12H2709 ‘big head’

[Stephen Aitken]

I managed to get for £50 a 12H2709 from an Austin 1800 as a bit of a punt. On receiving pleased that at least superficially there isn’t any cracks though it will be getting reconditioned shortly and converted to unleaded seats. Anyway I just wanted to get the stats on what this will give me over stock rubber bumper in terms of torque/bhp. I have the k and n filters and like the idea of adding hs6 plus a new exhaust.
I think from memory Vic has this type of setup?
Anyway keen to know the ins and outs.
Also what makes a stage 2 head does it help that it’s the larger head to begin with or doesn’t it matter
Thanks
S

[Vic Butler]

Afternoon Stephen.
Yes, my 77 BGT has a 12H2709 head. I think the definition of a Stage 2 head is gas flowed and ported and with 1.625" inlet valves which are standard on the 12H2709 head.
However my head is somewhat different. It's a 12H2709 that I found among a pile of heads that my mate had but the inlet valves are 1.75" from an MGC. This occurred because Brian, who modified the head (he was Downton Engineering's ace cylinder head tuner) found that one of the valve seats was damaged probably due to something being ingested so he carried out a "Downton Dodge" which was fitting MGC inlets.
It has also a pair of genuine BL Special Tuning HS6 1.75" carburettors and a stainless steel 2" bore 3 branch manifold and exhaust system with one silencer at the rear (enhances the bass.) The distributor was built by the Distributor Doctor to.match. The camshaft is standard as it's perfectly adequate.
It goes well and will rev well into the red zone if I'm not careful and is perfectly driveable in traffic despite the large inlet tract. Downton had a knack for getting the best of both worlds.
Unfortunately there is no rolling road nearby and I'm sure it needs a needle change but without a rolling road it's impossible to find the correct one.
I suppose my cylinder head is probably a Stage 3 and it would really benefit from a hotter cam.

[Stephen Aitken]

Thanks Vic do you know power increase stats?
Also I know the chat about hs6 being maybe too much carb but anyway I’m assuming with the right manifold there’s enough room etc given the bowl is to the side on a late B vs hifs

[Vic Butler]

I'd really like to know the actual power but driving it there is obviously considerably more power than a standard rubber bumper car which, must be admitted,are gutless.
Yes, I've heard tales of overcarbing but due to the Downton expertise I have a perfectly tractable engine even at lower revs with the larger than standard 1.75" inlet valves as opposed to the normal 1.625" of a standard Stage 2 head but drop a gear or 2 and off it goes.
There's plenty of room for the superior HS carburettor and I 've got a heat shield fitted. An advantage is that I can remove the manifold and leave the carburettors attached. Brass barrel nuts as were used by BL Special Tuning on the manifold to head studs make removal easy. My HS6'S are.mounted on to the manifold by genuine BL Special Tuning rubber mounts.
The throttle return springs attach to a curved strip of metal mounted on the carburettor to inlet manifold studs, again as was used by BL Special Tuning.
I forgot to mention air filters. I'm using MGC.K&N's with stub stacks.
Ignition timing too is guesswork. I'm going to try timing it by ear.
Your MG is a rubber bumper one isn't it? If so, the 12H2709 has a centre oil feed to the rear rocker shaft post whereas the rubber bumper head has an offset feed. You'll have to change the rear rocker post for a centre feed one or get the complete shaft assembly from the 1800. It'll fit because the shaft on my BGT is from an 1800.
The rubber bumper distributor having its vacuum advance take off point on the inlet manifold will not be suitable and will need to be recalibrated for the Stage 2 set up which will have the take off point off one of the carburettors in the proper place on the air filter side of the throttle disc.

[ChargedAutoGT]

If you substitute '6' carbs for '4' (hs or hif) on a stock head you will lose low end torque and power. you will gain a bit over 5krpm.

the rest of the engine has to be able to breath to make use of them.

regarding the increase in power by putting a 12H2709 head on, i'm afraid it's so minimal that you are not going to notice it. sorry it's not what you want to head but it takes a lot more than bigger valves to get a kick in the seat.

if you want to decide where to modify for some extra horses, take a read of Peter Burgess' book. Flowing the head would be a good start. Followed by a decent road cam. that is assuming the rest of the engine is in fine fettle.

G

[Vic Butler]

Are you going for a Stage 2, Stephen? If not keep the standard carburettors or swap for a pair of HS4's which are better.
To digress slightly, many years ago I came across a case of real overcarbing. Someone had fitted an HS6 to his 803cc Morris Minor. I'd imagine it went even worse than before.

[Stephen Aitken]

Have just got Peter Burgess book and will have a good read through though I think gas flowed would be the way to go plus a new cam.
On the carb front after doing some googling and getting quite a few views it would seem hs6 doesn’t really add much and the existing carbs are fine. Interesting though on your point Vic on the hs4s vs hifs being better - what I’m reading is that it is slightly inferior though maybe not from inspecting the float bowl etc but isn’t thought the hifs cope better with heat vs hs4s?
Thanks

[Vic Butler]

Stephen, I've had experience of both HIF and HS. The HIF's were introduced when emission regulations were being introduced and the jet is controlled by a bi metallic strip which weakens the mixture as the engine heats up. Replacement of the float chamber needle and the float requires removal of the carburettors from the manifold whereas HS's don't. There is a wax controlled jet on some HS's but this is easily replaced.
I've never had any fuel vaporization problems. I have fitted a heat shield designed for HS6's and the stainless steel exhaust manifold is wrapped. Cast iron manifolds must not be wrapped because it can cause fractures.