The Effects of Advanced Timing?

[Jeff_S]

My J is coming out of annual, and the A/P said one of the things he fixed was that my timing had gotten slightly advanced. Forgive me if I screw up the terminology here, but he said it was supposed to be 25 top dead center but had advanced to about 27 TDC.

I remember something in one of the Mike Busch webinars about the effects of timing differences, but don't recall exactly what they were. If my timing was slightly advanced, would that make my EGT/CHTs cooler, or hotter? I'm just wondering what I should expect to see when I take her out for the test drive after annual. My A/P said I might not see much change at all, since it wasn't off by that much, but I'm curious.

Thanks.

[rbridges]

I remember when I had my '89 Mustang, people would advance the timing.  IIRC, stock was 10* before TDC, but we'd try to squeeze 12-14 out of it.  Jim is correct about detonation.  The more advanced timing has a higher likelihood of causing detonation, especially on hot days.

[1964-M20E]

For what it is worth here are my experiences and knowledge on the subject.

When retarding the timing setting i.e. 25BTDC to 20BTDC you reduce the power output of the engine.  You are starting the burning process of the fuel air mixture 5 degrees later in the cycle and you have less time for complete combustion.  The advanced timing will give you higher cylinder temperatures and exhaust temperatures and thus a greater chance of pre-ignition.  Retarding the timing reduces the chance of pre-ignition and t reduces power output.  Unfortunately our engines are locked into only one timing setting for our entire range of operation.  Therefore we need to compromise on a timing setting.  We are also controlling the combustion event through RPM and mixture.

Outboard motors and automobiles have had variable timing for many years (mechanical, vacuum or electronic) even model Ts had variable timing although it was manual.  Today modern engines (except certified aircraft piston engines) have variable ignition timing and electronic fuel injection which makes for quicker starts, better fuel economy, more reliability and virtually no pre-ignition or knock.

Higher octane fuel slows the combustion event in the cylinder and allows for a slower cooler flame front.  The slower cooler flame front helps to reduce pre-ignition.

Higher compression ratios in engines will increase HP and will generally require higher octane fuel or timing adjustments to prevent pre-ignition.  Our engines I believe are around 9.5 to 1 compression ratio.   Our engines would most likely run quite well on 93 octane pump fuel but no one has taken the time and expense to show the FAA that it will work so we are still dependent on a supply of 100LL.

Our engines being air cooled are less efficient in keeping the cylinder temperatures uniform and are more susceptible to pre-ignition.

[LFOD]

One reason for the variable timing in cars is the wide range of RPM in which it operates. Unlike airplanes which essentially operates at one RPM setting. Adding variable timing to an A/C magneto would be of little benefit and would add a failure point.

José

[danb35]

Quote: 1964-M20E

Our engines I believe are around 9.5 to 1 compression ratio.

[danb35]

Quote: LFOD [or is it Piloto?]

Adding variable timing to an A/C magneto would be of little benefit and would add a failure point.

[Hank]

The optimal spark timing expressed in "Degrees Before Top Dead Center" varies with engine RPM; as RPM increases, timing should advance further before TDC, and it does so in your car but not in your Mooney or other certified aircraft [with a very, very few exceptions].

Igniting the fuel before the piston reaches full compression allows the fuel to burn so that maximum pressure is exerted during the power stroke [pushing the piston down; Cycle #3 in a four-cycle engine]. Thus advancing the timing in a fixed-timing engine can increase the power produced, but it also increases the opportunity for detonation, where the fuel burns and produces power [expanding against the piston] before the piston reaches Top Dead Center.

TDC is the point where the piston stops moving up and begins moving down. This is controlled by your cam shaft and the cam followers, and is forever fixed for a particular engine unless you change these parts. Sparking too early [advancing the spark] is bad, as the cam is still pushing the piston up while the burning fuel is pushing the piston down; this will quickly erode your cam shaft and followers.

Is 2º enough to make a difference in power produced or in cam wear? I can't say off the top of my head. Byron has generated data that significant additional power is created by advancing spark from 20º BTDC to 25º BTDC. To say whether 27ºBTDC is better or worse will require part prints for pistons, pushrods, cam shaft and some time digging through textbooks from too long ago, unless there is an A&P around who knows about our engines in detail.

What is the margin of safety required by the certification process? What was the margin of safety designed into the engine? Why was timing changed for many of them from 25 to 20 BTDC? I don't know.

BTW, I think our engines are either 8.5:1 or 8.7:1; my 180-hp is different from your injected model, too, I think.

[KSMooniac]

Quote: Hank

 

What is the margin of safety required by the certification process? What was the margin of safety designed into the engine? Why was timing changed for many of them from 25 to 20 BTDC? I don't know.

BTW, I think our engines are either 8.5:1 or 8.7:1; my 180-hp is different from your injected model, too, I think.

[jetdriven]

Quote: Hank

 

TDC is the point where the piston stops moving up and begins moving down. This is controlled by your cam shaft and the cam followers, and is forever fixed for a particular engine unless you change these parts. Sparking too early [advancing the spark] is bad, as the cam is still pushing the piston up while the burning fuel is pushing the piston down; this will quickly erode your cam shaft and followers.

Is 2º enough to make a difference in power produced or in cam wear? I can't say off the top of my head. Byron has generated data that significant additional power is created by advancing spark from 20º BTDC to 25º BTDC. To say whether 27ºBTDC is better or worse will require part prints for pistons, pushrods, cam shaft and some time digging through textbooks from too long ago, unless there is an A&P around who knows about our engines in detail.

What is the margin of safety required by the certification process? What was the margin of safety designed into the engine? Why was timing changed for many of them from 25 to 20 BTDC? I don't know.

BTW, I think our engines are either 8.5:1 or 8.7:1; my 180-hp is different from your injected model, too, I think.

[garytex]

Byron

Your new motor come with a 20 degree dataplate?

Gary

 

[Seth]

Quote: 1964-M20E

For what it is worth here are my experiences and knowledge on the subject.

 

When retarding the timing setting i.e. 25BTDC to 20BTDC you reduce the power output of the engine.  You are starting the burning process of the fuel air mixture 5 degrees later in the cycle and you have less time for complete combustion.  The advanced timing will give you higher cylinder temperatures and exhaust temperatures and thus a greater chance of pre-ignition.  Retarding the timing reduces the chance of pre-ignition and t reduces power output.  Unfortunately our engines are locked into only one timing setting for our entire range of operation.  Therefore we need to compromise on a timing setting.  We are also controlling the combustion event through RPM and mixture.

 

Outboard motors and automobiles have had variable timing for many years (mechanical, vacuum or electronic) even model Ts had variable timing although it was manual.  Today modern engines (except certified aircraft piston engines) have variable ignition timing and electronic fuel injection which makes for quicker starts, better fuel economy, more reliability and virtually no pre-ignition or knock.

 

Higher octane fuel slows the combustion event in the cylinder and allows for a slower cooler flame front.  The slower cooler flame front helps to reduce pre-ignition.

Higher compression ratios in engines will increase HP and will generally require higher octane fuel or timing adjustments to prevent pre-ignition.  Our engines I believe are around 9.5 to 1 compression ratio.   Our engines would most likely run quite well on 93 octane pump fuel but no one has taken the time and expense to show the FAA that it will work so we are still dependent on a supply of 100LL.

 

Our engines being air cooled are less efficient in keeping the cylinder temperatures uniform and are more susceptible to pre-ignition.

[jetdriven]

Quote: garytex

Byron

Your new motor come with a 20 degree dataplate?

Gary

 

[aviatoreb]

Quote: Seth

[carusoam]

More power is good....

Until the gas bill cometh.

Byron,  does the increased timing improve overall efficiency of the conversion of old dinosaurs?

Best regards,

-a-

[jetdriven]

Yes. It would deliver more speed for about the same FF and LOP, it would deliver 3-5 KTAS for the same GPH.  Around 3% more efficient.

[carusoam]

I have to look at what I have for timing on the IO-550G just to see what Continental was thinking on the topic...

I would like an extra 3-5% as well if I could....

Thanks,

-a-

[Jeff_S]

Well, since I started this whole thread I thought I'd report my results. I've taken two flights since the annual, both really just to test all the systems and see if I can notice any difference. It does seem like my CHTs are somewhat below what I was used to, when I go LOP at 9.5 GPH in my standard flight configurations. Of course, the outside air has been a good 15° cooler this weekend as well, and so I can't keep all the variables constant. But overall I can't say I've noticed any detrimental effects either. So those 2° may just be a non-issue.

[garytex]

Byron:

That's too bad.  It will be interesting to see how it runs at 20.  I think that I would prefer to have the different options that 25 affords, and monitor the engine, and then reduce power via MP or Fuel Flow when 25 results in an undesirable situation.

Faeries?

Gary

Gary

Gary

[John Pleisse]

Quote: Jeff_S

My J is coming out of annual, and the A/P said one of the things he fixed was that my timing had gotten slightly advanced. Forgive me if I screw up the terminology here, but he said it was supposed to be 25 top dead center but had advanced to about 27 TDC.

I remember something in one of the Mike Busch webinars about the effects of timing differences, but don't recall exactly what they were. If my timing was slightly advanced, would that make my EGT/CHTs cooler, or hotter? I'm just wondering what I should expect to see when I take her out for the test drive after annual. My A/P said I might not see much change at all, since it wasn't off by that much, but I'm curious.

Thanks.

[1TJ]

Quote: Jeff_S

My J is coming out of annual, and the A/P said one of the things he fixed was that my timing had gotten slightly advanced. Forgive me if I screw up the terminology here, but he said it was supposed to be 25 top dead center but had advanced to about 27 TDC.

I remember something in one of the Mike Busch webinars about the effects of timing differences, but don't recall exactly what they were. If my timing was slightly advanced, would that make my EGT/CHTs cooler, or hotter? I'm just wondering what I should expect to see when I take her out for the test drive after annual. My A/P said I might not see much change at all, since it wasn't off by that much, but I'm curious.

Thanks.

[DaV8or]

Quote: 1TJ

 

More octane is not cooler, hotter, faster, slower, higher energy or lower energy.  A lot of misconceptions out there.

Ed

Edward Kollin - Technical Director - Aircraft Specialties Lubricants

[Skybrd]

I used to have a homebuild airplane that had a Lycoming engine and Lightspeed ignition making the timing variable. It made a positive difference in starting, power levels and economy. I believe that the later cars improved their gas milage and power by varing timing and mixture. The cars used sensors to check for knocks, manifold absolute pressure and oxygen sensors to keep the system optimized. Our airplanes are designed for "reliability" at the expense of not using newer technology. Also our airplane engines can't take the stresses that the water cooled engines have since the cylinders develope cracks. If we could get our engines to fire near top dead center for starting and then modulate the timing at cruise, and climb I think we could get better efficiency. One problem with electronic ignition is reliability. My lightspeed ignition became unreliable and intermitten after a few years. Fortunately I had an extra magneto for backup.

[jetdriven]

IM going to have to write a book here, but our airplane set a new SARL class record at 193 MPH on 23 degrees of timing. 25 degrees was 4 MPH slower. We spent 500$ to prove that...

[DaV8or]

IM going to have to write a book here, but our airplane set a new SARL class record at 193 MPH on 23 degrees of timing. 25 degrees was 4 MPH slower. We spent 500$ to prove that...

Were the atmospheric conditions identical? Seems like 4mph is such a small difference that with all the variables, it's really hard to quantify how much the timing really matters.

[jetdriven]

Were the atmospheric conditions identical? Seems like 4mph is such a small difference that with all the variables, it's really hard to quantify how much the timing really matters.

Yes, that was the 2300' DA air race speed. The airplane previously posted a 187 MPH top speed. Now its 193 MPH. 5 days before the air race we did a 3-course NTPS speed run and recorded 176 MPH at 24.5 degrees. Three days later we recorded 179 at 23 degrees. The air race we pulled down a 193 MPH top speed at 3000' DA at 23 degrees timing. . Previous record was 187. Error of measurement but the airplane is faster at 23 degrees than 25.