[CFItom]

I recently bought a Thorp T18. It has dual Pmags on an O320 B1b. The previous builder rebuilt the engine but did not keep records well and his family may have thrown out the logbooks not realizing what they were or their value.

Anyhow I know it is 160HP. From what I understand 160hp can have either 8.5:1 or 9:1 compression pistons. I previously read that premium no ethanol auto fuel can be used with 8.5:1 or less compression but should not be used with the 9:1 pistons.

My question is: with dual electronic ignition does the variable timing offer additional protection from detonation to allow even 9:1 to used auto fuel?

From what I have read so far. The only way to determine if I have 8.5 or 9 to one pistons is to pull the cylinder and look at the part number that is located on the bottom of the piston.

I really don’t want to pull a cylinder just for this knowledge of there is no other reason to pull the cylinder.

The previous owner and owners son only burned 100LL as it was convenient at the airport.

Is it OK to burn auto fuel? Or is there another way to determine which pistons I actually have without pulling a cylinder?

It would be nice to burn auto fuel to save money. But not if it is unsafe or would do damage to the engine.

 

[Toobuilder]

….My question is: with dual electronic ignition does the variable timing offer additional protection from detonation to allow even 9:1 to used auto fuel?

This is a question that requires a nuanced answer. There is nothing inherent in an “electronic ignition” that differentiates it from a magneto with regards to igniting the mixture. It is the variable timing aspect of EI’s that drives detonation behavior in this context. Among the various EI products, some offer ignition curve tailoring and some are firmly locked down. In the context of your situation, the Pmag is both “locked down” AND has a very aggressive factory ignition slope.

There are people who likely have your exact combination and run auto fuel without issue, but strictly speaking to your question above, the PMag is among one of the worst choices if you are trying to tailor the curve for increased detonation margin.

And for the record, I have purchased many PMags and have many hours of flying time with them. I now have an ignition map tailored specifically to run auto fuel on an 8.5 CR PV Lycoming - and it isn’t a Pmag.

 

[DanH]

My question is: with dual electronic ignition does the variable timing offer additional protection from detonation to allow even 9:1 to used auto fuel?

There are owners successfully flying advancing ignitions with auto fuel, perhaps even with 9:1 pistons. However, technically speaking, ignition advance is pro-detonation, the opposite of additional protection.

As Mike noted, a P-mag needs additional attention. Without an owner-installed jumper between pins 2 and 3, and with timing set at TDC per the manual, the base timing at full power will be 30.8 BTDC, quite a lot more than the 320's standard 25 BTDC. Even with the jumper installed, base timing is 26.6 BTDC.

It is possible to clock the installation; a search will explain.

 

[lr172]

As Mike noted, a P-mag needs additional attention. Without an owner-installed jumper between pins 2 and 3, and with timing set at TDC per the manual, the base timing at full power will be 30.8 BTDC, quite a lot more than the 320's standard 25 BTDC. Even with the jumper installed, base timing is 26.6 BTDC.

WOW. I knew their curve was all messed up and too agressive, but didn't realize they were throwing 30* of advance at full MAP. That is Really bad.

 

[skirting_virga]

WOW. I knew their curve was all messed up and too agressive, but didn't realize they were throwing 30* of advance at full MAP. That is Really bad.

And more like 40° at "low" manifold pressure, where EMag decided that ~24" is "low". It is an absolutely bananas amount of timing for a relatively high load. If we were still in the early days of EMag, I would be incessantly lobbying the original guy to change it.

 

[Toobuilder]

…Is it OK to burn auto fuel?

What is your mission profile? Do you do 30 minutes of hard acro with all the knobs forward or do you do long cross countries high and LOP? If you only fly ROP then it’s an easier answer - since the PV Lycoming is relatively insensitive to timing when ROP, one can mechanically spoof the PMag by clocking the housing into an appropriate retard value. This will buy you a bunch of detonation margin with little to zero cost in power. This also assumes you have installed the jumper which forces the advance shift to a lower schedule. This will significantly erode your upper end advance if high and lean, however. If you want both high power detonation protection AND the high and lean advance capability, then you need to be looking at other ignition options.

 

[erich weaver]

one can mechanically spoof the PMag by clocking the housing into an appropriate retard value. This will buy you a bunch of detonation margin with little to zero cost in power. This also assumes you have installed the jumper which forces the advance shift to a lower schedule. This will significantly erode your upper end advance if high and lean

So what exactly should one be looking out for when high and lean?

I have dual pmags with the jumper installed and their timing set with the ring gear 1.5 - 2 teeth past TDC, which has eliminated kickbacks during startup as well as reduced CHTs while not noticeably affecting power. I always cruise LOP and typically fly at 7500-8500 feet, more rarely at 12,500 when heading over the Sierra to Tahoe. Haven’t noticed anything of concern. A lot of Pmag criticism of late, perhaps deserved, although mine have been performing flawlessly for years now.

Thanks for your insights

 

[DanH]

And more like 40° at "low" manifold pressure, where EMag decided that ~24" is "low". It is an absolutely bananas amount of timing for a relatively high load. If we were still in the early days of EMag, I would be incessantly lobbying the original guy to change it.

Some of us did. Brad would not do it. Heck, look how long it took to get an addition to the manual regarding angle valve timing.

 

[DanH]

So what exactly should one be looking out for when high and lean?

In the context of 9:1's and 100LL, mostly just whatever CHT you may find excessive. Opinions vary as to "excessive".

I have dual pmags with the jumper installed and their timing set with the ring gear 1.5 - 2 teeth past TDC, which has eliminated kickbacks during startup as well as reduced CHTs while not noticeably affecting power. I always cruise LOP and typically fly at 7500-8500 feet, more rarely at 12,500 when heading over the Sierra to Tahoe. Haven’t noticed anything of concern. A lot of Pmag criticism of late, perhaps deserved, although mine have been performing flawlessly for years now.

Great setup for a parallel valve. See Nigel Speedy's excellent article.

 

[skirting_virga]

Some of us did. Brad would not do it. Heck, look how long it took to get an addition to the manual regarding angle valve timing.

I've never talked to him or seen him on VAF as I didn't have a need to replace mags until the last year or so, but spending a decade ignoring key customer feedback before selling out to private equity is not what I would want to see in a business or a product. My impression is that the product was 90% of the way there and just needed a little work on bearing reliability and slightly more robust and customer-centric approach to firmware and programming for the remaining 10%. That Brad would have the insight, skill and grit to build a business and product like EMag and fumble the last 10% is truly frustrating.

 

[Rdog420]

WOW. I knew their curve was all messed up and too agressive, but didn't realize they were throwing 30* of advance at full MAP. That is Really bad.

Light speed says “hold my beer” at 40 degrees

 

[CFItom]

This is a question that requires a nuanced answer. There is nothing inherent in an “electronic ignition” that differentiates it from a magneto with regards to igniting the mixture. It is the variable timing aspect of EI’s that drives detonation behavior in this context. Among the various EI products, some offer ignition curve tailoring and some are firmly locked down. In the context of your situation, the Pmag is both “locked down” AND has a very aggressive factory ignition slope.

There are people who likely have your exact combination and run auto fuel without issue, but strictly speaking to your question above, the PMag is among one of the worst choices if you are trying to tailor the curve for increased detonation margin.

And for the record, I have purchased many PMags and have many hours of flying time with them. I now have an ignition map tailored specifically to run auto fuel on an 8.5 CR PV Lycoming - and it isn’t a Pmag.

Thank you for the response.

One question: what does “PV” stand for when you say “PV” Lycoming?

 

[CFItom]

So what exactly should one be looking out for when high and lean?

I have dual pmags with the jumper installed and their timing set with the ring gear 1.5 - 2 teeth past TDC, which has eliminated kickbacks during startup as well as reduced CHTs while not noticeably affecting power. I always cruise LOP and typically fly at 7500-8500 feet, more rarely at 12,500 when heading over the Sierra to Tahoe. Haven’t noticed anything of concern. A lot of Pmag criticism of late, perhaps deserved, although mine have been performing flawlessly for years now.

Thanks for your insights

How do you set the mag up 1.5 teeth past TDC? How do you get half a gear tooth?

I will have to look to see if there is this “jumper” between posts.

For mission profile. It honestly will be mostly relatively local flights. More occasional longer X-C flight. I would expect to run it 65% to 75% power except for take off and initial few minutes of climb.

No hard full throttle acro.

I have never prior run an engine LOP in the past. But so many people do it, I might be willing to try. But I don’t want to ruin my engine by doing it via detonation or any other cause whatever that might be.

I do have Dynon skyview with 4 cylinder EGT/CHT fuel flow and percent of power etc engine EMS.

 

[Dr_Dave_MT]

Thank you for the response.

One question: what does “PV” stand for when you say “PV” Lycoming?

Parallel Valve; verses Angled Valve.

 

[Scott Hersha]

Setting the Pmag up 1.5 teeth past TDC merely refers to timing the Pmag. Normally we set ignition timing via a mark on the starter ring gear lined up with the hole in the nose of the starter. Instead of doing that, you would line up the starter hole 1.5 gear teeth past the TDC mark on the ring gear. This offsets the entire curve by a fixed number of degrees. Typical starter ring gear has 149 teeth (but not all of them), or 2.4* of engine rotation per tooth. So 1.5 teeth would offset ignition timing by about 3.6* when setting your timing. In that case your retarded timing on that mag would fire the ignition during engine start at 3.6* after TDC, pretty much eliminating any possible kickback during engine start. It also sets the whole timing curve 3.6* later. Base timing with the jumper installed becomes about 23*, or without the jumper about 27* - still too aggressive, especially when running auto fuel. Unfortunately, there’s nothing you can do about WHEN Pmag starts advancing the timing (based on manifold pressure)……. except install a different ignition system.

 

[Toobuilder]

Thank you for the response.

One question: what does “PV” stand for when you say “PV” Lycoming?

In the context of this forum and Lycoming engines on RV’s, there are two distinct cylinder head types: Angle Valve and Parallel Valve (AV and PV). Though each is manufactured by Lycoming, their combustion chamber behavior (and by extension, ignition timing requirements) are vastly different. They might as well be considered a whole different breed of engine.

Why is this important? At a macro level it illustrates the fallacy that there can be a one size fits all variable ignition curve. Despite the fact that a PMag will physically bolt to both a AV and PV engine, the fact remains that the baked in ignition advance schedule is applied equally to both. At the micro level, we have discovered that there are many variables among engines that appear the “same”. these include induction air schemes, exhaust, mission profile, fuel type, etc. Exploring these variables is why you asked the question on this forum in the first place. It’s also why it’s widely accepted that the end user needs to take additional measures to make the PMag ignition curve work on an AV engine.

Back to you. You are concerned about detonation with auto gas with a carb and moderate compression pistons. You have an ignition with advanced base timing and an unnecessarily aggressive advance schedule (slope, curve, map, whatever jargon used on the forum) that can’t be modified. The “fix” (admitting that it’s a bandaid for a poorly thought out advance schedule) is to shift the whole advance schedule to retard with the jumper wire AND to further retard the advance by mechanically clocking the PMag housing itself on the accessory case (this is the “1.5 tooth” retard fix seen on the forums). These actions can get you to the timing you want at the detonation prone engine conditions of high MAP and RPM, but because you have to retard the entire ignition schedule to get it, you do so at the expense of optimizing the opposite end of the operating range “high and lean”. Think of it like a tablecloth that’s a few inches too short for the table - you can make one side look good, but the far side comes up short. Fix the far side, and the near side is short. It’s an apt analogy; for those of us who need detonation margin at high MAP, yet ALSO need optimal high and lean timing, the PMag comes up short. Fortunately for you, optimized LOP ops is not a priority so the “far side of the table” can be left uncovered with that too short tablecloth. You won’t have enough advance, but so what? It will only cost you a few knots and won’t hurt anything. I’d say that If you want to stick with the PMag then spoof the timing with the jumpers and housing clocking as required to satisfy your high MAP concerns and don’t worry about the high/ŁOP optimization. Worst likely scenario is that you leave a few knots on the table if/when you do decide to dip your toe in the LOP pool.

…Or buy an ignition that does it all.