# Spark redundancy.

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#### Dan Thomas

##### Well-Known Member
....but my understanding is they are in no way an example of efficiency.
Aviation is full of misconceptions. This may or may not be another one. I was a pilot for over 20 years before I was an aircraft mechanic, and when I took my training and got the required experience I found I had believed numerous aviation myths, passed around among pilots, that simply weren't true. Much of it was carried forward for decades, stuff that might have been true in 1925, and mistakenly applied to current engines, airframes and whatnot. That's where the common rant about "Lycosaurs" comes from. The current Lycomings and Continentals, even the carbureted and magnetoed versions, are not nearly the same thing as they were in the 1950s or 60s. They might look the same, but the metallurgy has changed a lot. Aircraft TBOs have been creeping up for a long time now, and some operators get 4000+ hours out of a 2000-hour TBO engine; operators like pipeline patrollers and fish spotters.

If I took all the plumbing and plastic off the engine of a new Chevy, I'd think that nothing has changed with that engine, either. Yet it's a different animal inside and lasts 300,000 miles instead of being a smoking, rattling wreck at 100,000 miles like they used to be when I was young.

#### dwalker

##### Well-Known Member
Aviation is full of misconceptions. This may or may not be another one. I was a pilot for over 20 years before I was an aircraft mechanic, and when I took my training and got the required experience I found I had believed numerous aviation myths, passed around among pilots, that simply weren't true. Much of it was carried forward for decades, stuff that might have been true in 1925, and mistakenly applied to current engines, airframes and whatnot. That's where the common rant about "Lycosaurs" comes from. The current Lycomings and Continentals, even the carbureted and magnetoed versions, are not nearly the same thing as they were in the 1950s or 60s. They might look the same, but the metallurgy has changed a lot. Aircraft TBOs have been creeping up for a long time now, and some operators get 4000+ hours out of a 2000-hour TBO engine; operators like pipeline patrollers and fish spotters.

If I took all the plumbing and plastic off the engine of a new Chevy, I'd think that nothing has changed with that engine, either. Yet it's a different animal inside and lasts 300,000 miles instead of being a smoking, rattling wreck at 100,000 miles like they used to be when I was young.
I spent a couple of decades breaking misconceptions and "common knowledge", so I can relate.

#### aeromomentum

##### Well-Known Member
You are suggesting that in an aviation application, an auto conversion, by nature of it's basic architecture (high compression, multi valve, etc) has a significantly better BSFC than a Lycoming? I'd like to see that data, because frankly, a Lycoming even with mechanical FI and a simple EI cranks out some very impressive BSFC numbers when LOP.
While cruise numbers are not total mission numbers let use use these for the moment. A modern FI, liquid cooled, high compression, etc does get 0.38 lb/hp/hr. We have actually seen slightly better on our dyno. I am sure Lycoming has also seen slightly better on their dyno but from lots of sources a Lycoming IO-360 is about 0.42 lb/hp/hr LOP in cruise. So about 10% different if you are very skillful and attentive leaning to LOP. Off cruise like that 20 min climb at rich of peak also has larger difference and this is not insignificant in total mission fuel consumption.

I am not berating Lycoming at all. But liquid cooling, higher compression, better volumetric efficiency (less pumping loss) and variable spark advance (optimized to manifold pressure for example) all have impact on efficiency and power. Even Lycoming rates engines with higher compression higher power with the same air/fuel flow.

#### rv6ejguy

##### Well-Known Member
Std. IO-550 Conti gets .38 LOP in cruise, Lycomings also pretty close to that if you get the RPMs below 2200, WOT, LOP.

We see some modified Lycs. with higher CR, EFI and EI get better than .35 and low rpm. Dave Anders has done a lot of work refining the intake system to improve the VE even more. He's in light aero diesel territory with his setup now. Likewise the new Adept engines are around these figures as well.

#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
While cruise numbers are not total mission numbers let use use these for the moment. A modern FI, liquid cooled, high compression, etc does get 0.38 lb/hp/hr. We have actually seen slightly better on our dyno. I am sure Lycoming has also seen slightly better on their dyno but from lots of sources a Lycoming IO-360 is about 0.42 lb/hp/hr LOP in cruise. So about 10% different if you are very skillful and attentive leaning to LOP. Off cruise like that 20 min climb at rich of peak also has larger difference and this is not insignificant in total mission fuel consumption.

I am not berating Lycoming at all. But liquid cooling, higher compression, better volumetric efficiency (less pumping loss) and variable spark advance (optimized to manifold pressure for example) all have impact on efficiency and power. Even Lycoming rates engines with higher compression higher power with the same air/fuel flow.
Much of the inefficiency of the Lycoming is related to the applied systems, not the basic architecture. Yes, the updraft induction, plenum area, port shape and lower compression are "sub optimal" (to put it mildly), but even then, the very narrow operating range of the engine eases some of these inequalities. After all, one does not "need" a radical intake port to fill a combustion chamber at 2300 RPM with the throttle wide open.

But even with the basic architecture remaining, one can gain substantial efficiency with the simple addition of an ignition system with variable timing. Add EFI on top of that and one can clean it up even more. Add tuned exhaust, decent induction, and suddenly the "typical" inefficient Lycoming dyno mule has been transformed...

My point is simply that the basic architecture differences between a lycoming and a modern liquid cooled multi valve marvel are not as much as a performance impact as most people think.

#### rv7charlie

Sub-.4 BSFC in a Lyc (even with electronic ignition, etc) at anywhere near 'standard' operating output (meaning >65% power) just doesn't pass the 'smell test' for me. Like Aeromomentem implied, BSFC @ 40% power & 16K feet altitude does not tell the whole story. Besides, by the time you've replaced literally everything except the long block, you've just about bought two $30K engines. ;-) Lycs do what they do, pretty well, and do it better when modified. But I don't see them being as good as modern tech, any more than a 1955 Chevy small block can compare to a late 2010s-era LS motor. #### Toobuilder ##### Well-Known Member HBA Supporter Log Member Sub-.4 BSFC in a Lyc (even with electronic ignition, etc) at anywhere near 'standard' operating output (meaning >65% power) just doesn't pass the 'smell test' for me. Like Aeromomentem implied, BSFC @ 40% power & 16K feet altitude does not tell the whole story. Besides, by the time you've replaced literally everything except the long block, you've just about bought two$30K engines...
The discussion concerns claimed superiority of one architecture vs. another architecture in a head to head comparison. Pick an altitude and speed to fly, remove the fuel and ignition variables - in most cases the Lyc is going to be very close to the BSFC of the theoretical "modern" auto based engine. This has been played out through formal test as well as impromptu head to head comparison.

As for cost to retrofit - A set of magnetos costs more than a replacement EI from SDS, and a new Bendix FI costs more than EFI from SDS. BTDT - I sold my used mags and FI for nearly enough to buy the stuff from Ross. The cost difference is insignificant.

#### rv6ejguy

##### Well-Known Member
Interesting how some folks here have seemingly never looked at dyno tests and BSFC figures put out right from Lycoming test cells...

I'm no big fan of Lycoming engines and fly a Subaru but the Lycoming works pretty well if mixture distribution and timing is optimized. I've seen it personally in flight testing, shared info with Dave Anders (who has probably the most efficient Lycoming on the planet) and done videos on the subject.

Piston engines, whether auto or aero, generally run pretty rich at full power to stay cool and/or keep the piston crowns alive, BSFC isn't great under these conditions on either, but in cruise, the slow turning direct drive aero engines have low frictional losses and perform quite well LOP. The modern OHC auto designs would be challenged to do significantly better under cruise conditions.

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#### rv7charlie

##### Well-Known Member
I haven't had access to Lycoming test cell data (would love to see it if you can attach a copy or link to it.. But I do have some Lycoming operating handbooks, and haven't seen any sub-.4 numbers at realistic power levels & altitudes likely to be used by most of us.

#### Dan Thomas

##### Well-Known Member
I haven't had access to Lycoming test cell data (would love to see it if you can attach a copy or link to it.. But I do have some Lycoming operating handbooks, and haven't seen any sub-.4 numbers at realistic power levels & altitudes likely to be used by most of us.
Lycoming recommends leaning to max power, which is richer than LOP.

##### Well-Known Member
We're a long way from spark redundancy, but I'll throw in my 2 cent's worth anyway -
Modern auto engines are optimised for car use.

Among other things, this means :
Low emissions (so no LOP because of NOx)
High efficiency at light throttle openings (irrelevant for aircraft use)
Wide range of operating power (irrelevant for aircraft use)
Wide range of operating speeds - my car cruises at under 3000 RPM, but spins to 8000 for max power.

So among other things: the VVT is to cover the vast range of engine speeds - not required for aircraft.
Overhead cams - needed for high revs, but not necessary below 5-6000 rpm
The multi-valve head lets it breathe at high revs - again irrelevant for aircraft.

That's not a criticism of auto engines, but they have been optimised for a different workload, so it's no surprise that a specialised aero engine is just as good in the role it's designed for .

I'd love to see what GM could do if they set out to build an aero engine based around the LS engine, but leaving out all the stuff planes don't need.

#### Geraldc

##### Well-Known Member
Modern auto engines are optimised for car use.

Among other things, this means :
Low emissions (so no LOP because of NOx)
High efficiency at light throttle openings (irrelevant for aircraft use)
Wide range of operating power (irrelevant for aircraft use)
Wide range of operating speeds - my car cruises at under 3000 RPM, but spins to 8000 for max power.
Most or all of this is sorted by using a properly tuned aftermarket computer.
So among other things: the VVT is to cover the vast range of engine speeds - not required for aircraft.
Overhead cams - needed for high revs, but not necessary below 5-6000 rpm
The multi-valve head lets it breathe at high revs - again irrelevant for aircraft.
There are still suitable motors available without vvt and multivalve.
Small auto conversions typically rev at about 5000 rpm for takeoff and a bit less for cruise.
I'd love to see what GM could do if they set out to build an aero engine based around the LS engine, but leaving out all the stuff planes don't need.
Most LS conversions leave out that stuff anyway.

##### Well-Known Member
Most or all of this is sorted by using a properly tuned aftermarket computer.

There are still suitable motors available without vvt and multivalve.
Small auto conversions typically rev at about 5000 rpm for takeoff and a bit less for cruise.

Most LS conversions leave out that stuff anyway.
I'm not arguing against auto conversions - if I were building a plane that's the way I'd go.

Just pointing out why the last 50 years' auto development hasn't resulted in a vastly superior aircraft engine.

#### Toobuilder

##### Well-Known Member
HBA Supporter
Log Member
Hey Ross -

Are you aware of any auto conversions that run LOP relaibly in an aviation application? LOP ops in a Lyc/Con engine is well understood and the sheer brute strength of the pistons seems to offer some measure of protection from the inattentive pilot as he passes through the "Red Box". It "seems" that the lighter automotive pistons with their significantly lower thermal mass would be less tolerant of extended visits to the Box.

I dream of a DD LS based aero conversion, but if it has to be run ROP to keep it alive then that takes some of the shine off the dream.

Id appreciate your insight on the LOP aspect of auto conversions.

#### rv7charlie

##### Well-Known Member
I'm not Ross (and still waiting on that Lyc dyno data), but rotary guys run lean of peak all the time in cruise. Of course, the NA rotary doesn't have a 'red box', so it doesn't really mean much. One builder I know even programmed his Megasquirt controller to be max lean until he reached WOT at about 1/2 of the throttle control lever travel, and then further lever travel enrichened until max power richness was achieved. The rotary is perfectly happy with that operation (along with the lowest octane mogas you can find). ~5-10% lower that should read, *higher* fuel burn than a Lyc; but do the math on fuel cost advantage....

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#### Toobuilder

HBA Supporter
Log Member
Well, the current price of 100LL is within pennies of the lowest grade auto gas at the corner station around me (all over $4 bucks per gallon) so the ability to run auto fuel has lost much of its luster. (And yes, my 540 eats the lowest octane car gas just fine). ...But gasoline pricing variables is another subject for another forum The auto conversion rotary is an interesting subject though, because I have seen a few head to head flights published over the years and it seems the rotary guys generally burn more gas than the Lycoming guys on the same mission with the same airfame (usually RV's). IIRC, the articles were typically alternative engine "friendly", and the fuel burn disparity was explained away by the often told "cost savings" of the rotary at overhaul. So once again, if we are comparing basic engine architecture as it relates to the aircraft mission, using fuel consumption as the metric, the Lycoming remains a tough contender. Last edited: #### AdrianS ##### Well-Known Member Well, the current price of 100LL is within pennies of the lowest grade auto gas at the corner station around me (all over$4 bucks per gallon) so the ability to run auto fuel has lost much of its luster. (And yes, my 540 eats the lowest octane car gas just fine).

...But gasoline pricing variables is another subject for another forum

The auto conversion rotary is an interesting subject though, because I have seen a few head to head flights published over the years and it seems the rotary guys generally burn more gas than the Lycoming guys on the same mission with the same airfame (usually RV's). IIRC, the articles were typically alternative engine "friendly", and the fuel burn disparity was explained away by the often told "cost savings" of the rotary at overhaul.

So once again, if we are comparing basic engine architecture as it relates to the aircraft mission, using fuel consumption as the metric, the Lycoming remains a tough contender.
I know it's a smaller engine, but how does the venerable VW flat 4 compare for fuel efficiency?

#### rv6ejguy

##### Well-Known Member
Hey Ross -

Are you aware of any auto conversions that run LOP relaibly in an aviation application? LOP ops in a Lyc/Con engine is well understood and the sheer brute strength of the pistons seems to offer some measure of protection from the inattentive pilot as he passes through the "Red Box". It "seems" that the lighter automotive pistons with their significantly lower thermal mass would be less tolerant of extended visits to the Box.

I dream of a DD LS based aero conversion, but if it has to be run ROP to keep it alive then that takes some of the shine off the dream.

Id appreciate your insight on the LOP aspect of auto conversions.

#### dwalker

##### Well-Known Member
The auto conversion rotary is an interesting subject though, because I have seen a few head to head flights published over the years and it seems the rotary guys generally burn more gas than the Lycoming guys on the same mission with the same airfame (usually RV's). IIRC, the articles were typically alternative engine "friendly", and the fuel burn disparity was explained away by the often told "cost savings" of the rotary at overhaul.

So once again, if we are comparing basic engine architecture as it relates to the aircraft mission, using fuel consumption as the metric, the Lycoming remains a tough contender.
I like the vagueness and "seems like". It IS hard to provide hard numbers to compare the rotary efficiency vs the Lyc because being honest, there is a serious lack of consistency between builds. MY build will very likely result in a configuration that compared with a Lyc of equal performance will burn at least a 1/3rd less fuel. Now, if you compare that motor against a smaller lyc and ignore all performance numbers, I am sure the lyc might perform the same or better. The biggest issue I have when people say its a "thirsty engine" is they are comparing say an 0-235that pushes the plane 170knts vs a rotary conversion that pushes the same type airframe around at 200knts, assuming similar cruise settings. If the Lyc has a CS or other in flight adjustable prop, the difference grows again. .
As noted before, you can SAY, all you want, that a choked down Lyc pushing along at 30% throttle will sip fuel, but that is not a valid comparison.

Now lets turn to the 4V/VVI etc. things that seem not important for flight engines.
4V technology can provide more power at higher RPM, which might be helpful if you are using a PSRU, but 4 or 5v per cylinder technology is mostly good to build power all throughout the range and NOT make a peaky high RPM motor. The person building the engine will need to understand which camshafts and valve sizing to use, but it is a fallacy, and a bad one, to dismiss multi-valve engines from flight engines.

VVTL (Variable Valve Timing and Lift) technology is absolutely the future of airplane engines if they want to increase power and efficiency. This again is not a high RPM technology, rather, it allows for a much broader powerband across its usable range. Yes, early versions were used to have a high-lift and duration cam profile which allowed for good power down low with a noticable "VTEC KICKED IN YO" kick in the pants at higher RPM, but modern and better programmed versions these days are amazing in operation.

VLI (Variable Length Intake) technology again provides a very broad powerband. When working with the Mazda Renesis we did a bunch of testing with the variable runner and valve arrangements- removing them, opening them earlier or later etc.- and honestly just leaving it alone with some minor tweaking provided the best powerband we could ask for. The same was true in the 996/997 Porsche and pretty much everything else.

Yes, these give high power are redline, but the most important thing they do is to broaden the powercurve so that the usable power is available early and carries throughout. In a flight engine this means that you can have your available cruise power- where your spending most of your time- at a lower rpm, with lower fuel use and higher efficiency than ever before.

ALL of the things that make auto engines better for cars make them better for flight. And you only have to look at the current marine engines and thier tech to figure that out. Once all two-stroke, the market is being dominated by 4-cycle, multivalve, VLI/VVTL engines that are not only more powerful, but more reliable and quieter on the water. The use of turbo and supercharging is changing the game yet again, with 400hp class engines available in outboards that are the size and weight factor of the old 250/300hp class engines. And these engines run in basically two positions- WOT and idle, for most of thier lives.

While I tend to enjoy watching circular arguments ebb and flow, this is one where

#### rv6ejguy

##### Well-Known Member
Hey Ross -

Are you aware of any auto conversions that run LOP relaibly in an aviation application? LOP ops in a Lyc/Con engine is well understood and the sheer brute strength of the pistons seems to offer some measure of protection from the inattentive pilot as he passes through the "Red Box". It "seems" that the lighter automotive pistons with their significantly lower thermal mass would be less tolerant of extended visits to the Box.

I dream of a DD LS based aero conversion, but if it has to be run ROP to keep it alive then that takes some of the shine off the dream.

Id appreciate your insight on the LOP aspect of auto conversions.
I only know one guy who did some hours on a Subaru LOP but it was at below 50% power. That didn't seem to hurt it. Auto engine have double to triple the specific output of a Lycoming so heat loads on the piston crowns are much higher. With oil jets and forged pistons, probably very doable. Quite a few HP auto engines have oil jets and a few even have factory forged pistons. The latter are easy enough to install. My EJ22T has both, the LS9 from the factory has both.