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one or two?
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Norseman



Joined: 09 Jan 2019
Posts: 60
Location: Essex UK

PostPosted: Sat Feb 09, 2019 11:07 pm    Post subject: one or two? Reply with quote

Hi folks, here's one for enthusiasts (such as myself) of Rover classic.

Q1: why did the P6 2000/2200 in automatic spec. only have one SU when the manual versions had two ?

Q2: why was the P5 (auto or manual) only fitted with a single SU at a time when rivals such as BMC, Rootes & Triumph all fitted twin carbs to their straight six's of the same cyl. capacity. Although Ford used downdraft carbs the six pots were usually of the twin-choke variety.

nb: out of interest, the automatic P4 105R DID have twin SU's !

I have James Taylors's 1934-77 collectors guide, but cannot find an explanation for the P6 difference in spec.
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Rick
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Joined: 27 Apr 2005
Posts: 19794
Location: Cheshire

PostPosted: Sat Feb 09, 2019 11:22 pm    Post subject: Reply with quote

I can't answer the question, but iirc the VdP 1300 came with a single carb in auto configuration, and twin with the manual gearbox.

RJ
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Various 1920s-1960s - Austin, Morris, Commer, Dodge etc.
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47Jag



Joined: 26 Jun 2008
Posts: 1429
Location: Bothwell, Scotland

PostPosted: Sat Feb 09, 2019 11:36 pm    Post subject: Reply with quote

Norseman

A possible answer to your question could be the autos used the Borg Warner type 35 which probably couldn’t handle the increased torque of the twin carb engines? The older Rover 105 would have used the older DG gearbox which was stronger.

Art
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Ray White



Joined: 02 Dec 2014
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Location: Derby

PostPosted: Sun Feb 10, 2019 1:17 am    Post subject: Reply with quote

I thought it was due to fuel consumption. The automatic is thirsty enough with just one carb. !
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roverdriver



Joined: 18 Oct 2008
Posts: 1063
Location: 100 miles from Melbourne, Victoria, Australia

PostPosted: Sun Feb 10, 2019 6:37 am    Post subject: Reply with quote

The standard Rover 2000 had only a single SU. The more powerful 2000 TC had twin SU's as a more 'sporty' version of the car. You are right the 2000 automatic had a single SU. In my experience the standard 2000 had very performance that I would describe as more than adequate for the circumstances for which is was made.

The first of the P4's, the Rover 75 had twin SU's as it was considered that the engine was a little underpowered with a single carburettor. The later P4 engines with a bigger capacity reverted to a single carby, but twins were used later on certain models.

I cannot speak for the P5 as I have never owned one.
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mikeC



Joined: 31 Jul 2009
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PostPosted: Sun Feb 10, 2019 9:24 am    Post subject: Reply with quote

The fitting of a larger carburettor, or in this case fitting twin carbs, usually results in more power, but produced at higher revs. A small carburettor, or a single in place of twins, will produce greater low-speed power and torque. The automatic gearbox of the period was usually a 3-speed (and in some cases only a 2-speed), so the greater flexibility of the single-carb engine was needed to mask the limitations of the transmission.
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Last edited by mikeC on Sun Feb 10, 2019 1:13 pm; edited 1 time in total
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alastairq



Joined: 14 Oct 2016
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Location: East Yorkshire

PostPosted: Sun Feb 10, 2019 10:47 am    Post subject: Reply with quote

I agree with mikeC....Also, was the final drive ratio on automatics raised at all? [To compensate for loss of performance due to fewer gear ratios?]
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alanb



Joined: 10 Sep 2012
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Location: Berkshire.

PostPosted: Sun Feb 10, 2019 2:16 pm    Post subject: Reply with quote

Spot on MikeC, the single carb produced a more flexible engine and better torque at Lower engine speeds
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Penman



Joined: 23 Nov 2007
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Location: Lancashire

PostPosted: Sun Feb 10, 2019 2:43 pm    Post subject: Reply with quote

Hi
My father had a twin carb MG 1100.
They had 2 different performance outcomes compared with the single carb 1100s
I got places quicker, and my father used far less fuel.

Whether driving style produced those differences on other engines I can't say, but I suspect it would.
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Ashley



Joined: 02 Jan 2008
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Location: Near Stroud, Glos

PostPosted: Tue Feb 12, 2019 3:03 pm    Post subject: Reply with quote

In simple terms two Carburettors give more top end power at the expense of low end torque. How much torque depends on the capacity of the engine and size of the carburettors.

Typically the kind of cars most members on here are interested in will produce peak torque at around 2000 revs, while sports cars like TRs, MGs, Healey’s etc at nearer to 3000 because it’s assumed that owners are happy to change gear a little more often.

Big engines cars like Jaguars and Bentleys etc need two carbs because more air flows through bigger engines so are still very torquey. For example an S1 Bentley has two 2 x 2" SUs and still produces max torque at about 2000rpm.

Hope this helps.
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alastairq



Joined: 14 Oct 2016
Posts: 984
Location: East Yorkshire

PostPosted: Tue Feb 12, 2019 5:32 pm    Post subject: Reply with quote

Inline sixes work better with multiple carbs, compared to one carb, due to the distance to the end cylinders? This can lead to very uneven mixture strengths along the line of cylinders.
{US Ford inline sixes a case in point....together with the integrally-cast inlet manifold & cylinder head......Offenhauser produced [still do] a piggy-back manifold which houses 3 carbs.....and a nicely complex linkage which meant that on light throttle, only the centre carb was 'working'...floor it, and the other two came on line]

Changing to a vee cylinder formation made fuelling [and emissions] far easier to deal with.

If the choke size of the [twin] carb setup is appropriately small enough, so low down torque can be improved.

Hence, I have a twin SU set-up, which are 1 1/8th inch....for a sidevalve Ford...also, an 1 1/4 twin SU set-up, for a later[??] sidevalve Ford engine.

Camshaft shape does more to dictate how far up the rev range power comes in.....hence, modified cams can lead to rough idling [or a higher-than-normal idle speed] and a loss of low-down torque.

When I trialled a Skoda Estelle [with 1300cc engine] I had, at one time, a swan neck inlet manifold, with 40 DCOE Weber jug clamped on.....a session on a local [long screwdriver type] rolling road tuner, led to the fitment of 26mm chokes..to get the torque back up at tickover and just above.

Latest trials Skoda has 4 bike carbs....
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Dellow Mk2, 1951 built, reg 1952.
Ford Mustang coupe, 1967, 6 cylinder auto.
Fiat 126 BIS
Cannon special [1996 registered. Built in 1950's]
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Ford Pop chassis, Ashley 1172 bodyshell, in pieces.
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emmerson



Joined: 30 Sep 2008
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PostPosted: Tue Feb 12, 2019 7:44 pm    Post subject: Reply with quote

My old Dad used to say that anything which came in twos was double trouble!
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alastairq



Joined: 14 Oct 2016
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Location: East Yorkshire

PostPosted: Tue Feb 12, 2019 8:02 pm    Post subject: Reply with quote

Bike carbs are very reliable...don't go out of tune , unlike SU's....

After all, look at the popularity of Japanese 4 cylinder motorcycles over the decades?
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Dellow Mk2, 1951 built, reg 1952.
Ford Mustang coupe, 1967, 6 cylinder auto.
Fiat 126 BIS
Cannon special [1996 registered. Built in 1950's]
----------------------------------------------
Ford Pop chassis, Ashley 1172 bodyshell, in pieces.
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Norseman



Joined: 09 Jan 2019
Posts: 60
Location: Essex UK

PostPosted: Wed Feb 13, 2019 10:03 pm    Post subject: Reply with quote

Many thanks for the very comprehensive replies, for which I will query just two.

47 jag : the BW35 was the only option on the P5B whose V8 produced greater power & torque than the Rover IOE engines.

mike C : still leaves the question as to why all Rover's main competitors chose to use twin carbs on their auto's (some of which I've owned)

Cheers to all, interesting viewpoints Smile
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Many makes & models have served as my transport since 1964, all old & some of them older than me.
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Mog



Joined: 30 Dec 2007
Posts: 609
Location: Sydney

PostPosted: Thu Feb 14, 2019 3:58 am    Post subject: Reply with quote

I remember seeing a Mini pick up with 4 Amal Mono Blocs fitted .

I think it belonged to Tony Marsh . He lived not far from me .
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