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MG MGA - RPMs With 5-Speed and 3.9 Diff
|I have been looking through the archives for data on RPMs/Speed for the 5-speed conversion and 3.9 diff. I did not find anything there, although I came across the 4.1 diff figures I posted a few years back, so thought I would offer my findings for anyone who is interested.|
Top (5th) gear gives 23.5 mph per 1000 RPM.
1000 rpm = 23.5 mph
1275 rpm = 30 mph
1700 rpm = 40 mph
2000 rpm = 47 mph
2125 rpm = 50 mph
2550 rpm = 60 mph
3000 rpm = 70.5 mph
3400 rpm = 80 mph
3800 rpm = 90 mph
4000 rpm = 94 mph
4250 rpm = 100 mph
The combination seems to sit very comfortably at about the 3400 - 3600 rpm mark, allowing the car to maintain a good competitive station with other motorway traffic.
|That sounds good Steve - better than my 4000 rpm at 65 mph!! Now to look for a 3.9 diff = cheers Cam|
Have you done the 5-speed box conversion yet?
3.9 diffs are becoming quite sought after and fetching high prices. Only very occasionally coming onto ebay. The implanting process into a standard MGA axle is relatively straight forward and well documented by Barney. It may require a special home made tool (as described in the Haynes Manual Translation Thread!!!) This is needed if you wish to change the oil seal while you have the diff housing out. See the picture in my website (scroll half way down): http://www.mgaroadster.co.uk/odds_and_sods.htm
You can borrow my version.
|I'd be interested to find out. I do remember reading it somewhere, but it escapes me. |
I've decided (at least in the short term) to stick wit the good old 4.3:1 for acceleration, as high speed cruising doesn't really appeal to me - I much prefer high speed bends!
The only difference I really notice over the 4-speed box is the slightly higher geared 1st gear. A bit more revs as I pull away from stationary, but I can hold 1st longer than a standard box. Third is also slightly higher I think. I also run the 1800 which has a bit more grunt, so the difference to a 4.3:1 is quite manageable. I bet mine would match and probably beat a standard 1800 with 5-speed and 4.3:1 throughout the range on acceleration.
I like the bends too!
PS. I meant 4.3:1 in my opening post.
that's very useful to know. I too have the 5 speed, but married to a 2.1L. All I need to do now is fit the wheels, steering wheel, seats and interior, get an MOT, etc.
Knowing that a 5 speed adds that extra bit of acceleration is great news. I can't wait to unleash my MGA from its garage and give it a good blast around the country lanes!
Rereading my last post I think I may have confused myself a bit - probably memory fading a bit. It is about 8 years since I fitted the 5-speed box and 2 years since going the 3.9 route.
The 5-speed box has, I believe, a similar 1st gear ratio to the standard MGA box. However, I recall it did give a longer acceleration (higher speed). May be this was due to better engineering, allowing higher gearbox revs - none of that 1st gear whine.
With the 3.9 diff it is obviously higher geared in first and requires a bit more 'welly' on start off. But after that it is just a case of getting used to the different ratios and appropriate mph/rpm matching. Generally speaking, for any particular speed, you can stick comfortably in a lower gear than the 4-speed box, but at higher revs where the engine is performing best but not being stretched. 4th gear is lovely round the country roads.
|Steve, This is very close to my calculations for my five speed conversion. I calculated 23 miles per hour per 1000 rpm in fifth gear. |
This is using 165SR15 tires with an effective rolling diameter of 24.8 inches.
What size tires are you using?
BTW your statement of motorway cruising indicates that the general speed is 80 to 85 mph. That is quite brisk!
Yes, that is my tyre size.
The engine has been singing since I started commuting 17 miles on the motorway, instead of 8 on restricted A roads 3 months ago. The speed is a bit high, but at the time of day I travel, the inside lanes are clogged with lorries. The easiest thing is to move out and keep up with the company cars. At least then you don't need to be constantly changing lanes. In a strange sort of way it feels safer. Our motorway limit is 70mph, but the outside lanes usually sit at about 80 to 85, some times faster by a few cars if traffic is lighter. Our stretch of motorway round Preston is one of the busiest in the UK.
What are you using to measure RPM? Your figures are high compared to mine 65mph @ 2200rpm
|I used a GPS and recorded at 55mph the motor was turning 2500 rpm with a five speed conversion. |
While Steve's records show: 2550 rpm = 60 mph with a 3.9 rear end installed.
One thing I enjoy about the 5 speed conversion is the taller second gear.
Traffic in the UK is indeed brisk. Reminds me of traffice around Toronto. Drivers around Chicago (and Detroit, St. Louis, Indianapolis, Minneapolis) are a bit slower making usually about 75 in the left lane but are far less courteous or cautious than the Toronto drivers. In Toronto, the Canadians maintain distance and use the flasher when approaching from the rear. In Chicago, it is a more like NASCAR!--drafting and quick lane changes are the order of the day.
While Steves figures are a wee low from my calculations, your 2200 at 65 mph seems unusually low. The tire revolution at 60 mph for a 165SR15 should be about 813 revolutions per mile. Multiply times the 3.91 axle ratio and the answer should be 3180. This is in fourth gear at 60 mph. Now factor for 65 mph 3180 x 65/60 = 3445 and reduce for the overdrive 3445 x .82 = 2824 rpms. Using Steves 60 mph rpm figure, rpm at 60 and factoring for 65 works out like this: 2550 x 65/60 = 2763. The rpm goes up if using a 4.3 rear axle ratio by about 10%. In order to achieve 2200 rpm at 65 mpg, one would need huge tires or a greatly reduced ratio rear axle, some where in the near 3.1. Might you be using an MGC 3.07 gearset?
I took the figures straight off my electronic tacho. I will check it against my combi dwell meter over the week end. I think the mph is spot on. I had it re-calibrated when I installed the 3.9 and it ties in with the sat nav.
I agree with you about second gear. I was monitoring my gear changes this morning on the way to work. Without forcing the engine on acceleration I was going from 1st to 2nd at about 18mph, 2nd to 3rd at 40mph, 3rd to 4th at 55mph and 4th to 5th any time thereafter.
On the way back I achieved my 4250rpm in 5th (see my figures in the opening post). I had a good look round first; the road was fairly empty for a change. Brisk 15 knot head wind, but she just crept over. Yippee!!
|James and others|
You mentioned the effective rolling diameter of your 165/15 tyres is 24.8 inches. Can you explain this to me?
I checked out a website that stated the following:
diameter = wheel diameter + (tire width x aspect ratio / 1270)
This gives: 15 + (165 x 80 / 1270) = 25.39 inches.
This would appear to bear no relationship to the measurement I have just taken from my car where the centre of the spinner to the ground is 11.75 inches = 23.50 inches diameter.
Can someone educate me?
Barney talks about all of this somewhere on his vast website, mentioning rolling radius and tire squirm along with the effective revolutions per mile for a tire. On some tire websites such as the Tire Rack they will quote tire revolutions per mile for different brands and sizes of tires. Tires of the same brand and size can exhibit slightly different revolutions per mile and some may conclude that this is due to different rubber compounds and tread patterns. While this may be true the biggest contributor to the smaller rolling diameter verses the overall tire diameter is the squish caused by the load of car on the tire. That being said, I find your measurement, which was done properly by measuring the distance from the hub center to the ground, on the small side.
This is great fun! Barney, are you out there? Time for your wisdom!
Where the tyre sits on the ground it flattens. What you have measured is from the centre of the wheel to the middle of the flat bit. This will be the smaller than the average radius of the wheel/tyre. If you want to measure the rolling diameter, a rough way is to mark the tyre with tape or chalk and push the car forward for 3 or 4 revolutions and measure the distance travelled. If the wheel has done say 4 revs you now have 4 times the rolling circumference and so you can calculate the diameter. Or, you should be able to get a figure from the tyre maker. The effective diameter also changes with speed but I have no idea if this is significant.
|Okay, that's fine so far, However! What about tyre pressure? Not a mention yet, nor in the formula I posted. I run 27 all round. To take it to a logical conclusion, if I had zero pressure I would be running little more than the 15" diameter wheel plus the width of a scrunched up carcass of a tyre.|
This thread was discussed between 24/09/2009 and 25/09/2009
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