Classic cars forum & vehicle restoration.
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Uncle Joe
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 Posted: Mon Jan 14, 2008 1:21 pm Post subject: Bristol automatic overdrive conversion |
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I know that there are a couple of Bristol owners on the forum, so a question to them (or anyone else for that matter.
I have been told that there is a company in the UK that manufactures an overdrive unit that can be fitted to the automatic gearbox fitted to the Mopar V8 engined Bristols.
Is there anyone who knows anything, or has any experience of these units? |
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Penman
Joined: 23 Nov 2007
Posts: 4759
Location: Swindon, Wilts.
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| Posted: Mon Jan 14, 2008 1:56 pm Post subject: |
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Hi UJ
I'm just a BOC member, a wannabe rather than an owner.
I will C&P your enquiry to the BOC forums, meanwhile Ashley may be along soon he is a 6cyl owner, but may have heard about this.
Edit
I have found this in the BOC wikki
| Quote: | ACCS at Kings Cross fitted a 4-speed Chrysler A518 automatic transmission to my 603 in place of the standard 3-speed 727 transmission about 12 months/20,000 miles ago. I am very happy with the work they did for me.
The A518 has the same ratios as the 727 for the first three gears and an overdrive 4th gear (0.66 if my memory serves me correctly) as well as the capability to run a lock-up torque convertor (I don't think it is possible to fit a lock-up with the 727, but someone may correct me). The A518 is essentially a more modern version of the same gearbox. The overdrive and lock-up are controlled by an additional column switch in my car, although ACCS can do a speed related automatic switch if you prefer. I think the A518(?) in a Blenheim is controlled by the ECU in a more sophisticated way (e.g. speed and load related).
The biggest difference is the car is now a great deal quieter at motorway speeds (e.g. 80mph is approx 2000rpm rather than 3000rpm). You really notice this if you switch the overdrive off at 80mph. It may also get you a few mpg more (my record is 25mpg on a steady 70-80mph motorway run, and my daily commuting average is 18.5mpg, up from 16.5mpg), but my experience is that fuel is one of the lesser costs of running a Bristol on a daily basis. I was a bit concerned that this ("interstellar") gearing would make acceleration in overdrive unacceptably slow, but the good low rpm torque of the V8s means this hasn't been an issue for me in practice, although obviously you will want to knock it off to get the quickest overtaking when desired.
I believe the A518 bolts up to the big block and small block engines as standard, but is a few inches longer in view of the overdrive unit at the rear end and so you need a shorter drive shaft and possibily some modification to one of the cross members to give some extra space. I think you also need to be careful about changing the driveshaft angles. Martin Barnes at ACCS would be able to tell you more. I think Martin carries the A518 in stock as he gets them from the US a few at a time and has done several of these conversions.
Regarding other modifications to go with the transmission swap, one thing you could consider is changing the rear differential ratio. I think most Bristols are just under or just over 3:1. You could change to say 3.55:1 giving a proportional boost in acceleration in the first three gears whilst the overdrive still gives you a very reasonable, and quieter than standard, approx 2400rpm at 80mph on the motorway. I haven't done this myself, but am giving it serious consideration as and when I need some work doing on the differential for other reasons.
Another option when changing the transmission (either 727 or A518) is to fit a "shift kit". These reprogram the gearbox with respect to shift points and speed of shift. They are usually fitted to give quicker (and firmer) than standard shifts. I have one myself and like it, but it would be a matter of personal preference. Again, Martin knows all about these.
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Uncle Joe
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| Posted: Mon Jan 14, 2008 2:47 pm Post subject: |
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I have heard about the A518 conversion. The best way to control it is by using two oil pressure switches, the first one engages the overdrive, and the second, at a slightly higher pressure locks the converter. The trouble with this unit is that I understand that it has never been fitted to an RB block, it is longer than the A727, but also fatter, so needs a new gearbox crossmember, as well as a bit of transmission tunnel bashing.
The system that I was refering to is, I believe, based on the old Laycock de Normanville unit that was popular on british cars once upon a time. It should therefore be a bolt on conversion, even if it does need a new tailshaft.
Thanks for posting on the BOC forum, I'll be interested to hear any answers. |
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Penman
Joined: 23 Nov 2007
Posts: 4759
Location: Swindon, Wilts.
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| Posted: Mon Jan 14, 2008 10:50 pm Post subject: |
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Hi
I haven't had anything back yet and when looking through the Bristol Cars site I could only find a reference to overdrives for the 6cyl cars.
I did have another thought though, what about a 2speed axle?
http://www.flatheadv8.org/columbia/columbia2.htm
Last edited by Penman on Tue Jan 15, 2008 8:22 pm; edited 1 time in total |
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Uncle Joe
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| Posted: Tue Jan 15, 2008 8:14 pm Post subject: |
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On this particular car, I've got a Sure Track (LSD) so I'm not keen to swap that.
You did make me think of something though. A two speed might just be the thing for the E83W owners on the forum. |
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Penman
Joined: 23 Nov 2007
Posts: 4759
Location: Swindon, Wilts.
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| Posted: Tue Jan 15, 2008 10:00 pm Post subject: |
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Hi UJ
This later post on BOC seems to confirm your comment re. gearbox size.
| Quote: | I was talking to Martin Barnes at ACCS this very afternoon about four speed boxes for my 410. A modern Torqueflight four speed box (technically a three speed with overdrive) with a lock-up torque converter would be around £4400 fitted. Interestingly, Martin told me that the 410 is one of the easiest to convert, but Brian Marelli has told me it's tricky compared to the later cars. The four speed box is larger than the 3-speed, so some surgery to chassis may be required, it certainly is on a 410.
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Uncle Joe
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| Posted: Wed Jan 16, 2008 9:17 am Post subject: |
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Yesterday, I was reliably informed that the A518 is 3½" longer than the A727. The gearbox crossmember mounting is also lower, so therefore a thinner rubber mounting is required, as well as I said previously, a bit of tunnel bashing.
I'm curious as to ACCS. Do they do their own ECU's? I did have a look for them on internet, but it doesnt seem that they have a website. Pity! |
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Penman
Joined: 23 Nov 2007
Posts: 4759
Location: Swindon, Wilts.
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Uncle Joe
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| Posted: Thu Jan 17, 2008 9:14 am Post subject: |
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Thanks for that penman. I'll post a letter to them, see what they say.
I hope BrianM doesnt read this, or I'll never live it down, but apparently, the Laycock unit is a modified version of the one that Volvo use... |
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Uncle Joe
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| Posted: Thu Apr 17, 2008 1:57 pm Post subject: |
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After doing some research on this, I'm going to go this way. It should be of use to any Bristol owners, as well as any other owners of cars with Chrysler motors, that want to replace the A727 with an A518 overdrive transmission.
A few minutes ago, I finished a CAD programme that can be put in a CNC miller. This programme will allow for the accurate removal of the A518 bellhousing, which is part of the gearbox casting. A new aftermarket bellhousing that will fit the big block Chrysler engine will then be bolted on. The propshaft will have to be shortened by 3 1/2" as the 518 is this much longer. A new gearbox crossmember needs to be made up.
To cope with the big block engine, a number of upgrades must be done to the A518.
Oil pressure switches will control the overdrive engagement, plus locking up the converter. This eliminates the need for electronics. |
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Brian M
Joined: 23 Nov 2007
Posts: 783
Location: Leigh-on-Sea, Essex
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| Posted: Tue Apr 22, 2008 8:21 am Post subject: |
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| Uncle Joe wrote: | Thanks for that penman. I'll post a letter to them, see what they say.
I hope BrianM doesnt read this, or I'll never live it down, but apparently, the Laycock unit is a modified version of the one that Volvo use... |
UJ
Somewhat belatedly I have read this post. I will refrain from any comment 
_________________
Brian
1970 Volvo Amazon and 1978 Safari 15-4 Caravan
Classic Safari Forum: www.classicsafaris.co.uk |
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Uncle Joe
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| Posted: Tue Apr 22, 2008 8:48 am Post subject: |
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| BrianM, did you see the posting I made regarding the Amazon bodyshells? |
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