Twin plugs do reduce the need for ignition advance & therefore reduce full throttle fuel octane requirement at a given compression ratio and bore size… Its a common aftermarket and recent factory mod for large bore motorcycle engines, making them like aero-engines in that regard, and increases their performance and fuel flexibility. However the reduced advance with twin plugs (and the 23-26 degree number) is for high manifold pressure operation and in most applications electronic ignition does more than control advance relative to rpm, it also adjusts for manifold pressure. That means variation of advance with either rpm plus manifold pressure directly (like Lightspeed) or rpm plus throttle position combined with ambient pressure, the so-called Alpha-N system. The intent of either is to beneficially increase advance when rpm is normal/high and manifold pressure is low – up to 50+ degrees of advance is common and often useful when MP is low enough that detonation is not the dominant issue. For a ground based engine that means the advance is retarded back to (say) 23-26 degrees whenever the throttle is snapped open but for a normally aspirated aircraft it translates to increasing SFC at high altitude with low ambient pressure and normal/high RPM, in the low percent power configuration that may still make enough power to pull the plane through thin air at good TAS. At lower altitude there is not so much fuel efficiency to be gained with electronic ignition over fixed ignition advance because engine efficiency is even more improved when higher MP in available (with necessarily retarded ignition, driven by fuel quality) combined with lower rpm… even while airframe efficiency may be much lower in thick low altitude air, and the combined effect decreases low altitude aircraft mpg as a result.
I must admit to doubting to claims made for electronic ignition having, in the past, changed from a points ignition engine to an updated version of the same unit.
Admitedly these are the smaller Rotax two stroke engines but there was no noticable difference in performance or fuel consumtion. The gains that I experianced were a reduction in maintenace and having twin ignition when I had a coil failure I found it only on shut down checks. That said I see that there is now an MDP on the latger Rotax units concerning the failure of the ignition units, fautly units I believe.From BeechTalk (mounted on a Baron):
“I’ll say right off the bat that electroair is awesome. Starting is easy, the engines are turboprop smooth at any power setting, and the extra performance is noticeable. That being said, all of this comes at a price of higher CHTs.
First, let’s talk about the installation:
1. The timing on my engines is set correctly. I was present during the timing, it was done with a digital sensor, and it was done by a very proficient A&P.
2. I have the crankshaft trigger wheels on my system because there was room for it to fit behind the propeller flanges. This reduces the timing errors that have been associated with the magneto timing housings.
3. The electroair installation was done at an approved electroair installation facility with A&Ps who attended the electroair installation courses. Furthermore, much of the installation was directly supervised by Mike at electroair.
4. The biggest fuel spread between the richest and leanest cylinders on both engines is 0.3 GPH with GAMI injectors.
5. I have new baffling and it was light checked and adjusted accordingly for a snug fit in the cowl.
I have flown the airplane for 90 hours now with the electroair system. Below 10,000 feet my new normal CHT is 400. Above 10,000 feet my new normal CHT is 415. We were concerned that the electroair system was advancing timing too much and that’s why the temps were high. We reviewed the engine data with Mike at electroair, and he said the system was working perfectly. He told me flat out that I should expect a new normal CHT of 415.
These are the numbers that I have recently seen:
13,000 feet, MP 18", OAT 44F, 2450 RPM, 11 GPH/side, cowl flaps cracked open 3 seconds: 176 ktas and hottest CHT 410. (70 ROP)
10,000 feet, MP 20", 51F, 2450 RPM, 11.3 GPH/side, cowl flaps closed: 182 ktas and hottest CHT 400. (30 LOP)
6,500 feet, MP 23", OAT 72F, 2500 RPM, 12 GPH/side, cowl flaps closed: 191 ktas and hottest CHT 390. (LOP)
Below MP 20", the electroair aggressively advances timing. This drops EGTs and increases CHTs. At lower altitudes, reducing MP from 23" to 20" will help to bring CHTs down slightly because there is enough cooling air flow to overcome the moderate timing advance. At higher altitudes, reducing MP usually means bringing it below 20", which is where the aggressive timing advance kicks in and the CHTs rise because the effects of the timing advance exceeds the available cooling air. So, as a general rule, WOT is where I see the lowest CHTs on a flight above 8,000 feet. Essentially the electroair has turned my airplane into a cowl flaps airplane – I climb to altitude, set the power where I want ROP or LOP, and then dial in the cowl flaps to balance the speed penalty and CHTs.
Is it possible to keep the CHTs at or below 380 in level cruise flight? Sometimes, and it’s not easy. At MP’s of 20 or higher, bring the mixtures back to 100 LOP and you’ll see CHTs around 380. My engines don’t like to run that far LOP and that makes sense because 100 LOP is on the edge of the EGT cliff. The penalty is a speed loss (expect 170 ktas) and the benefit is a fuel savings of 1-2 GPH. Depending on OAT and IAS at altitude, sometimes I still have to open the cowl flaps and then I’m flying at 165 ktas. At MP’s below 20", reducing power alone will not work. I have tried every RPM and power setting between 2300-2500 RPM and 100 LOP to 100 ROP. I concluded that cowl flaps are necessary. So, I usually pick a ROP mixture and crack the cowl flaps to keep the hottest CHT temps below 415. If I open the cowl flaps all the way, I can get the temps below 380 but again – I end up at 165 ktas and that’s too slow to justify. With the MP below 20", I just don’t see a feasible LOP setting in the hot southwest air that will give me an acceptable speed because the cowl flaps have to be open and it’s too much of a drag penalty (maybe that’ll change in the winter – we’ll see).
I live in AZ and it’s a literal oven right now, so others may have better numbers in cooler parts of the country. When I flew to Washington state this past weekend, I had no issue keeping my CHTs below 410 with the cowl flaps closed at 12,500 feet (LOP). The cooler OAT helped, and in the winter it’ll probably be even better.
Also, consider that air flow through the cowl is different in a single versus a twin, so singles might have better results.
I like the electroair and I don’t have any regrets. I think the system is great for somebody with a lot of Midwest flying where they can cruise along below 10,000 feet. Out here on the west coast where the mountains force you higher, you have to accept the higher CHTs or slow down and negate the performance gain."
The system only replaces one magneto which means the other spark plugs fire at a random time (timing tends to shift). I assume that can cause side effects like higher CHT.
Obviously the CHT is lower if you don’t burn all fuel (ROP) or burn the fuel outside the cylinder (late timing) but that just means your engine becomes very inefficient.
I wonder if CHT 415 is OK long term for a Lycoming. My POH says the limit is 500°F but that was written in 1979. On my last trip at ISA+20, I could not always keep the CHT below 380°F. It won’t be a problem for NA airplanes because they’re out of power above FL100 anyway.
That report doesn’t sound good.
I don’t know anything Beech aircraft (some of the engine installations I have seen do look very tightly packed-in) but those CHTs are very high for cruise.
The system only replaces one magneto which means the other spark plugs fire at a random time (timing tends to shift). I assume that can cause side effects like higher CHT.
It isn’t any more “random” than what you have now 
I thought the point of electronic ignition (leaving one old mag in place) is that it must advance the timing, at all times, because obviously it can’t retard it since the old mag is going to fire anyway! Or, as a part of the installation, does one retard the timing on the old mag, to give the electronic one more room to play?
100F LOP is way too far LOP. A non-turbo engine will be making very little power down there, especially at the higher altitudes, where you are MP limited. And a turbo engine will be useful at 100F LOP only if you wind the MP way up (the recommended mode for e.g. the PA46, IIRC, which is another tight engine installation).
I wonder if CHT 415 is OK long term for a Lycoming
Probably not, but then your turbo shortens your engine life anyway 
Very few turbo engines make 2000hrs without cylinder replacements. I recall having debates on some site about this years ago… some pilots there did not regard cracked cylinders as “engine not making TBO”. IMHO they should go into politics (a job at the ECB, perhaps?).
Peter, I promise that after one flight in a TB21, you wouldn’t even look at your TB20 anymore…
I haven’t really seen any conclusive evidence establishing that a certain CHT is a sensible upper limit. Personally I try to stay below 380°F but my POH says 500°F. Lycoming generally allows higher CHT than Continental and also has fewer cylinder issues in my experience.
Efficient combustion with high power just means high CHT, that is pure logic. LOP only means low power and ROP means pouring fuel out the exhaust. Therefore this is the expected result of the electronic ignition.
I promise that after one flight in a TB21, you wouldn’t even look at your TB20 anymore
You may be right
Efficient combustion with high power just means high CHT, that is pure logic. LOP only means low power and ROP means pouring fuel out the exhaust. Therefore this is the expected result of the electronic ignition.
Well, you can’t have your cake and eat it.
If you generate more HP then the STC involves recertifying the whole aircraft.
If you generate the same HP but with a lower fuel flow, you avoid recertification (the change in the POH data i.e. lower fuel flows can be taken care of by the STC) but the CHTs don’t need to be higher.
Or do they?
Peter wrote:
If you generate more HP then the STC involves recertifying the whole aircraft.
I don’t think it’s that exact. Nobody will measure this higher HP output and I believe the regulations give you a 10% buffer anyway. If you look at e.g. the JetProp STC which is super sloppy, you see statements like “will match or exceed original performance” all over. Same for the MT props, the STCs say “as good or better” everywhere.
If the Baron owner configured his aircraft for the same CHT in LOP as before, I would assume he gets the same speed as before but with a lower fuel flow.
The Mooney Rocket puts out a lot more HP (305) than the original airframe (200)and they certainly didnt recertify it as a whole.
