Dual Pmags and auto fuel use

[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