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MG Midget and Sprite Technical - More About Exhaust Manifolds

Apologies to our new pal Jan for hijacking his thread on headers earlier. I've moved this to a new thread because I really would welcome some more learned advice on exhaust manifold performance.

I'm comparing a maniflow 1 1/4" unit with the (probably) Speedwell 1 1/4" that came with the car - see picture. The Speedwell is more elegant in its passage from cylinder head to exhaust pipe - it doesn't hit anything - but the Maniflow looks as if it might have better internal cross-sectional area where it matters. The next two posts will show screen shots from my bendy camera, inside the Speedwell.



Nick and Cherry Scoop

This is at the Y-joint where the two outer branches meet. It isn't a cut-to-fit joint of two similar branches, but the right branch cut off more or less straight and the other shaped to snug onto it. Perhaps only a minor obstruction.


Nick and Cherry Scoop

This is just as the long centre branch comes in on the right, and we can see that the next bit of pipe is smaller diameter. This was pretty clear from the outside, but here confirmed. I would not be surprised if the lower part has suffered damage at some time, and been patched up.

My question is - does any of this matter? Once the long centre branch has solved the problem Paul Walbran described, of exhaust gas entering an adjacent cylinder on its inlet stroke, what makes the rest of the downstream system perform well or badly?



Nick and Cherry Scoop

Maniflow pipe joints look like this.


Nick and Cherry Scoop

Here's a template I made from the Maniflow ports, transferred to the cylinder head. Nice positioning for the most part - putting the 'non-return' steps in substantially the right places.

Sorry this has turned into an epic. But I would like to hear your opinions on exhaust sizes and so on. Dave Vizard is great on this, provided you know your exhaust gas flow rate, but I don't.

994cc, Peter Burgess fast road head, 266 cam, twin 1 1/8" carbs, by the way.


Nick and Cherry Scoop

I am not convinced by the "non return" step theory!

I did a lot of research many years ago that showed that a smooth transition from one shape / diameter to the next gave better flow characteristics than steps or sudden changes in shape - these caused eddies that interfered with the boundary layer laminar flow.

You might find this article interesting -

http://www.enginelabs.com/engine-tech/exhaust/performance-exhaust-system-design-and-theory/
Chris at Octarine Services

Blimey, Chris! Vizard would disagree absolutely. His exhaust chapter seems to say that laminar flow on the inside of the bend will likely be disrupted anyway. I read the article you attached - thanks - and what with that and Vizard, my head hurts. I would go and lie down in a darkened room, if I didn't have to cook the dinner.
Nick and Cherry Scoop

I'm with Chris - any sudden step change will cause turbulent flow at that step, increasing drag and fluid resistance, and thus impeding satisfactory egress of the exhaust gases from the combustion chamber. The essence of any exhaust system is to remove those gases as quickly as possible with no resistance to flow.
Oggers

There is no such thing as laminar flow in an IC engine, gas speed too high. The flow is turbulent or fully developed, more like a plug of cotton wool going through the engine. We have found steps over 1mm detrimental either way in the inlet port, less of an effect in the ex port. Not sure of anti flow reversal technology but the ex manifold seems to have less effect. On a race B with a daftly exaggerated step 34 mm inlet manifold exit to a 43mm inlet port on the head the engine actually misfired above 5000 rpm, made 20 bhp more and no misfires when matched. Mechanic who fitted it told owner a step is beneficial if in right direction, a difference twixt step and Grand Canyon methinks!
Peter
Peter Burgess Tuning

Thanks all. I'm going with Vizard. He seems to have carried out thoughtful, methodical and well-documented tests, and he considers mid rev range driving, and economy, as well as racing. Going with Peter as well, because a very experienced man who says 'not sure' when he doesn't know the answer is unusual and reassuring.

Decades ago, when the first 1500 midget was rolling off the line, I worked on the design of a manufacturing plant for Kodak, where steady laminar air flow was essential for a process involving wet emulsion on celuloid. The principle was, in simple terms, filtering upstream, vacuum downstream, just like the inlet tract of a Frogeye. Pressure upstream inevitably produced turbulence, even at very low air speed.

In light of what Peter said, what do we think about the Titan 38mm inlet manifold? There's a lip that you have to file off if you want the 44mm aperture. But if you go with the 38mm HIF, and hence the lip, would you call that a performance-inhibiting step, or are steps less important further upstream?
Nick and Cherry Scoop

Interesting
We had some headers made up for the Chev once by Schoenfield in the States to suit our very worked over Brownfield "D" port heads
They were the best guys in the business at the time and wanted to know everything about the engine,car rev. range gearing etc
They came with an adapter plate to fit the heads that continued out in the D shape , The headers had round 2" primary pipes which matched the dia. of the plate port perfectly but the bottom ,probably 1/4" was left as a D shaped anti reversion step
Having the step makes the engine think it has a smaller dia exhaust and retains some of the low down power that would normally be lost with an oversize exhaust while not being a long enough restriction to cause a drop off in top end power
The solo bike guys used to experiment with them as well
They would run a little pipe ,about4-5" long and about 1 1/4"dia and that would let them run a 2" exhaust over the top of it for top end power
Without the little pipe in there they couldn't run the larger pipe, they just lost all their torque
willy


William Revit

According to Vizard, Janspeed's controlled vortex system used a very short stub at inlet port diameter, then a bigger manifold. But the very short stub was only about 1/4", not 4-5" like you describe, Willy.
Nick and Cherry Scoop

This thread is of interest to me as I need to get a new exhaust manifold over the winter but am unsure of what to go for as opinions seem to differ. Its for a 1098 and the Maniflow website suggests a small bore but Magicmidget and most of the other protaganists suggest a medium bore. Any words of wisdom from anyone else with a 1098.

Trev
T Mason

Trev
It's a horses for courses decision
If it's basically std. in tune and driven as a road going car at normal revs then go little but the further up the rev range and the higher the tune then the larger you go
Putting big pipes on a smaller engine that isn't capable of feeding them simply robs you of any useable low/mid range power that you had- My advice would be to err on the side of small and have a good think about the rev range that you normally drive and want to feel the benefit from your choice
If you're 'not' fanging it out there all the time, go small and use the power you gain at the mid range where you normally drive
Nick
As I understand it, Anti reversion exhaust manifolds give no advantage at all untill you get into serious competition engines with very large inlet ports, monster camshafts and very large primary exhaust tubing and the design was introduced to stop pumping in the ports with the lack of air speed with BIG camshafts at lower revs----for example
Our Chevvy ran out to 7800-8000 regularly and never came under 4500--- It was at the 3500 (where the cam just started to come in) through to around 5500 that the porting and headers were designed in the D port configuration to help the exhaust flow
With normal matched ports and 1 3/4" headers worked fine at these lower revs but they would hit a wall at 7500, hence the need for 2" primaries and then the D ports were needed to retain some bottom end power
For a road car or club/ comp/road car they would be a complete waste of time and money
willy
William Revit

Willy, that sounds like common sense. I don't want power only up at the top: we are mainly tourers now, and maybe we would set a greater value on accelerating up a Pennine Hill at 3500-4000 revs. I'm now tempted to try the old Speedwell despite its slight obstructions. Maybe a slight hooding as the LCB comes in (pic A) is beneficial for extraction! (clutching at straws now)

I will make another ports template and see how it lines up at the head. And it's only five posts ago that I said I'm going with Vizard! Still, if you can't be capricious in your hobbies, where can you?

Trev, I've looked at the Maniflow site too. When I bought mine twelve years ago their advice was to go for small bore, but the smallest they seem to quote now is 1 1/2", whereas the ID of the one they sold me is 1 1/4". I'm not in the market for a new one, so I haven't phoned them.



Nick and Cherry Scoop

normally exhaust pipe sizes are quoted as OD-
William Revit

I didn't know that, Willy. My Maniflow is 35mm LCB, 30mm side branches, 38mm after LCB entry. Pipe is 42mm. So perhaps it's the 1 5/8" model.

Speedwell is 35mm everywhere.

Here's the ports template. So, the steps are there, whichever I use. Those rectangular exhaust ports are 22 / 25 / 22 wide and 27 high. Who's got a manifold which matches that exactly?


Nick and Cherry Scoop

Thanks Willy. That ties in with my thinking of going small, mainly because although it has had some porting done and has a Weber carb, it has a standard cam. I was just caught in two minds because most of the suppliers stated medium, presumably on the misunderstanding that bigger is better.

Trev
T Mason

I've had a nice chat with Maniflow, and 1 5/8" is their smallest, but they will do a 1 1/2" system for the same price. But apparently the 1000cc Minis use 1 3/4" manifolds, and that's what the RC40 box was.

I'll shut up now.
Nick and Cherry Scoop

Nick
Don't shut up, it's how we all find out about things
Earlier when talking about smaller being better I was refering to primary pipe sizes not the outlet pipe size
After the exhaust has left the primary pipes and collector,--from then on the least restriction in the exhaust system the better,
willy
William Revit

Glad to hear that, Willy. Saves me the cost of a new pipe, if I can feed 35mm into 42mm somehow. Or I might sort out the bottom end of the Speedwell so that the last 6" is sleeved OVER the upper pipe rather than being inside it. I'm not a welder or a pipe bender, but maybe I'll go and talk to the blacksmith.
Nick and Cherry Scoop

Can someone clarify please, when talking of different sized systems, where are these measured? I understand that these are taken as external, but where abouts on the system?
I have just measured my aged Maniflow as 38mm external on the central downpipe, 35mm on the #1 and #4 pipes and 42mm at the outlet end under the car. So what size would that be quoted as?
GuyW

Just spoke to Kim at Magic Midget. As others have said, very helpful, nice person to do business with. Seems they are measured at the exit pipe so mine is a 42mm (1 5/8") one.
GuyW

Has that Maniflow been on your blue car, Guy, with those tiny ports throwing gas into the huge manifold diameters? Was your mid-range grunt poor?
Nick and Cherry Scoop

Both cars are now blue so that's confusing Nick. Even more so when you add in the blue Volvo!

The manifold has been on the 1275 Sprite. Which has an MG Metro head with bigish valves It seemed happy enough with it.

Not so happy yesterday on flooded roads. After about the 7th flooded section it got doused enough to splutter and give up. But then so had several much bigger cars, including several pseudo 4 X 4s. The difference was that they sat forlornly waiting for the breakdown man in steamed up cabs full of crying and arguing children (afternoon school run time,) , whilst I lept out (well, crawled out really) with my old towel, wiped over the HT leads, fired it up within a couple of minutes, and sped off again with a cheery wave!
GuyW

This thread was discussed between 19/11/2017 and 24/11/2017

MG Midget and Sprite Technical index

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