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MG MGA - 5 spd. Sierra and RPMs

I'm thinking about the Sierra conversion and was wondering what RPMs I'd be turning with the stock 4.3 Diferential at say, 50, 60, and 70 mph. Is there a chart posted somewhere? Is a different rear end suggested? Would there be an advantage to lightening the flywheel? I'd be using the 1622 cc engine most likely.
Thanks.
Al
A. Tirella

My RPM is about 3300 at 80 mph. This is with a 3.9 and in 5 th gear on the fwy. This makes it a real cruiser
JEFF BECKER

Al

This is what my car is giving me:

40mph = 1880-1900rpm
50mph = 2350-2400rpm
60mph = 2820-2900rpm
70mph = 3300-3350rpm
80mph = 3750-3850rpm
90mph = 4230-4350rpm
100mph = 4700-4800rpm

The 100mph is an assumption, only got it up to 98mph indicated so far.

Steve
Steve Gyles

Al,

It all depends on the tyre diameter you have.
also called Rolling diameter of the tyre.

You can sometimes find this from the tyre companies web site.

I have the mga twin cam calculator / excel Spreadsheet

Email me if you want it.

Also I saw a very good calculator on an MX5/Miata site once. Google that.

Barney Offers this:
http://mgaguru.com/mgtech/tires/tt103.htm



Go for a 3.7 banjo Diff Newly available.
or the old MGB 3.9.... then you don't need to chop the car about and can easily go back to standard.
1st gear is a waste of time with the standard diff, so becomes more usable with these diffs, but you don't get a 1st gear syncro though.

Stick in a LSD or Phantom grip centre whilst your there too.

Hope this helps
<MARK>
Mark Hester

Ops, email address above
Mark Hester

From the twin cam calculator with 24.9 inch diameter wheels,,,, which I think are slightly taller on twin cams.

4.3:1 diff gives approx.

40mph = 2300rpm
50mph = 2900rpm
60mph = 3500rpm
70mph = 4050rpm
80mph = 4650rpm
90mph = 5200rpm
100mph = 5800rpm


I know on the way back from Le Mans last year on French roads when anything goes.... I tried to stay at 5000 RPM and got overtaken by everything and the Speedo +-5MPH was on about 90MPH... with 4.3 diff


Not sure what Diff ratio Steve was quoting, but he's 1000RPM out somehow
Mark Hester

Mark

Are you sure about no syncro on 1st? Mine changes sweetly into that gear on the move. I had always assumed it had syncro.

Steve
Steve Gyles

Steve and Mark, Maybe you guys are not on the same page. I think Steve is referring to the 5 speed Sierra transmission, and Mark, aren't you talking about the stock MGA transmission ( with no synchro on first, of course) ?
A. Tirella

Al

Yes, I was referring to the Ford Type 9 box. My car was a standard 1500. I transplanted a late 1800 (electric tacho) to mate with the 5-speed box. The gearing certainly makes the car more practical in today's traffic. A 3.9 differential is a viable modification. We had quite a discussion about it a couple of months ago. Worth a check in the archives. It's a modification high up on my list.

Steve
Steve Gyles

I believe that Steve's figures are correct.
The Hi-Gear company that supplies the Sierra Type 9 gearbox conversion gives the ratios as:
3.65
1.97
1.37
1.00
0.82

http://www.hi-gearengineering.co.uk/frames.htm

Using a MGA wheel diameter of 24.9 inches and a rear axle of 4.3 I calculate fifth gear as:
50 MPH = 2400 RPM
60 MPH = 2800 RPM
70 MPH = 3400 RPM
80 MPH = 3800 RPM
90 MPH = 4300 RPM
100 MPH = 4800 RPM

The diameter of the tyre is important. I should start a thread on how it is measured, not as simple as first appears!

Mick
Mick Anderson

Sorry though Steves were for standard.

OK so to cruze at 70 MPH with the standard MGA gearbox.

4.3:1 Diff = 4050 RPM

3.9:1 Diff = 3700 RPM

3.7:1 Diff = 3500 RPM


With standard 4.3:1 diff and a 0.82 5th gear ratio at 70 MPH also.

4.3:1 diff = 3350 RPM


So 550 RPM less with 3.7 diff
and 700 RPM less with 5 speed.


One is about £200 in parts and the other is about £770 plus

Plus one can be reversed in about 1-2 hours and the other you can't

Only my 0.02c worth

But I do have a 5 speed in my MGB GT MK1

Cheers <MARK>
Mark Hester

This thread was discussed between 16/06/2006 and 17/06/2006

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