How much of a difference is there really between .40 and .46 size? - RC Groups
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Sep 01, 2009, 04:35 AM
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planebldr's Avatar

How much of a difference is there really between .40 and .46 size?

Ok, yet another newbie question here. I'm making the transition from electric to glow...and am looking at a aircraft that suggests a .40-.46 size engine. Now, given my financial status, every dollar counts. That being said, most engines that I find, the .40 costs less than the .46 (no real suprise really there). Now, the aircraft I am looking at is a T-6 Texan, with a wingspan of around 60 inches, and has a flying weight of 6.4lbs, and wing loading of 22.6 oz/sqft. Would there really be much that much of a difference between a .40 and .46 on this airplane? My flying style is more that of a Sunday flyer...nice circuits with a few loops and rolls here and there. Nothing wild and crazy. Thanks in advance for the help!
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Sep 01, 2009, 05:29 AM
Bombs away! Err...landing
Ira NZ's Avatar
It's only going to be about $10 difference between them, .46 is pretty much the standard small/mid engine size, there aren't many 40s left. But with something that heavy the xtra little power will be good.
Sep 01, 2009, 05:42 AM
Will fly for food
15% displacement difference.
Sep 01, 2009, 09:20 AM
Registered User
I think there would be a noticeable difference between a 40 and a 46. If you want good power cheap, get the gms .47. Once you seal the carb up with silicone it will run great. I think it is only like $60 in the US.
Sep 01, 2009, 10:14 AM
Chief Mechanic for my son
The bigger difference is between a bushinged engine and a double ball bearing engine. Make sure you consider this in your price/performance comparisons.

Sep 01, 2009, 10:23 AM
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earlwb's Avatar
Well it depends. if a .40 is merely bored out to a .46, you will likely only see a 200 to 300 rpm increase in power.

But in the vast majority of cases a .46 is designed to use a larger crankcase, a larger carb, and more radical port timing as well. So the .46 develops a lot more power than a .40 would. Something like a 1,200 to over 2,500 improvement in RPMs. Now sometimes the .46 large case engine might have a longer piston stroke than the 40 does as well, as the larger case allows for a larger crankshaft, so they can increase the stroke easily. I see they have bored out some of these engines to .52 and even .55 size now as well.

So then we see two types of .40 class engines, the small crankcase version and a large crankcase version. Plus some .40's are entry level, easy to use engines, with a bushing supported crankshaft. other 40's have larger crankshafts supported by ball bearings. So one needs to determine which type of engine they are trying to compare to.
Sep 01, 2009, 10:35 AM
Registered User
The .46 is considerably more powerful than a .40. And an engine with a crankshaft supported by ball bearings will turn a little faster. With an airplane is heavy as the one you are considering I would really go for a Super Tigre .51 or a Magnum .52 since price is of concern. Those engines are not pricey as an OS.

But there is no doubt that you should shy away from a .40. And they are hard to find now days new.


Sep 01, 2009, 11:03 AM
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planebldr's Avatar
Thanks for the tips....I was leaning towards the .46 or larger...but now I am certain.
Sep 01, 2009, 02:40 PM
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For a Sunday flier (what ever that is) give the ST 51 a good hard look!
Friends don't let friends fly nickel,
Sep 01, 2009, 06:12 PM
Larry Jasmann
Aside from the technical stuff... more power is better. You can always cut the throttle back, but in a tight spot when you need the extra power, it is either there or it isn't. The "little bit extra" has saved my planes many times.

L. Jasmann
Sep 01, 2009, 07:10 PM
Registered User
I agree with Chip...I have also had very good luck with Sig Aviastar .46 (takes a while to break-in and needs tuning) and the magnum .46 (also needs break in and tuning). I think that I paid $50 for the SIG and it really runs well (after half a gallon of break in fuel!). Any of these motors run as well as my OS 46AX but the OS is a bit more plug and play.
Sep 01, 2009, 07:57 PM
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I TOBOR's Avatar
One thought that got missed here is that when all things are equal except one engine has ball bearings and the other has bushings they will have identical power, same as when ringed or ABC there will be no difference.

Konrad, I missed your post, the SuperTigre .51 seems to be a well kept secret for some reason, I have two, an ancient one converted to Diesel and a new one from the Chinese factory and both sweethearts. The Diesel turns a 12x6 three blade at 10,100 no muss no fuss.
Sep 01, 2009, 09:52 PM
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Originally Posted by I TOBOR
One thought that got missed here is that when all things are equal except one engine has ball bearings and the other has bushings they will have identical power, same as when ringed or ABC there will be no difference.
At sport power levels ABC piston and sleeves might not show much of a power level advantage.
But with the ABC engine the cylinder ports are more free flowing as there isn't as much webbing to support the ring as it passes the port. Also there is less frictional losses as there is no ring drag. Both the flow and frictional loss don't become advantageous until higher RPMs or loads are reached.
I have been known to change out the ABC P&L in my AMA Turnaround engines (Webra long strokes) to rings for my practice ships. And only used the ABC liner in competitions. These were 10k to 11K rpm engines. I have to admit I would have to look real close to see any RPM boost in that band with the ABC P&Ls. I seemed to have better pipe transition on the ABC as I think I could run the engine a little leaner before they started to sag (loose rpm).
So the short of it is Yes ABC is better than the ringed engine. Would the sport flier see it? That is open to debate. One could make an argument that the ringed engine is better from a cost stand point.

Friends don't let friends fly nickel,

I think Chip01 pointed out the ST51 first. I was just piling on
Sep 02, 2009, 01:34 AM
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planebldr's Avatar
Well, now I have a new development.....I went to the club today and a fellow I purchased an Extra 300 from has a 4 stroke .70 size for sale. He said it was barely used, and would sell it to me for $50 (not sure of make yet). So I am going to take a look at it next time we are both out at the field. I would tend to believe this guy because he is very meticulous. The plane I purchased from him was mint, and well cared for. So, now 4 stroke or 2 stroke.....great more questions/decisions.......
Sep 02, 2009, 05:21 AM
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RocketMan09's Avatar
ST suggest a 10x6 on the 51...seriously? At what, 15,000 RPM? What prop does it realistically use?

The only concerns I can see with the 4 stroke are: it being used, maybe too heavy, and prop clearance.

However, it would sound cool, uhhh...and, it would be quieter.

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