52 replies to this topic

[Spider]

On the contrary, I'd sooner pick up an SU for just about anything over a Weber, including Performance, though for an all out Track Car, I would (and have done so) go for the Weber. Back to back testing I did on the Dyno years ago showed the Weber to be fractionally better than SUs but this was at the very Peak of the curve, everywhere else, the SUs were fractionally better.

 

The Weber (and similar Carbs) fall down in the economy stakes when compared to an SU for two reasons. Fuel atomisation on an SU is way better than a Weber, until you get up in to the 70% and higher end of it's Air Flow capabilities, when they improve significantly and the Weber has a very coarsely metered accelerator pump circuit. They just dump loads of fuel down the throat with next to no atomisation.

 

You can always pick a Mini Engine that's been run with SU(s) over Webers as regardless of manifolds, when the head is lifted, Cylinders 1 & 4 always blacker (richer) than cylinders 2 & 3 with a Weber engined car due to the atomisation issues. You need to fit a very long manifold on an A Series Engine to get the best possible from a Weber and in a Mini, that means hacking out the firewall.

[carbon]

Some really good insights from Moke Spider and Retroman about performance SUs.

 

Is there any recent definitive work/post anywhere about modifying HS2s or HS4s for fast road? Vizard is very good but does not go into detail about modifying piston leading/trailing edge or allowing it to raise higher by machining the dashpot (as covered by Hammill). And is there room for flow improvements to both HS2 and HS4 by boring out the intake before the piston (HS4 in particular, why is this 35mm diameter and not 38mm?).

 

I guess SU factory found a set-up that worked well for economy and reasonable performance, just curious to know what might be possible.

[Spider]

Carbon, honestly, these days, just go up a carb size - if warranted - rather than mess around modifying the carb. As we've both found, getting them right (for street cars) afterwards is problematic and the overall results are not reflected.

[Retroman]

I can agree with the going up a size, but if you are running a 44HIF, and need to or want to get more torque and power then you have to modify the internals, and will compare very well with a Weber, and you don't have to fit an airbox either as a 2inch carb on an A series is not needed...

As the whole topic started, its a good method to get more out of a small carb if thats what you want to stick to.

I have done a few for historic racers where they have to retain the standard carbs and manifolds with good results.

[Minigman]

So an update on the original post. How many bhp from a HS2?

Following a visit to Slarks for a RR session I have no idea due to other restrictions, namely the exhaust, possibly.

The engine build was completed and installed a few weeks ago and has now done about 700 miles.

The spec ended up as;

1132cc (+40 thou)
Ported flowed 202 head with 295 valves.
MG1100 cam
Lightened flywheel
Std 850 exhaust
Std HS2 carb with airbox with K&N
4 synchro box with 3.4FD.
Dizzy is the original DM2 rebuilt and curved with Petronix electronic kit.

On the RR it made just 42.5bhp but had 60ft-lb of torque.

Slarks reckon the exhaust is the limiting factor.

So who knows what the max power is from a HS2!

Mods needed to the exiting of gases before I can answer my original question.

Edited by Minigman, 16 June 2018 - 05:55 PM.

[Spider]

Thanks for the update.

 

Respectfully, with a number like that, while it could be the exhaust and all these parts do work together as a 'package, I'd be looking again at the cylinder head.

 

A stock 1100 makes circa 50 - 55 HP (at the flywheel) and a 998 Cooper made similar numbers though a different torque curve.

 

What exhaust have you got?

 

On one of my 1275's, a stock engine, running short 3 in to 1 headers but the pipe is only 1-1/2" though the muffler is a high flow type. That's putting out over 60 HP but is 'set' for all bottom end power.

[Minigman]

What would you be looking at the cylinder head for, as in what could be wrong with it. Built by Paul Inch so can’t imagine too much wrong.

It’s a standard 850 single box pea shooter exhaust.

[Retroman]

So an update on the original post. How many bhp from a HS2?

Following a visit to Slarks for a RR session I have no idea due to other restrictions, namely the exhaust, possibly.

The engine build was completed and installed a few weeks ago and has now done about 700 miles.

The spec ended up as;

1132cc (+40 thou)
Ported flowed 202 head with 295 valves.
MG1100 cam
Lightened flywheel
Std 850 exhaust
Std HS2 carb with airbox with K&N
4 synchro box with 3.4FD.
Dizzy is the original DM2 rebuilt and curved with Petronix electronic kit.

On the RR it made just 42.5bhp but had 60ft-lb of torque.

Slarks reckon the exhaust is the limiting factor.

So who knows what the max power is from a HS2!

Mods needed to the exiting of gases before I can answer my original question.

 

Just modified an HS2 for a 750MC member - running the 850 reliant engine, modded head and high lift cam.

 

The only work was a vizarded carb and stubstack - peak torque up by 22% to 55lbft

 

In a 305kg car even more fun

[Spider]

What would you be looking at the cylinder head for, as in what could be wrong with it. Built by Paul Inch so can’t imagine too much wrong.

It’s a standard 850 single box pea shooter exhaust.

 

Nothing 'wrong' with the head per se, but just a hunch that it's not allowing the engine to develop better power.

 

The exhaust could well be an issue too and perhaps given what it is, that could be your next step.

[Minigman]

Built by Paul Inch and rolling road tuned by Slarks.

I don’t really think it’s the head, but I’ll take a look if I get no joy after sorting the breathing bit out.

[Retroman]

Have to agree with spider

 

What compression are you running?

 

Was the cam timed in or just dots lined up?

 

I assume you are set on keeping it looking as standard as possible from the outside...

 

The manifolds would benefit from separating to get rid of the hot spot and both cleaning up inside as there can be all sorts of flaws and casting flash, non of it helps. 

 

Sometimes too the exhaust downpipe flange can be restrictive and sharp edged....

again needs measuring / sorting, sometimes they also seat off-centre.

Be wary smoothing the inside of the downpipe flange as the taper is welded on some and needs a run of weld on the outside...which may need grinding off to return it back to sealing enough (if that makes sense)

[Minigman]

I don’t know the CR. I asked for 9.5:1.

I’d hope the cam was timed properly but I am beginning to think that it’s not been done well.

I can’t get any info out of Paul Inch either.

It wasn’t a cheapo build either.

Slarks are pretty good at what they do and if Neil thinks it’s the exhaust I’ll go with that for now. Considering the magic pipe plus single box RC40 in order to keep the look correct ish.

If that fails it will be a full on twin 1.25” SUs with freeflow manifold and the RC40.

To be honest, even though it’s only putting out 43hp the 60ftlb with the 3.4FD is still quite fun to drive.

Edited by Minigman, 18 June 2018 - 12:05 PM.

[carbon]

RR result of 42.5bhp and 60ft-lb of torque suggests the torque has dropped off a lot by 5,250rpm

 

The twin carb MG1100 (ADO16) was reckoned to put out 61ftlbs at 2,750 rpm, the factory also quoted 58bhp for this unit at 5,500rpm.

 

Is your 42.5bhp measured at the wheels or the crank?

[carbon]

Back to the original question 'what is the maximum power with HS2 carb?'

 

My experience is with running twin HS2s on a 1293. Originally this was using the factory alloy twin manifold with the large cast-in balance pipe, and I could understand the comments about 'you can pull air through both carbs, so you get twice the area of single carb'.

 

But I then change to using steel fabricated manifold, similar to Maniflow, and the performance is a lot better. However the balance pipe on this steel manifold is less than 1/2 inch internal diameter, so there is no way it is pulling much air through the balance pipe at full throttle. Most of the air that goes into any cylinder must be drawn through just one of the HS2s at a time.

 

And in my experience the HS2 steel twin inlet manifold with a tiny balance pipe definitely outperforms the factory alloy HS2 twin inlet with big balance pipe...

[Minigman]

42.5bhp was achieved at 4518rpm at the crank.
60lbs/ft torque at 2870rpm.

The torque figure is pretty much what was expected with a MG1100 cam. The bhp however is about 1000rpm down on where the ADO16 early 1100 made 48bhp with a single HS2 and then later 55bhp on twin SUs with the same spec engine. So basically adding a second carb gets you a 7bhp increase without doing anything else.

Also just found that the ADO16 1100 std exhaust bore was 1 1/2”, where as std 850 bore is just 1 1/4” external diameter. So the exhaust being the limiting factor kind of stacks up.

Edited by Minigman, 18 June 2018 - 10:29 PM.