The Pport is at a disadvantage at idle in that the inlet tract is so large that air velocity in any situation is low. Great for high power (Low flow drag). So fuel injected for idle more than a few MM from the port into the block will surely return to droplets resulting in erratic idle and surging. There is far less of a problem at close to 2,000 RPM.

 

The side port block is usually run only on the inner or primary runners that are deliberately  

quite small and have higher velocity in any situation. In the car the primary ports have a separate butterfly, so the only air available to the engine is through the primary runners.

This gives a very satisfactory idle even when cold. With injection or a carb the engine runs only on the primaries until higher power is demanded.

 

The exact idle mixture will vary according to conditions. No thought is given to producing best cylinder pressure at 50 degrees after TDC as you will at some higher power setting.

 

So on a cold morning you may need 11.7:1 to keep it running, and on a warm morning perhaps 12.7:1 will be enough. But no two mornings will be exactly the same, so the mixture knob must be adjusted in probably every case. The more sophisticated the brain box the less adjusting will be required. In effect, the brain box just fiddles with the mixture knob for you.

 

On timing.

 

For best power above 8,000 RPM with car gas 25 to 26 degrees. For avgas 27 to 28 degrees.

 

For best power at 5,000 to 5,500 RPM, 22 degrees for car gas and 24 for avgas.

 

The function of advance is to provide enough time to produce  highest cylinder pressure

at 50 degrees after TDC. Or better yet, since I have never even seen a dyno pass below 7,000 RPM, because we need best power at 9,400 RPM (My engines had 244 HP at 9,400), So I am guessing on this.

 

If the EGT does not exceed 1650 in any case then no damage is usually possible.

So if you keep that in mind  and fiddle with the mixture at cruise to get best airspeed, the go back a bit richer on the figure, all will be fine. So you are rich of best power by a readable margin.  Or you can go over 100 degrees lean of peak EGT for economy cruise and add advance as the over-lean mixture burns slower. EGT will be much lower and the timing can be adjusted to get highest air speed without fear of damage.  

 

The airplane is just an expensive dyno. So much research can be done.

 

The wide range of data is a result of the many intake and exhaust designs being produced be so many folks building installations. If you have a dismal intake system that limits cylinder filling then you can get away with lots of advance. If you have a great system that really shines at 5,500 RPM then you cannot run as much advance. So when another builder says

this or that as a fact, don't crank that data into your engine without lots of thinking first.

Your engine acts only like your engine and not the same as any other engine.

 

A Pport would more likely have much better cylinder filling than a side port. So, its effective compression ratio would be higher at any RPM than a side port engine. So, higher compression means higher flame speed requiring less advance to get best pressure at 50 degrees ATC.  At idle it does not matter.

 

Any question, anytime.

 

Lynn E. Hanover 

 

In a message dated 1/16/2012 6:53:37 P.M. Eastern Standard Time, msteitle@gmail.com writes:

Lynn, 

I know this is an old posting, but I save all of your posts for future reference.  I was wondering if you could you comment on the amount of advance appropriate for a p-port engine?  What is the max advance I should run with 100LL?  Do the numbers you quoted below apply the same to p-ports as to side port engines? 

As it relates to a p-port engine, what MAP readings should I get at idle (1800-2000 rpm)? 

Thanks,

Mark