LoPrestis Cowl on a M20J

[A64Pilot]

The dirt thing is sort of a red herring. you don’t use ram down low where the dirt is, but let’s run some numbers anyway. Check my numbers I sometimes make mistakes.

We have a 360 cu in engine, it’s a four stroke so it pulls in 180 cu in of air each rotation, but it’s not 100% volumetric efficient so we will say it’s 85%, probably better but I’ll use 85% anyway.

So 180 x .85 = I think 153, so 153 cu in per revolution, if we turn 2500 RPM that’s 382,500 cu in per min.

382,500 cu in is 221 cu ft, so 221 cu ft per min, I change my filter at every Annual, if I fly 100 hours that’s 6,000 min x 221 cu ft = 1.326 million cu ft of air.

So that filter has passed 1.326 Million cu ft of air and it’s not very dirty, then as most of that dirt was gotten on the ground very little of it came from in flight.

Now anyone who has flown has seen that nasty layer of air sometimes that you don’t see from the ground, so the air isn’t “pure” but I think those particles are so small that it passes through the filter anyway, a filter the size of ours that filtered 221 cu ft of air a minute down to micron size would be real restrictive and ours isn’t that bad, so we aren’t filtering that nasty brown / yellow SMOG anyway.

Which takes us back to the factory ram air, in that less than .5” I get, how much of that’s air filter restriction that goes away when you select ram air?

Maybe the filter is why some get higher boost from ram air, their filter is more restrictive?

[1980Mooney]

My Missile Conversion has a ram air on the left side of the cowl.  Rocket Engineering said it adds 0.5 psi or about 1" to the manifold pressure while at speed.  I have confirmed it by looking at and plotting fuel flow at WOT (full rich/max RPM with Auto-Lean fuel pump) and using your rule of thumb regarding 1000 ft. (plotting fuel consumption from SL to 10,000 ft. by climbing leveling off every thousand to build speed back up to about 180 kts and then recording engine data.)  I have all the Continental engine performance curves.  At SL I am set up for book fuel flow and pressures.  For instance 6,000 ft I am approx. consuming fuel and producing HP equivalent to a non-ram air IO-550A at 5,000 ft.

[A64Pilot]

4 hours ago, 1980Mooney said:

My Missile Conversion has a ram air on the left side of the cowl.  Rocket Engineering said it adds 0.5 psi or about 1" to the manifold pressure while at speed.  I have confirmed it by looking at and plotting fuel flow at WOT (full rich/max RPM with Auto-Lean fuel pump) and using your rule of thumb regarding 1000 ft. (plotting fuel consumption from SL to 10,000 ft. by climbing leveling off every thousand to build speed back up to about 180 kts and then recording engine data.)  I have all the Continental engine performance curves.  At SL I am set up for book fuel flow and pressures.  For instance 6,000 ft I am approx. consuming fuel and producing HP equivalent to a non-ram air IO-550A at 5,000 ft.

I don’t doubt at 180 kts you get an inch, but us J model types aren’t anywhere near that speed, and again that inch is bypassing a filter and it has to have some restriction, how much I have no idea, depends on type and how old it is, whether it’s wet or not etc. I guess.

Us J models at 7500 ft are more likely indicating 135 kts and trueing 156 kts or so. I’m not smart enough to calculate pressure as a function of speed, but I know it’s not linear.

But to me the fact Mooney deleted the thing says it can’t do much, I can’t image they knowingly took performance away from their aircraft.

[philiplane]

you can generally get one inch more MP by using a Donaldson (cellulose media, AKA paper) filter, instead of an oiled gauze (K&N, Challenger) or an oiled foam (Brackett) filter.

[A64Pilot]

The inch per thousand roughly holds to about 10,000 beyond that it’s less than 1 inch. I believe by 18,000 it’s 15 inches where it would be 12 if it kept the inch per thousand. At least if memory serves 18,000 is half pressure.

Standard adiabatic lapse rate is 3F per thousand or 2C, another rough rule of thumb. 

[A64Pilot]

1 minute ago, philiplane said:

you can generally get one inch more MP by using a Donaldson (cellulose media, AKA paper) filter, instead of an oiled gauze (K&N, Challenger) or an oiled foam (Brackett) filter.

I’m thinking that I’ll pull the filter completely and see, I run Bracket but I’d be surprised if there is an inch difference, if so then zero ram air would give us an inch of increase just by bypassing the filter. Surely no filter is less restriction than any body’s filter?

Only way to know is to try it and see.

But you never know, back in 1980 or so I was building drag bikes and we had a flow bench that we used developing head porting, we found out that for some reason the cone type K&N filters flowed slightly better than open velocity stacks, telling me that the velocity stacks weren’t properly designed.

[Tx_Aggie]

Late to the party here but I agree with the Lopresti requests. If I wanted to be economical I wouldn’t have bought an airplane. 
 

I sent an email today to Whelen asking them to consider restarting the Lopresti cowl.  I bet if a few of us requested it they’d start it back up?

[Tx_Aggie]

On 12/15/2022 at 6:49 PM, philiplane said:

you can generally get one inch more MP by using a Donaldson (cellulose media, AKA paper) filter, instead of an oiled gauze (K&N, Challenger) or an oiled foam (Brackett) filter.

Did you see an additional inch switching to the Donaldson?