields, and a small perspex windscreen. It doesn't say.

Now I need a graph, or conversion formaula, showing the B.H.P. available at the wheels for the different MGA engines at different RPMs. I've got a power curve for the later 1500 engine which is superimposed on the twin cam's.

Blake
Bullwinkle

The MGA 1500 is credited, according to Haynes MG "contemporary source", as reaching 99mph with 72 bhp; The 1600 as doing 100 mph with 79.5 bhp, 1600mkII 103 mph with 90 bhp and the twin cam 115 mph with 108 bhp (Yeah, right!).

Anyway, even with optimism, these figures do not necessarily mean max bhp at max speed due to the gearing (I assume all at the flywheel.)

Also, from my experience of my own MGAs, if I had the various powers above, from Mark, and my car correctly geared to give max speed about at max bhp, I would expect to reach close to the corresponding speeds. (I might want to grease my kingpins before doing 127 mph though LOL)

Therefore, I would conclude that those figures are likely pretty accurate.
JN McGurk

You could do 122mph this way:-

http://www.mgcars.org.uk/cgi-bin/gen5?runprog=mgoc&p=emg/mgaspo.htm

I'd like to.

Derek Nicholson

Barney I am afraid I do not know who makes a 3.3 diff for a bango axle but this company,
""http://www.autogear.co.uk/""
Make ratios as low as 3.7 to 1 which is pretty good.

Bullwinkle without going to deep you need to consider WHERE in the rev range an engine develops most power. With the MGA it is NOT at max RPM so therefore the power figures quoted by the factory are not the values needed to drive the car at its top speed. In fact if the ratios are tuned to make max power available when needed, (top Speed) then top speed would increase.
Bob (robert) yes I own an MG Dwarf!

Looking at the photos of this 3.3 diff in the eBay listing, the crown wheel doesn't look like a 43 tooth gear, the teeth seem too beefy. With a 13 tooth pinion the pinion teeth would be rather slender and so should the crown gear teeth. I know the carrier is marked 13/43 but that gear looks more like the 39 tooth crown gear on the 4.87 rear that I have. Did anyone actually get to examine this banjo and count gear teeth?

Frank
Frank Graham

Bob:

I know all about the fact that many engines don't develop maxim hp at maximum rpm.

I am looking at the acutal power curves showing bhp at given rpms.

What you need to know is the bhp the engine is developing at a given rpm and see if with the gearing used that the bhp at that rpm is greater the the drag bhp.

Suppose you use that 3.3 final drive.

Using Derick's 92mph@4000 rpm. That creates the following:

92mph@4000
69mph@3000
46mph@2000
23mph@1000

The graph I consulted says the mga drag needs:
20mph about 2bhp
45mph about 8bhp
70mph about 23bhp
90mph about 48bhp
100mph about 70bhp

Thus you have to know what hp is being delivered to the wheels when the engine is turning 4000 rpm.
If the BHP at the wheels doesn't equal or exceed the drag BHP you can't pull that gearing at that RPM.

A 1500 power curve shows the engine putting out 58 bhp at 4000 rpm. The drag graph says you need at least 48BHP to go 90mph. So you MAY have a 10 BHP excess which all may be internally consumed due to ancillaries, transmission and diff bearing drag, rolling resistance (tire internal friction), etc. It would be my guess that you couldn't get a 1500 up to 90 mph with this gearing.

Blake
Bullwinkle

This thread was discussed between 27/11/2006 and 08/12/2006

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