Overdrive and Engine RPM
Discussion
well It sort of does both,
from this I mean when you engage the o/d the engine revs drop by a certain amount then as you accelerate to the point where the revs were before you engaged the o/d you will be travelling that bit faster on the speedo.
simply put when o/d is engaged for every .75 of a rotation on the input shaft you get 1.0 rotation out to the propshaft.
at least that's the way I look at it.
from this I mean when you engage the o/d the engine revs drop by a certain amount then as you accelerate to the point where the revs were before you engaged the o/d you will be travelling that bit faster on the speedo.
simply put when o/d is engaged for every .75 of a rotation on the input shaft you get 1.0 rotation out to the propshaft.
at least that's the way I look at it.
Thanks for your reply.
I suspect that a 25% o/d reduces engine speed by 20% for a given road speed.
My guess is that if a car is geared for 20 mph/1000 rpm in direct 4th, at 80 mph its doing 4000 rpm. If you engage a 25% o/d then bring the revs back up to 4000 I think the car would be travelling at 100 mph. If you then bring it down to 80 mph in o/d the revs would be 80% of 4000 which is 3200. The drop from 4000 to 3200 at 80 mph is 20%. If you then disengage the o/d but maintain 80 mph, the engine would be doing 4000 rpm which is an increase of 25%.
The gearing stated in the handbook for my car states that 4th is 1.0 and o/d 4th is 0.82. I've not heard of an 18% overdrive but I think 18% roughtly corresponds to 22% in the same way that 20% equates to 25% in the above example. The Autocar roadtest of the Triumph 2500S also states the gearing of o/d top is 0.82.
None of this is hugely important its just something that's intrigued me for a while.
People I've spoken to so far assume that a 25% o/d reduces engine speed by 25% which I suspect is wrong.
If anyone could clarify I'd be grateful.
Thanks,
Chris
I suspect that a 25% o/d reduces engine speed by 20% for a given road speed.
My guess is that if a car is geared for 20 mph/1000 rpm in direct 4th, at 80 mph its doing 4000 rpm. If you engage a 25% o/d then bring the revs back up to 4000 I think the car would be travelling at 100 mph. If you then bring it down to 80 mph in o/d the revs would be 80% of 4000 which is 3200. The drop from 4000 to 3200 at 80 mph is 20%. If you then disengage the o/d but maintain 80 mph, the engine would be doing 4000 rpm which is an increase of 25%.
The gearing stated in the handbook for my car states that 4th is 1.0 and o/d 4th is 0.82. I've not heard of an 18% overdrive but I think 18% roughtly corresponds to 22% in the same way that 20% equates to 25% in the above example. The Autocar roadtest of the Triumph 2500S also states the gearing of o/d top is 0.82.
None of this is hugely important its just something that's intrigued me for a while.
People I've spoken to so far assume that a 25% o/d reduces engine speed by 25% which I suspect is wrong.
If anyone could clarify I'd be grateful.
Thanks,
Chris
The overdrives fitted (A and J types) were both close to 0.8 : 1, I think, which is the almost same as the Ford type 9 5 speed box (0.82 : 1) as fitted to many Sierras and so on.
so this means that for each 0.8 rotation of the engine, the propshaft (driveshaft) turns once. this represents a ratio of 1 divided by 0.8, which is the correct way to do the maths. 1 /0.8 = 1.25 which is where the 25 percent comes from.
Or another way - if your diff ratio is 3.45 (Triumph saloon) then the effective diff ratio is as follows -
In 4th gear (1:1) the engine will turn 3.45 times for one turn of the rear wheels. The road speed is dependent upon the rolling circumference of the wheels and tyres. (and yes, you CAN calculate this pretty closely from the tyre size)
with overdrive in, the diff ratio becomes 3.45 *0.8 = 2.76 so it's like having a 2.76 : 1 diff ratio.
I know some big Jags and some old V8 USA cruisers, and early V6 Ford Capri had a 3.08 : 1
If it's of interest, you can calculate max speed and mph/kph per rpm in each gear if you know gear ratios and rear tyre sizes, in something like an excel
spreadsheet. It's not actually that hard.
so this means that for each 0.8 rotation of the engine, the propshaft (driveshaft) turns once. this represents a ratio of 1 divided by 0.8, which is the correct way to do the maths. 1 /0.8 = 1.25 which is where the 25 percent comes from.
Or another way - if your diff ratio is 3.45 (Triumph saloon) then the effective diff ratio is as follows -
In 4th gear (1:1) the engine will turn 3.45 times for one turn of the rear wheels. The road speed is dependent upon the rolling circumference of the wheels and tyres. (and yes, you CAN calculate this pretty closely from the tyre size)
with overdrive in, the diff ratio becomes 3.45 *0.8 = 2.76 so it's like having a 2.76 : 1 diff ratio.
I know some big Jags and some old V8 USA cruisers, and early V6 Ford Capri had a 3.08 : 1
If it's of interest, you can calculate max speed and mph/kph per rpm in each gear if you know gear ratios and rear tyre sizes, in something like an excel
spreadsheet. It's not actually that hard.
Edited by RCK974X on Thursday 29th June 06:13
Just looked at web, to reminisce my old Triumph days ... and get reminder of overdrives
'A' type o/d (early Triumph TR4,5,6, saloons) was 0.82:1 - exactly same as Ford 5 speeders.
'J' type o/d (late TR6 and 2500 saloon) was 0.78 : 1 - A little taller.
so an example calc -
Triumph 2500 saloon -
185 x 13 tyre (original fitment) turns 845 times per mile.
In 4th gear (which is 1:1 in gearbox) the engine would turn 845 * 3.45 times, = 2915.25 for 1 mile
so at 2915 rpm the car will travel at 1 mile per minute, which is 60 miles per hour.
with overdrive in, engine turns 845 * 3.45 * 0.82 = 2390.5
so 60mph is 3000 rpm 4th, 2400 rpm o/d (rounded, as on tacho)
with later odrive second number becomes 2275 rpm (2300 rpm) possibly enough to notice
a 195.70 x 13 tyre turns 875 times per mile, so engine would be turning a little faster, and you would go slightly slower for same rpm.
As I think you are looking at a TR6 with 15 inch wheels, this becomes
185x15 is 780 turns per mile , so with 3.45 diff, the numbers become
60mph 4th 2690 rpm 'A' o/d = 2205 rpm. 'J' o/d = 2100 rpm.
'A' type o/d (early Triumph TR4,5,6, saloons) was 0.82:1 - exactly same as Ford 5 speeders.
'J' type o/d (late TR6 and 2500 saloon) was 0.78 : 1 - A little taller.
so an example calc -
Triumph 2500 saloon -
185 x 13 tyre (original fitment) turns 845 times per mile.
In 4th gear (which is 1:1 in gearbox) the engine would turn 845 * 3.45 times, = 2915.25 for 1 mile
so at 2915 rpm the car will travel at 1 mile per minute, which is 60 miles per hour.
with overdrive in, engine turns 845 * 3.45 * 0.82 = 2390.5
so 60mph is 3000 rpm 4th, 2400 rpm o/d (rounded, as on tacho)
with later odrive second number becomes 2275 rpm (2300 rpm) possibly enough to notice
a 195.70 x 13 tyre turns 875 times per mile, so engine would be turning a little faster, and you would go slightly slower for same rpm.
As I think you are looking at a TR6 with 15 inch wheels, this becomes
185x15 is 780 turns per mile , so with 3.45 diff, the numbers become
60mph 4th 2690 rpm 'A' o/d = 2205 rpm. 'J' o/d = 2100 rpm.
Edited by RCK974X on Thursday 29th June 07:28
Thanks RCK for lots of great information. I appreciate the time you have taken to put all this together. I now know why a 25% o/d is called 25% and its not all that obvious - not to me at least. If I've understood this it seems a 25% o/d does reduce engine speed by 20% for the same road speed (and increase road speed by 25% for the same engine speed). Its something I've suspected for a while but you have now explained it in terms I can understand. Thank you.
Slightly off topic but I believe there are a couple of TVR 2500s in New Zealand. Its the car I'm trying to understand in a round about sort of way.
Thanks also to //j17. Mintylamb is the sort of site I can lose myself in for hours.
Cheers,
Chris
Slightly off topic but I believe there are a couple of TVR 2500s in New Zealand. Its the car I'm trying to understand in a round about sort of way.
Thanks also to //j17. Mintylamb is the sort of site I can lose myself in for hours.
Cheers,
Chris
I can give you the mathematic formula for turns per mile. If you want just ask ! No its NOT complicated....
Glad that helps - I did a rpm vs roadspeed spreadsheet a long time ago for kit cars.
Yes, there are a few TVRs around here in NZ, of all types. A few Vixens, 1600 and 2500, a couple of 'M' , and a handful of wedges.
There are more of the later chimps and grifs, and I do see even speed sixes for sale (via Japan).
Have a look in here (pistonheads) for TVR/classics blog some great info on all pre wedge cars.
NZ made it harder to import 'special' and 'classic' cars, and it became very expensive as they had to have special compliance inspections, and even though
it's easier today (as I understand, not tried it out), there are still some stupid rules in there. But some dealers specialise in this type of thing.
For example (I have been told) it's not allowed to have copper alloy brake pipes (kunifer) in a rebuild, or the Goodrich braided brake hoses.
No idea why....
I could not fit adjustable spring height dampers to my wedge, as this classes as a 'serious modification' and costs equiv of 1500 quid to inspect and recertify ....as does an engine change, or even a gearbox change (i.e. 4 to 5 speed)
Glad that helps - I did a rpm vs roadspeed spreadsheet a long time ago for kit cars.
Yes, there are a few TVRs around here in NZ, of all types. A few Vixens, 1600 and 2500, a couple of 'M' , and a handful of wedges.
There are more of the later chimps and grifs, and I do see even speed sixes for sale (via Japan).
Have a look in here (pistonheads) for TVR/classics blog some great info on all pre wedge cars.
NZ made it harder to import 'special' and 'classic' cars, and it became very expensive as they had to have special compliance inspections, and even though
it's easier today (as I understand, not tried it out), there are still some stupid rules in there. But some dealers specialise in this type of thing.
For example (I have been told) it's not allowed to have copper alloy brake pipes (kunifer) in a rebuild, or the Goodrich braided brake hoses.
No idea why....
I could not fit adjustable spring height dampers to my wedge, as this classes as a 'serious modification' and costs equiv of 1500 quid to inspect and recertify ....as does an engine change, or even a gearbox change (i.e. 4 to 5 speed)
Edited by RCK974X on Thursday 29th June 21:47
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