Carb Sizes for Zephyr

[aex125]

I have a couple of options available for the 2.6 Zephyr engine I am looking for and one of the people also has a set of 1.25" SUs available. According to the reference material I have found, the 2.6 engines came with 1.75" SUs and so the 1.25" seem to be undersized. However, when I do the math, it seems the 2.6 will only need ~255 CFM@6000 RPM, which is well within the capability of the triple 1.25" setup. Does anyone have any experience using the 1.25" carbs on a 2.6 Motor? It seems for a street driven car, the smaller carbs might actually provide better throttle response at lower RPMs.
   
   Thanks,
   Jay

[nikbj68]

We have just gone down this route with the Greyhound, which had it`s Bristol engine replaced by a Triumph 2.5 before our ownership, and was fitted with a pair of (1.25")HS4`s. The engine is just about to be reassembled after a major overhaul, so a true back-to-back comparison isn`t going to be possible I`m afraid.
   This is the HS6(1.75") set-up that will be going on:
   
   
   
   These should push 420cfm compared with 266cfm of the HS4`s.
   In conjunction with a slightly worked inlet manifold, 6-3-1 extractor exhaust manifold, skimmed head with new valve guides & seats, better cam & 8lb-lightened flywheel. The target is an early torque band for road rather than track use, without having to keep the rev range high for spirited progress!

[aex125]

Thanks for the info Nick. You have somewhat confirmed what I had calculated and I am thinking 3 HS4s will be adequate for a 2.6. For the 2.6 I had calculated ~255CFM required at 6000RPM (not that I would ever run at 6000, but I used it for the upper limit), and had an HS4 flowing 145CFM. This is slightly different than your 133CFM, but either way with 3 of the HS4s I get 399cfm or 435CFM which should be adequate for a naturally aspirated 2.6. In your case where you are only using 2, the swap to the HS6s makes sense, but I think I would be OK unless I am missing something.
   The one thing I am still not sure of is throttle response for SUs. For down draft carbs, (which is where my experience is) if you run lower CFM carbs that still meet the upper RPM range, you typically get better throttle response than running an oversized carb. However, I am not sure if this holds true for SUs.
   
   Jay

[3.8Jaguar]

You have probably already have this info but I will post it anyway for interest.Are you on a Raymond Mays Cylinder head?
   
                                 year          Carb spec.     Type No.  Needles Rich, std,  weak,   spring colour
   Zephyr-Aquaplane (series I) 1954/7          AUC 753      3 H4                4      3     L      RED CTR YELLOW FRONT AND REAR
   Zephyr-W.H.M.B LTD          1955/7          AUC 820      3 H2                EM     ES    AP     RED
   Zephyr series I             1953            AUC 733      3 H2DD                  AUD1412WX       AUC1167
   Raymond Mays                1954            AUC 724      3 H4 F&R                AUD1127DJ       AUC1167 F/R
                                                              H4                    AUD1127DJ       AUC4387  C
   Raymond Mays SI             1954-56         AUC 771      2 H4                CN      5    GE     YELLOW
   Raymond Mays SII            1957/62         AUC 836      2 H4                MME     7    AO     YELLOW
   Raymond Mays SIII           1957/62         AUC 836      2 H4                A0  AUD10067  MME   AUC1167
   
   
   I hope this is of help or interest.
   
   Regards
   James Harris
   www.bristolengineering.co.uk

[3.8Jaguar]

sorry for the jumble of numbers, for some reason the format is changed from the draft to the post, should any one need better copy please contact me by email and I will return better copy.
   
   Kind regards
   James
   
   www.bristolengineering.co.uk

[aex125]

Thanks for the info James. I am understanding the listing correctly in that all of the above applications (except 1) used H4s and not H6s? I thought all of the 2.6 Aces used the 1.75" H6s.
   
   Jay

[3.8Jaguar]

Humm I have to admit that we have seen them on H6's as well though I have given you data from a period SU Data book as well as a contemporary SU data Book.
   
   Make of it what you can but I would say that it is accurate period information . Though you do get cross over between carburettors dependent on capacity i.e. 2 x HS 6 carbs will breath 3000 cc as will 2 HD 8 due to the different set ups under there differing part numbers.

[3.8Jaguar]

There is another point in the fact that the stats I have given refer to 'Modified Ford' Maybe other members might be able to cast some light on any further modifications AC and notably Ken Rudd made for the production 2.6 AC's?