Go Faster Carbs

[mike.perry]

Whilst my MM engine is out I may take the opportunity of trying to extract a little more power. The reprofiled cam I have in a spare engine will go in and I already have a lightened flywheel and possibly slightly larger valves which I will check when I lift the head.
This brings me around to the subject of carbs. Having heard a quote of £193 for rebuilding a carb at that well known establishment in Salisbury I thought that was a little costly to rebuild one of my spares to make up a pair, and I would still have to find a decent twin carb manifold and the linkage. I could almost buy a Morano kit for that money.
The other alternative which occured to me was to fit an H4 (not HS4) and enlarge the throat of the inlet manifold to match. An H2 would probably not be worth the effort. If anyone produces a 4 branch exhaust manifold I could cut off the inlet manifold and use it with the new exhaust. The carb to manifold fixing might be a problem.
Any comments?

[RobThomas]

I'd actually put money on the fact that a bigger carb wouldn't make the slightest improvement. The exhasut system on the MM is an embarrasement at best and is, IMHO, the biggest destroyer of power output on the car. A bigger carb might allow more air in but it couldn't get it out fast enough.
A 4 branch exhaust (Keeeeeeeith!) with a longer (and separated) long tract inlet manifold onto an H2 with a radiused belmouth ought to make a vast improvement. The current intake takes heat off the exhaust and pre-heats the air, thus sapping potential power. This is the same for the later cars, as an anti-icing method, but was reduced over the years of development. By the MG Metro, BL were separating the manifolds and using radiator heat to keep the ice at bay. THIS is what we need. (Keeeeeeith!)
A 4-2-1 exhaust helps to draw exhaust gasses out of the cylinders by creating a constant flow that helps suck gasses out through the exhaust valves. These 'tuned' exhausts on a VW aero engine are worth about 8% power over a stubby open pipe. Imagine the improvement over the MM manifold!

The H4 is so big that the port velocity would be greatly reduced to the point that it might even struggle at low throttle to draw the fuel up the jet (if you see what I mean) since the small airflow would be going through a large gap. Effectively, you are losing the 'signal' strength in the same way that a fart through clenched buttocks gives the best signal strength!!! Big bumhole and it whimers into a 'fluff' instead of a manly trumpet.[Couldn't think of a better analogy] The carb might be bigger but you then have to draw the air round a tight turn into the block and through a tiny 1" inlet port in the block, past a big valve guide and then round some more tight turns. This is where the ALTA makes such an improvement (Keeeeeeith!)

Power comes from buring fuel and air at the correct ration. Lots of air in PLUS lots of air OUT.

Might the MMOC get some income out of comissioning MM exhaust manifolds?<br><br>

[alanworland]

Lets hope Keeeeeeith1 reads the above,(speaking on behalf of all MM owners) (cos I got it going in but not coming out!!)
Mike, have you got the original small bore pipe? ie 1 inch?

[mike.perry]

I have the later M1000 bore exhaust, not the original pea shooter.
My theory was that provided I fitted a decent exhaust manifold then I could fit a single large carb (H4) which may be more efficient than 2 small carbs (H1s) in the same way that an HIF38/44 is reconned by some to be better on a 1275 than 2 HS2s. There is not much room on the standard inlet manifold to enlarge the throat so I think a larger manifold will be needed
The ports on my engine have been cleaned up a bit and there is certainly something not standard on my engine, judging by its performance when built. I hope to find out exactly what has been done to it when it is rebuilt.

[RobThomas]

The Weber 40 DCOE is a possible option. 28mm chokes are about the smallest I can find here and they ought to be about the right size. Trial fitting one under the bonnet showed that the ports are not too far out of line and it should all clear an exhaust manifold with ease.

[mike.perry]

I want to keep it looking as period as possible which is why I want to stick to SU H carbs

[Kevin]

quote]
in the same way that an HIF38/44 is reconned by some to be better on a 1275 than 2 HS2s.
[/quote]
Quite right Mike, as in David Vizards book the 1 1/2" produces equal performance to the 1 3/4" on the 1275cc and is only disavantaged at speeds over 70 mph, that's why I used an HS4 on my 1275cc.

[Keith]

I hear you Alan....things are progressing....


Keith

[RobThomas]

Keith. I'd even settle for a set of bent pipes! There isn't anyone round here that I know to be able to bend small bore pipes but I can get a backplate cut quite easily (I have 2 already done).

How about 2 at just under 90 degrees, 2 at just about 80 degrees and a pair of short ones that can be swivelled to align with 2x SU or a single 40DCOE?

[Keith]

Rob, I'll leave things as they are at the moment, the chap is making a copy of my Derrington, minus the carb pipes...

regards Keith

[Keith]

Rob, Any chance I could borrow the backplate, makes it easier to get a laser copies made if you have an example to copy. Thinking in advance once the sample has passed Alan's approval...
regards Keith

[alanworland]

Corr, sounds like things are a moving!
Mike, I believe there were two types of MM manifold? the early one with abrupt right angles which looked like it had been made from gas barrel! and the later one which was more kind to gas flow but still fairly contorted for mixture to flow through. I would think you would be better off starting from scratch?
I initially started with an early Sprite manifold (for H1's) but realised it wasn't really what I wanted.

[RobThomas]

Keith. I just gave them an exhaust/inlet gasket and said "Copy that!". They stuffed it into their machine and out came a blank plate with the holes cut out. 1/4 inch plate. Cost me £10 for 3 of them. Start to collection was about 2 hrs due to me disappearing off to do some shopping

If anyone wants to scare themselves, look inside an MM exhaust manifold. It is hard to believe that anyone could actually makes something to mind-numbingly cr4p. The inside is as smooth and aerodynamic as an oilrig!

[alanworland]

Not the best are they?

[tortron]

Not the best are they?

id say sand was on short supply so they used gravel to cast them

I too had exhaust flanges cut when i had my twin carb flanges cut.
Of course making an exhaust up is somewhere on the long list of things to do