pre-heating the fuel/air mix - questions

[djc]

Sorry to bother you all. So I now have an SU, and I've read quite a bit here about the switch (a '91 413) and one thing seems a bit odd to me. The Aisin has coolant plumbed through its casting I believe- and that must be to heat the carb, either so it works right or to pre-heat the mix (I guess).
I've seen a nice SU carb>manifold adaptor here somewhere with a heater jacket incorporated.
BUT, why? Surely cold air is good (more dense, more oxygen). I assume one benefit of the SU switch is better power or torque. I'm also puzzled how effective a heated adaptor can be given the rate of flow through the adaptor. Basically - why bother with heat into the carb body or adaptor at all??

[twiss]

Hi,

Basically the mixing of fuel vapour and air is an endothermic reaction. This means that it reduces the temperature (dramatically!) of the mixing location... with an SU adapter, the fuel and air mixes inside the carb, then gets sucked down through the bent pipe into the manifold

when the mixture hits the metal it freezes it, causing "carb icing" which is literally exactly what it says on the tin
my friend had mini sheets of ice forming around his adapter!
this changes the properties of the fuel/air mix going into the engine causing bad running

the carb will get cold but shouldn't freeze up because the fuel air mixture goes straight out of it without cooling it down too much

simplest way is wrapping some copper tubing around the adapter, then connecting it inline into one of the hot water pipes from the manifold
not the heater pipe though, as it will be shut when the heater is set to cold

[djc]

Thanks Twiss, got it and makes sense. I assume it's the adaptor that introduces this issue. I had thought of the copper tube idea (brake pipe maybe, bends nice?) but had been puzzled by the need. I'm thinking of getting a spare inlet manifold and doing everything remotely then bolting all the new bits on. There are a lot of tubes in and out of the Aisin which is off-putting but plenty here re SU Conversion so it should be OK. Who was it here who said a 20 minute job not round my way.

[Jordi]

The heater pipes are for auto choke.

It's the long draw of the elbow adaptor that gives carb icing. No carb icing on modded manifolds.

[twiss]

Oooh something I forgot to mention... you can use ali adapters with more success.
The grade of steel that most people make adapters out of (T304 I think) does not conduct heat anywhere near as well as the ali the manifold is made of
With an ali adapter, the heat from the engine is able to pass through the intake manifold, and then through the adapter
Problem is with the ali adapter - they are hard to get hold of and cost almost as much as having your manifold welded so suit SU!

[ScottieJ]

No carb icing on modded manifolds.

Depends on your intake set up, mine suffers from carb icing on very cold winter days. I have pulled off the dash pot before and there has been a thin layer of ice on the piston.

[twiss]

Its probably cos of your massive air filter mate... never had a problem with mine (with your current manifold, or my current one)

Things that will effectively decrease intake air temperature...
> making the air intake system shorter
> reducing turbulence in the intake system (i.e. not running an airbox)
> larger/free flow air filter (less resistance)
> ram-air effect (i.e snorkel top pointing forwards into the wind)

[ScottieJ]

As said it depends on your intake set up

Its fine once I move my air filter to a warmer part of the engine bay

[twiss]

When I had the 1.3 I found 45mm aquarium hose seemed to work alright...
Snorkel top pointing forwards, but the tubing was ribbed so it created a lot of turbulence...

Even when the air going in was freezing cold in the winter I still managed to avoid icing!
Just not enough airflow capacity for the 1.6