# GM 3.6L variants FYI

Discussion in 'General Auto Conversion Discussion' started by maticulus, Aug 11, 2019.

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1. Aug 15, 2019

### rv6ejguy

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4 valve engines give the best of both worlds. Can give good fuel economy at low rpm and part throttle and make good power up high when you need it. Look at the Gen 3 Coyote engines compared to older 2 valve V8s. Pretty impressive hp and torque. BTW, these have both DI and PI like Lexus to solve the oily valve issues.

2. Aug 15, 2019

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Yes.
But the TPS says full throttle, while the MAP says 1/2 throttle. Which sensor does the (std) ECU trust ?
At least some ECUs would trust throttle position over MAP.

Does it tune for light-throttle cruise (typically lean) or WOT (typically rich)?

3. Aug 15, 2019

### wsimpso1

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I have several topics to cover here...

Direct injection engine and intake valve coking - It seems all brands of DI engines have the issues, it throttles the engine by restricting flow through the intake ports and eventually prevents valves from closing. Sounds bad for airplanes. There is no clear consensus on what the source is, although everyone seems to lay the coking on the lack of port injectors washing the intake valves in fuel. Sources for material that burns to leave the coke are cited variously as fuel and oil mist from PCV systems, oil coming past the valve stems, and combustion chamber gases. The folks who do understand the phenomena are not talking. Toyota is solving it by adding port injectors to their DI engines and presumably injecting modest amounts of fuel that way. I admit that PCV effluent seems the most likely culprit here while having no evidence to point to. Since we routinely vent our crankcases overboard in airplanes, it strikes me that crankcase mist as a source will be avoided by following airplane practice when converting DI engines for airplanes.

Sure, DI may allow higher compression ratios and thus somewhat increased thermodynamic efficiencies if you can maintain spark timing. The reliance upon spark retard driven by knock sensors to protect the engine does indicate that the actual improvements at high power settings may not be realizable - when you increase compression ratio but then have to retard the spark, you will typically lose as much or more than you gained in both power and efficiency.

Evaluating engines based upon torque curves is another area for applying some prop science. At any prop size and pitch rate, props can absorb energy based primarily upon RPM squared and by forward velocity. When you vary the blade pitch, you can shorten or lengthen the curve, but it is still dependant upon rpm squared to make torque and rpm cubed to make power. As such, any given prop will make almost all of its usuable power between a max rpm point (wide open throttle) and about 75% of the max rpm. 75% rpm is about 40% power. So, in comparing engines, select the PSRU gear ratio and max prop rpm, figure out the 100% engine rpm, and then look primarily at curve between 75% and 100% of that value. Power over the rest of the range will generally not be expressible through the prop.

The OP has mentioned power per unit displacement as being attractive with this engine. I will emphasize that WEIGHT IS THE ENEMY in airplanes. Power per unit weight is the metric that is most useful in airplanes, and many of us will suggest that as the more critical point in engine selection. This is probably why NA LS versions have been suitable and increasingly popular engines in homebuilts of that horsepower range. Included in the weight equation is the fuel at takeoff to fly the intended mission and duty cycle.

PSRU's have a been a favorite topic of mine, being as I spent 23+ years on automotive powertrain engineering and one airplane project, with a lot of it on vibration management, a prime issue in PSRU. Perhaps one of the airboat drives successfully used on V-8's could be adapted to this engine. The isolation systems may need to be adjusted for lower firing rates and lower rotating part inertia as compared to the V-8's. This adaptation will not be trivial, but it might be straightforward. Yep, real engineering to accomplish it. Then you add the PSRU weight to the engine, induction, exhaust, and heat exchangers, and fuel load.

Variable Valve Timing has been discussed heavily. It still looks to me like something that could be established for cruise and left there, either through mechanical locking (with weight reduction through part omissions) or by programming (for simplicity). After all, the DI will be absorbing a lot of effort to get right, why keep VVT in there too if it is giving little benefit on airplane type duty cycle and complicates DI calibration?

In cars, the GTDI engines took an order of magnitude more engineering effort to sort out than previous engines, and then the tranny calibrators got in the act and a bunch more engine cal work occurs compared to previous programs. We sometimes had to wait months for updated engine cal so trans cal could proceed, and this from dedicated teams of knowledgeable engine geeks doing the engine cals. Now, if someone still wants to select a GTDI engine, a PSRU, and then go about adapting it for airplanes, I suggest that it will require a lot of test stand time to sort the engine, PSRU, ECU, and prop. Some luck would be good too.

Billski

Last edited: Aug 15, 2019
4. Aug 15, 2019

### rv6ejguy

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Our ECU doesn't care about the TPS at all for main mapping, only rate of change for acceleration enrichment. The engine doesn't care where the throttle is- only MAP or MAF to supply the correct amount of fuel. Throttle angle mapping can be used on ground based applications but obviously wouldn't be suitable for aircraft.

5. Aug 15, 2019

### maticulus

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Yes that's exactly what I meant. I don't doubt that some DI engines have had problematic coking, but it varies by manufacturer and application. My focus is on the GM 3.6L where that has not been a sufficient enough concern beyond dealers offering a top engine cleaning service and that's the reason I have not previously spent much time looking into it. google will always turn up any number of threads/info to support, or oppose a subject of discussion.

You having been looking down the intake runners of auto engines for 40+ yrs is probably well aware of the existence of intake valve sludging from the good old days of carburetors. You are probably also aware that many DIY enthusiast have remedied any chance of an issue from occurring by implementing a simple oil catch can to remove much of the oil mist from crank case gases before entering the intake and as a boosting connoisseur I'm surprised you didn't mention it.

If acft motors have a PCV system arranged similarly, their valves shouldn't be all that clean either. If they vent to open air then the same can be done for the auto motor for similar cleanliness. Chances are that maintenance intervals would address this area long before the potential to be a concern could arise given the 3.6L runs well beyond 100k miles and more without valve coking concerns, however many acft hours that translates into.

I do use symbolic gestures from time to time and only meant to indicate that the stock rods in the naturally aspirated 3.6L have come about as the first weak point under boost. That is how the ~500 hp was arrived at following a couple 600ish hp turbocharged failures. Still that's a pretty darn stout spread across 6 rods and 3.6 litres. Those who keep it around 500 hp to the wheels are still driving. Every couple years or so, the stock equipment is continuing to be upgraded bringing many crucial OE components to high performance level quality. Lighter valves and increasingly durable valve seats would not have me concerned about them.

I did manage to find a good info source for some insight on acft engines, particularly Lycoming from the same page I linked to above. At a glance, they appear to be highly inefficient relative to what they could be and by the appropriate class comparison such as what you provided. 6,7 and 8:1 compression on 100+ octane fuel! My immediate thoughts were, antiquated engine design (old combustion chamber design and low compression tends to need high octane to prevent poor efficiency related spark knock) and engine management if any. After searching with a question regarding the low octane levels, it wasn't long before I came across evidence for what I also suspected from the beginning, the cost of improving the design which someone put into perspective by mentioning the very expensive certification.

With that info, and given the 3.6L mind you as an example, weighs far less than the closest Lycoming to its power level, it's hard to ignore the potential overall performance and efficiency capability of an auto motor including the 3.6L with practical thinking in place. If it's significantly more efficient by comparison on any fuel it uses, regular, or 100CLL which opens the possibility for it to run even more efficient using more aggressive timing advance, to me it's worth considering and using.

I would suspect the high torque at low rpm capability provided by VVT could be capitalized on best with a constant velocity prop of appropriate pitch angle and not need gear reduction since prop rpm range is well within its sustainable capability. Auto engine in house durability tests are brutal and multidimensional

There are many parameters untouched in discussion that I'm sure you are very knowledgeable of, like camshaft specifications best suitable for the conditions for example, that should go without being said. I just don't see the "cons" list approaching the level of pros. Imagine carrying 20-30% less fuel because of an automotive engine in place of an acft engine. Backup systems work on modern engines to, for those concerned about the sophisticated technology of auto engine management.

Another compelling point would be to estimate how many decades backward one would have to go to reach the equivalent level of engine efficiency and performance in an automobile.

6. Aug 15, 2019

### pictsidhe

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Maticulus. Look up the BSFC of an aircraft engine, then look at the BSFC map of a typical car engine.

7. Aug 15, 2019

### rv6ejguy

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You might want to do more research about aircraft engines before making these statements. With modern EFI and EI systems like we install on Lycoming and Continental engines, they become quite efficient when running Lean of peak (LOP) in cruise, despite relatively low CRs. We see BSFC figures of better than .38 on pretty standard Continentals in cruise and on one Lycoming with high CR pistons, better than .36 which is near light diesel territory. You're saying the 3.6 V6 will do .27 to .30? Never going to happen.

Speculation is easy and we get lots of that here with regard to new engines and airframes from new folks who've never put their theories into practice by actually building them up and flight testing them. That's why I encouraged you to go ahead with your project and show us.

I'm trying to be courteous here but until you've done it, you don't know what you don't know. You've been given some tangible reasons why DI won't make much difference in WOT fuel economy but choose to disregard that information.

I am no big fan of traditional aircraft engines which is why I fly an automotive engine but those old beasts can generally take a pounding and have decent life and good reliability. Yes, they have warts as almost all engines do, but they do the job and do it pretty well.

You're going to learn a lot if you actually go ahead and fit this engine in a plane a fly it. Guaranteed it will be very educational. I'd love to see one of these engines flying successfully in an aircraft.

Last edited: Aug 15, 2019
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8. Aug 15, 2019

### maticulus

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More research on my part is an absolute must without question.

"Imagine carrying 20-30% less fuel because of an automotive engine in place of an acft engine."

That's what I said, being careful not to attribute it to a specific motor. I don't know what the current BSFC figures are for the 3.6L, or its capabilities with the correct manipulation to say what it is and would never be capable of. I am curious about the displacement and resulting compression ratio of the engine you mentioned that netted .36 or better with the high compression pistons.

Seeing that installing higher compression pistons in a Lycoming delivers the kind of improvement that made me question the low compression examples, I would expect to see whatever the current BSFC is for the 3.6L increase above its current rating also, if its compression ratio were increased considerably to take advantage of what 100 CLL would afford it. I am not sure what the results of an automotive engine in flight could accomplish if it were also run under LOP conditions. I do know that DI motors favor lean in that they tend to misfire under rich conditions that port injected and carb motors have no trouble with.

A concern I have encountered in my most recent research is a problem to running without a gear drive, thrust bearing forces on an engine not designed for prop loads.
I'm not sure if it is legal and therefore active now due to technological advances, but early in automotive engine management, a subroutine referred to as "Lean engine run" at light cruise speed was present in programming, but sidelined from use by the EPA although still present in the programming and capable of being activated.

I'd like to view my interest in this subject as more of a quest for knowledge and better understanding in an effort to make a more informed decision, more so than mere speculation.

"I am no big fan of traditional aircraft engines which is why I fly an automotive engine but those old beasts can generally take a pounding and have decent life and good reliability. Yes, they have warts as almost all engines do, but they do the job and do it pretty well."

That fits quite well into my mindset and interest in seeking out something more practical if not also better.

9. Aug 15, 2019

### rv6ejguy

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The low BSFC engine was a Lyc IO-360 angle valve with 12 to 1 pistons. There is a diminishing return on both power and BSFC as CR is raised. HUCR is around 16 to 1 for SI engines and some of the latest Toyota and Mazda economy engines are almost there now, being over 14 stock. If you're already at 11.5 or 12, you'll only net a couple percent lower BSFC going to 14. When you go from 8.5 to 12 you gain a lot more.

You need to understand that ignition timing is retarded considerably on high CR engines running low octane fuel at high MAP. They will detonate otherwise, even with DI. In cars at part throttle, the high CR pays off well to boost thermal efficiency and lower BSFC at these low charge densities because timing can be optimized to maximize cylinder pressure without much chance of detonation.

We fly Lycomings usually at or near WOT for lowest pumping losses. As air density drops with altitude increase, cylinder pressure diminishes. The high CR has the same benefits here. High altitude has a similar effect as part throttle down near sea level in a car.

You'll need a PSRU to use a prop on the 3.6L, both to handle gyroscopic and thrust loads which the crank isn't designed to handle and be able to extract good power from the engine without causing a loss in prop efficiency with too much rpm and high tip speeds.

Most modern auto engines run leaner than stoich under light throttle cruise conditions since the advent of wideband O2 sensors. The EPA doesn't care what you do as long as you meet the emissions specs. Modern catalysts and EGR can still clean up things lean of stoich to pass the regs. Most cars are cruising light load at around 17 to 1 AFR. This is also where we find best economy in aircraft engines running LOP.

Frictional losses increasingly impact BSFC as rpm is raised above about 2000 rpm. It's a fight between these losses and the gains in volumetric efficiency which is highest at torque peak rpm. This is one reason why most auto conversions don't do any better with cruise BSFC than a Lycosaurus which is turning 2200-2500 usually in cruise. The modern auto engine has lots of advances in lowering frictional losses over traditional aircraft engines but loses out with higher frictional losses because of the higher rpms and most 4 valve engines have higher valve train friction than a Lycoming pushrod engine with roller lifters.

Last edited: Aug 16, 2019
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10. Aug 15, 2019

### pictsidhe

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My first girlfirend had an economy car. it had a CR a point higher than the regular model, lost some hp due to the ignition retard necessary to keep it alive, but gained a very decent amount of mpg. A CR increase really boosts part throttle economy. At part throttle, you only get a fraction of the expansion ratio, which is main factor in determining thermal efficiency. Even small increases at low ratios, such as happen at part throttle, give a big bsfc increase. Aircraft engines just don't get used at low throttle, so optimising for those conditions is a waste of time, complexity and weight.

If you'd bothered to look up a BSFC map for a modern car engine, you'll see that the BSFC peaks a little under half the piston speed that an LGX pumps out peak power at. It goes downhill above that. You'll be cruising in a high BSFC part of the map and will be very unlikely to even match aircaft engine BSFC without significant detuning (less revs), let alone beat it by any amount. The data is already out there, you just need to google it. Maybe you will reasses the project then. You should consider listening to the people who have a lot of relevant engineering experience, too. If the pros think this won't work without a lot of work, how likely is it that someone with no experience is to pull it off? Have a look at the Raptor project for a great example of that.

For the record, I'm not going to be using a conventional aviation engine either. But I'll be very pleased if I get close to the BSFC of one. I won't get near the specific power. I am curious about the potential of turbo-diesels, but doubt that they will currently match the specific power of aviation engines without being fragile. strangely enough, the engine manufacturers are actually quite good at building engines for a specific purpose.

If you want an example of a modern aviation engine, have a look at the Rotax 9xx series. The have their pros and cons as well as their fans and detractors compared to lycosaurii.

11. Aug 16, 2019

### BoKu

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Start by looking at your prospective motor's torque curve at 2700 RPM. Because what you see there is pretty much what you get unless you have a PSRU. Down at that sort of RPM, few of the standard hot rodding tricks do much good, except adding lots of cubes.

As I've written time and again, looking at air-cooled direct-drive aircraft engines and thinking that they're dinosaurs because they have such low HP per cubic inch fetishizes a metric that is meaningless in and of itself. As Billski has pointed out here already, the metric that actually has meaning is installed-and-running weight for a dependable and reliable unit that delivers power at an RPM an actual propeller can use. And for a car motor, that means the engine, the gearbox, the cooling system with coolant and all plumbing, etc, etc, assembled with good hardware to aeronautic standards of workship.

Jack Kane wrote probably the best sermons on this topic, disregard them at your peril.

12. Aug 16, 2019

### wsimpso1

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Maticulus,

To the point of your last couple of posts, we have some knowledge to fill in.
• Automotive engine use as an airplane engine usually requires either a PSRU or a prop bearing:
• Prop tip speeds must be kept to no more than about 0.8 Mach. Drag due to sonic effects is huge, so props are generally sized so that the vector sum of speed due to rotation and due to forward speed stays below 0.8 Mach. So direct drive means that you either run in the conventional range for prop size and rpm - and extract much lower power - or you run up into the "good" range for this engine and run really small prop diameter - much smaller static and low speed thrust, poorer cruise efficiencies as you shove a small diameter propwash cylinder to much higher speeds;
• The PSRU is to take power at rpm too high for a practical prop and convert it to higher shaft torque and lower shaft rpm for much better prop results;
• The prop bearing is necessary. Conventional aircraft engines have big thrust elements and a beefy bearing and crank end to carry the very substantial loads imposed by the prop. These loads come from power converted to thrust, P-factor due to the sometimes off axis operation of the prop, and due to gyroscopic moments imposed by spinning the prop while rotating the airplane in pitch and yaw axes. Car engine crankshafts are designed around smaller thrust loads, smaller inertia, and smaller combined yaw/pitch axes rotation rates and usually will fail when a prop is hung on them. PSRU's have prop bearing function built in. And if you go automotive with direct drive, you will have to come up with a coupling and prop bearing set too.
• The specific fuel burn is very pertinent here. Look up the fuel burn maps and note the specific fuel burn of some common auto engines. The plot axes are usually engine torque (or BMEP) and rpm. They typically have the best (lowest) lb/hr/hp at medium torque and rpm and that is called the fuel island.
• For production road vehicles, the operation areas within the fuel economy/emissions cycles are almost entirely at stoich for low emissions. This may seem like rich ground for running LOP for efficiency in airplanes. Maybe it is a zone of opportunity;
• In high torque/high rpm areas, almost all of these engines use mixture enrichment to keep the exhaust valves and piston crowns in the engine. Those areas are outside of fuel economy/emissions cycles. Similarly, spark may be severely retarded in high power regions to prevent knock, preignition, and detonation. Both mixture enrichment and spark retard sharply reduce efficiency, and can be part of why you see big increases in specific fuel use at higher rpm and torque. These areas may be particularly bad places to run LOP;
• You will be hard pressed to run much of any airplane's duty cycle near the fuel island in anything but descent power settings or very near Vy. It sounds nice to run at very fuel efficient settings. Do you drive 50 mph on the highway to conserve fuel? Neither do we, and we fly at higher cruise speeds than the best economy speed too.
Billski

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13. Aug 16, 2019

### Winginitt

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I think something that should be noted here is that 2700 rpms is not something defined by physics as being the absolute best rpm for propeller operation, but simply the optimum operating rpm for the design of current common certified aircraft engines. Based on the physical limitations due to the size and weight of the rotating assembly, the engines produce a sufficient amount of power and are very reliable when confined to that rpm. Then propellors are designed to provide the best efficiency at that rpm. The engines can be operated at higher rpms but generally at a loss in reliability, or somewhat lower rpms with a concurrent loss of power. So, 2700 is not a magic number. Within reason, if an engine can produce sufficiently more HP for its weight when operating at a higher rpm, a prop designed to work with that rpm could provide an improved result. In other words , a Continental GO-300 only provides a net increase of 30HP by operating at 3200 rpms (175hp). If someone forgoes the added weight and frictional loss in a reduction drive and produces an engine capable of even more HP ( say 200/225).....then optimizing a propeller for that hp/rpm might be a better combination. If that person can build that same engine to provide even 175 HP at the proverbial 2700 rpms, then they have achieved quite a lot.
Engines aren't built to meet propeller size limitations, the propellors are built to optimize an engines characteristics. The Curtiss 0X operates at 1400 rpms and the Rolls Royce Hawk operates at 1350 rpms. Reno racers push their engines to higher rpms effectively.
So I think one premise that should be dealt with is that the most effective operating rpm for all engines doesn't have to be 2700.

14. Aug 16, 2019

### pfarber

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So a highly modified engine? How long did it last? I'm a firm believer of EFI/FI/Higher compression to make certified engines make more power, but also concerned about popping a cylinder or simply melting it.

The only hard limit for RPMs would be at the prop tip.

PSRUs have gotten a bad name for numerous reasons. But certified engines snap cranks, blow jugs and do all sort of things that you'd never expect a certified aircraft (or heck, any $45k engine) to do. Read up on some airboat forums. Blowing PSRUs is not that common. If you're concerned about PSRUs, then why don't you care about all the crap that goes wrong with certified engines? As for BSFC.. it should not be a primary concern if you're looking at auto conversions. From what I can deduce, the major reasons for an auto conversion (not an overpriced FWF package, but a true E/AB install) Lower cost to install ($10k-$20k less for used,$30-40k less for new)
Lower maintenance costs per hour
Lower rebuild ($1000-ish vs$10k-$20k+) Higher performance Just the savings on ONE rebuild would more than cover and fuel use/cost for the TBO interval. Yes, you could argue that an auto engine may not be a efficient as a tuned up/tricked out certified motor, but not your talking certified motor costs PLUS mods. The PPlonk upgrades to the O-470 for example. But the fact is a lot of airplanes never get built, or finished, because the money needed forward of the firewall. 15. Aug 16, 2019 ### BJC ### BJC #### Well-Known Member Joined: Oct 7, 2013 Messages: 9,289 Likes Received: 6,059 Location: 97FL, Florida, USA That may be true, but a used Lycoming can be purchased, rebuilt, installed and flown for more hours than most HBA amass over 50 years for no more than the cost of an equivalent HP auto engine / PSRU / liquid cooling system. BJC akwrencher, wsimpso1 and BoKu like this. 16. Aug 16, 2019 ### pictsidhe ### pictsidhe #### Well-Known Member Joined: Jul 15, 2014 Messages: 6,399 Likes Received: 1,660 Location: North Carolina For each flight speed and hp, there is a range of optimum prop diameters. Lycontisaurs run at suitable RPM to drive prop sizes typical of private aircraft ata psiton speed that minimises BSFC. They were designed that way. The physics that those designs are based on has not changed. That is why aircraft are still using an old design. It was done well way back when. Car engines are not optimised for aircraft direct drive useage. You can get a car engine to efficiently power an aircraft, but nowhere near max power RPM or generally direct drive. Yes, EFI is a modern improvement that can be applied. But VVT and DI do not give worthwhile gains and have complexity, weight and reliability issues compared to simpler engines. 4 valve heads are good for high piston speeds, way above the economical range. OHV 2 valves breathe fine at economical piston speeds and are lighter 17. Aug 17, 2019 ### Hot Wings ### Hot Wings #### Well-Known MemberHBA Supporter Joined: Nov 14, 2009 Messages: 6,198 Likes Received: 2,168 Location: Rocky Mountains And we even have one current production flat head that seems to work. A 4 valve push rod engine might be worth the trouble? VW's water-cooled 4 valve had much better low end torque than the equivalent 2 valve it replaced. Restating the rest of your post: Making power anyway you can is a lot different than making reliable power for direct drive. 18. Aug 17, 2019 ### Winginitt ### Winginitt #### Well-Known Member Joined: Apr 5, 2019 Messages: 74 Likes Received: 18 I think that's an oversimplification. There are a lot of ancillary factors that come into play when making that kind of comparison.There were a lot of auto conversions in the 90s and early 00s that were cheap to build and assemble and lots of belt driven redrives were used. Much of the comparison depends on the abilities or lack of ability of each home builder. Once a reasonably able builder assembles a proven ( or well constructed) conversion with a little grease under their fingernails along the way, the cost of flying and maintaining will most likely be far less than the cost to maintain a "formerly certified" engine. The builder who chooses to install a used certified will normally be someone uncomfortable with building their own engine. They will need advice ($) and inspection($) before buying or using an engine. They will need someone to actually perform that rebuild($) and they will need parts($). If they don't fly a lot, they will need more parts. Even though it is/isn't flown it will need inspections ($) even though they aren't required. To think that inspections aren't still needed even though it's in an experimental is a mistake. The cost for oil,sparkplugs,gas,and magneto's will be more. There are many things which contribute to the cost of buying,owning,and flying a formerly certified engine. That being said, there are many builders who want a plug and play conversion and end up spending an exorbitant amount of money. Every time they need anything done, they will pony up ($) just like the guys who know little or nothing about their LyCons. There have been many successful and reasonably inexpensively conversions of Subarus,V6 Chevy's,Buicks,Ford's over the years. They were cost effective conversions. Names like Jess Myers, Been Hass, Ross Farnham, quickly come to mind as cutting edge developers who have proven conversions are viable and affordable. Again, the oversimplification is that you have to consider not only the abilities of a builder, but their level of interest and commitment. 19. Aug 17, 2019 ### maticulus ### maticulus #### Member Joined: Aug 11, 2019 Messages: 6 Likes Received: 1 Thanks again for the input/good reads. I'm traveling at the moment, but quite intrigued at the increasing depth of knowledge that is resulting. I'd like to reiterate that I'm taking into consideration many more parameters than actually being mentioned given that a good bit should go without saying. I have no issues with the dedicated acft engines aside from the static state of improvement in performance efficiency, no doubt largely the result of economics related to any change and subsequent re certification, as the cost is not only born by the manufacturer, but also the customer in what is already at the extreme end of "pay to play". Keeping a dynamic approach in mind is important in that to get the most out of an automotive motor, it has to be made more like the acft engine barring efficiency short comings. That's an easy path for auto engines, because of interchangeability of parts between existing versions of engine lines, and availability of performance enhancing upgrades. My fixation on compression ratio increase, is because it increases hp, torque and efficiency over the entire rpm range with little to no increase in fuel cost, depending on the goal point relative to the fuel grade and whether it needs to change. The most recent info I have found suggests that the current BSFC of the 3.6L is very good by approximation relative to production motors I could actually find a BSFC rating for and that bumping the compression ratio would make it even better although returns are diminishing at the upper levels of possible compression ratio that can be achieved. Keeping in focus the effort to maximize existing efficiency in the auto motor, I understand that the apparent possibility of sub .40 BSFC under normal operating, un manipulated (leaning efforts) engine conditions, would have to be normalized to acft engine and flight related conditions for the net effect of the initial gains. Understandably, road BSFC does not directly translate into flight BSFC. I've consumed quite a bit of energy trying to normalize data I've encountered due to different units, while trying to get a better picture of the likely results of my compression ratio goals. This "wicked pedia" link lists some BSFC rates for several engine examples with Rotax and Lycoming on the list with some automotive engines that run typical stock compression ratios well below the goal I have in mind. There appears to be a lot going on to determine such values given the wide range of displacements relative to the values. https://en.wikipedia.org/wiki/Brake-specific_fuel_consumption 20. Aug 17, 2019 ### pfarber ### pfarber #### Well-Known Member Joined: Feb 21, 2019 Messages: 191 Likes Received: 9 Location: Pennsylvania My search has not show this to be true. A used O-360 with about 300-500 hours on it is$10k-$20k depending on model and hours. And if not flown, that motor will degrade quickly, so it would most likely not reach 2000 hours before needing a$20k field rebuild. So you could say a valid 10 year budget on a used certified motor is $30k-40k. Look up the cost of the mandatory replacement items for an O-360. That's$5k-ish just to crack the case.

Is ebay full of $5k cores? Sure. That prop strike O-360 that 'dialed in' at .004 might seem like a good deal, but you still need to crack it open and check all those parts. No, there is no mandatory prop strike inspection (Service bulletins are not mandatory), but you really have to push the limits of sanity to run a prop strike motor, off ebay, without a thorough inspection. So now you have a prop-strike motor, inspected for$10k. Is there any doubt that auto conversion can be done for less than $10k? I think there is no question that it can be. Unless you go with a FWF package or 'crate' motor, you are never going to be close to that with a auto conversion. Are there$20k+ PSRUs? Yes. Are they better than the $5k Stingers? Well, there are vastly more stinger units in operation than high end units. Also even though that O-360 is in a E/AB, you still need to comply with AD's if issues. More cost. Auto conversions are like real estate. The money is made at the buy. You knock$20k-40k off your E/AB with a car motor if you do it yourself. Now if you drop $60k on a kit, then maybe$20k is not a huge stretch. My BD-4B cost $3k, and will fly for less than$10k total.

The only downsides to an auto conversion: more work on your part, 10-20% more weight, but usually 25-50% more horsepower. Even if you burn 100LL not mogas, you're still SIGNIFICANTLY ahead of the game as your rebuild is going to be less than $2k and you already saved$20k.

The days of needing an all aluminum custom race block/heads is long gone. There are so many all aluminum, high HP motors for less than \$2k.