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MG Midget and Sprite Technical - Inlet manifold design

Whilst mucking about sorting some actual problems I have been thinking a bit about inlet manifold design. Dangerous, I know.

My current manifold is a "crossover" type (pictured), where one carb barrel feeds cylinders 1 and 4 and the other feeds 2 and 3. I guess the idea behind it is to smooth out the flow through each carb barrel, i.e. the firing order is 1-3-4-2, so the "sucks" on each barrel are more evenly spaced. 1-3-4-2, barrel 1, barrel 2, barrel 1, barrel 2. Make sense?!

The real world driving benefit of this is a little unclear to me though.

The downsides are clear though:-

1. It is pretty bulky therefore I can't fit a tubular exhaust with this manifold (I tried, it is way too big).
2. As all the runners are different lengths, especially no. 4 (shortest) and no. 1 (longest) there is quite an imbalance on the fueling, which I think manifests itself as a poor idle and probably no. 1 running quite lean at higher RPM and no. 4 running rich.

Can anyone shed any light on this type of manifold and its supposed benefits?

Malcolm Le Chevalier

I could look to find a "traditional" style manifold, this would solve the exhaust clashing issue.

Or... what would happen if you were to make a manifold where both carb barrels feed into a single plenum that feeds all four cylinders? (Like my attached drawing).

I can't think of a good reason why this wouldn't work, but maybe I have had too much of Props moonshine! :-D

Cheers,
Malcolm

Malcolm Le Chevalier

looks to me like the one you've drawn in your second post works the same way as the first one. In your first pic it looks like there isn't enough space for that red/blue crossover, leading to the conclusion that the second picture is actually how the first one works.

Rob Armstrong

Hi Rob,

The two runners in picture one are defiantly separate.

Dunno if I have a picture to prove it though...

Cheers,
Malcolm
Malcolm Le Chevalier

Malcolm
The first manifold looks the same as the Lynx crossover design we have here
If you actually get a measuring tape in through the ports and measure the length of the runners you will find they are reasonably equal
The advantage of this design is as you have pointed out with more even pulses than traditional straight manifolds
Your second pic. would work with port injection BUT fuel distribution with a carburettor would be not so happy at all
If you really want to fit a tubular exhaust you might have to get a custom job done on the centre pipework OR space the inlet manifold out further from the head
There's a project for you and you can gain some torque at the same time
If you have an idle problem with this manifold, it will be the carburettor; or a vac. leak, or whatever you have hooked up to that vac. port leaking as I suspect that it only feeds into cyls.2/3
If you have a crankcase breather or something plugged on there that flows air, that is where your problem is

willy
William Revit

Hi Willy,

Thanks for the input, I appreciate it.

Sorry, the vacuum port is a red herring. That was a picture from a while back when I was experimenting with PCV valves.

It will idle, just not steady or at a consistent level whilst driving about. But perhaps I am expecting too much of such an old car?

Cheers,
Malcolm
Malcolm Le Chevalier

Malcolm,

I've done something like what you're considering for the A series and it seemed to work well, looked something like a X chromosome. The main driving factor was I had 6 40 DHLA and didn't want to buy a 45 or 48 to use on my 1420cc engine build. The Dellorto people said it wouldn't make peak horsepower but that wasn't what I was after. They did give me a good starting point regarding jets and venturi as with both sides feeding the ports the settings are quite different.
David Billington

Very interesting David, do you have a picture?

Mal.
Malcolm Le Chevalier

Malcolm
Inconsistent idling is usually a result of too much air
Either an air leak of some description OR the actual idle mixture is set too lean
I would suggest around 13;1-12 1/2;1 for a good consistent idle

willy
William Revit

Malcolm,

I forgot you wanted a picture so have just taken one. Top and bottom halves were beaten from 2mm steel and welded together. Carb mounts are standard Alfa Romeo bonded rubber items.

David Billington

Thanks David, that is a funny looking thing!

I think I am going down the root of modifying what I have, rather than starting fresh. The thread in the general section about welding aluminium explains all!

Cheers,
Malcolm
Malcolm Le Chevalier

This thread was discussed between 30/01/2016 and 16/02/2016

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