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Twin Hs2's - Try A New Needle Or Swtich To Twin Hs4's?


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#1 stevegrabba

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Posted 16 October 2017 - 02:05 PM

Hi Guys,

 

One of my mini's has a MK3 'S' engine fitted, the basic spec is as follows.

 

1293 cc

GTS Box, 3.44 diff ratio, on 12" rims

10.2 CR ratio

MK3 12G940 head

Standard Cam

Maniflow Exhaust Manifold

RC40 twin box 1" 7/8 bore

Twin HS2, red springs, 'M' needle, chamfered piston, reduced butterfly spindle and butterfly

Reworked BMC 12G264 inlet manifold

Accuspark Ignition in a 45D4.

 

 

So I'm going to say it should be about 80bhp at the flywheel...ish!

 

I'm toying with the idea of switching to 'AH2' needles or switching to Twin HS4's and re-working them also?

 

Question

 

Can you recommend a needle change to stick with the HS2's?  Or a switch to Twin HS4's and a needle and spring change also?

 

Cheers!!

 

 



#2 Wim Fournier

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Posted 16 October 2017 - 04:34 PM

Why should you? What is your goal? What are you using this car for?

There is a great difference between daily road use or circuit racing.

Only when you are driving 'wide open throttle' at 6500 rpm in the highest gear it is possible that the throats of two HS2 's are to narrow to let pass all the air your engine needs to sustain this speed for several minutes.

With the wide throats of two Hs 4's the airspeed in the carbs will be a lot slower, so the next question rises if the airstream has speed enough to fill the cilinder each time at 6500 rpm enough to have enough oxigen there to burn the fuel that you need at that speed and rpm.

Where do you find the balance? Read the book from David Vizard. He will tell you.

What is the openings sequence of your inletvalves? How many overlap between outlet and inlet opening times?


Edited by Wim Fournier, 16 October 2017 - 07:10 PM.


#3 greenwheels

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Posted 16 October 2017 - 04:53 PM

Wouldn't a rolling road session tell you all you need to know?



#4 stevegrabba

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Posted 17 October 2017 - 09:32 AM

I appreciate the replies guys, and as ever some food for thought.

 

I know I'll end up giving the HS4's a try, so I may as well just crack on with it.

 

Rolling roads are good, however when trying different components it's a never ending, expensive and time consuming scenario.  So rarely 'a rolling road session'.....Instead more of continuation of session after session after introducing each new tweak, or recommended component.  Before you know it you've spent hundreds, and months have gone by between appointments.

 

So I think I'll be doing it the old fashioned way, no point inventing a new wheel!

 

Thanks anyway!!



#5 Wim Fournier

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Posted 17 October 2017 - 10:20 AM

Yes, you're right in avoiding the rolling road tests. But, what about answering the questions that are put. Or do you prefer thinking on your own? No interference.



#6 Cooperman

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Posted 17 October 2017 - 10:35 AM

Twin HS2's are fine for up to around 85 bhp on a 1275 engine. 

Really HS4's are better on higher revving engines. You don't need a carb size which exceeds the inlet gas flow requirements of the engine taking into account the cam, head (gas-flowing & valve sizes), compression ratio, etc.

It is easy to 'over-carb' an engine thinking there will be an improvement, but the important thing is for the carb(s) to be able to supply the volume of mixture at the optimum revs for peak power and torque.

I run twin H4's on my 1310 cc Cooper 'S' because it is running a 286 cam with 1.5 roller rockers, 11.1:1 static CR and gives peak power at 6400 rpm. Twein HS2's would not supply sufficient mixture at those revs.

In the past I tried twin HS6's on a 1330 'S' with a 649 cam, but there was no improvement at the top end and I lost a bit low down because the inlet flow velocity was too low and it 'bogged down' below about 3200 rpm.

You do not need twin HS4's for an 80 bhp 1275 engine, but flowing the carbs might give a bit of further improvement as will improving the inlet manifold a bit.



#7 stevegrabba

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Posted 17 October 2017 - 11:10 AM

Hi Cooperman....the HS2's are already flowed with the vizard basics around the butterfly & shaft, and piston, full radius rolled ram stack etc. 

 

Your comments regarding twin H4 and Twin HS6 are interesting.  For 30 years and more I've always question the correct balance between inlet port size and carb size.  We do so much to reduce airflow obstructions inside the carb, and inlet manifold, and I then often see them mated to an inlet port with maybe 40% more flow capacity.

 

I've always thought....why not use a carb with the same volumentric flow efficiency/size as the inlet port??  Otherwise it surely restricts the flow potential that we worked so hard to create by re-working the inlet port of the head?

 

Funny old things these mini engines!!

 

 

Yes Wim.....I am guilty of being a tinkerer!!



#8 nicklouse

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Posted 17 October 2017 - 11:17 AM

personally I would fit a HIF44. went down the twins setups in the early 90s and for a road going car saw next to no difference over a good single HIF on a sorted inlet.

 

but a set of twins always wins on the looks.



#9 Cooperman

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Posted 17 October 2017 - 11:24 AM

personally I would fit a HIF44. went down the twins setups in the early 90s and for a road going car saw next to no difference over a good single HIF on a sorted inlet.

 

but a set of twins always wins on the looks.

Yes, I agree.

I use twins because they are on a genuine 'S' which is used for competition and with my 100 bhp+ engine 1.5" SU's are needed.

On an 80 bhp road Mini a single H|IF44 will be an excellent choice. Twins do tend to give better initial throttle response when set-up accurately.

With the OP's engine it is simply a question of optimising the needle choice to get it right.



#10 stevegrabba

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Posted 17 October 2017 - 12:20 PM

 

personally I would fit a HIF44. went down the twins setups in the early 90s and for a road going car saw next to no difference over a good single HIF on a sorted inlet.

 

but a set of twins always wins on the looks.

Yes, I agree.

I use twins because they are on a genuine 'S' which is used for competition and with my 100 bhp+ engine 1.5" SU's are needed.

On an 80 bhp road Mini a single H|IF44 will be an excellent choice. Twins do tend to give better initial throttle response when set-up accurately.

With the OP's engine it is simply a question of optimising the needle choice to get it right.

 

I appreciate what you are saying about a HIF44.  However, I have a lot of twins kicking around from my minis of yesteryear (15 or 20 sets)....and I'm trying to remain in Keeping with it being a SA GTS.  Not everyone knows the GTS which is why i mentioned it to be a MK3 'S' engine.  They came with twin HS2's, but with a GY needle and anti-over runs on the butterfly so heavily throttled back/restricted.  I agree that twins always look the part and I can poetically justify HS2's or HS4's and be almost in original keeping, but a HIF just wouldn't look right.  Technically a good train of thought though...cheers.



#11 stevegrabba

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Posted 08 November 2017 - 04:03 PM

I'm sure this question goes through quite a few peoples minds so, I'll update from my own perspective.  I fitted the twin HS4's, red springs, SAE 20 dashpot oil, and they were the fixed needle type with new 0.9 jets.  The existing D3 needles were way to lean and there were masses of hesitation when driving.  AH2, were a definite step in the right direction, but still lean on mid to higher rpms and load, giving a bit of hesitation when under load.  AAA needles ironed out the mid rpm hesitation and seem to be running really well.

 

So in conclusion for me, on my spec of car as given at the beginning of the post .....I have no hesitation in saying that twin HS2's, with AH2 needles were holding the car back.  Switching to twin HS4's has free'd up a lot of potential and are much smoother throughout the entire rev range.  Definitely feels like it was the right move in making the switch.  I hope that this helps others.



#12 carbon

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Posted 08 November 2017 - 06:02 PM

In defence of the twin HS2s... ff these were using GY needles, blue springs, and the anti-over-run valves on the butterflies on a standard manifold then I am not too surprised that they were sluggish compared to a set of twin HS4s.

 

With twin HS2s a good set-up for a fast 1275-1330 motor would be:

- plain butterflies with the small bleed hole

- throttle spindle waisted down, remove legs from spindle screws and fix with Loctite

- blend in carb body sharp internal edges just after piston (see Vizard for details)

- old red springs (heavier than blue, not as strong as new red springs)

- probably M, AH2 or QA fixed needles depending on cam & rockers being used

- put on a good steel inlet manifold

- fit a decent pair of filters such as K&N with stub stacks or similar

 

I have not done a back to back but this set-up should not give away much to twin HS4s until you get up to very respectable power outputs.






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