DarkWingDuck

My Feedback: (1)

Some engines like Brison use a throttle coupled spark advance system instead of an electronic advance system built into the module. Is the mechanical advance system obsolete? What are the pros and cons between the two systems?

Dave G.

GalenB

My Feedback: (18)

I have one of these "obsolete" Brison 3.2 engines and I can tell you it's a great engine! I see the mechanical advance as being simple and reliable. It's also my understanding that by adjusting the length of the linkage you can alter/adjust the timing -- although I've never had to do this to my Brison. As to cons -- I cannot think of any as mine has performed flawlessly for many years...

Ch0pp3r

My Feedback: (3)

Having a Mechanical TCSA is an advantage over an electronic advance in so much as you can tune your spark curve. You can bring any amount of advance in at any RPM you so desire to tune the idle, midrange and top end timing. But, if the electronic advance is perfect for your setup than you can't get any better than that...

Electronic:
Pro;
No moving parts
Can move the total advance 'window' (usually 26 degrees) up or down a few degrees.
Con;
Can't control the timing curve or total timing. Not very tunable.

TCSA:
Pro;
Totally adjustable timing for total advance and the curve at which it comes in. Tuneable.
Con;
Moving parts, they don't really wear out but can get bent or bumped if hit hard enough.

That's how I see it. Again, if the electronic ingnition were set up properly for a given motor and flying type then that would be my choice due to simplicity and that you can't get any better than perfect once it's right. If you are a serious gear-head than go with the TCSA.

DarkWingDuck

My Feedback: (1)

Hello GalenB,

Perhaps the word "obsolete" was a little inappropriate. Some of my club member friends own Brison engines and they would agree with you. It is a very good engine.

Thanks for the input.

DWG

GalenB

My Feedback: (18)

No worries... I like my Brison 3.2 and have a NIB 2.4 sitting around that I am thinking of putting in a Cermark Pitts S2B. FWIW -- I think Brison has pretty much dropped the mechanical advance on their engines in favor of the simpler one size fits all electronic advance. Customers perceive the electronic advance as being better and Brison responed to this perception. That is if you can still get a Brison... It is not clear what the future of Brison holds after the Kange deal and its aftermath...

Diablo-RCU

My Feedback: (1)

You need to be a little more careful with the mechanical advance. Never choke the engine at any throttle position other than idle with the ignition on. Otherwise, your hand is going to get smacked hard.

Volfy

My Feedback: (23)

This is true only because the current commercially available computer advance systems do not have provisions for user-reprogramming. It is technically simple to build in an interface so one can tune the ignition timing map, just like the automotive world have been able to do for years now. I can certainly understand it from the ign system manuf's point of view. It's just like the RC TX makers not allowing user-reprogrammability - they are concerned about the support nightmares once they open up the pandora's box.

The mechanical advance systems work extremely well for simple small engines like our RC gassers, but I would not call it superior in any technical sense to an electronic advance sytem. Spark timing advance curve should be mapped to RPM - plain and simple. Mechanical TCSA system can get away with "mapping" it to the throttle position mostly because throttle position typically has a good correlation to RPM. That correlation, however, is neither linear nor invariant. TCSA is a rough approximation at best. For example, many of the high performance engines that sport big-bore carbs will have a very non-linear throttle vs. RPM curve, such that RPM builds quickly through the first half of throttle opening and the second half toward WOT produces relatively small gains in RPM. A simple TCSA system is incapable of compensating for such nonlinearity. Plus many engines have significant throttle reponse delays.

Mechanical advance systems are indeed "obsolete" in 1:1 automotive engines. When was the last time anybody has seen vaccum advance mechanism on a distributor? Hmm.. come to think of it, when was the last time anybody has seen a distributor on a late model car?

For the RC gassers, however, TCSA will continue to provide a very functional and cost-effective alternative for quite some time to come.

Rcpilot

My Feedback: (78)

I've never understood what was meant by "tuning" the mechanical advance system.

Sure--you can advance the timing in the middle of the RPM range by adjusting the rod length.

But, then you've screwed up the timing at all the other spots in the curve.[]

For example:
Your idleing at 6* BTDC and your at max advance--or 28*--at 5500RPM. Everything from 5500RPM and up to max RPM is at 28*. And thats fine. Maybe you don't need a bunch more advance from that point on.

But--lets say you want to increase or decrease the timing in the middle--around 3800RPM. Maybe the advance is currently set at 14* at 3800RPM and you want to set it up to 16* for better throttle transition. So, you lengthen or shorten the rod to gain some advance in the middle.

Haven't you just screwed up the idle and top end timing????

Geistware

My Feedback: (16)

On my Patty I adjusted the advance to get 7600 on a 20x8 APC propeller.
It ran great and I could probably have done better but I didn't want to over heat a little 39cc engine.

Tired Old Man

My Feedback: (1)

Truth be told, some auto advance systems can be modified.

Antique

My Feedback: (4)

Syncro spark ignition control can be configured a few different ways..
TOTAL advance can be either 20 or 26 degrees, starting wherever you want..
TOTAL adance can occur at 4000 or 6000 rpm..No other options...
I know of no foreign made ignitions that can be user configured at all, except for where full advance occurs, usually at 28 or 30 degrees BTDC...
You have to be pretty critical to even tell the difference between ANY of these settings, except when starting the engine...Too much initial advance will get your hand slapped..
Pro Spark had an adjustable pot that could change the timing at WOT, I think they're not around any more...
Without a degree wheel and/or a dial indicator, you can't very well tell the difference of a few degrees either way anyhow...Try it and see[8D]

rmh

No one mentioned that the mechanical setup does NOT necessarily follow the engine thru the rev up .
The Falkons etc., start low and you simply accept the high - at our altitude -you can bump em a bit -
When we tried the mechanical setups -they worked ok -just a bit dated now.
The Synchro Spark - in comparing - is hard to beat - The extra features in the foreign ones are very nice tho -the tach features auto shutoff etc..

Antique

My Feedback: (4)

I think mine is linear...the arm on the carb moves about 45 degrees each way from center, and the ring on the hub does the same...

Wings-RCU

My Feedback: (2)

The throttle coupled advance is based on the throttle position & not the actual RPM. I know this has already been mentioned but from a theoretical standpoint this may be as good as or better than advance based on actual rpm. Autos makers of yesteryear realized this and included vacuum advance (i.e. throttle position). Today timing in an auto is set based on throttle position, RPM, O2 & nock sensors, octane level, etc.
I'm pretty sure there is no performance advantage to either setup in our engines. An electronic advance should actually be less expensive than a mechanical setup. The component cost is less. The premium cost you are paying for electronic advance comes from "development." Because the cost is less for an electronic advance system, all future systems will eventually go that way.

rmh

ab so friken lutely- like trying to stop the sun from coming up

Antique

My Feedback: (4)

Also ab so friken lutelyWings-----Cheaper ? Really ? The "premium cost is from the syncro module..I would really like to see an electronic chip for spark control that costs less than the 2 DuBro links and the $2.50 bearing used for the mechanical advance...CAN'T be done, at least I haven't found one..I use the same syncro module as C&H, and it is WAY more than $2.50...
Your facts are not the same as reality in this case.....
Show me where I can get electronic advance for $3.00....

Wings-RCU

My Feedback: (2)

Good one..dick - like teaching an old dog new tricks.
RC, I'm sure the syncro-spark module is more expensive. The actual electronic components necessary to add the advance to the ignition module however is not. The cost you are paying for is in the programming and flashing of the ECM (i.e. development). Some of the modules, like the new Falkon are now adding features like in-flight RPM recording. There is really little or no cost in "parts" to accomplish this, just additional programming added to the flash.

tkg

My Feedback: (1)

Changing the length of the push rod changes WHEN the spark happens. Changing the points on the bell crank change HOW MUCH total timing curve there is.
For the creative you can set up the linkage so the bell crank ALMOST goes over center, then you can get the timing curve to come in faster at a lower RPM. OR is you really want to be creative use another servo slaved to the throttle to run the timing..... get any curve you want that way.

Forgues Research

My Feedback: (7)

Mechanical advance does work but then so did the Model -T

Roger

Forgues Forgues

Antique

My Feedback: (4)

And you could buy a brand new one for $295....
Almost anyone could repair it, try that on your electronic advance

Diablo-RCU

My Feedback: (1)

Throttle couple advance is not the best way to control spark timing on an engine, but it's good enough on a model plane motor. The reason the old cars had vacuum advance as well as mechanical advance (rpm advance), was because when the vacuum is high, the engine load is low, and power and economy improve with more advance. The model plane electronic ignitions have an advance curve based on rpm (like the centrifugal mechanical advance in the old cars). For steady state running (not accelerating or decelerating) at a fixed rpm, the engine load doesn't change a lot while flying. It changes a little with flying conditions and if you change the prop. If you added a load sensor (MAP sensor or vacuum sensor) to an electronic ignition, all it would do would be to decrease the advance on acceleration when you slammed the throttle open, and gradually the advance would be added back as the vacuum increased.

The throttle coupled advance is only related to rpm under steady state conditions. On acceleration, it gives more advance than is required. If you slam the throttle to full open, the ignition advances fully before the engine can reach max steady state rpm.

Either of these systems provides a timing curve that is closest to optimum if you roll the throttle on slowly, so the engine rpm can catch up with the throttle opening (and steady state vacuum).

Next we can talk about the lack of accelerator pumps on the diaphragm carbs, and how the mixture has to go lean on fast acceleration unless the carb is set richer than required for steady state running.....

Antique

My Feedback: (4)

There are Walbro carbs that DO have accelerator pumps, 2 types...One is a plunger that operates from a cam on the throttle shaft, and the other is just a few holes drilled through the shaft and carb body that uncover and let more fuel in...Electronic advance is full at anything over 4000 (syncro spark, don't know about others) so mechanical advance is slower over 4000 rpm...
Syncro spark CAN be configured for full advance occurs at 6000 instead of 4000...There is no detectable difference either way....

Forgues Research

My Feedback: (7)

hahaha good one.

Roger

Forgues Research

Flypaper 2

There is an old walbro with an accererator pump on the side that pumps when the vacuum drops on acceleration. Fuel cavity on the side with a diaphram spring loaded in one direction and held back with high vacuum. Throttle opens, vacuum drops, fuel squirts in. Sounds strange idling in complete two stroke smooth. Dousn't hesitate when you crank the throttle open. Landing aproach sounds the same, smooth 2 stroking all the way in.