Rover P6 and SD1 gearbox info needed.

Rick-Manta · Post by **Rick-Manta** » Sun Aug 19, 2007 1:42 pm

I'm currently rebuilding a Rover V8 engine from an early SD1 to fit into an Opel Manta B-series. Previously running an automatic gearbox I'm loooking to fit a manual and have been offered a 4 speed box from a Rover P6, firstly does anyone know if the flywheel and clutch kit are the same for the Rover P6 4 speed and the SD1 5 speed?

Secondly, I've been told this idea is crap as I'll be much better off using the SD1 gearbox despite the price difference and this is from people who have done manta V8 conversions before, just wondering if any of you guys have tried the 4 speed with a 3.44:1 diff or similar?

Thanks. Rick

Post by **katanaman** » Sun Aug 19, 2007 6:25 pm

Use a 5 speed box, a 4 speed on a 3.44 will be very revvy on cruse.

darren peart · Post by **darren peart** » Sun Aug 19, 2007 8:31 pm

I would go for the five speed mate you will get much better economy if your bothered about that, i am past caring about mpg now but i used a 3.5 sd1 engine and 5 speed box when i first did mine and it was spot on with the standard gte axle.

Rick-Manta · Post by **Rick-Manta** » Mon Aug 20, 2007 4:00 pm

Thanks. I don't fully understand why the 4 speed would be revvy as I thought those of the Rover P6 persuasion fit the 5 speed SD1 box to replace their original 4 speed as an upgrade. Wouldn't this make then make their P6 do 180mph but 0-60 in 2 hours (just an example) if the ratios and final drive are that different betwwen the two boxes?

Post by **katanaman** » Mon Aug 20, 2007 4:22 pm

The fifth gear is an overdrive which will drop the revs by around 1000-1200 rpm depending on your tyre diameter/diff ratio. If you want to supply me with your tyre size I can plot a graph between the two that will show you the difference.

Rick-Manta · Post by **Rick-Manta** » Mon Aug 20, 2007 4:54 pm

Thanks mate, only if you've got the time.

Tyres are 205/45/17

I was previously using a 3 speed auto from an SD1, that really revved it's nuts off! Great acceleration though!

Post by **katanaman** » Mon Aug 20, 2007 5:07 pm

A 4 speed box has the same top gear as your auto box which is 1:1. The SD1 5 speed top gear is 0.792:1. I dont have the ratios for the lower gears of the 4 speed but in the following you can take the 4 speed and the 5 speed to be the same in 4th gear and 5th will be the difference.

-------------------------------------------------------------------------------------------
Top Gear gives 26.495 MPH/1000 RPM and a top speed of 145.721 MPH at 5500 RPM

Engine speeds in top gear:-
30 MPH = 1132 RPM 40 MPH = 1510 RPM 50 MPH = 1887 RPM 60 MPH = 2265 RPM
70 MPH = 2642 RPM 80 MPH = 3019 RPM 90 MPH = 3397 RPM 100 MPH = 3774 RPM

Top Speed in 1 gear = 34.752 MPH
And changes into 2 gear at 3456 RPM dropping 2044 RPM
Top Speed in 2 gear = 55.300 MPH
And changes into 3 gear at 3679 RPM dropping 1821 RPM
Top Speed in 3 gear = 82.673 MPH
And changes into 4 gear at 3940 RPM dropping 1560 RPM
Top Speed in 4 gear = 115.411 MPH
And changes into 5 gear at 4356 RPM dropping 1144 RPM
Top Speed in 5 gear = 145.721 MPH
******************************************************************************************

SO when you select 5th you would drop 1144 RPM and gain a theoretical 30 MPH. Course in the real world your car probably wont do 145 but the lower revs on cruse is well worth having.

JC. · Post by **JC.** » Wed Aug 22, 2007 8:12 am

Hi,

The fly wheel is the same for both boxes.
The clutch cover plate is the same however, the clutch friction plate is different as their are a different number of splines on the input shaft.
Release bearing is also specific to each gearbox.

An LT77 is very much superior to a Rover 4 speed though. I wouldnt even use one as a last resort. Infact, its a popular conversion to dump the 4 speed box and fit an LT77 to the original P6 cars.

Rick-Manta · Post by **Rick-Manta** » Wed Aug 22, 2007 7:34 pm

Thanks very much for the info and also big thanks for the graph Marki. Are you sure the autobox 3rd was 1:1 ? I could only do 50mph at just over 3000rpm, then I had valve bounce at 80mph

Good news to me that the flywheel is the same and so would the starter be in that case. It just means I can order and fit them before making the final decision on the gearbox.

Post by **katanaman** » Wed Aug 22, 2007 7:49 pm

so far as I know its 1:1 . Errors come into the equation like were you running the proper speedo drive gear, how accurate is your rev counter and were your tyres at the correct pressure. This was worked out on your 3.44 dif ratio as well. I dont know if this is what you have fitted at mo or your going to fit. If its fitted are you sure its 3.44?

Either way the idea was to show you the advantage of the 5th gear which it does.

ChrisJC · Post by **ChrisJC** » Wed Aug 22, 2007 7:57 pm

I'm pretty sure the SD1 autobox was 1:1 which was why the back axle had a very tall gearing , about 2.8:1??

Chris.

Rick-Manta · Post by **Rick-Manta** » Wed Aug 22, 2007 8:28 pm

The speedo was pretty accurate as I got a mate to follow to check my speed against his modern car speedo. I fitted a sports rev counter properly calibrated also. The rear axle ratio is definatlely 3.44:1 (a standard Manta axle) but there is a destinct possibilty the autobox isn't SD1 as it has a GM bellhousing which I'm not cetain but I thought a borg-warner jobbie would have been original to the engine. Additionally, I didn't fit the gearbox.

Currently I'm rebuilding the V8, selling my 1988 Manta shell (to instead have a mid 70's model) which I had with the autobox and was impossibly revvy. With the auto junked I've been shopping around for a manual and would actually prefer to have a P6 4 speed as they're only around £50 off ebay and I actually like to have a 'four on the floor' setup in my more retro 70's Manta. However everyone is advising the 5 speed and I should follow the advise of those in the know.

Appreciate the advise, thanks again.

Post by **katanaman** » Wed Aug 22, 2007 9:41 pm

There is a reason why the P6 box is 50 quid

Maybe your auto is slipping or something but anyway your junking it so its not a problem. You could use the P box but as chris said you need to change your diff to around 2.8 ish. You would then get gearing something like:

Tyre:- 205/45 x 17
-------------------------------------------------------------------------------------------
Top Gear gives 25.780 MPH/1000 RPM and a top speed of 141.791 MPH at 5500 RPM

Engine speeds in top gear:-
30 MPH = 1164 RPM 40 MPH = 1552 RPM 50 MPH = 1939 RPM 60 MPH = 2327 RPM
70 MPH = 2715 RPM 80 MPH = 3103 RPM 90 MPH = 3491 RPM 100 MPH = 3879 RPM

Top Speed in 1 gear = 42.695 MPH
And changes into 2 gear at 3456 RPM dropping 2044 RPM
Top Speed in 2 gear = 67.940 MPH
And changes into 3 gear at 3679 RPM dropping 1821 RPM
Top Speed in 3 gear = 101.569 MPH
And changes into 4 gear at 3940 RPM dropping 1560 RPM
Top Speed in 4 gear = 141.791 MPH
******************************************************************************************

CastleMGBV8 · Post by **CastleMGBV8** » Sat Aug 25, 2007 7:23 pm

Forget the 4 speed box, it is actually a Moss gearbox and dates back to at least the 50's and was use in jags up to the series 1 E-Types.

I had one in a 3500S and whilst it is quite strong it seems like it takes about 5 mins to change gear and of course you have no overdrive 5th so cruise economy is poor.
I have a D suffix LT77 in my V8 MGB and the 1st to second change with that is pretty awful, and the box was rebuilt when I did the conversion.

How do you post pictures on this BBS, can't work out the system.

Kevin.

Post by **katanaman** » Sat Aug 25, 2007 7:45 pm

you have to host the pictures somewhere and then you just wrap the URL in img tags. Click the img button in the text editor, paste the url then click the closed tags text. Instructions are in the FAQ.