Amazon: - How many revs?

Giovanni · Sep 19th, 2009, 20:26

I have a 122s fitted with a B20B engine (twin SU's) on OD model, I recently fitted a digital rev counter to the car. Driving on the motorway at 70MPH and the rev counter reads just under 5000 revs, this seem a bit excessive. Does any one have any Idea how many RPM the car should be doing at 70MPh or 80MPH and how much difference would an OD gearbox make to this figure?

Regards

Giovanni

222s · Sep 19th, 2009, 20:47

For an overdrive equipped car with the correct o/d diff ratio, 80mph is 5000rpm in 4th, and 84 mph is 4000rpm in o/d.

Moosejaw · Sep 20th, 2009, 00:30

Quote:

Originally Posted by 222s

For an overdrive equipped car with the correct o/d diff ratio, 80mph is 5000rpm in 4th, and 84 mph is 4000rpm in o/d.

Huh? I thought the O/D ratio was far higher than that. I get about 19mph/1000rpm from my non O/D car. At 80 mph mine sits around the 4000rpm mark in 4th.

Giovanni · Sep 20th, 2009, 20:27

Given the results you quote, I suspect my rev counyter may be lying. The rev counter I have is digital and takes its reading fron the -ve side of the coil, the old type are probably more accurate.

Regards

Giovanni

Derek UK · Sep 20th, 2009, 22:05

Quote:

Originally Posted by Moosejaw

Huh? I thought the O/D ratio was far higher than that. I get about 19mph/1000rpm from my non O/D car. At 80 mph mine sits around the 4000rpm mark in 4th.

Cars that left the factory with OD had the 4.56 diff. No B20 cars were fitted with OD as standard and had the 4.10 diff except a few of the very last ones that had a 4.30. Means that retro fitting an OD puts it with the 4.10 axle (or 4.30) which is fine for cruising and economy but the higher axle ratio takes the edge off the acceleration. You can also rev it out to max speed in 4th with the 4.56 and maybe even get a few more mph in OD under favourable conditions. Not so easy with the 4.10. You can replace it with a 4.56 of course which will help if you tow as well as the odd track day!
Is this useful?

Giovanni · Sep 20th, 2009, 23:15

Very interesting chart, there appears to be very little difference between a later 4 gear car without OD and an early model with OD. Not really worth changing to OD and putting additional stress on the engine and pocket.

Giovanni

Moosejaw · Sep 20th, 2009, 23:30

Interesting chart. Looking at that, my car's closest to the blue 'J' O/D line, maybe a shade under it but not much.

222s · Sep 21st, 2009, 12:47

Quote:

Originally Posted by Derek UK

You can also rev it out to max speed in 4th with the 4.56 and maybe even get a few more mph in OD under favourable conditions.

That may be the case with a standard twin carb B18, but the twin carb B20 in my old bus has enough power & torque to enable the o/d to be treated as a fifth gear rather than just a cruising ratio, and as such I very much doubt that it would be able to rev high enough in 4th to any where near its maximum speed when in o/d.

Quote:

Originally Posted by Moosejaw

Huh? I thought the O/D ratio was far higher than that. I get about 19mph/1000rpm from my non O/D car. At 80 mph mine sits around the 4000rpm mark in 4th.

Probably either your speedo over-reading, perhaps you have the 4.30 diff that Derek refers to, or possibly a combination of both. The gearing calcs that I refered to (admittedly give or take a couple of mph) and those in Derek's graph are calculated figures from the gearing, correct 165R15 tyre diameter etc rather than indicated ones on the speedo.

Btw, Derek, your graph appears to be missing the line for the 4.10 axle. I'd be interested to see that, as I've only worked out the calculations for the 4.56 axle & o/d gearing.

Derek UK · Sep 22nd, 2009, 00:30

Paul, yes with a 4.56 it's more like a proper 5 sp box but with the 4.10 is more of the modern over geared type which means you might have to drop out of it more often than with the 4.56. B20 torque does help though.

Not my graph, just grabbed off the net somewhere, Brickboard I think. 4.10 is written there as dropping the revs by 10% so you could plot a new line based on that.
4.30 plus J type OD might be a nice compromise with a strong B20. Would be sporty rather than cruisy if that's what you prefer.

classicswede · Sep 24th, 2009, 11:16

To me the 4.1 diff seems quite nice with a B20 and will still pull away from a stop in 4th. With a 4.1 diff you don't realy need an od unless you do a lot of high speed motorway driving. I'll be putting an OD box in the old man's 131 because he does then to run out of revs in top gear (6000 rpm)

I personlay like high gearing but its just a case of personal preferance.

That chart is quite usefull thanks for posting it up Derek

Sep 19th, 2009, 20:26	#1
Giovanni I've Been Banned Last Online: Nov 16th, 2009 22:22 Join Date: Dec 2008 Location: Glasgow	How many revs? I have a 122s fitted with a B20B engine (twin SU's) on OD model, I recently fitted a digital rev counter to the car. Driving on the motorway at 70MPH and the rev counter reads just under 5000 revs, this seem a bit excessive. Does any one have any Idea how many RPM the car should be doing at 70MPh or 80MPH and how much difference would an OD gearbox make to this figure? Regards Giovanni

Sep 19th, 2009, 20:47	#2
222s Amazoniste Last Online: Sep 19th, 2016 21:52 Join Date: Nov 2002 Location: Emsworth View Photos By: 222s	For an overdrive equipped car with the correct o/d diff ratio, 80mph is 5000rpm in 4th, and 84 mph is 4000rpm in o/d. __________________ Paul - 1967 Amazon 222S B20 o/d Estate & 1961 A-H Sprite Mk2 948cc WANTED - For '67 Amazon estate - offside rear quarter, preferably new old stock.

Sep 24th, 2009, 11:16	#10
classicswede Trader Volvo in my veins Last Online: Yesterday 19:12 Join Date: Dec 2004 Location: Anglesey	To me the 4.1 diff seems quite nice with a B20 and will still pull away from a stop in 4th. With a 4.1 diff you don't realy need an od unless you do a lot of high speed motorway driving. I'll be putting an OD box in the old man's 131 because he does then to run out of revs in top gear (6000 rpm) I personlay like high gearing but its just a case of personal preferance. That chart is quite usefull thanks for posting it up Derek __________________ http://www.classicswede.org www.classicswede.co.uk

Sep 20th, 2009, 20:27	#4
Giovanni I've Been Banned Last Online: Nov 16th, 2009 22:22 Join Date: Dec 2008 Location: Glasgow	Given the results you quote, I suspect my rev counyter may be lying. The rev counter I have is digital and takes its reading fron the -ve side of the coil, the old type are probably more accurate. Regards Giovanni

Sep 20th, 2009, 23:15	#6
Giovanni I've Been Banned Last Online: Nov 16th, 2009 22:22 Join Date: Dec 2008 Location: Glasgow	Very interesting chart, there appears to be very little difference between a later 4 gear car without OD and an early model with OD. Not really worth changing to OD and putting additional stress on the engine and pocket. Giovanni

Sep 20th, 2009, 23:30	#7
Moosejaw Premier Member Last Online: Jun 29th, 2012 19:44 Join Date: Aug 2008 Location: Glasgow	Interesting chart. Looking at that, my car's closest to the blue 'J' O/D line, maybe a shade under it but not much.

Sep 22nd, 2009, 00:30	#9
Derek UK VOC Member Last Online: Yesterday 11:16 Join Date: Jul 2006 Location: Chatham	Paul, yes with a 4.56 it's more like a proper 5 sp box but with the 4.10 is more of the modern over geared type which means you might have to drop out of it more often than with the 4.56. B20 torque does help though. Not my graph, just grabbed off the net somewhere, Brickboard I think. 4.10 is written there as dropping the revs by 10% so you could plot a new line based on that. 4.30 plus J type OD might be a nice compromise with a strong B20. Would be sporty rather than cruisy if that's what you prefer.

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