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MG MGB GT V8 Factory Originals Technical - Which axle ?
I'm in the position of having two axles to choose from - a 3.307 MGC unit and a 3.07 MGB V8 unit.
Assuming they are both servicable which would you recommend I use?
Awaiting your opinions with great interest!
|I'm sorry for posting this twice, unintended.|
|What trans. are you going to use? With a .63 O/D that is in a T-5 I would use the 3.3. With a rover .8 you will need the 3.0.|
Its a standard SD1 5 speed LT77.
The SD1 LT77 can have a 0.792:1 or 0.833:1 depending on the year, later cars having the higher ratio. Although either will work fine with the 3.07:1 axle, you might find the 3.307:1 a bit too, low especially if you have an early (pre D suffix) box.
|It's a 1985 SD1 and the box serial number is xxxxx....xD so I assume it has the later ratio.|
|Yes, a D suffix box should have a 0.792:1 5th gear ratio.|
|So the advice is to go with the 3.07? |
How much difference in practice would I notice between that and the 3.3?
I have the 3.07 with the standard overdrive and can just get 27mpg on a cruise and about 24mpg otherwise. Given the price of fuel, I would opt for the higher ratio, it's also more relaxed at any given speed and puts less use on your engine.
|J E G Eastwood 1|
|Of course you might be putting a V8 into your car for the breathtaking accelleration rather than economy.|
I kept the 3.9 and I love it. I take off in second gear like a catapult!
There are online calculators to help work out things like RPM at given speed for various axle ratios.
Also it is worth taking into account what type of driving environment and you particular style to help make the choice.
For long distance driving the lower numerical ratio will use fewer revs at a given road speed while the higher numerical ratio will give a higher rate of accelleration but also more revs at the same given speed.
This is one but there are others:
The case you are looking at the 3.07 with the 5th. gear of 0.792 will give about 2140 RPM where as the 3.308 diff will give around 2305 at 100 KPH (62 MPH) both assuming you have 14 inch tyres of 165-75/14 size.
Cheers , Pete.
when I did my conversion in the early 1980is, it was hard to find an axle and I had to take what I could find. Mine is a 3.3.
As the RV8 (3.9 FI) was introduced, the 3.3 was chosen by Rover, helping to reeve out in 5th.
Remember, the works V8 did not use the SD1 engine that came later, have a look at the reevcounter of the BGT V8, it is scaled to 6000 and should not be taken above 5000, maybe another reason for a 3.07 as Abingdon had to look at their competitors for the V8 in those days (Capri 3.0, Datsun Z 260, Reliant Scimitar GTE, Stag ...).
Belive it or not, the BGT V8 is capable for more than 130Mph with the 3.3 setup although it feels very unsafe at speeds in exess of 125Mph, becoming even worse bejond the final 140 figure on the speedo.
The other point is the style of driving you prefer.
With the 3.3 ratio and the 0.83 5th, it is fun to drive around town on idle reevs.
I was only able to compare it to the works V8 (mine is a SD1/Holley/JWR/manyfolds throug the inner wings) and found the original V8 not comparable. (Not very much quicker than my stage 6 roadster.)
Concerning fuel economy the 3.07 axle should be better than 3.3 for motorway work, for a mix the 3.3 will give you nearly equal results but with better response.
I realy love my old MGC 3.3 axle!
|Keep in mind that you might not improve the economy by going for the 3.07. I have heard complaints that it got worse in some instances. This is because the motor has an optimum powerband/rev range. You might be reving lower, but you're pressing the acellerator alot further. |
For my motor it appears be be around 2700 plus that it really starts to get lively. A small increase on the acellerator gets you a big surge of power.
|I have a little different question relating to this post.|
I am running 3.07 with the LT77 but am not seeing the RPM levels mentioned above.
At 70 MPH, I am turning around 3100-3200 RPM. This is higher than I expected.
I discussed this with Clive Wheatley and he thought I might have an LT77 with a 1.00 5th gear ratio. I checked the serial number on the transmission, but it had been partially scratched out (at the time of the rebuild, I guess). I believe the last digit is a C.
I am using the RV8 VDO tachometer.
The LT77 never had a 1:1 5th gear. 4th is direct drive and 5th will be 0.792:1 or 0.833:1. The C suffix box is probably the lower 0.833:1 ratio. 3100-3200 rpm will be closer to 90mph than 70.
|Maybe the 3.07 ratio really isn't!!|
|There have been several excellent articles in the V8 newsletter on transmission and axle selection. It has to do with a lot more than just the overdrive ratio. Having the proper 1st gear ratio is just as important, as well as a useful spread between the gears. The T-50 articles, while directed towards an old and less than ideal transmission for a V8 swap, gave lots of useful information for choosing good gearing across the board. Several other articles addressed the more common choices.|
|Scott, I would be suspicious of the tach.|
If all is as you say then your RPM at 70 MPH should be about 2500 if you have the 14 tyres mentioned in my earlier post.
Either that or you have some item in the mix which is not as it pretends to be.
Just a thought...try the online calculator for all the gears and see how it matches up to your car.
HTH , Pete.
Although I have not read the article, I am in complete agreement. Most people only look at what the final gearing is going to do with their 5th gear ratio. This is a poor way of looking at things.
1st gear and 4th gear are things I ask people to consider as well- most Still overlook this from everything I see.
Also, if you have a high compression ratio with a high lift cam producing lots of upper end power and a very limited torque band, a close ratio gear set is needed with one final ratio while a standard motor or slightly built motor may be Far better with a wide ratio and a completely different rear axle ratio. Its funny how most people associate a "close ratio 'box" with going faster when some cars are actually slower because they do not have the peak power but have far more wide band...
When you have a choice in Total gearbox ratios, look at that first to match your motor, then once that is matched as best available, then look at rear end ratios.
Sorry, ranting a bit.
|BMC Brian McCullough|
|Thanks everyone for all the useful information, and the very interesting debate.|
I have gone with the 3.3.
This thread was discussed between 22/04/2006 and 28/04/2006
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