Lude Generation

So,

The geek in me escaped this evening and I did some maths on the 'ram air' effect

Obviously to get the numbers there is a link between engine speed and car speed, it also takes no account of drag in the air intake path or losses from the air filter but I've got the following;

If you have an M2B4 box with 215/45/17's you'd have a 5th gear of circa 20mph / 1000rpm

for every 1000 rpm a H22 needs 2.157 m^3 / min of air.

a cold air intake of 72mm ID can provide 2.39 m^3 / min of air in 5th gear which gives approx +13% more airflow than the motor needs so essentially it's adding a little extra pressure to the air intake so it's doing what it's supposed to eventually. This bore only offers benefit in 5th gear, it cannot provide sufficient flow in the lower gears.

Continuing with the above 'car' (215/45/17's, M2B4 box) and changing the air feed bore you get the following numbers

80mm ID
1st gear 33% of air required
2nd gear 56% of air required
3rd gear 80% of air required
4th gear 106% of air required
5th gear 139% of air required

90mm ID
1st gear 42% of air required
2nd gear 71% of air required
3rd gear 102% of air required
4th gear 134% of air required
5th gear 176% of air required

100mm ID
1st gear 52% of air required
2nd gear 90% of air required
3rd gear 126% of air required
4th gear 166% of air required
5th gear 218% of air required

110mm ID
1st gear 63% of air required
2nd gear 109% of air required
3rd gear 152% of air required
4th gear 200% of air required
5th gear 264% of air required

Think of these numbers what you will, to me for a car used as a road car to get any benefit from the ram air effect the intake needs to be 100mm ID or greater to see some benefit at the top end of 3rd gear.

Do you have the results of what a non-ram % is for each? for comparison.

What size is the standard air intake?

well there is no ram effect for the standard intake as its taken from behind a cover in the front wheel well

Very true

That's why you go custom

Isn't there also an airflow generated by the engine intake itself, ie: sucking air in? Or is that just carbs?

Yes, as the pistons draw down.

I must be misunderstanding what is being displayed here

I took it that where he said...

A1ex wrote:80mm ID
1st gear 33% of air required

It meant the engine was having to suck to make up the additional 67%. Basically anything under 100% the engine has to draw it's own air. Anything over 100% and it's getting force-fed.