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David C 07-06-21 09:33 PM

85A / 85C rebuild.
I am rebuilding an SU carburated 85A engine for my 400 but have a spare 85C head and Solex carbs that I could use to upgrade. Does anyone have informed opinion of which is better? As the workshop manual has detailed instructions for retro-fitting Solex I imagine this was the preferred option back in the day. Also is the original 85A cam profile OK or is there something better available now?

No need for crazy speed but I am considering fitting an overdrive and this may need a bit more grunt. I know some on this forum are not keen on this but I fitted overdrive on my 403 long before the gearbox adapter kit was available and although it was a lot of work it was well worth it for me.

Geoff Dowdle 07-06-21 11:48 PM

I have owned my first 400 since 1976 and it has been fitted with three different engines over those years. During the restoration I fitted a 100D2 engine , then 85A with 405 camshaft , 8.5 comp pistons and 100 head , I estimate about 100 - 105 bhp(I tried 3 SU , 3 Solex and 3 Weber on this engine)
The SU was the smoothest running but seemed to lack a little bit of power above 4000 rpm . For general touring between 60 - 80mph I suggest you stay with the SU.
Regarding the overdrive I dont think they are worth considering unless you plan to tour at 90 - 100mph . Many owners fit them to reduce engine revs and fuel consumption. They would have been better rebuilding & tuning their engines , rebuilding or replacing the distributor , getting the valve timing correct with an adjustable camshaft sprocket .
All 6 cylinder engines are designed to rev higher than modern engines , and will hapilly tour at 70 - 80 mph (3500 - 4000rpm) and achieve 28 - 30 mpg if in good condition.

The engine that is currently fitted in my 400 is a combination of parts , 100B2 block , 406 crank & rods , compression approx 9.0 to 1 , Sports Camshaft , 85C head modified to 6 inlet ports fitted with 3 twin throat Weber carbies with accelerator pumps . It his a strong engine, but can achieve 28 mpg on a long trip crusing at about 70 - 75 mph .
A STD 85A can do the same or better except it wont have the accelerating power.
I'm in the process of securing and rebuilding the original 85A engine for this car to get it back to original.

I suppose my recommendation is to build an engine suitable to your planned useage . No point in building a 170 bhp engine to tour at 60 mph.


David C 08-06-21 07:47 AM

Thank you, Geoff.

Hard to decide, then, but the SUs may win the day for smooth running.
Originality is not a worry as I will keep the single Solex 85 engine as a spare.
Clearly a bit more urge is required as the first owner found this lacking and Bristol swapped his diff for a 405 ratio. Fine for running about locally, but not so clever on the motorway.

I think we will have to agree to disagree on overdrives. In my opinion the 403 is far better to drive since I fitted it. It does need the 405 back axle ratio to go with it, though.

I plan to fit a 123 distributor to get around mechanical wear issues and to benefit from vacuum advance. Any thoughts on whether pumped carburettors would be better in this instance?

David C 08-06-21 07:33 PM

I have just been chatting to another long time 400 owner who had an 85a engined 400 back in the sixties. He found the SUs would perform beautifully for a month or two but then go out of tune and he was forever going back to the factory. He then bought another 400 with Solex carbs and had several tens of thousands of miles without having to do more than clean them every year or so.

Does anyone else have experience to help me decide the best way forward?

Geoff Dowdle 08-06-21 11:40 PM

David ,
when BCL fitted overdrives to 405 and 406 they used a 28% reduction overdrive and fitted the 4.2 ratio diff (approx 9% difference) , so the overall
ratoi difference was about 19%.

I've been told the current overdrive units used are about 18% reduction which equates to about the same .

With my second 400 I have been trialling a Ford type 9 5 speed gear box .The 5th gear is 18% reduction to 4th . I have only covered about 6,000 miles in this car but found at the 70mph max speed limit in Australia the engine is happier in 4th gear than 5th . Later this year I will be changing this box for a B/W CR5 with remote shift as they seem to be a better box than the Ford.
The B/W wont have the 5th gear but for my useage it is no loss.

Your 400 with a 4.2 ratio 405 diff will need either the original 3.9 diff fitted or an overdrive.

If you have a complete 85C engine , rebuild that and keep your original 85 as a spare.

David C 09-06-21 10:06 AM

The plan has always been to keep the 85 for originality.

I have an 85A engine complete and a head and Solex carbs from an 85C The decision is whether to update the 85A block with 85C head etc.

David C 09-06-21 10:16 AM

I'm interested in the 18% ratio for current conversion. My understanding is that the kit available works with the J type Laycock overdrive. I believe this was supplied as 27% for both Triumph Stag and Volvo. Any idea which car used the 18% ratio or is this a special for the Bristol mod?

Stefan 10-06-21 11:08 AM

85c head and OD mods?
Stefan Cembrowicz wrote:
Good mods- the 85c with Solex carbs is a better bet than the original For breathing.But check both heads carefully to see which has the most meat left on it and which is in the best condition. Some of the very early heads were porous and the factory had to replace a number of them at the time. My 110 cylinder head has been pressure treated with resin to seal any any incipient fissures.
There are sports cams around, but consider one of the new “high torque” camshafts, which adds to midrange grunt which is where our lovely long stroke engines are lacking. Will help with your OD too.
OD is an excellent mod- protects our Bristol engines at their
Natural cruising speed of 80 mph on today’s motorways. 3750rpm (2500 piston ft/ min) was IIRC the recommended max revs for continuous running back in the day , so motorway speeds (and we had no Mroads back in 1950s) are rather too close to that.
Good luck!

Stefan Cembrowicz

David C 10-06-21 11:42 AM

Good advice thank you, Stefan.

I may even have too much material on the 85A for modern fuel as the engine has had very little use. I plan to get the chosen head fully tested. I assume I will need new exhaust valves for unleaded fuel. What is the thinking on the valve seats? Were the originals good enough even if re-surfaced?

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