Manifold pressure

[Russ]

Ask....and I will find.

 

At wide open throttle my manifold press guage reads around 28, reduce revs to cruise it shows around 20 ish

 

Fuel consumption at full power reads 35 LPH ....cruise reads 22 LPH.

 

This is my gyro ( Subaru ej25 engine ) .....I'm interested to know what can be concluded from these numbers ...eg, is this engine fully performing at full power, and just how hard is it working at cruise.

 

Full power revs 5300, cruise revs 4200

 

Thanks....Russ.

 

 

[mnewbery]

International standard atmosphere ir 29.92 inches of mercury. When the engine is turned off the manifold pressure will read about this or a bit less if your field elevation is a bit higher. 28 inches is 948 HPa which you will get around 1000-1500 feet AMSL.

 

So if the engine is stopped and you are standing at 1500 feet above the sea on a nice day with the grass airfield begging to be used ... Etc etc ... 28 inches it shall read.

 

This is the theoretical limit for the engine at wide open throttle (WOT) too, in the absence of forced air flow and pumping losses. The higher altitude you go, the lower the WOT value will be. Anyone with a constant speed propellor feature endorsement goes through this learning.

 

The EJ25 will shift 2.5L of air every two crank revolutions. So at 6000RPM (just a number for the ease of math) with no pumping losses the engine will pass 7500 Litres of air a minute, mix it with liquid fuel and poop out a bit more because now it's got combustion products mixed in with it.

 

At sea level that would be 7.5 Kg of air. At 948 HPa or 28 inches of mercury it's 7.0 ish kilograms per minute or 420 Kg per hour.

 

Now we look at kilograms of air plus kilograms of petrol. Conventional wisdom says the ratio of air in kilograms to petrol is 14.7 : 1 if it's pure octane e.g. Like 100 Low Lead.

 

Let's say the engine runs at 5300 rpm and 28 inches as suggested. That gives us 6.2 Kg of air per minute or 372 Kg/hr with an expectation that 25 Kg of 100 octane petrol would give the best mixture. In practice, it's a bit richer as the engine produces more power more safely at WOT if the mixture is rich. Lets go for 30Kg per hour.

 

Convert 0.72 Kg of petrol to one litre and you get 34 litres for 25 Kg and 41.6 litres for 30 Kg.

 

The engine is doing fine but probably could be richer at WOT if it is often operated at sea level on hot days. The extra fuel will help to keep the head temperatures down.

 

22 litres per hour is 63% of the WOT value therefore the same percentage of the WOT power

 

 

[Russ]

See.......ask, and you will find..........thankyou.