My Head is Spinning! (Long)

[David Lockhart]

A lot of good points and discussion being made in this thread - and nice to see no "feuding" amongst brands.  It truly is nice that options are out there.

A couple more items to consider -

Initial price vs. secondary costs -
- fuel - the average 2C is running on lower nitro than the average 4C (do the math based on the number of times you fly in a year).
- vibration - the average 4C vibrates more than the average 2C (makes your own estimates for airplane longevity, servo gears, servo pots, etc - and don't forget to include your time for completing the maintenance).
- maintenance cost of engine - parts  is parts, they all wear, and all cost money to replace/fix.  4Cs have more parts.  Again, based on the number of flights you fly in a year, add up the cost for parts and the time to make the repairs.
- Use the number of posts on this list as resource to gauge what engines need more repairs.
- when I do the math, the 2C wins.

Throttle linearity and consistency -
- In my opinion, throttle linearity is far over-rated - everyone has there own idea on what a linear throttle is - is 50% stick = 50% RPM and 90% stick = 90% RPM?  Linear to one person is not linear to another - which is why you see many identical setups using different throttle curves in the hands of different pilots.
- Throttle consistency is what we really care about - if the throttle is consistent and predictable, it is easy to fly - and if it doesn't feel linear, the "curve" can be adjusted to make it feel "more linear" - the curve can be adjusted mechanically or with the radio.
- The other thing we care about is that the throttle is symmetrical and without hysteresis - meaning throttling up to 50% stick will get you the same RPM as throttling down to 50%.  The biggest instigator of hysteresis is a poorly setup piped 2C (but other things on both 2Cs and 4Cs can and do cause hysteresis).
- I've not had a problem getting 2Cs or 4Cs to run consistently, so for me, this is a wash.

Influence of exhaust system on engine
- We all know the exhaust system is primarily to quiet the engine, and secondarily to not detract from the power the engine makes (or  maybe increase the power the engine makes).
- How the exhaust system effects a 2C is very different than a 4C - primarily because a 4C has dedicated intake and exhaust phases, while a 2C shares them (gross simplification).  Keeping things simple, in a 4C, the best the exhaust system can do is scavenge the exhaust gases from the combustion chamber and efficiently exhaust them out the tailpipe, and depending on the design of the exhaust system, the torque curve might be enhanced in some RPM ranges to a small extent.  In a 2C, the exhaust system can have a far greater influence on how the engine runs - it can not only scavenge exhaust gases, but it can increase the volume/density of the intake charge, AND it can affect the pressure (or vacuum) at the carb itself.  And with the 2C, the specific design of the exhaust system can have a huge effect on the torque curve, peak power, and "linearity" of the throttle.
- If you understand the above, you will also understand that 2Cs do not suffer from lack of midrange torque and do not suffer from poor throttle response - a poorly setup 2C may lack midrange torque and throttle response, but so will a poorly setup 4C.

If you think your engine has a linear throttle AND consistent throttle, and you aren't using a throttle curve, an EFI, or an MC carb (or the OS equivalent carb), then you don't need a throttle curve, an EFI, or an MC carb (or the OS equivalent carb).  If you don't think digital servos work better than analog, you don't need digital servos either.  Point being, people (myself included) often don't realize the flaws in a current system until they try a "better" one - nothing more than adaptive human nature at work.

Many of the 2Cs available today and in recent years have been labeled as poorly carbureted - because the carb appeared to have rich/lean spots, flat spots, hesitations, etc.  The majority of the time, it is not the carb that has the problem - it is the fault of a poorly setup or designed exhaust system (or one that is designed for max power, not throttle response).  Because it is possible for the exhaust system to influence the pressure (vacuum) at the carb inlet, it can also affect mixture at the carb.  So, if your 2C carb has rich/lean spots, there are 2 ways to fix it - re-tune the exhaust system, OR, adjust the carb mixture at that throttle setting.  Fixing with the exhaust system is where the whole art of 2C tuning comes into play - Odds are, if you copy the setup (prop, plug, fuel, pipe, tune length, header) recommended by the manufacturer, you won't have any carb issues - the manufacturer has already done the 2C tuning so you don't have too.

At this point, I haven't been brand specific.........
- The most linear consistent throttling engine I have ever run was a prototype KB150 using a Bolly 590 muffler - 20% nitro and NO PUMP - just muffler pressure (yes, I did have to change the jetting in the carb several times to get it perfect, but the production version wouldn't need that tweaking).  A shame the engine never made production.
- I've run and seen countless examples of Bully145s, Webra145s, OS140RXs, OS160FXs, OS140/160 EFIs, Mintor 140/170s, and most of the YS 4Cs that all had very consistent throttle response using a variety of exhaust systems and fuel.
- I've also run and seen all of the above engines running poorly with a poorly setup exhaust system.

Why use an EFI?  Why use an MC?  Because it allows the carburetion to be matched to a greater variety of exhaust setups - and it does increase the linearity and consistency compared to the majority of setups being used (but you might not believe it until you try it, and maybe the amount of difference won't convince you to change).  The OS EFI system allows the mixture to be set at 3 points - idle, midrange, and high speed - and then it self regulates the mixture (to an extent, only effectively within a limited regime).  The MC (and OS equivalent carb) allows the setup to be adjusted just about anywhere (8 points is easy with a JR 10X, I can't speak for other TXs, but figure they are close to the same).  And as has been pointed out, both systems have some advantages so far as no needle valves.

A 140 EFI is $1,000.00 and while the mechanicals are very durable (with the SS rear bearing), the injectors can be a problem.

A 160 MC is under $500 and should be very durable (based on the track record of the Webra145s).

Regards,

Dave Lockhart
DaveL322 at comcast.net
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