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Can ram air be used to increase manifold pressure?

This came up on another, unrelated, thread.

It ought to be possible because the air inlet path can definitely increase the air temperature. That was proven here where a delta T of 10C (10K using the correct physics notation) was measured. But maybe an ability to warm up the air is not the same as the ability to compress it. Anyone here who knows the physics of gases?

It would be a cheap and reliable form of turbo normalisation.

Shoreham EGKA, United Kingdom

Formula 1 racers and Mooneys had a ram air control which in theory adds some HP using unfiltered air

Enstone (EGTN), Oxford (EGTK)

Mooney have tried that before they went to turbocharged engines (M20K variants),
- M20F had useful uptick in performance from unfiltered cold air straight to engine using ram air intakes (roughly +10HPs/+5TAS/+1"MP)
- M20J not much difference from ram air, so that is best kept close all time and the design was abandoned for next variants (getting unfiltered air while taxing is not very good to the engine life, so ram air is only opened at high cruise clean air)

Also, it has not much effect when the aircraft is standstill as you need speed for the “ram effect”… local copy

Last Edited by Ibra at 04 Jun 17:41
ESSEX, United Kingdom

Why unfiltered air? The MP change between filtered and alternate air, on the TB20, is about 0.3" which is insignificant. That is incidentally why the K&N filter STC mod was not worth doing (apart from the STC writer not having tested it in the TB20/21 so it didn’t fit).

Shoreham EGKA, United Kingdom

Surprised a 10 degree ram rise at piston speeds. That seems high. Obviously there will be some ram rise with an efficient ram inlet.

EGTK Oxford

Your maximum static pressure rise is 1/2 rho v^2, that leaves you at SL and 150KTAS at roughly 3500 Pa or around 1 inch mercury. Your 150ktas in FL100 calculates to around .34 inches mercury. Lossless.

Inside the sky.
EDXE, EDXF, Germany

High performance motorcyles typically use it.

Sports car tinkers are always messing around with induction system designs to try to combine a ram-air effect with getting cold air ducted through the warm engine bay, smoothing the flow of that air into the inlet manifold and compromising on how much (if any) filtration to employ.

At the other end of the system they mess around with tubular exhaust manifold designs to try and create some sort of ‘active scavenging’ system where the exhaust pulse from one cylinder helps to suck out the exhaust from the next to fire.


They are using it but it’s 5hp over 150km/h on a 180cv 1.0l engine which has a very direct intake. Actually I think it works best with a high volume plenum…

In NA car engine, acoustic tuning works better but only acts for some reduced RPM range. This is where variable intake length plenum succeed to have 1:1 intake ratio at low/mid range where cylinder air fill efficiency is not good. It actually uses the vacuum wave of the intake valve opening that reflects into the plenum (which acts as a helmhotz resonator) and return to the valve on the next cycle as a shockwave, creating a pressure wave when valves open and fills more air.

LFMD, France

Your maximum static pressure rise is 1/2 rho v^2, that leaves you at SL and 150KTAS at roughly 3500 Pa or around 1 inch mercury. Your 150ktas in FL100 calculates to around .34 inches mercury.

Is there a proof of this formula which is applicable to every possible air inlet geometry?

I believe somebody also proved that the 10K delta I measured is impossible.

Shoreham EGKA, United Kingdom

It should be valid for any intake as long air does not stall (Reynold number) or compress (Mach number) too much inside (longitidunal tube geometry and laminar uncompressed airflow) but that is the max increase in MP you can get from ram air in theory, anything else give you lower values

On racing cars/motorcycles you can add anything as long as it looks cool and has some nice science story without having to justify teh associtaed performance !

Ram jets is different story, that seems to work well (tough some have stretch the concept a lot in YH-32 Hornet) but does not offer takeoffs…

Yes, 10K is probably a lot unless you have around 350kts on the ASI but not sure where it comes from? sun heat?

Last Edited by Ibra at 04 Jun 21:32
ESSEX, United Kingdom
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