HS6 carbs + accompanying modifications.

Technical MGB discussion
Hazza1190
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HS6 carbs + accompanying modifications.

Post by Hazza1190 » Mon May 20, 2019 5:10 pm

Hi guys, me again!

So I am currently restoring a '73 MGB GT. The other week I picked up some HS6 carburettors with a correct MGB manifold so they are ready to put on when the car is done. One of my jobs along the way will be to rebuild the engine. I will be replacing the camshaft with a fast road one (not sure which one yet) and I have also picked up a nice free-flowing stainless steel manifold too. I will be using the car for some fast road driving and possibly some sprints if I decide to; However, I will also be using it on the other side of the spectrum which will be road trips through Europe with the possibility of towing a tiny teardrop camper.
Will the HS6's be too much 'carb' for the engine? What other modifications should I be thinking of doing to make the HS6's worth it?

Furthermore, as this is my first restoration, it will also be my first engine rebuild. I would love some pointers, and links to good threads that i may find helpful!

TIA,
Harry.

Ian Fozzard
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Re: HS6 carbs + accompanying modifications.

Post by Ian Fozzard » Mon May 20, 2019 8:08 pm

Hi Harry,
good to see you getting on with your restoration! In my opinion HS4's should be adequate for most stages of tune, and larger carbs may result in some low down "driveability".

Best advice is to get Peter Burgess's book on tuning MGB engines. Especially valuable if you are doing the work yourself - good practical advice from an expert in the field.

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

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Peter Cresswell
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Re: HS6 carbs + accompanying modifications.

Post by Peter Cresswell » Tue May 21, 2019 10:28 pm

Hi Harry.
I would tend to agree with Ian and stay with the HS4s with the state of tune you are proposing. This was also the advice given by Gerry Brown when he did a talk to the MGB Register 'MGB Tuning for Fast Road or Track Days' in November 2006. The larger HS6 carbs lose power at the bottom end for a gain at the top end, and will also giver higher fuel consumption. Unless you use a cam like the Piper 285 Magnum and have a very modified head with a compression ratio approaching 10:1 the benefits may be marginal and the downsides quite noticeable. The Piper 285 is considered a fast road/mild rally cam, but it does give a lumpy tickover. This will be even more obvious if you use a lightweight flywheel.
If you are thinking of doing some Sprints (and Hillclimbs) with the car, you would be best to compete where other MGBs are likely to enter (or you will be very uncompetitive in an open class running up against the likes of Honda S2000s with around 220bhp in the upto 2000cc class). Read the Spec for the MGCC Speed Championship. If you want to keep away from some very expensive mods, Group 2 Road Going A is the most suitable and you have to use HS4s. You can get all the details from here:https://www.mgcc.co.uk/speedchampionship/wp-content/uploads/sites/57/2019/02/MGCCSC-Regs-2019-V1.2.pdf
Pete
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965

Vic Butler
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Re: HS6 carbs + accompanying modifications.

Post by Vic Butler » Wed May 22, 2019 9:34 pm

I've had HS6's on my 77 BGT since 1981. They are genuine BL Special Tuning AUD505F & R on the matching inlet manifold. The camshaft is the standard one which is more than adequate for normal road use. At the same time I was given a cylinder head from a Mk3 Austin 1800. This head had been modified and when I had it unleaded it was confirmed as a Downton head by a former Downton employee who carried out the unleaded conversion.
The head developed a crack so I got a 12H2709 and had that modified by the same Downton man. One of the valve seats was damaged so oversize MGC inlet valves were fitted.
With MGC K&N filters and a 2 inch bore single silencer exhaust and matching manifold, the car is perfectly driveable at low speeds and it will quickly reach the red line when dropped down a gear or 2. Downton modified heads had a reputation of being driveable at low speeds. Despite the larger inlet tract which slows the air speed into the cylinders I can potter along country roads in top gear or cruise at the legal speed on motorways, so I don't see any problem with the larger carburettors.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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Peter Cresswell
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Re: HS6 carbs + accompanying modifications.

Post by Peter Cresswell » Wed May 22, 2019 10:16 pm

I knew it would be controversial recommending that!
Yes Vic is right. Some cylinder head modifiers can preserve the tractability low down (another is Peter Burgess with his High Torque heads), the key is to keep the inlet port gas speed up and at the same time ensure the cylinders get a good fill of mixture. I also think in Vic's engine the standard cam helps as it doesn't have a great deal of overlap. In fact it is an exceptionally good cam and modern road cam profiles struggle to give anything like the same characteristics at low and high revs as the standard cam.
I will probably go to 1.75in carbs myself as I have a pair of 1.75in bore HS4s specially built by SU for BL Special Tuning to fit to a 1293cc Cooper S. They came off an international spec rally car, so I suspect they were something of cheating carb as the Cooper S wasn't homologated with HS6 carbs. I want to get the car on a rolling road and set up with the HS4s (standard 1.5in bore) I am currently using and then later swap to the 1.75in bore HS4s and do a back to back test. It will be interesting to see the results side by side, and I will then be able to fit either set of carbs depending on what I want to do with car.
Pete
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965

Hazza1190
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Re: HS6 carbs + accompanying modifications.

Post by Hazza1190 » Thu May 23, 2019 2:41 pm

Thank you for the replies!

I have done a bit more investigating and taken into account your suggestions. I think i am going to flog the HS6s (even after many hours of making them look pretty) as low down driveability will be an important factor. I understand that these can be overcome with the right mods, however the benefits do not (in my case) outweigh the costs. Instead, I will be putting the money towards a Peter Burgess head, rebuild kit for my current HS4's and also a fast road cam - this will be a while down the road yet though as i have so much to do beforehand!

Thanks again!
Harry.

Ian Fozzard
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Re: HS6 carbs + accompanying modifications.

Post by Ian Fozzard » Thu May 23, 2019 8:53 pm

Hi Harry,

this sounds like a good plan, and if you aren't satisfied with the result you can always spend more and upgrade later. A Peter Burgess head would be a very good investment I think

I modified the head on my BGT myself after reading Peter Burgess's book and following his advice and methods. My '72 BGT already had the larger valves so I re-used them after a little modification, and with springs and valve guides from Peter Burgess. I concentrated on work on the ports, the valve throats and the combustion chambers. I got the chambers all to the same volume and skimmed the head to give a 9.5:1 compression ratio. The bores were overbored to give just under 1900cc, and I used high quality Nural pistons (from Brown and Gammons).
The cam is a Piper HR270 which pulls in top gear from around 1500rpm, but really "comes on cam" at about 2000 - 2500rpm. This gives a really tractable engine with plenty of "grunt" low down. I think this is quite attractive in a road car, and much more useful than peak BHP at 6000rpm!

I have standard distributor and HS4 carbs, but rebuilt to as new condition. Air filters are K&N with the "MGB" alloy plate. Runs really well on standard needles in the carb. Don't overlook the distributor - it will run the engine even if quite worn - but a rebuilt original Lucas distributor (I used the Distributor Doctor for this) will make a big difference to the performance.

Each to his own of course, and my set-up may not please anyone else, just got to decide what you want and how much you want to pay!

Ian F
1972 BGT, Blaze, Navy trim, recessed grill
1961 Midget, 948cc, Clipper Blue, Blue trim and weather gear

Hazza1190
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Re: HS6 carbs + accompanying modifications.

Post by Hazza1190 » Fri May 24, 2019 5:29 pm

Hi Ian,

Yeah, my thoughts exactly!

I have Peter Burgess' book arriving next week so im looking forward to giving it a read! Did you physically do the porting yourself? if so, was it somewhat easy to do? What sort of tools did you need? (im sure it will say in Peter's book but may as well ask whilst we're here!)

I was looking at the Piper HR270 as it looks perfect for what i need. Low down 'grunt' is very important in my eyes, as lets be honest, that's where most of the driving time is spent!

What are your thoughts on electronic ignition? Would you suggest i just stay with the standard distributor (which is what i was going to do anyway) or go with an electronic ignition of the likes of Pertronix or 123 (not sure that i can afford 123 to be honest).

Thanks!
Harry.

Vic Butler
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Re: HS6 carbs + accompanying modifications.

Post by Vic Butler » Fri May 24, 2019 6:00 pm

When you've sorted out the state of tune of your engine the Distributor Doctor can build a distributor to suit if you tell him the mods.
I've got Lumenition Optronic on my Stage 2 BGT and Pertronix on the Land Rover. Both had their distributors rebuilt to suit by the Doctor.
1977 Stage 2 MGB GT
1975 SWB Series 3 Land Rover with a later 2.5 petrol engine

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Peter Cresswell
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Re: HS6 carbs + accompanying modifications.

Post by Peter Cresswell » Fri May 24, 2019 6:51 pm

Hi Harry,
Peter Burgess is quite a fan of the Piper HR270 Magnum cam, describing it as 'an excellent all-round performance cam'. He does warn that the HR270/2 and the BP270 profiles don't seen to offer any real gains over the original (HR270 Magnum).
Pete
1969 MGB Roadster
2011 MG 6 TSE Magnette
2007 Mercedes SLK
Plus 32 other cars since 1965

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