How do you set your cruise?

[EricJ]

Many (most, all?) carburetors for these engines have an "economizer" circuit that increases richness of the fuel/air mix at WOT.    Usually with a carb pulling it a little off WOT gets it off the "economizer" circuit and into what is supposed to be the more efficient cruise mode.

I don't know for 100% certain that typical Mooney carbs do or don't have this, but my understanding has been that they generally do.

So from that standpoint a C model would do better to not cruise at WOT, as others have suggested.

Fuel injected engines, at least the naturally aspirated ones, otoh, prefer WOT for cruise, hence the confusion.

 

[McMooney]

Opinion: What price speed? - AOPA  

 

[0TreeLemur]

Rules I was taught by the CFII that I bought our C from:

Above 5000 ft, WOT.  Always.

When leveled off in cruise at WOT above 5000 ft, when you lean the engine it will start to run a little rough- pull the throttle a little bit until it stops running rough.  This happens because you're getting enough throttle plate engaged to help mix/atomize the fuel.  Continue leaning.

Contemporary literature gives the max power mixture for the O-360 at 50F ROP.   Hardest on the engine.   I lean for 80-100 ROP.

A couple of times in cruise I've experimented and got our C to run smoothly LOP.  Our engine has the induction tubes that pass through the oil pan like @PT20J mentioned.  Fuel consumption down near 7 GPH is pretty cool.  But it's slower. I don't fly to save money or go slower.  So I don't do that very often.

+10 for engine monitor.  Best first investment you can make to modernize an older Mooney, in my opinion.   Gets you so much more awareness about that all important engine.  A certified primary engine monitor lets you remove those six tiny little indicators down by your co-pilots left knee, MP/FP and Tach steam gauges, plus it gets the flammables back outside the firewall where they belong.  Plus you can download the data and get analytics run on them regarding engine health compared to the fleet!

 

[kortopates]

4 hours ago, EricJ said:

So from that standpoint a C model would do better to not cruise at WOT, as others have suggested.

Typo? I assume you meant do better to cruise at WOT which is true for all engines, subject to limitations.

All the marvel-schebler carbs have the Economizer circuit that enriches at WOT but it only works down low and probably does nothing by around 3K and higher. 

[Hank]

2 hours ago, kortopates said:

Typo? I assume you meant do better to cruise at WOT which is true for all engines, subject to limitations.

All the marvel-schebler carbs have the Economizer circuit that enriches at WOT but it only works down low and probably does nothing by around 3K and higher. 

Your injected engine runs best WOT at all altitudes.

Our carbureted engines run best with the throttle body cooked inside the carb to create turbulent flow inside, so we have to back off on the throttle some. 

Really.

[kortopates]

5 hours ago, Hank said:

Our carbureted engines run best with the throttle body cooked inside the carb to create turbulent flow inside, so we have to back off on the throttle some. 

Of course, the throttle plates are backed off just enough to add turbulent air flow to improve mixture and even out EGTs but not enough to significantly reduce MAP, so still in the regimen of WOT.

[Hank]

14 minutes ago, kortopates said:

Of course, the throttle plates are backed off just enough to add turbulent air flow to improve mixture and even out EGTs but not enough to significantly reduce MAP, so still in the regimen of WOT.

Right. But I don't want to confuse a new operator into thinking he should leave the throttle fully forward, because it should be pulled back just enough to make the MP needle wiggle.

That's how you tell it's far enough back. And it can be a surprising distance. 

[kortopates]

7 minutes ago, Hank said:

That's how you tell it's far enough back. And it can be a surprising distance. 

Agreed, the throttle distance is proportional to the altitude or ambient atmospheric pressure.  It just really isn't possible to explain how to set optimum WOT in Carb'd C/G model in cruise without writing a paragraph to explain objective and technique.  

[bcg]

Thank you all for the in depth responses here, I've learned a lot. I'm looking forward to putting this knowledge into action. PPI is this weekend, we're going to just do an annual for that. Fingers crossed it all goes well!

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