Lotus Elan and Lotus Elan Plus 2 Forums

As I have been trying to install an original type air cleaner, I am struck by the difference in size between the trunking outlet on the air cleaner and the two pipes that the air enters through. A quick calculation which I hope I got correct tells me that the cross sectional area of the outlet is 2.56 times the cross sectional area of the inlet. Does anyone know why this is?

Richard Hawkins

Hi Richard,
I don`t know the answer to your question but I do know that when calculating flow rate through a ducting, a water pipe or electrical cable you have to take the length into account. The further you want to go, the bigger the diameter of the tube or cable has to be to keep the resistance low enough for practical use.
Electrical cable manufacturers provide tables for this.
What if the carburettor duct length is 2.5 times the inlet pipe length. I am sure that is too simplistic but you should get my drift.
Eric in Burnley
1967 S3SE DHC

RichardHawkins wrote:As I have been trying to install an original type air cleaner, I am struck by the difference in size between the trunking outlet on the air cleaner and the two pipes that the air enters through. A quick calculation which I hope I got correct tells me that the cross sectional area of the outlet is 2.56 times the cross sectional area of the inlet. Does anyone know why this is?

Richard Hawkins

Read up on the theory of inducing cold air intake.

HCA wrote:

RichardHawkins wrote:As I have been trying to install an original type air cleaner, I am struck by the difference in size between the trunking outlet on the air cleaner and the two pipes that the air enters through. A quick calculation which I hope I got correct tells me that the cross sectional area of the outlet is 2.56 times the cross sectional area of the inlet. Does anyone know why this is?

Richard Hawkins

Read up on the theory of inducing cold air intake.

It is actually a good question. Taken further, there is only one inlet port sucking at any one time, so arguably the ratio to consider would be between a single inlet port CSA and the trunking CSA. My Wife's 3.5L V6 SUV has a much smaller inlet pipe between the air filter and MAF sensor than the Elan.

Its no bad thing to have a decent size pipe, any restriction will reduce max HP, but as with many things on the car my suspicion is that price and availability had something to do with the choice. I believe the pipe is the same type and diameter as was widely used on MGs and other cars of the period for the heater air inlet.

Actually Richard’s original question was about the different sizes of the INLET pipes and outlet duct on the filter. The same logic will apply of wanting a low pressure loss down the duct to carb inlet.

Does anyone know the original application of the air filter?

Part of the reason that most filter inlets have multiple smaller diameter pipes is around silencing the inlet from the induction noise of the carbs. An Elan with just open trumpets generates more noise from the carbs than from a TTR race exhaust

cheers
Rohan

Mine has ‘AC made in England’ stamped on it, so probably a General Motors car of the period. Something like a Vauxhall Victor?

The outlet duct is probably sized to bolt directly on to a GM downdraft carburettor and nothing to do with air flow.

Sized to fit the original donor car seems a much better reason to me. What car was it fitted to?
Eric in Burnley

Original application was Vuxhall Cresta PB series 3.3l

Donels wrote:Mine has ‘AC made in England’ stamped on it, so probably a General Motors car of the period. Something like a Vauxhall Victor?

The outlet duct is probably sized to bolt directly on to a GM downdraft carburettor and nothing to do with air flow.

I beg to differ! I think it is all to do with airflow - they calculated that a 3.5" duct was necessary and then hunted around for a suitable filter can. By having a smaller inlet meant that the air was cooled that little bit more. Had the car been designed today, I guess they would have used a K&N cold air conical filter.

Drifting the thread a bit, I am not sure how effective though the original cold air set-up was. The duct is ribbed and soon became deformed that must have made some difference to the theoretical airflow. I wonder if it would not be better to have a K&N cone at the end of the airbag, in the engine bay, with a narrower pipe down into the nose? Having said that, the other question needs answering: in reality, how effective are cold air intakes..?

Hi Hal,

Do you have any reference links to support your theory? Google "theory of inducing cold air intake" brings up references to simple CAI systems but nothing about a cooling effect (below ambient temp) that I can find.

The general idea of cold air inlet is that the colder the air going into the carburettor it will combine with more fuel and give you a bigger bang. Air in the nose of the car is cooler than the air in the engine compartment and will thus have a higher density which gives you a greater air mass flow going into the engine. Without additional cooling you cannot create air colder than ambient. Water injection is one way of doing this.

I don’t think it has anything to do with airflow. As pointed out 1 cylinder is breathing in at any one time. The carb chokes are 32mm so theoretically all it needs is a 32mm dia pipe. Yes this would be inefficient with losses etc, but you get the point.

Hi Mike,

Completely agree with you that the cooler the air the more dense it is.

Unless I'm reading it wrong Hal seems to indicate that having 2 small ID tubes at the beginning of the system somehow increases cooling and power. I am curious what the theory behind this is.

I am taking the theory to the limit here - the original question, that the outlet is two and a half times larger than the inlet, will cause a rush of air through the inlet that will be cooled, albeit by a small amount. If it is enough to be significant or not, I stand corrected, but I do remember way back someone explaining it...