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MG Midget and Sprite Technical - HIF44 inlets

I have read a lot of stuff in the archives about inlet manifolds, but still don't have the answer to my question. What I am interested in is manifold length. The A Series gurus say that a long inlet tract works wonders for torque, and what matters most is the length from the carburettor to the cylinder head. What I have is not quite right, because it's an MG Metro manifold. For a start, it's not right for a Sprite as it's angled upwards too far and the dashpot touches the bonnet. It's not quite as bad on a small bore engine, which is not as tall as a big bore one, and I have done what a lot of posters in the archives have done, which is to chamfer the mating face on the manifold to drop the angle. This works perfectly well for the clearance, but I'm still left with a manifold that is shorter than ideal. Years ago I tried to make extensions for the two branches of the manifold, out of brazed aluminium, but I could not make these sufficiently accurate or robust.

OK I could shell out £225 for the Maniflow inlet, but I am far too mean for that. The Oselli and Titan manifolds seem to have disappeared. So what I'm thinking of now is to make an adapter to lengthen the manifold just behind the carb. This will be far easier to make accurately than two at the other end of the manifold. So my question is, will this lengthening of the inlet tract work as well as having two extensions at the other end? My guess is it won't be quite as good but will be better than the short manifold.

The reason the Metro manifold is so short is of course that it's for a transverse engine. There is a lot more room for a long inlet on a Sprite. At present I have a long ram pipe on the carb mouth and my setup is really the wrong way round. I should have the length behind the carb, not in front of it.
Les Rose

Les
In my past, i've done quite a bit with manifolds on racing chev v8's. Basically, the longer the primary runner the better the torque and horsepower in the lower rev range <6000 and the larger the plenum area of the manifold the higher up the rev range for more horsepower
Fitting say, a 2"open spacer between the base of a 4 barrel carb and the manifold on a v8 makes a noticeable differance to top end horsepower but needs more fuel down low to get clean acceleration---There is also a 2" spacer with individual bores for each barrel of 4bbl carbs, these improve low down power but don't do anything for top end power.
Your SU being a single barrel, by having a spacer between the manifold and the carb will increase vac. (might reduce carb fluttering at low speeds) and should give the mixture more time to atomize and 'should' give you some improvement in upper rev range power.
If you're after more performance down low in the revs it's the primary runners that need to be longer
A mate stuck 1 3/4" su's on his B once and just tapered out the std MGB phenolic spacers to match it all up--it went ok but not mindblowing
When I did a set for mine there was going to be a bonnet clearance issue so with some measuring up the final setup was MGB manifold ans mgb spacers then 50mm curved/tapered alloy spacers(that was fun) to get the carbs down horizontal and then another pair of phenolic spacers---The final result was a manifold probbably 3" longer than std MGB and it had noticeably better power throughout the whole revrange than mate's car--I've still got the adapters but no use to you as they are 1 1/2 one end and 1 3/4 the other
Just for a try how would you go with long studs and a stack of phenolic spacers
William Revit

Had a hunt around and found my old spacers --actually 2 1/2" long---plus phenolic spacers each end

And just remembering back to my old midget hillclimber, it had 2" su's on long manifolds, the carbs were right out as far as possible in the engine bay, ended up swapping over to a 48Dellorto and had it mounted out under the edge of the mudguard with the longest runners i could fit---Just kept going longer and longer and it went better and better





William Revit

Thanks Willy. I am more interested in low rev torque with this small bore engine. It might be more practical for me to reproduce the Maniflow steel inlet, as my steel welding is a lot better than my aluminium brazing. Just need to get the pipe bent properly.
Les Rose

Les
Not much use me going on about my old car, it feels like 100 years ago now, but I can remember when i was fiddling with it trying everything i could imagine, just to see what happened---The car was just a guinea pig with the plan to build a propper one when it was all sorted--Talked to Peter B. at the time and it was his suggestion to give longer runners with my manifold a try just to see--He did give me a measurement back then that I questioned thinking too long but this is when i found longer and longer was better and better and seems to suit these engines---If you're going to fab. a manifold I'd go as long as you can possibly fit.
Nothing wrong with steel-
This is what I ended up with for the dellorto, and it poked out good power at that----



William Revit

Ha! I recall reading somewhere that if you don't mind a big hole in the bulkhead you can have an inlet on a Mini which puts the carb in the back seat. And the torque keeps going up the longer you make it.

I just looked again at your aluminium extension. Very like the two I made, although mine were of course smaller. The problem with the aluminium brazing is that I could braze one end very successfully, but when I did the other end I got the whole thing so hot that the first one fell off. Also they were a swine to fit, with twice as many flanges to match up, and nuts and bolts to do up with little space. Steel is the way to go.
Les Rose

Les,

When you say aluminium brazing are you talking about the likes of Muggy weld aluminium repair rods or proper aluminium brazing/welding.

I was thinking extensions to fit between manifold and head as well but I can see you would have to make the faces non parallel in the vertical plane to keep carb top away from the bonnet as you say it's already close and the bonnet is coming down as you go further out. I've made a few manifolds in steel and it's an easy material to work with. Maybe make one plate for head side, one for manifold side, weld in tubes then cut in 2 to make a pair with the same angle on each.
David Billington

Yes Dave, I mean the widely available repair rods. I've had good results with them for other jobs, for example repairing a broken windscreen stanchion. But for this I am looking at steel.
Les Rose

This thread was discussed between 09/04/2024 and 11/04/2024

MG Midget and Sprite Technical index

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