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Single HS4 vs pair of HS2 on a 1275?

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CraigZ

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Title says it all really...my first Midget had the chopped manifold off an Austin America and its HS4. This was around 1986 and if I recall the car ran pretty well. And I still have that manifold/carb. My current Midget has the usual HS2 pair.

I honestly can't recall how that single carb ran - a fellow locally swears it's the bee's knees...so I'd like to solicit comments from any users here who've done this swap...and thanks!
 
My initial gut reaction is that feeding two with two is a better bid for volumetric efficiency than a single throat supplying four holes... but that's just my WAG. :wink:
 
Read your Vizard - four valves sucking at a single plenum through a single large carb makes more than two valves working on a small cabr via an inefficiently-designed manifold. Viva el HIF 44!
 
is the HIF44 the MG Metro carb?
 
As Bugger said...
I've heard good things about the single carb set up. Just because of the design of the dual carb intake manifold.
 
I haven't tried it, but I want to.

My initial reaction is that it's probably the same as with a turbocharger. Namely that pulsing flow is very bad for a turbo and most certainly not a good thing on a "continuous velocity" carburetor (SU, Stromberg et al).

Single carb means "more" continuous flow. Double carb on a siamese port engine definitely means pulsing flow.

How could they ever expect pulsing flow to give any kind of uniform air/fuel ratio?
 
JP, my HIF44 came from a Mini but it also is on the Metro. I bought mine on Ebay under Mini parts and assy. and the intake I bought there also.
 
This doesn't make any sense to me. Wide open throttle on two HS2s gives you an carb throat area of 2.45 sq ins and the same on a single HS4 gives you 1.77 sq ins. Wouldn't WOT on the single HS4 be the same thing as running at part throttle on the dual HS2s.

The HIF44 on the other hand has a carb throat area of 2.41 sq ins, almost as great as the twin HS2s.
 
Single SUs were used for years on the 1275 Mini. There are tradeoffs. I believe the twin HS2 are typically attributed with more power down low. That will translate to a bit better acceleration. However, at speed I think most people find little difference between a single 1-1/2" carb and two 1-1/4" carbs.

The obvious advantage of a single carb is much easier tuning. So... if you want an easy to tune car for around town, a single HS4 (or HIF equivalent) will be fine and easy to work with. If you want the most acceleration and don't mind the added synchronization and tuning steps, go with the dual HS2s.

My Mini has dual HS2s. I'm trying to keep the car more or less original and have left the duals in place. Were I not concerned with originality, I would probably run a single carb. My project Spitfire has a single Stromberg ZS carb which has its own set of problems. I have a pair of HS4s and manifold to install "one of these days". However, I also have fabricated some parts to fit a single HS4 carb in place of the Stromberg. I anticipate using the single HS4 on the Spitfire long before attempting to fit and tune the duals.

EDIT: Regarding the flow areas and such, don't get too focused exclusively on the throat areas of one vs two carbs. Vizard made some observations in his books about the impact of air velocity on performance and economy. There are instances where the higher velocity through a smaller carb throat is not a hindrance but a benefit. Sorry, I can't give you specifics except to suggest reading Vizard.
 
There's an HIF6 up on ebay now, with very cheap asking price. Tough part now is the manifold, I bought what may have been APT's last Titan a couple of years ago. Depending on the car and bonnet fit a mini 30 degree tilt manifold may or may not work, but they're relatively cheap and you could make an adapter to lower the top of the carb. Throw in a rebuild kit and a proper baseline needle (likely BDL, I found with my tuning I needed a BDK to get stoichiometric), plus the heat shield/throttle cable holder, and you're in business. PS I made up an even larger heat shield out of sheet aluminum and thermal insulator sheet, and I never (knock wood) have any trouble with hot-soak starting. Plus, the enrichment device on the HIF is a superior cold start device to the moving jet on the HS2, and it never sticks.
 
I like twin HS2s, plus you should already have the cores, which is normally less expensive to to totally rebuild than buy the other stuff.
 
davester said:
This doesn't make any sense to me. Wide open throttle on two HS2s gives you an carb throat area of 2.45 sq ins and the same on a single HS4 gives you 1.77 sq ins. Wouldn't WOT on the single HS4 be the same thing as running at part throttle on the dual HS2s.

The HIF44 on the other hand has a carb throat area of 2.41 sq ins, almost as great as the twin HS2s.
Click to expand...

That would be the initial thought, but WOT isn't WOT on an SU type carb. The piston height is determined by the difference between carburetor vacuum and atmospheric pressure, effective limiting the throttle size to the point that the venturi velocity is constant regardless of the engine speed.

Also, the throat area of 2.45 would assume excellent flow characteristics through both carbs to each and every cylinder, but one at a time. There is simply not good flow from the front carb to numbers 3 and 4 cylinders, or from the rear carb to numbers 1 and 2 cylinders.
 
Speaking of the single vs dual carbs.
I currently run the HIF6 SU. It works well. Smooth and
responsive. In looking at the Mini Mania site I noticed they sell a stub stack for my set up. Is this a waste of money?
Sure seems short and I wonder if such a short stack would make
any difference. It is designed to fit in my K&N cone filter.
Do rams, even short ones, make much difference?
 

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Gundy said:
.
Do rams, even short ones, make much difference?
Click to expand...


Short answer, YES!
 
Again, "Read your Vizard". On pg 59 you'll see in the illustration that the best flowing ram pipe, no. 11, is also the shortest with the widest radius.

JACK
 
lesingepsycho said:
Again, "Read your Vizard". On pg 59 you'll see in the illustration that the best flowing ram pipe, no. 11, is also the shortest with the widest radius.

JACK
Click to expand...


Thanks! I placed my order this morning.
The pic above is not the correct stack however.
That's for a HS6.
This is the one for my HIF6.
 

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HIF6 Stub Stack on 1380

Me likey! I installed the stub stack this morning.
Noticeable improvement in performance. Motor sounds /runs smoother too. I had to use a razor knife to trim a little
on the K&N's rubber trim to get the edge to fit.
Still, it only took about 15-20 minutes to install.
:thumbsup:

Side note: Top off in 49 degree weather is shall we say "brisk"
driving.
 
Re: HIF6 Stub Stack on 1380

I've come in late on this thread, but for what it's worth..
I replaced the ZS with a single SU HIF4 on my 79' 1500 engine and love it!
This setup is good to 100 HP, so I've been told by a racer. The change over was a snap with few mods. Manual choke is the only way to go, at least for me.
 
Re: HIF6 Stub Stack on 1380

Maddy,
I have a 1980 Spitfire 1500 that I need to change over from a ZS to a SU HIF4 for Solo 2. Would you please describe what mods you made to make the changeover. Thanks
 
I'm running twin HS4 SUs on my 1275 Sprite and that seems to be the best combination I've found for the mods I've made to the engine. The HS2s were just not doing the job. Just a thought.
 
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