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Superspec Gearbox/diff Ratios


alanrichey

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AndyW and I have been comparing notes on our Superspecs and among a lot of other differences we have found quite an appreciable difference between our speed/RPM ratios.

 

For the test we both used a GPS-based Speedo, as the speedo in the Superspec is known to over read because the wrong speedo drive wheel is fitted to the gearbox

 

To maintain a true 60mph in 5th gear I need 2500rpm. Andy has to maintain 3100 rpm.

 

(As an aside, at a true 60mph I see 66mph indicated and Andy is seeing 72mph)

 

We are both using the same wheels, and we assumed we were both using the same drive train, MT75 (off a LHD V6) and a standard drum brake based Sierra diff, but obviously not.

 

The difference in indicated speed would lead us to believe we have different gearbox ratios (although maybe only in 5th?) but could different Diff ratios have the same symptoms ? Or maybe a combination of both ?

 

Anyone any thoughts ? Any other Superspec owners who could do the same test ?

 

Al

 

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Sierra diffs will come in 3,14 3,38 3,65 and 3,89 i.m guessing you have 3,65 and the other a 3.89. my car pulls 3k at 70mph on 3.88 diff with a type 9 .82 5th

 

Found this www.super7thheaven.co.uk/components/mt75-gearbox/

Shows different ratios in the mt 75

Edited by DanE
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Al, as a first step we both need to check our diff ratios. I'm going to look see if I still have the Id tag on the diff back plate, or if not I'll do the rotating wheel test to get a rough calc of the ratio.

 

Bob, coincidence! I've found GearCalc earlier on and started playing with it. First impression is that it doesn't have the MT75 gearbox as on option so we're going to have to try to input some appropriate gear ratios.

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Sierra diffs will come in 3,14 3,38 3,65 and 3,89 i.m guessing you have 3,65 and the other a 3.89. my car pulls 3k at 70mph on 3.88 diff with a type 9 .82 5th

 

Found this www.super7thheaven.co.uk/components/mt75-gearbox/

Shows different ratios in the mt 75

 

Same table is at http://www.fordscorpio.co.uk/mt75.htm so I guess the ratios are

 

1st 3.61

2nd 2.08

3rd 1.44

4th 1.00

5th 0.83

 

My other data point is 2000rpm in 3rd gear gives me 30 mph. Very useful for avoiding speeding tickets when my speedo cable broke.

 

Putting all that into the two calculators shows I might have a 3.38 ratio, Like Andy I need to get underneath to see if I have an ID Tag.

 

Fascinating stuff.

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No Id tag on my back plate, so I did the wheel rotation test with just one side jacked up. 10 turns of the road wheel gave 19.5 rotations of the propshaft. So 19.5/10 x2 = 3.9 (I believe I have to double the answer to account for only one wheel moving?)

 

So DanE and Bob are correct in guessing that I have a 3.89 diff.

 

Different from you then Al! Both Superspecs, both different. :-)

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Correction. I just looked at my diff again and there IS an id tag after all. Hadn't spotted it the first time as it was painted same as the diff and stuck flat to the casing at 11 o'clock.

 

Through the paint I think I can just make out the lettering - 83BG JD 392 / 4A 1G05 1 224

 

So that makes it a 3.92 ratio.

 

And from the data on super7thheaven website, it also indicates it's a 7" 83BG casing, made 5th July 1991, tripod pushin type.

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Found mine as well. Needs a lot of cleaning but I can make out 3.62, so Bob was correct. But it does explain all the speed differences. Have to do some more cleaning as the only other thing I can see is 687 where you have 224.

 

Where is the breakdown of the numbers on the super7thheaven site ?

 

So I assume Andy has better acceleration than me, but I have a higher top speed (never been close to that so it's a bit academic), and I have a more relaxing time at cruising speed ?

 

Al

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http://www.fordscorpio.co.uk/mt75.htm

 

Interestingly the MT 75 gearbox ratios are lower than the equivelant Type 9 so the V6 MT 75 is similar to the standard Type 9 meaning the V6 Type 9 which I use has the closest ratios, the only Ford gearbox with better ratios is the Cosworth T5 box

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Where is the breakdown of the numbers on the super7thheaven site ?

 

http://www.super7thheaven.co.uk/components/sierra_rear_differential/

 

But it doesn't explain all the numbers on the tags. From mine I deduced:

 

83BG = casing type

JD = ?

392 = ratio

4A = assembly plant?

1G05 = manufacture date starting with 4=1984, G=sequential month, 05 day in month

1 224 = ?

 

There's also another code cast into the rear cover to the right of the fill plug, but I'm not sure this gives much useful info. Mine has:

 

83BG-4033-BA D 04 BA

 

Andy

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