28 vs. 24 DBTC Timing

28 vs. 24 DBTC Timing

During my annual, my mechanic noted that my 0-200 (130 smoh) timing was set at 24 DBTC.  I have millenium cylinders on the plane.  With these new cylinders, the timing can now be reset to to @8 BDTC.  The question I have is will I notice a significant increase in engine power and smoothness?

Thanks

Mike

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Re: 28 vs. 24 DBTC Timing

Mike:

I am unaware of any engineering reason that a change to Millineum cylinders allows any change in timing.  I strongly recommend against advancing the timing.  It will make the internal cylinder pressures and therefore CHTs a lot higher.

Leave the timing where the Type Certificate has it set.  Do NOT advance it.  This opinnion is based on the observations I have made on the test stand.

(BTW, advancing the timing will have very, very little effect on the HP and may actually cause the HP to be reduced!

Walter Atkinson
Advanced Pilot Seminars

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Re: 28 vs. 24 DBTC Timing

Walter,

I suspect you're not familiar with TCM SB 94-8 and AD 96-12-06, which require setting the magneto timing of both magnetos on O-200 engines with one or more cylinders having part numbers lower than 641917 to 24° BTC, but for engines with all four cylinder having part number 641917 or higher, authorizing resetting the  magneto timing of both magnetos to 28° BTC. The change to 24° BTC had been made to prevent cylinder cracking.

When the TCDS was originally changed to show 24° BTC on both mags, it resulted in a loss of power. The authorization IAW AD 96-12-06 to change the timing to 28° BTC if all cylinders are p/n 641917 or later restores that lost power.

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Re: 28 vs. 24 DBTC Timing

Stan:

Yep, I'd fogotten about that SB.  Thanks.  The issue is the same, the retarded timing resulted in lower ICPs and protected the heads from high pressures.  I'll bet that the later timing resulted in HIGHER power during takeoff but lower power in cruise.  I'd have to put the engine on a test stand to prove that, but that's a common result in most aviation engines.  The advanced timing would probably result in very slightly lower TO power, but higher cruise power.  The 24 degree timing will definitely result in lower ICPs and lower CHTs.

Walter

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Re: 28 vs. 24 DBTC Timing

The o-200 was certified for 80/87 fuel. 100LL burns slower so I think the engine needs more advance if you use 100LL all the time. This alone suggests  tending towards the 28 dg setting. TCM must know best & I just can't see them suggesting going to 28 unless they are sure and  feel it is ok for the engine.

I've just changed to 28 deg & will monitor CHTs to see if there is a difference. Engine certainly feels more purposeful / better on the ground, if a little lumpy at tickover - I suspect it is a little too advanced at low speed now & thus  should be better at 23-2400 where I cruise most.

Regards
Steve

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Re: 28 vs. 24 DBTC Timing

Steve:

** 100LL burns slower so I think the engine needs more advance if you use 100LL all the time.**

Well, almost, but not quite. In an effort to be accurate, 100LL and 80/87 actually burn at very, very nearly the same rate.  The latency period is longer in 100LL and that *operationally* makes it seem like it burns slower.  It really doesn't BURN slower...  it takes it a little longer for the flame front to become organized than with the lower octane fuels.

That does not alter the basic premis of your message which is that the earlier timing is OK for that reason when using 100LL.  In that, you are correct.  I would think that it would not be as good an idea if one were using a lower octane fuel.

Walter Atkinson

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Re: 28 vs. 24 DBTC Timing

There is no way in the world you could possibly say this with a straight face, you are wrong as night is to day. Where do you work at Fisher-Price? Who ever reads this..... the total opposite is true....Why anyone would say this is totally a mystery

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Re: 28 vs. 24 DBTC Timing

Just what is the "this" you are referring to?  If you can back up your rebuttal with years of experience, a logical statement of your argument and most importantly, some hard test data, I would appreciate the comments you make.  In the meantime, perhaps YOU should go to the Fisher Price site and make childish arguments to their readers.  I've been reading this site for years and have never found anything Walter said to violate any of the above conditions. Let's try to keep our comments on a factual non-opinionated level to advance aviation understanding.  Thanks  Steve VW

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Re: 28 vs. 24 DBTC Timing

Hi,
so based on the discussion in this forum. It sounds like the AD96-12-06 addresses the issue of head pressure causing cracks. Improved cylinders are the answer per the AD. I'm only asking this question as a matter of mechanics. Using 100LL versus 80/87 will also HELP in preventing cylinder cracking? Ok let's say I buy into that rationale, will only running 100LL with the original 28' timing be enough to mitigate cracking?
I appreciate the perspective. Thank you, Marty

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