C

Catbird

Well Known Member
I?m in the final stages of finishing up the first annual inspection on N562E and am adjusting the propeller blade pitch for optimum performance. The PAP prescribes a static wide open throttle (WOT) range of 4,870 to 4,930. VAF forums related to this topic suggest 4,950 rpm as the sweet spot, and that is my target.
Last night my Carbmate synchronizer was connected to the carburetors as I ran up the throttle and adjusted the prop pitch through three iterations. After 60 minutes, I had finally achieved 4,930 rpm with the blades matched to within 0.1 degree of each other. The carbs were in perfect pneumatic balance, so I figured that was more than good enough for me. I disconnected the Carbmate hoses from the carbs and reconnected the balance tube to return the engine to its standard configuration. I then ran the engine one more time to double check the WOT rpm. To my dismay, with the balance tube connected, my maximum rpm had dropped from 4,930 to 4,850 rpm, below the low end of the range.
Is it normal for the connection of the balance tube to reduce power output, even though the engine runs smoother?
 
A bit confusing....

Carb synch adjustments and prop pitch adjustments are done separately and should have no effect on each other, other than the idle speed can be effected slightly if you make a real big prop pitch change (not typical when doing final tweaking)

The carb synch adjustment is primarily for the low speed end (idle stops to about 2500 RPM). Don't be trying to adjust the synch up near full throttle, the balance tube will take care of that. Once you get it as good as you can, remove the gage and reattach the balance tube.

Now do the final tweaking for your static RPM. As long as you were pretty close with the prop pitch when you started, it will have very little effect on idle RPM.
 
Jerry,

I wouldn't stress out over the small change. Fly it and see if you like the performance. Then tweak it

Rich
 
Hey Jerry - -

temperature, humidity, how warm the engine was at each test, all affect the RPM you see from time to time. Test it in a month and worry about it then if you feel like it.
 
Perhaps you had a leak and rather than running as rich as it is designed to, it was leaner, closer to where peak power is achieved.

Do your prop adjustments without the balance tube removed.

By the wy make sure nothing is rubbing on that tube as it is important. Wearing a hole in it in flight =:eek:
 
I appreciate the feedback. For me, part of the learning process is to experiment with things and check the results. I understand now that the carbs must be balanced mechanically and pneumatically before trying to adjust the prop pitch for optimum static WOT rpm. I also understand that the purpose of the balance tube is to dampen out manifold pressure variations at idle and off-idle where manifold vacuuum is high to make the engine run smoother. I've figured out through trial and error that the balance tube has little or no effect at WOT for two reasons: 1) manifold pressure is approaching atmospheric pressure with very little or no vacuum; 2) both carburetor venturis are opened against the stops and thereby balanced.

My real question is: Why would the engine generate more power at WOT when the intake manifolds are pneumatically isolated from each other than when they're joined with the balance tube? I agree that the rpm drop is insignificant in the real world. Just can't come up with a compelling reason for the rpm drop.
 
Catbird said:
My real question is: Why would the engine generate more power at WOT when the intake manifolds are pneumatically isolated from each other than when they're joined with the balance tube?
Click to expand...

A reasonable speculation would be intake manifold resonant wave effects...in plain language, a change in intake manifold tuning by connecting or disconnecting the balance tube. We're seeing all kinds of variable intake systems on cars these days, and simply opening/closing a connecting tube via ECM command is one of them. You can read all about it with a quick Google search. Here's a good example:

http://www.autozine.org/technical_school/engine/Intake_exhaust.html
 
Interesting article. Sorta like velocity stacks, which were in vogue with hot rodders during the 1950's and 60's.
 
My career was in nuclear power. When one of these interesting insignificant anomalies came up the grey beards would shrug and say "nuclear phenomenon". Which meant "who cares". This must be an internal combustion phenomenon! :)
 
Catbird said:
Interesting article. Sorta like velocity stacks, which were in vogue with hot rodders during the 1950's and 60's.
Click to expand...

Yes and no. Varying the velocity stack length and diameter is a pressure wave tuning factor, but the primary interest was increasing airflow by minimizing separation at the entry lip. The wave tuning is only perfect at one RPM while clean entry airflow pays out across the entire RPM range.
 
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