Diff 3.7 to 3.45

[Steve P]

Morning,

Has anyone gone from a 3.7 diff to a 3.45 diff. If so do you feel about the change? Was it a positive or negative effect?

 

Also looking for a replacement diff there are a range of suppliers and prices to choose from. I would be happy to hear of peoples experiences with sourcing a replacement diff.

 

Cheers

Steve

[Andrew Smith]

Morning, Has anyone gone from a 3.7 diff to a 3.45 diff. If so do you feel about the change? Was it a positive or negative effect? Also looking for a replacement diff there are a range of suppliers and prices to choose from. I would be happy to hear of peoples experiences with sourcing a replacement diff. Cheers Steve

 

Steve,

 

When I bought VUX (my 72 CP) last September it was fitted with a 3.7 (US spec) diff. Granted, it made the car very quick off the line and a bit quicker mid-range but I felt it didn't suit the gearbox or match the overdrive (A-type) it seemed to be revving too high at motorways speeds of 70 - 80mph (approx 2,300) . Luckily for me, Pete Cox (Moss London) rebuilt me a good UK spec 3.45 diff very quickly and VUX now cruises comfortably at 70 - 80mph (approx 2,000 rpm) and I returned an average of 31mpg on the 550 mile return trip to Harrogate.

 

Cheers

Andrew

 

Edit:

 

Yep, as I said approx 2,000 at 70mph in o/d - but will take VUX out and check though as I may be having a grey moment!

........a grey moment - my 320 oil burner beemer sits at 70-80 at <2,000rpm not VUX !! But the 3.7 diff still didn't match the A-type or gearbox as it revved at 300-500 too high to be comfortable. The 3:45 is much better suited by far and clearly that was the view Triumph Motor Company and British Leyland in their day.

 

Cheers

Andrew

Edited September 7, 2011 by Andrew Smith

[Pete R]

The 1969 CP I bought recently has a 4.1 diff and does 3500 revs at 70mph in overdrive!! Was the standard ratio 3.45 as I want to change mine to get more economy?

 

However still can't believe it will go down to 2000 revs at 70 will it? Others I have driven seem to do about 2800 revs at 70?

 

Pete

[Steve P]

Pete,

A 3.7 diff should do about 2800 rpm at 70 mph where as a 3.45 diff should do about 2650 rpm at 70 mph.

 

Steve

[TONYL]

The 1969 CP I bought recently has a 4.1 diff and does 3500 revs at 70mph in overdrive!! Was the standard ratio 3.45 as I want to change mine to get more economy?

 

However still can't believe it will go down to 2000 revs at 70 will it? Others I have driven seem to do about 2800 revs at 70?

 

Pete

 

Hi Pete

My 72 PI fitted with non standard J type does 2300 rpm @ 70 mph

Still thirsty though!!

Tony

[Mike Collins]

My CP PI with 3.45 diff and 28% saloon overdrive gives me 70mph at around 3000, 4000 in direct top. Speedometer is accurate, rev counter can't be that far out, can it?

[kiwican]

You can check speed at various revs here, just plug in the appropriate information.

 

http://www.mintylamb.co.uk/gearspeed/

[Andrew Smith]

Yep, as I said approx 2,000 at 70mph in o/d - but will take VUX out and check though as I may be having a grey moment!

 

Cheers

Andrew

[Tom Fremont]

It's all in the books, lads!

 

With original size tyres, the numbers are ( drumroll ):

 

385 rpm / 10 mph in 4th O/D for P.I. cars with 165 tyres and 3.45:1 diffs. Hence 2695 rpm @ 70 mph

 

395 rpm / 10 mph in 4th O/D for U.S. cars with 185 tyres and 3.7:1 diffs. Hence 2765 rpm @ 70 mph

 

Switching 185s for 195/65s and 3.45 for 3.7 gears kept the overall ratio just about the same for me So no change, just modern tyres for vintage ones

Edited August 15, 2011 by Tom Fremont

[88V8]

My tyres are rather large, but I have the equivalent of a 3.7 which replaced the equivalent of a 3.45.

3.45 was fine for cruising and economy, but it's supposed to be a sports car.

Wouldn't go back to a 3.45.

 

Play around with ratios and tyre sizes here.

http://www.mintylamb.co.uk/gearspeed/

You can overtype the figures in the dropdowns.

 

Ivor

[EliTR6]

 

It's supposed to be a sports car.

 

 

 

I agree! A bruiser not a cruiser...

[Guest ntc]

A 3.45 gives you both

[Steve P]

Thanks for all the responses. I've decided to go for the 3.45.

 

Any suggestions as to a good place to source said item?

 

Cheers

Steve

[rpurchon]

Thanks for all the responses. I've decided to go for the 3.45.

 

Any suggestions as to a good place to source said item?

 

Cheers

Steve

 

you dont have to get a tr6 one. saloons and stag will fit if you swap the front and rear covers.

also you can tell whether its a 3.7 or 3.45 by the paint on the half shaft nuts. i think red 3.45 yellow 3.7

im sure some one will chip in here !

richard

[Dex]

Thanks for all the responses. I've decided to go for the 3.45.

 

Any suggestions as to a good place to source said item?

 

Cheers

Steve

 

 

Steve

 

TR Bitz has them

 

Regards

Dex

[Alec Pringle]

"A bruiser not a cruiser"

 

Novel description of a TR6, if ever I heard one, I'll give you that. I can't recall even BL's infamous apology for a PR department ever coming out with quite such a daft slogan, not even after several G&Ts followed by the proverbial bottle of Scotch with the top off . . . .

 

Meanwhile, a CP-engined TR6 will handle a 3.45 diff all day long, unless there's something drastically adrift with the engine. Or, of course, with the driver. And it doesn't need an overdrive sapping power in favour of eking out the fuel either. Just four on the floor and a hefty right clog. If fuel consumption is an issue, don't buy a PI - it was meant for ragging, not posing.

 

Cheers,

 

Alec

[TIMS]

Guys

 

Sorry to disagree with many of you but 3.7 it is for me.

 

The engine in my 6 is mildly breathed upon and runs at its smoothest from about 2,800 rpm upwards. With a 3.45 cruising is below that point and the engine sounds unhappy. But on French motorways at 2,900 rpm in overdrive top with a 3.7 the car sounds mighty sweet and will pull from that point to upset many a BMW.

 

And then on UK "B" roads the extra wizz with the 3.7 gives a transformed car from the 3.45.

 

Before I changed to the 3.7 I spoke to various "guru's", including The diff builder, who all seemed to express the opinion that the 3.45 was used by Triumph to give longer legs and quieter cruising to the car, but that it took away from the sports car essence. Having made the change I can totally see where they are coming from.

 

Fuel economy!!!! that's what I have a diesel road car doing 50mpg for. The TR is for driving and fun!

 

Just my views.

 

Regards

 

Tim

[EliTR6]

"A bruiser not a cruiser"

 

Novel description of a TR6, if ever I heard one, I'll give you that. I can't recall even BL's infamous apology for a PR department ever coming out with quite such a daft slogan, not even after several G&Ts followed by the proverbial bottle of Scotch with the top off . . . .

 

 

 

 

Best I could come up with on short notice!

[Andrew Smith]

Yep, as I said approx 2,000 at 70mph in o/d - but will take VUX out and check though as I may be having a grey moment!

........a grey moment - my 320 oil burner beemer sits at 70-80 at <2,000rpm not VUX !! But the 3.7 diff still didn’t match the A-type or gearbox as it revved at 300-500 too high to be comfortable. The 3:45 is much better suited by far and clearly that was the view Triumph Motor Company and British Leyland in their day.

 

Cheers

Andrew

[88V8]

....the 3.7 diff still didn't match the A-type or gearbox as it revved at 300-500 too high to be comfortable. The 3:45 is much better suited by far and clearly that was the view Triumph Motor Company and British Leyland in their day.

 

In their day one could sit at 90mph with little interference from the law nor obstruction by traffic. No longer. Now, the pleasure is in acceleration, for which the 3.7 is much better suited. and still gives 24mph/1000 which is not exactly undergeared.

 

Ivor

[Steve P]

Just a quick update on the topic. I eventually fitted the 3.45 diff in place of the 3.7 diff and must say that I can barely tell the difference between the two. From some of the comments made in the previous posts I guess I was expecting a significant change. Ah well one lives and learns.

 

Cheers

Steve

[tjs]

The 1969 CP I bought recently has a 4.1 diff and does 3500 revs at 70mph in overdrive!! Was the standard ratio 3.45 as I want to change mine to get more economy?

 

However still can't believe it will go down to 2000 revs at 70 will it? Others I have driven seem to do about 2800 revs at 70?

 

Pete

 

 

Same.

 

Learn to love it, and take the B-roads!

[Alec Pringle]

There is no mystery about rpm and road speed.

 

Too many folks fondly believe their speedo and/or tacho, neither of which are particularly likely to be anywhere near accurate

.

Accurate calculation is not that difficult – you need the rolling circumference of the tyre, precisely.

 

Car on a flat piece of road. Drop a plumb line from road wheel centre to tarmac, marking tarmac and sidewall precisely. Push the car for one complete revolution of the wheel, until the plumb line again matches the sidewall mark, mark the tarmac. Measure the distance between the two tarmac points to 1/10”. 78” might be typical, so let’s use it as an example.

 

Divide that figure into 73920. That’s the number of wheel revolutions at 70mph. 948 to the nearest whole number. Multiply that by the diff ratio – 3.45, 3.7 or 4.1 – to find engine rpm in direct top. 3271, 3508 or 3887 respectively.

 

For overdrive top, multiply that rpm figure by 0.82 for A-type (normally 22%), or by 0.8 for J-type (normally 25%). Exceptionally you may have a different overdrive ratio if the unit has originated from a saloon, eg 28% o/d in which case the figure is 0.78.

 

A CP TR6, 3.45 diff with A-type o/d would be 3271 multiplied by 0.82, 2682rpm at 70mph, whereas a CR car with J-type would be doing only 3271 x 0.8, 2617rpm.

 

The CP car would see its o/d top revs increase to 2876 with a 3.7 rear end, or 3187 with a 4.1.

 

OK, I appreciate there are other factors involved, like more than one decimal point in the ratios, and tyre growth at speed, but the above calculations should have you accurate within 1% if your initial circumference measurement is precise.

 

Now we know what lots of TR men will be experimenting with next weekend . . . .

 

Cheers,

 

Alec

[PJM]

Mike Papworth provided me with good service, highly recommended. NFI etc

[jellison]

I agree! A bruiser not a cruiser...

 

Exactly.

The StandardUK stpec 3.45 is ****. Zero performance. These things are sportscars and hardly likely to be stad drivers.

 

The most logical change for a UK road car is to from a 3.45 to a US Std (as strangled power) 3.7, this give a bit of go without sacrificing much at M-way speeds. This is why you have an OD that drops it from 1:1 to 0.80. The reve you say you are doing on the M way is that with the OD in (has to be surely?).

3.45 might be fine with 400bhp but with something like 140 or less!<

3.7 is best for the road, well 3.9 is better but I'd presume you'd think that raving mad!!! Make even a std motor feel reasobly quick.

 

How goes the build Eli?

 

I have not worked out my MPG on 4.1 and rev'ing it to 7500

Edited March 20, 2012 by jellison