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Old 13th July 2014, 05:42 PM
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Default 6.9 with 450 differential

Some of you may have noticed a brief mention by SEL69L that he's had a 450 differential fitted to his car. This step was recommended by a friend of his that's quite heavily into American muscle cars, and he sort of specializes in making cars go 'harder'. That of course was the motive of fitting the 450 diff to David's car, but it was also suggested that the whole drivetrain would run more efficiently. Less torque converter slippage, more definite top gear engagement, it should run more economically and obviously accelerate a bit harder.

Possibly the conversion has achieved the last named improvement, but my 'seat of the pants' experience tells me that there are one or two facets that I find dissatisfactory, especially when comparing with the driving characteristics of Gleaming Beauty and also my 420 SEL.I am primarily referring to top gear driving. In fact, that's what I look for in a car - its ability to waft along without much prompting and maintain a good, steady gait. With the 3.08 diff fitted to David's car, it becomes somewhat mandatory to keep your foot on the accelerator to maintain speed. The moment one eases off, the car slows down noticeably. Others may perhaps prefer a car with those characteristics, but I tend to lean the other way - that is the reason why I have a 2.46 (?) diff in readiness to be installed in Gleaming Beauty. Regards Styria

Last edited by Styria; 14th July 2014 at 09:46 AM.
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Old 14th July 2014, 11:31 AM
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Default Re: 6.9 with 450 differential

The diff. was replaced because although not VERY noisy, was starting to develop a whine. A complete 450 diff. in good condition was available for free, so it was installed.

I have found that the fuel consumption for normal driving strictly within legal limits has not materially changed. The diff. specialist said that would be due to much less torque slippage in the range of normal suburban driving speeds, that is, up to about 100 km/hr.

The car is now more throttle responsive (if the vehicle ever needed that!). I will now have to be a little more conscious of rear end breakaway on wet surfaces, especially when negotiating roundabouts. The car is certainly sharper off the line.
Before, I used to get the car rolling, then just 'plant it', up to 60 km/hr. Now, I just accelerate it nicely, making sure to avoid wheelspin, which otherwise would just waste the capacity of the 6.9 engine to produce torque.

The taller diff. ratio would be more suited to German roads and autobahns of the 1980's, for which the car was obviously designed. Perhaps the shorter diff. is more suited to current Australian driving conditions and speed limits.

I would have to expect that the car now has a lower top speed, I but I can assure all that I will never be tempted to test it.

The car also had a 1 3/4 inch rear exhaust pipe from the forward muffler to the end.
It was replaced with a 2 1/4 stainless steel system.
The current tyres 225 x 60 15 VR's are nearly 1% smaller in rolling diameter than the original 215 x 70 x 14 VR's. That would alter the final engine to road ratio by a similar amount, i.e. 1%.

The air cleaner box was replaced with an all alloy air cleaner box from a 560 SL, but with the induct trumpets removed. The holes left by the trumpets were grilled. The airway area into the air cleaner element is now four times greater that that of the original 6.9, so the engine can breathe a little better. The engine bay now looks a little brighter with the alloy air cleaner box.

All of these minor changes have resulted in a car which probably performs similarly as to how a new 6.9 would perform, considering the fact that most older engines loose power over a lifetime of decades.



Anybody else in TK know of a 6.9 with a 450 differential? It would be good to share experiences here on TK.

Last edited by SEL_69L; 14th July 2014 at 11:54 AM.
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Old 14th July 2014, 12:00 PM
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Default Re: 6.9 with 450 differential

Hi David your answer is already here as SELfor50 had the same diff ratio in his 450SE 6.9 race car at Bathurst. Not restricted on speed at all Michel. Only restriction was the 450 diff I have installed (3.07:1) so my top speed was only about 210 at redline (maybe more, but I was too sh!tscared to look at my speedo just before entering the chase).
Already arranged for different gearing for next year.
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Old 14th July 2014, 02:08 PM
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Default Re: 6.9 with 450 differential

Who needs a top speed of 260kmh anyway?? Unless you're headed to the salt flats....

Personally I'd much prefer a car that's more responsive than one that just coasts in OD or top.

Is the 450 diff LSD? That's what I'll look for... And the W126 diffs with the flanges on the driveshafts near the inner CVs
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Old 14th July 2014, 02:36 PM
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Default Re: 6.9 with 450 differential

It was an option but a rare one Mark. So spend money on Quaife Diff.
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Old 14th July 2014, 04:48 PM
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Default Re: 6.9 with 450 differential

If you already have a 6.9 diff, by using the 6.9 casing, the limited slip clutch feature can be retained. Regards Styria
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Old 14th July 2014, 04:50 PM
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Default Re: 6.9 with 450 differential

I was informed by the diff specialist that the limited slip feature IS incorporated in the replacement differential.
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Old 15th July 2014, 10:48 AM
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Default Re: 6.9 with 450 differential

I am thinking along similar lines even going to 350 diff ratio if I get down over drive road.
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Old 15th July 2014, 11:19 AM
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Default Re: 6.9 with 450 differential

It's all a fine balance between acceleration, top end speed and fuel consumption...

All you have to do is prioritise what you want.
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Old 15th July 2014, 03:48 PM
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Default Re: 6.9 with 450 differential

Both I want both. Because I want it all and I want it now.
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Old 15th July 2014, 04:06 PM
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Default Re: 6.9 with 450 differential

Quality, service, price...

Choose any two
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Old 15th July 2014, 04:14 PM
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Default Re: 6.9 with 450 differential

if you remove wind resistance my car recorded 270kmh on the dyno when ignition cut out came into effect.
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Old 15th July 2014, 05:06 PM
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Default Re: 6.9 with 450 differential

Wind resistance increases proportionately with the cube of the speed.

That is why land speed record cars need many thousands of horsepower to reach their maximum speed.
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Old 15th July 2014, 05:41 PM
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Default Re: 6.9 with 450 differential

Hi David yes I am aware of those details . I have seen pictures of 6.9 speedos at 250kmh which with a Euro in good tune is possible. My point was that even with a lot more power without changing gearbox or rear ratio any claim of higher speed is just that. As my car hit over rev at that speed. And stock dyno run says my car was 2.9hp above a new Euro spec 6.9 so very healthy. But I would be willing to lose a little top end like you for more low end response. But seeing as my tyres go to smoke on take of that probably is not really required.
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Old 15th July 2014, 09:31 PM
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Default Re: 6.9 with 450 differential

My 450SE track car "whitey" had a 350SE diff, 3.46:1 diff installed on purpose because most of the time lowering the diff by around 0.5 a ratio you get about 50HP in real push-in-the back acceleration.

Whiteys D-Jet 450SE was tired and could hardly pull a skin off a custard, but with the 350SE diff it gave it a whole new lease on life... those who met me at Carl's Richmond workshop a few years will recall how easily it fishtailed half the way up the street as I left. For normal day to day driving, I didn't notice any difference in petrol consumption. What I did notice was if I was going 100 on the freeway and needed to overtake someone, it was like driving a completely different V8. I never measured the 100-130 acceleration time but it felt like it had been halved compared to the standard diff.

Changing the diff has been a well established performance upgrade for engines which have peaky torque and power curves, such as rotary engines. Virtually no one who is building a track or drag RX2,3,4 or 7 use the standard 3.92:1 diff, they go for a 4.1, 4.4 or 4.8:1

For the type of driving I do with my 6.9 these days, I would prefer the 450SE 3.08 diff, for added sizzle off the line. But it's diff is in great condition, and unlikely to need any attention... ever...

I.
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