Upgrading the 410 Final Drive Ratio

John Keighley · #1 (**permalink**) 04-10-23, 01:21 AM

We are in the final phase of major work to our 410. Among work being completed is: Engine rebuild including upgrade of the camshaft, electronic ignition, replacement of the Carter AFB with an Edelbrock AVS2 500. Addition of aircon (410s have no adequate internal ventilation and are utterly suffocating in Australian Summer conditions) and a long list of other improvements. With the engine upgrade, I want to raise the final gearing to lower the cruising engine speeds as to me, for an 8 cylinder, they seem ridiculously high. Current differential ratio is 3:07 (same as the 411, I believe). Has anyone in the group with a 410 raised their final drive gearing and if so, what specification?
Thanks, John K.

AndrewA · #2 (**permalink**) 04-10-23, 05:47 AM

Morning John
Thanks for pictures - looks great. To answer your question I've got 2.8 in mine - done by previous owner. I drove it back to back with another 410 which is ( I believe) on same ratios as yours last summer. My subjective memory is sort of what you'd expect - on rapid starts the 3.07 picks up quicker but once you're rolling the difference in performance is not really noticeable.
Once up to cruising speed the 2.8 makes a noticeable difference but in my case previous Owner also installed later gearbox with overdrive and lockup which is excellent particularly on long runs.
If you need any stats am out and about in car so happy to report back.
Cheers
Andrew

David C · #3 (**permalink**) 04-10-23, 08:36 AM

Hello John.

I'm nearing the end of an overdrive installation, keeping the original back axle ratio. Happy to share details if that helps.

If you are happy to trade acceleration for slightly lower cruising RPM a differential swap could well be worth a try. It shouldn't be too hard to find a 2.8 and you can upgrade to limited slip diff while you are at it.

Another option you might consider is finding an A999 transmission. This is non overdrive with lock up torque converter so would fit without the modification to crossmember and tunnel required by overdrive. Removing the slip would probably give a similar drop in revs as the diff swap and it would save plenty of fuel as well. You could go for broke and do both.

Please can you post pictures of how your air conditioning compressor is mounted? I am considering a Vintage Air system. Original Mopar air compressors were fitted above the alternator and I don't think will fit under the 410 bonnet. Sam Frost has kindly offered to help with details of what I believe is his factory installed system with the compressor mounted above the power steering pump.
I would like to compare all options before committing.

John Keighley · #4 (**permalink**) 04-10-23, 10:42 AM

Thank you Andrew for your reply. This is the information I am looking for. sounds as if the 2.8 ratio may be what we need as the car generally will do longer runs. Fuel economy in the car's original state gave us 21-22 mpg but I hope we can improve on that, particularly as the tank is not that big!
Many thanks!
John K.

AndrewA · #5 (**permalink**) 04-10-23, 01:20 PM

Well if you think of anything you need - let me know. Re the attachment, Kevin who kindly looks after this forum will probably sort it out for you if you ask nicely!
I think - but could be wrong - that once you go into the 2's as it were on the ratios you need a different carrier to the 3's - worth checking.

David C · #6 (**permalink**) 04-10-23, 01:41 PM

I have just spotted that you say the fuel tank is not very big. Has your car had a smaller tank fitted? The standard tank seems huge when I'm paying to fill it!

AndrewA · #7 (**permalink**) 05-10-23, 11:04 AM

This might help you.
Went for a drive - just relying on car's speedo and rev counter and with the 2.8 axle at a steady 70 mph.
Just axle 2200 rpm
With overdrive engaged 1800 rpm
With overdrive and lockup engaged 1600/1650.
Cheers
Andrew

David C · #8 (**permalink**) 05-10-23, 04:53 PM

It would be interesting to know how Andrew's speedo reading compares with GPS.

For a standard 410 with 3.07 final drive if maximum maximum speed 130MPH coincides with red line at 5000 ( I stand to be corrected...) then 70MPH will be at 2700RPM.
A change to 2.88 final drive will give 70MPH at 2532RPM.
So a drop of about 170RPM. Very similar to that achieved with a locking torque converter.

Change from 3.07 to 2.88 is a reduction of about 0.94:1 whereas standard Laycock overdrive and most modern 5th gears will give a reduction of about 0.75:1 to 0.8:1.
The Mopar overdrive really overdoes it with a drop of 0.69:1.

Applying a 0.69:1 overdrive to Andrew's 2200 RPM gives 1520 RPM and not the 1800 reported. As far as I'm aware Mopar didn't offer a different overdrive ratio so I'm quite confused.

AndrewA · #9 (**permalink**) 05-10-23, 05:32 PM

I've a decent GPS speedo around somewhere so will have another go tomorrow and report back.

David C · #10 (**permalink**) 06-10-23, 02:56 PM

Thank you for that. Loving the exhaust manifolds. Who made or supplied those?

I'm still keen to see some pictures of aircon compressor mounted on 410 engine as this is very different from the 411.

Thomas591 · #11 (**permalink**) 06-10-23, 03:07 PM

Uups, sorry that was a bit stupid from my side, of cause I am aware that your engine is different. Sorry for that.

The exhaust manifolds are from Spencer Lane Jones.

Regards

Thomas

AndrewA · #12 (**permalink**) 06-10-23, 03:21 PM

Updated figures (!) with GPS and a co pilot/scribe/second pair of eyes.
No o/d or lockup 72.1 mph 2400 rpm
O/d only 71.4 mph 1850 rpm
o/d and lockup 72.1mph 1650 rpm

David C · #13 (**permalink**) 06-10-23, 04:48 PM

That's a really good result but very puzzling. I have always been told that there was only one overdrive ratio for the 46RH of 0.69:1 Yours seems to be 0.77:1 which is much better. Next time you are under the car please can you look out for a serial number? I will do a bit of research and try to find out where to look.

David C · #14 (**permalink**) 06-10-23, 04:52 PM

I just calculated the ratio including lockup and it comes out at 0.68:1. Starting to make more sense...

David C · #15 (**permalink**) 06-10-23, 09:26 PM

Further investigation tells me that there was only one ratio for the 46RH and this was indeed 0.69:1

This suggests that in Andrew's test there was far more slippage at 1850RPM than at 2400RPM. If anyone out there understands torque converters, slip, stall speeds etc they are probably marvelling at my lack of understanding and screaming "of course!" at the screen. If so, please enlighten us. i need to understand in order to arrange automatic switching of overdrive and lockup. It seems to me that in Andrew's example that switching in overdrive without lockup will introduce significant slippage with attendant heating of transmission fluid and loss of MPG.

Andrew -if possible at some stage please can you briefly switch in lockup only at both 1850RPM and 2400RPM and let me know how much the revs drop?

AndrewA · #16 (**permalink**) 07-10-23, 07:46 AM

Of course re lockup and hunting for numbers if necessary!
May not be today as it entails trip to motorway - half hour each way - as no way can I (or even wish to try to) maintain steady 70 mph on New Forest roads !!

Kevin H · #17 (**permalink**) 07-10-23, 01:50 PM

Given that comments relating to air conditioning have nothing to do with the 410 final drive ratio, I have transferred those comments to this new thread http://www.bristolcars.info/forums/8...ditioning.html

John Keighley · #18 (**permalink**) 10-10-23, 10:16 AM

Quote:

Originally Posted by David C

I have just spotted that you say the fuel tank is not very big. Has your car had a smaller tank fitted? The standard tank seems huge when I'm paying to fill it!

David, it's a standard tank - 18 gal but given the distances we sometimes travel, the contents disappear rapidly.

AndrewA · #19 (**permalink**) 10-10-23, 12:31 PM

Sorry to fail you but it's been wired that you can't engage lockup unless overdrive also engaged - I did try.

David C · #20 (**permalink**) 10-10-23, 07:02 PM

No worries, Andrew. Having had a bit more time to think I'm not sure how much it would tell us. I was thinking of the torque converter purely as a fluid coupling but there's more to it than that. At lower speeds my understanding is that input RPM and output RPM can be quite different with attendant torque multiplication but I don't understand is how this relates to efficiency and therefore MPG.
Enough pontification, it's time for me to hurry up that gearbox installation and then report before and after EFI MPG figures.