Anyone checked rpm before and after advancing the timing ? - RC Groups
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Dec 13, 2001, 10:49 AM
small electrics r BIG FUN

Anyone checked rpm before and after advancing the timing ?

I finally bought the tool for adjusting 400 motor timing. Until now, I thought I could make one. Right. So last night I take a new 400 and advance it 1/8 inch as directed for "sport" flying. I did this as it was running and on two D cells I could hear the rpm's increase. This is BEFORE break-in. Break-in is another story: it took over an hour to properly seat the brushes under water, but also audibly increased the rpms.

My question: has anyone measured rpm of a 6v 400 on 8 cells before and after advancing the timing ?

Thanks in advance

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Dec 13, 2001, 11:08 AM
Registered User
Andy W's Avatar
Speed 400's are not exactly precision motors, so anything you do will help improve performance a little. Yes, unloaded RPM is supposed to increase as you advance the timing on this or any other motor. Keep in mind, advancing the timing optimizes the motor for full-power operation, so it can be less efficient at low power. This is often desirable, however - racing or sailplane where only full throttle is needed.
Dec 14, 2001, 01:31 AM
Registered User
be sure to use a smaller prop than with an untimed motor to let it wind up to the new rpm.

Otherwise, it will just draw more current and get hotter! I have been told by friend who was racing S400 pylon that the advancing only worked with fast, low drag planes that would note load the prop down too much.

I think they all used 5.5 x 5.5 Graupner props with the funny little tips.
Dec 14, 2001, 01:54 AM
Speed Demon
GregG's Avatar
I did do this test on a 6V Speed 400 before and after running in under water and retiming. This was tested with a 6V power supply so the voltage was steady. There was a measured 1200 RPM increase afterward!
Dec 15, 2001, 10:08 PM
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poobs's Avatar
A motor book by Astroflight decribes one method of adjusting the timing. In essence, he says you advance the timing to obtain the desired amp draw using the intended prop, battery etc.

Yes, the RPM do increase as does current draw.

The relationship between current and RPM is not linear since the motor looses eficiency as it heats up.

With this bench method (Yes I know that the prop should unload in flight) you can tweak the timing for the best ratio of RPM to current for a aproximate efficiency peak setting. OR - consider the current limit of a) the motor b) the battery c) desired run time and just advance to your hearts desire.

The above should be simple enough to proove/disprove on the bench with the timing tool and an AMP meter. The RPM increase is quite audible so a Tachomemter is not a must.

Keep us posted
Dec 16, 2001, 08:56 AM
small electrics r BIG FUN
Are you saying I can drop back to my 6 cell packs and advance the timing even more ? It would seem that after a point, more timing would just mean less efficiency and more heat/sparking ? This is interesting.

Dec 16, 2001, 09:27 AM
Registered User
poobs's Avatar
Al P

It is "very"

That is not exactly what I said but it sounds like it is worth trying. I gather you have some 6 cell packs that you wish to use but with the same performance as 7 cell packs.

I honestly don't know if a motor with the timing advanced behaves exactly as a high KV motor.

Remember that in your case, as you advance the timing using the 6 cell pack (increasing current draw) you will also get to the BEC voltage cut-off faster/sooner than with a 7 cell pack.

Please keep us posted
Dec 16, 2001, 12:52 PM
small electrics r BIG FUN
Well, it sounds like it is FINALLY time to construct a test stand in the house, to avoid wasting precious flying time at the field. It's just that I was happy with my brushless motors, setting the 400's aside. Now I have a renewed interest in the 6v 400 since it works so well in the Simple with 8 of the 1000 Nimh cells that are the same size and weight as 600ae cells.

It appears that there is a little more performance to be had by "tweaking" things, and why not ? It is basicly "free" for the small investment of time. Even without the test stand, I can already notice an improvement with the last flight, and flight times are a bit longer. Stay tuned...