20 or 25 Degrees BTDC, That Is The Question

Geico266

Geico266

Touchdown! Greaser!
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Geico
In the other mag thread I stated I attended a forum with Mike Bush talking about CHTs. He stated if the CHTs are high retard the timing from 25 to 22.5 D BTDC. I gave it a try and it worked! My CHTs are lower with a higher power setting!

If you have 25 degree timing are having high CHTs try retarding the timing a few degrees, up to 5 and see if that helps.

Seems Lycombing went to 25 degrees then decided some should stay at 20. Mike contends they all could run at 20 all day and be happy.
 
How big of a CHT drop did you see?
 
Not an engine expert but all things have consequences the trick is finding the best balance for you. The consequences of retarding the timing likely means exhaust valves getting hotter with more of a chance of them getting trashed from head. In this case, is that a risk, I have no idea? :)
 
jesse said:
Not an engine expert but all things have consequences the trick is finding the best balance for you. The consequences of retarding the timing likely means exhaust valves getting hotter with more of a chance of them getting trashed from head. In this case, is that a risk, I have no idea? :)
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Think about the 0-200 the engine was cracking cylinders, the Manufacturer requested an AD to retard the timing from 28 to 24 degrees. it stopped the cracking, But it took an AD to gain approval to do that.

How does Mike Bush get the authority to change the engine design?

With the 4 degree timing change on the 0-200 the dyno showed about a 10 horse loss of horse power. Will we see that in all engines? I don't know but it is not some thing I'd not like to see.
 
jesse said:
Not an engine expert but all things have consequences the trick is finding the best balance for you. The consequences of retarding the timing likely means exhaust valves getting hotter with more of a chance of them getting trashed from head. In this case, is that a risk, I have no idea? :)
Click to expand...

The EGTs are easily monitored and did not rise to unacceptable levels. I was able to run the engine 24 squared rop & lop with the same 40 degree CHT drop.
 
Geico266 said:
In the other mag thread I stated I attended a forum with Mike Bush talking about CHTs. He stated if the CHTs are high retard the timing from 25 to 22.5 D BTDC. I gave it a try and it worked! My CHTs are lower with a higher power setting!

If you have 25 degree timing are having high CHTs try retarding the timing a few degrees, up to 5 and see if that helps.

Seems Lycombing went to 25 degrees then decided some should stay at 20. Mike contends they all could run at 20 all day and be happy.
Click to expand...

You're not necessarily at a higher power setting. Did you gain TAS? You may see the same RPM and MP but that doesn't mean you're making the same power at the prop. You lose CHT because you transfer less energy into the combustion chamber and more out the exhaust.
 
Tom-D said:
Think about the 0-200 the engine was cracking cylinders, the Manufacturer requested an AD to retard the timing from 28 to 24 degrees. it stopped the cracking, But it took an AD to gain approval to do that.

How does Mike Bush get the authority to change the engine design?

With the 4 degree timing change on the 0-200 the dyno showed about a 10 horse loss of horse power. Will we see that in all engines? I don't know but it is not some thing I'd not like to see.
Click to expand...

Mike Bush didn't change the engine design. He recognized that Lycoming had to change the timing in some engine installations from 25 to20 degrees to reduce CHTs.

All Lycoming have the #1 cylinder timing set off top dead center. Retarding timing means the burn happens closer to TDC. This puts more stress and heat on the cylinder. Retarding the timing by a couple of degrees (as your example shows) puts less stress on the engine under full power.

The adjustable timing systems Lycoming uses on the same engine design. The trick with a fixed timing system is finding the timing setting that is a balance of the particular installation.
 
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why the hell would anybody want to give away horsepower? thats all that you do when you reduce timing. thats why the cht's go down. first step to getting lower chts is to fix the problems with baffling. most aircraft i have seen have crappy baffling. your engine should be set at the timing called for in the TC.

bob
 
Geico266 said:
Mike Bush didn't change the engine design. He recognized that Lycoming had to change the timing in some engine installations from 25 to20 degrees to reduce CHTs.

All Lycoming have the #1 cylinder timing set off top dead center. Increasing timing means the burn happens closer to TDC. This puts more stress and heat on the cylinder. Retarding the timing by a couple of degrees (as your example shows) puts less stress on the engine under full power.

The adjustable timing systems Lycoming uses on the same engine design. The trick with a fixed timing system is finding the timing setting that is a balance of the particular installation.
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Huh.....

Increasing timing moves the firing event further away from TDC, not closer...:confused:
 
unsafervguy said:
why the hell would anybody want to give away horsepower? thats all that you do when you reduce timing. thats why the cht's go down. first step to getting lower chts is to fix the problems with baffling. most aircraft i have seen have crappy baffling. your engine should be set at the timing called for in the TC.

bob
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Baffeling is good and tight.

I can now increase speed to 200 mph @ 25.5 IMP over 2400 RPM instead of 175. Before I had to reduce power to keep CHTs under 375.

The timing was 26 degrees, now 22.5.
 
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All you do by retarding the spark is reducing power. No wonder it is cooler!

Mike Busch is only a magazine salesman, not an engineer.
 
kgruber said:
All you do by retarding the spark is reducing power. No wonder it is cooler!

Mike Busch is only a magazine salesman, not an engineer.
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It if the timing is too advanced it heats the cylinders & piston tops up.

Mike Busch's advice worked for me. I'm running 40 degrees cooler and going 25 MPH faster. :dunno:
 
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Geico266 said:
Mike Bush didn't change the engine design.
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Exactly right, he would require at least a STC to do that.

Telling anyone to retard timing is very poor advice.

Timing is a requirement of certification, to change it will require proper testing and re-certification just like running auto gas, TCM used the AD method.

Arbitrarily changing the timing to fit your needs is as illegal as it gets.

How would you make the maintenance records entry and comply with 43.13? among others.
 
Tom-D said:
Exactly right, he would require at least a STC to do that.

Telling anyone to retard timing is very poor advice.

Timing is a requirement of certification, to change it will require proper testing and re-certification just like running auto gas, TCM used the AD method.

Arbitrarily changing the timing to fit your needs is as illegal as it gets.

How would you make the maintenance records entry and comply with 43.86?
Click to expand...

Geico flys a EXPERIMENTAL....

We don't need no stinkin STC.....:no::no::no::D
 
N801BH said:
Geico flys a EXPERIMENTAL....

We don't need no stinkin STC.....:no::no::no::D
Click to expand...

Do you want the engine to stay a certified engine ? Read the letter of limitations for his aircraft.
 
Geico266 said:
It if the timing is too advanced it heats the cylinders & piston tops up.

Mike Busch's advice worked for me. I'm running 40 degrees cooler and going 25 MPH faster. :dunno:
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You picked up 25 mph? In what? What were you getting before?

Considering that drag increases at the square of speed, that's a lot of HP gain. If you picked up that much horsepower by retarding the timing a few degrees, I'd be looking at my pistons because you were operating in perpetual detonation before.
 
Tom-D said:
Exactly right, he would require at least a STC to do that.

Telling anyone to retard timing is very poor advice.

Timing is a requirement of certification, to change it will require proper testing and re-certification just like running auto gas, TCM used the AD method.

Arbitrarily changing the timing to fit your needs is as illegal as it gets.

How would you make the maintenance records entry and comply with 43.13? among others.
Click to expand...

Seriously? :rofl::rofl::rofl:
 
Henning said:
You picked up 25 mph? In what? What were you getting before?

Considering that drag increases at the square of speed, that's a lot of HP gain. If you picked up that much horsepower by retarding the timing a few degrees, I'd be looking at my pistons because you were operating in perpetual detonation before.
Click to expand...

Yeppers. RV-8.

Ran at 18" of MP. Now I can run 25.5 @ 2400 RPM.
 
Tom-D said:
Do you want the engine to stay a certified engine ? Read the letter of limitations for his aircraft.
Click to expand...

Why would I care about certified engines? :dunno:

How can Lycoming have the same engine timed at 20 and 25 BTDC? It's the same exact engine, pistons, cranks, and cams?
 
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Geico266 said:
Yeppers. RV-8.

Ran at 18" of MP. Now I can run 25.5 @ 2400 RPM.
Click to expand...

By what regards are you at the same power setting? If you could only pull 18" of MP before without busting CHT, I reiterate, look at your piston tops.
 
Henning said:
By what regards are you at the same power setting?
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I never said I was at the same power setting. I always had to throttle back to keep the CHTs in the green. Now, no problem.

I look at the pistons every annual. Not sure what I'm looking at, but no holes! :lol:
 
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Geico266 said:
I never said I was at the same power setting. I always had to throttle back to keep the CHTs in the green. Now, no problem.
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Ok, but still, if you had those problems before, you were operating in a really destructive regime. Peak in plug holes with a cheapie device to make sure you don't have damage.
 
You may have achieved your goal by altering ignition timing but you haven't addressed the real problem. Something else is going on because there are thousands of engines out there running at 25 deg advance that don't have CHT problems.
 
Silvaire said:
You may have achieved your goal by altering ignition timing but you haven't addressed the real problem. Something else is going on because there are thousands of engines out there running at 25 deg advance that don't have CHT problems.
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A few Lycs had their timing marks off a few degrees. He might have had his mags set at something like 30° when the timings marks indicated 25. He'd have to use the spinner-mounted timing indicator to find TDC accurately.
 
Dan Thomas said:
A few Lycs had their timing marks off a few degrees. He might have had his mags set at something like 30° when the timings marks indicated 25. He'd have to use the spinner-mounted timing indicator to find TDC accurately.
Click to expand...

Or a pencil with eraser in the top of the #1 cylinder to find TDC and an iphone with angle meter rubber-banded to the prop to do the same thing a lot easier.

Jim
 
Or a tailor tape and a scribe and you can degree out the flywheel (metric tape makes life a lot easier. :lol:) Whatever, but there's something not right here.
 
Personally, I always use the "top stop" method to find TDC....
 
N801BH said:
Personally, I always use the "top stop" method to find TDC....
Click to expand...

Yep, and you can measure the circumference in mm, divide by 360, and use that as the starting point to measure and mark accurate degrees on flywheel. Something's not right if you can't pull more than 18" @ 2400, and backing off from 25° to 22.5° made the difference, not unless he has a load of really bad gas in his tank.
 
Henning said:
Yep, and you can measure the circumference in mm, divide by 360, and use that as the starting point to measure and mark accurate degrees on flywheel. Something's not right if you can't pull more than 18" @ 2400, and backing off from 25° to 22.5° made the difference, not unless he has a load of really bad gas in his tank.
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Or the motor was assembled with the crank / cam gear one tooth off...:redface:
 
N801BH said:
Or the motor was assembled with the crank / cam gear one tooth off...:redface:
Click to expand...

That is the other place I was hoping not to go. I don't know if this is a new to him plane that's always had the problem, or one that just developed. But there were problems, all of the possibilities quite destructive in potential.
 
Easy to troubleshoot though... Use the top stop method, find the exact TDC and degree in the cam and mags... Take maybe an hour or so..
 
Henning said:
Yep, and you can measure the circumference in mm, divide by 360, and use that as the starting point to measure and mark accurate degrees on flywheel. Something's not right if you can't pull more than 18" @ 2400, and backing off from 25° to 22.5° made the difference, not unless he has a load of really bad gas in his tank.
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With a constant speed prop, you can arrive at any RPM / MP number.. Within reason of course... Remember , Geico did say he throttled back to keep the CHT's under control so 18" @ 2400 is a viable number..
 
N801BH said:
With a constant speed prop, you can arrive at any RPM / MP number.. Within reason of course... Remember , Geico did say he throttled back to keep the CHT's under control so 18" @ 2400 is a viable number..
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It's not a viable number because he's making half the horsepower he should be for the heat.
 
Something I haven't seen mentioned: what were the CHT's? Normal would be 350 to 435 with a max of 500. Secondly the first step in trouble shooting CHT's is to verify your instrument is correct, so was that done? Finally in regards to baffles, it's not enough to just look at the seals. It's a matter of differential pressure between the upper and lower zones and things such as the lip on the lower cowl outlets can have a critical effect. As an RV8 builder I'm pretty sure you'd be aware of all that but the other relevant question is - has this been an issue since day one or did it suddenly start?

I still contend that if the data plate on the engine says 25 deg then that is not the cause of your problem.
 
if you are dealing with an RV i would suggest you go to VAF and read everything that has been written about cooling and CHT's. you should be able to run 25 degrees in an RV and keep temps below 400 in the climb and in the 370 or so range in cruise no problem with a good baffle system. there is no reason to cut the horsepower of your engine (which is what you are doing with less timing) for that aircraft. find what is wrong with your baffle system.

bob
 
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