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Heavy breathing on the V8
Peter Laidler (Harvest Gold 0811) from Oxfordshire provides an interesting article which explains how the V8 inlet manifold works and which carburettor feeds which cylinders. (Jan 80) Uploaded: 250212

The carburettors used on the MGBGTV8 are SU HIF6 units and although they are not foolproof by any means, they are possibly the best choice of the easily available carburettors for this engine. Unlike the old HS type, the HIF6 type are integral and the spindles have airtight seals, and leaks are probably non-existent. However having said that, by now most MGBGTV8s are getting old and I suggest that as soon as the carburettors are removed from the engine for any reason, take the opportunity to refurbish them with two sets of seals available under part number GSU500. The inlet manifold BHH1017 sat on top of the adaptor carburettor BHH988, then the two trusty HIF6 SUs.

It is generally accepted that where there are two carburettors per engine as on the MGB then the front carburettor feeds the front two cylinders and the rear carburettor feeds the rear two cylinders. Likewise on the old Cooper S model, the left hand carburettor feeds the two left hand cylinders and the right hand carburettor feeds the right hand cylinders. Yes before anyone tells me there is a balance pipe and one carburettor can subsidise another, I accept this but what I have said is basically correct. Now on this basis you would feel it would correct to assume that with the mighty V8 that the left hand carburettor feeds the left hand bank of cylinders and the right hand carburettor feeds the right hand bank of cylinders. But you would be wrong. Before I go any further I must clarify "left" and "right" on the V8 - they are left and right as viewed from the rear of the car. I am clarifying this now as I have seen someone come drastically unstuck while viewing the engine bay the other way round!

If anyone has seen the V8 inlet manifold you will have noticed two big holes below where the adapter is bolted on. These holes are called the plenum chambers. Two dictionaries give two definitions of the word plenum - one states that it is a "place for conditioned air" and the other states that it is "a gathering chamber". They sum up the word plenum in its V8 context - a gathering chamber for conditioned air, in this case the 14:1 air petrol mix.

Let us now turn to these two chambers on the V8 unit, left and right. At the outset I should stress that these are not connected as the MGB unit and consequently one carburettor cannot subsidise the other on the V8 unit. The left bank of cylinders are numbered 1, 3, 5 and 7 and the right bank 2, 4, 6 and 8. The left hand carburettor has two feed tracts leading from the carburettor directly to cylinders 1 and 7. Via number 1 cylinder tract there is a takeoff to feed number 4 cylinder on the right bank and via number 7 cylinder tract a takeoff to feed number 6 cylinder on the right hand bank. That has dealt with the left hand carburettor. The right hand is similar in that it has two feed tracts direct to number 2 and 8 cylinders on the right hand side. Via the number 2 cylinder tract is a takeoff to feed number 3 cylinder on the left hand side. From number 8 cylinder feed tract there is a takeoff to
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feed number 5 cylinder on the left hand side.

I will briefly explain that I set up my carburettors with a Colourtune, simply because I believe the saying "the only way you can tell what is happening in the cylinders, is to look inside" is true and that this enables you to do just that. However this is only my personal view. I say this as I refer to this item of equipment in the next paragraph. I also use the Colourtune to set up the Webers on our racing Midget.

You can now see that for the purpose of setting up the air/fuel ration of the left carburettor, it is essential to use the Colourtune plug and observe the colour in the plug of cylinders number 1 or 7 as cylinders 4 and 6 are second best. Likewise to set up the mixture of the right hand carburettor, use cylinders number 2 or 8 and regard 3 and 5 as second best.

The observant of you will now say "it doesn't look that the V8 manifold is very efficient" and at first glance I would have to agree, but if that were the case when you had set up cylinders 1, 7 2, and 8 correctly, then cylinders 3, 5, 4 and 6 would be on the weak side. But strangely enough this is not so - or as near as makes no difference and must show credit to the designers who did their homework with this manifold set up.

As an afterthought, those of you who set up their mixture ration by looking at the colour of the plug, then do not fall into the trap of looking at the colour of the plugs in the drive at home. That will simply show the condition of the plugs and the air/fuel mixture "off load". Instead take the car out for a thrash and at about 70 mph on a clear stretch of road, switch off the ignition and coast to a stop and then inspect the plugs which will record in terms of colour the condition "on load" and adjust the carburettors accordingly. Do not coast to a stop with the engine running out of gear as this will the same as running "off load" on your drive.

SU booklets on the HIF carburettors

On the subject of carburettors, there are two publications available from SU on the HIF4 and HIF6 carburettors. The part numbers of the publications are:
BLMC AKD7521 SU AUC9940
BLMC AKD7902 SU SUC9939A
Copies of both are available on the V8 website at:
SU publications


Tuning SU carburettors
There is also a useful book, Tuning SU Carburettors from Speedsport ISBN 85113-0720-0 More
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