Hi Chris,
Chris Dimmock wrote:
> Just in case - can I just add that you should NEVER make a cold air box
> which is sealed, and try to 'ram' or 'force' air into your carburettors.
> Many people have tried this in the past - hoping that fresh air being forced
> into the carbs will give a 'poor mans supercharger' effect & improve
> performance.
There will not be any "ram" pressure below around 100 mph, or more,
anyway. Ram pressure on the average car is largely imaginary. The main
benefit is that HP rises about 2% with each 10 degrees F drop of intake
temperature. If ambient air intake of 90 degrees F is used & the under
hood temp is is 200 there will be a 22% increase in air density & power
with the cold air intake, if the jetting is changed to match.
You may have noted that the Healey 100M vents the float chambers into
the "pressurized" air intake, not to atmospheric pressure. In effect
this puts the same extra pressure on the fuel as it does on the carb air
intake & automatically enrichens the fuel mixture if intake pressure
rises. This effect could not take place if the box was not sealed.
> If you 'force' air into SUs at greater than atmmospheric pressure, there
> will be one result - melted pistons from running too lean, usually resulting
> in a complete disaster. A supercharger compresses the already mixed
> air/fuel - i.e. the carb is first, then the supercharger, then the engine.
> Forcing air into the SU at higher than atmospheric pressure just makes the
> car run lean - too much air, not enough fuel. Remember the discussions on
> richening SUs before Lake Tahoe because of the altitude?
No I don't remember the "discussion" in detail, it seems it was pretty
convoluted & led to no agreement. The reality is that a car that was
optimumly jetted at sea level, would require 18% leaner jetting at 6,000
ft. to be the same. Air density drops about 3% per each 1,000 ft rise in
elevation.
Some earlier centrifugal superchargers DID force "air only" into the
carb & relied on a pressurized float chamber connected to the same
pressurized air source to compensate the mixture. They actually worked
quite well. As the carb entry pressure rose so did the pressure in the
float bowls. The carbs don't run lean if the float bowls are pressurized
as above. They did require a fuel pressure regulator that could
compensate for the float bowl pressure changes.
> A cold air box should have holes at the back to ensure it isn't pressurised.
> This is critical.
I doubt it, if the above principle is adhered to. Any changes noted by
this practice were likely from some other cause.
> Modern motorcycles that benefit from 'ram' effect have computer controlled
> ignition/fuel injection systems to prevent the engine from running too lean
> at high speed - ie they change the air/ fuel ratio depending on air
> volume/pressure.
Very true, but modern motorcycles are capable of speeds far beyond the
average Healey, & may benefit from some ram effect.
> Best regards
>
> Chris
Dave Russell
BN2
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