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Why is the air intake above the exhaust?
Posted: Sat Apr 13, 2019 5:47 pm
by KeithL
On other forums I am subscribed to, such as MX5 and Jaguar, there always seems to be a quest to get colder, and therefore denser, air into the engine, and people are always coming up with ways to reroute the air intake to just behind the radiator or into the wing in front of the engine where it can collect cool air.
Why therefore, is the air intake on the Minor positioned just above the hot exhaust manifold?
Re: Why is the air intake above the exhaust?
Posted: Sat Apr 13, 2019 7:10 pm
by pde2000
On my renault 4 the air intake could be positioned either over the manifold or away from it, and this was for summer or winter. My MMs2 has the old silencer type air intake, and there must have been many variations since then.
If the weather is too cold i imagine that could cause problems.
Re: Why is the air intake above the exhaust?
Posted: Sat Apr 13, 2019 7:20 pm
by liammonty
It’s to prevent carb icing in cold, damp weather. You can rotate the filter housing intake away from the manifold in warmer weather to get colder intake air, which will give slightly better power (due to colder air being denser).
Re: Why is the air intake above the exhaust?
Posted: Sat Apr 13, 2019 9:53 pm
by cococola
I have a pancake filter on both my Moggies, is this likely to cause me longer term problems.

Re: Why is the air intake above the exhaust?
Posted: Sat Apr 13, 2019 10:30 pm
by paul 300358
A lot have changed the single manifold assembly for separate inlet and exhaust manifolds (including me) to remove the hotspot between the two. According to Vizard etc the cooler and denser air will give more power. Actually the hot spot was there for a reason, it allows the engine to pull with less choke when cold, it will also run smoother. Mine is now really lumpy, misses and underpowered until warn, then it goes great but I now wish that I had used the full inlet and exhaust mariner manifold.
Re: Why is the air intake above the exhaust?
Posted: Sun Apr 14, 2019 8:40 am
by ManyMinors
pde2000 wrote: ↑Sat Apr 13, 2019 7:10 pm
On my renault 4 the air intake could be positioned either over the manifold or away from it, and this was for summer or winter. My MMs2 has the old silencer type air intake, and there must have been many variations since then.
If the weather is too cold i imagine that could cause problems.
The Minor 1000 is exactly the same. As the owners handbook states: "To obviate the possibility of the carburettor icing up when the vehicle is operated under cold conditions the air intake should be positioned adjacent to the exhaust manifold. During warmer weather it is advisable
to move the intake away from the manifold.
Like the Renault 4, the Morris Minor is a nice simple and effective design in its original form!
Re: Why is the air intake above the exhaust?
Posted: Sun Apr 14, 2019 9:55 am
by RobThomas
Moisture will precipitate out when the pressure drops so an SU carb (or pretty much any) will cause ice to form just down from the dashpot. Injection engines don't suffer this as much since the fuel droplets are created right close to the valves and thus don't add to the cooling effect in the intake manifold. Aircraft have hot air supplies for when the throttle is closed during descents to stop ice forming but not on injection engines, for the same basic reason.
The heated air is an attempt to allow the owner to adjust the intake temperature if icing starts to form. Scandinavian and Canadian cars had optional dashpot heaters to heat the base of the dashpot to keep ice formation down and also a spacer between carb and manifold with an electric heater to shift ice that formed on the butterfly.
Reece Fish carbs had a gadget similar to a Kettle heater element that sat in the mouth of the carb to keep ice away.
https://www.google.com/url?sa=i&rct=j&q ... 8317778645
https://publicapps.caa.co.uk/docs/33/20130121SSL14.pdf
Re: Why is the air intake above the exhaust?
Posted: Sun Apr 14, 2019 12:08 pm
by KeithL
Thank you everyone - that's very interesting. I'm glad I asked the question now.
Re: Why is the air intake above the exhaust?
Posted: Sun Apr 14, 2019 7:33 pm
by philthehill
These are the SU carb intake heaters:-

- Carb heaters.jpg (239.65 KiB) Viewed 3583 times
The dashpot heater can be viewed in the pistonheads link above.
Only engines fitted with the air cleaner Pt No: 12G316 (1098cc) could be turned to enable warm air to enter the carb/engine.
948cc engines with the saucepan type air cleaner Pt No: 12A243 could not be turned or enabled to draw in warm air. Likewise those engines fitted with the oil bath or oiled gauze air filter also could not draw warm air.
When I had the 28/36DCD carb fitted to my A35 the water heated manifold had to have warm coolant passing through it otherwise I would get icing in the carb/inlet tract.
Re: Why is the air intake above the exhaust?
Posted: Sun Apr 14, 2019 8:59 pm
by JOWETTJAVELIN
Carb icing is rare in this country I doubt you would experience it unless you were in the wilds of Scotland.
Re: Why is the air intake above the exhaust?
Posted: Sun Apr 14, 2019 10:36 pm
by liammonty
JOWETTJAVELIN wrote: ↑Sun Apr 14, 2019 8:59 pm
Carb icing is rare in this country I doubt you would experience it unless you were in the wilds of Scotland.
I’ve experienced it several times over the years, in the barmy heat of Oxfordshire, and latterly Dartmoor - cold damp weather does it. Some cars are more susceptible than others - particularly bad culprits were a Volvo 340 and a mark 2 VW Polo. My 948 Minor (which as Phil pointed out doesn’t have the facility for a warm air intake position on the air filter housing) has also experienced carb icing from time to time.
Re: Why is the air intake above the exhaust?
Posted: Mon Apr 15, 2019 10:21 am
by myoldjalopy
But you can point the intake of the saucepan type air filter towards the radiator - wouldn't that help a bit?
Re: Why is the air intake above the exhaust?
Posted: Mon Apr 15, 2019 12:54 pm
by philthehill
May help a little but not as good as air directly passing over the exhaust manifold.
Re: Why is the air intake above the exhaust?
Posted: Thu Apr 18, 2019 4:30 pm
by MorrisMinion
My 1965 convertible only gets out in sunny weather and I also wondered why the air intake was sitting on top of the manifold. The Owners Handbook (page 51) suggests winter and summer positions for the intake - winter on the manifold and in summer pointing down. The only thing preventing me moving the air intake to the summer position was, as you appear to have, a very short breather tube between the rocker cover and the air filter housing. The problem was solved by purchasing a new longer breather tube from ESM.
I've got an awful lot to learn about these wonderful cars, so thank you for the opportunity to offer a solution to one of the very few problems that I've solved on my car to date. As you can see below, the ESM breather tube is long enough to allow the summer position of the air intake which has help my car to run better in the summer months.
Re: Why is the air intake above the exhaust?
Posted: Thu Apr 18, 2019 6:25 pm
by KeithL
Thanks MorrisMinion. I was wondering how I was going to get around the problem of my hose being too short. Now I have the answer.
Re: Why is the air intake above the exhaust?
Posted: Fri Apr 19, 2019 7:42 am
by RobThomas
Carb icing is rare in this country I doubt you would experience it unless you were in the wilds of Scotland.
Strangely, the UK is ideal for forming carb ice. The drop in pressure down from the carb and the latent heat of vapourisition (Is that right?) drop the air temp in the manifold by maybe 15 degrees and dumps you right in the area for forming ice. You'd need at leat 15 degrees hotter air to get back out, hence the need for exhaust heat.
Cessna heat exchanger. Basically a silencer with an alloy heat shield wrapped round with 2 air ducts to draw hot air round the silencer to heat it.
https://awi-ami.com/650-43-cessna-182-r ... anger.html
Re: Why is the air intake above the exhaust?
Posted: Fri Apr 19, 2019 9:40 am
by oliver90owner
Some ice will form before the addition of atomised fuel (which undoubtedly requires heat to fully vapourise). Even with no fuel to evaporate, the air entering the choke tube will change in pressure and this alone will change its temperature - sufficient to drop out water as ice in some marginal circumstances. Think here - compressing air heats it up (diesel engine?), icing on plane wings, etc.