longdan

I may be reviving an old subject, but I looked and couldn't find much info on this.
Many engine manufacturers produce 2 engines with the same crankcase size, like .20 & .25. 40 & .46, .61 & .75 etc.
Often the larger displacement engines weigh less than then the smaller displacement ones due to there being less metal and more volume.
What I'm wondering about is, do the larger displacement engines supercede the smaller displacement ones?
Why would I by, say, an OS FX 40 rather than an FX.46.
I own a GMS .61, which has the same outside dimensions as a GMS .75. Should I be feeling that I own a big old clunker?

Mr67Stang

Yes.

Could not help myself

earlwb

No you shouldn't feel that way. The engine manufacturers tend to do things like increase the engine displacement a little as a marketing ploy to get you to buy newer, the latest and greatest and so on. Now if your plane is flying great with your current engine, you do not need to "upgrade" just because they came out with a newer, bigger engine. Now if you were in the market for a new engine for a plane, then yes the latest and greatest may be the way to go. Unless they have a good sale on the older model going on though.

I am still using and flying a engine on a plane made back in the late 1960's. It still runs good and pulls the plane around just fine. So since it hasn't worn out why replace it, if I don't need to. Yeah I could have put in a new fancy modern Schnuerle ported wonder engine, but it wouldn't gain me anything except making my billfold more light.

spaceworm

Super Tigre (Como) produced a .60 a .75 and a .90 in the same basic case by adding a little metal on the bottom of the case to clear the longer rod(s) and then boring the cylinder and thinning the sleeve to accommodate the larger bore pistons. Evolution increased the .46NT to make the .60 NX. I am guessing that most of the .50 and .55 engines were once .46 size and the .46 likely .40. Bigger is better, right? My facts on some of these may be wrong, but of good intention.

longdan

But what about all those poor people that purchased the engines at the bottom end of the scale, only to find that another engine is soon available which is a direct drop in replacement, and most spares are interchangeable.

When I bought my .61, it was 2nd hand and both (61 & 75) engines were and still are available. I'm just wondering if I would have been better off paying the extra 20 bucks and getting a .75. And does anyone actually bother to buy the smaller displacement engies.

sandal

I'm one of those few (?) who like the smaller displacement engines. If there is a 40 and a 46, I would usually buy the 40. There might not be any difference, but I have a feeling that the thicker metal in the smaller engine will provide better cooling and a more "stable environment". In addition, it produces less power, but has the same crankshaft and bearings, so it will most likely last longer. It would be great if someone with a little more knowledge than me could comment on this. I also have to say that I usually fly on the wing, I'm not into 3D and hanging by the prop. Several of my planes have/had smaller engines than "required". An Irvine 25 on a 40-size trainer was fun, OS 46LA or FS-52S on the 60-90 size BTE Flyin' King works great etc...

Truckracer

My Feedback: (19)

Its really not as simple as 2 or more engine sizes that share the same basic case where more displacement is better. For example, The ST GS40 loves higher RPMs and really comes alive when allowed to turn up .... but the 45 in the same case is more of a torque engine and likes larger props turning slower than one might expect. Both are great engines but with very different personalities. The same situation exists comparing the ST 61, 75 and 91 within the same basic case size. The 61 and 75 thrive at higher RPMs where the 91 likes a fairly heavy prop load. Same again for some of the OS 40 / 46 engines and a bunch of other engines that share the same case family ... I just used ST as an example because they are still being produced. The cylinder port timing can vary considerably between these engines making them have very different personalities. One engine in the family may be considered a "performance" engine where another might be just a "sport" engine. You have to evaluate every engine for its performance potential and whether it fits your application? Maybe the larger prop required for the larger displacement engine won't allow for enough ground clearance if mounted in a plane designed around the smaller engine. You get the picture here ..... larger is not always better. If a high performance, smooth running, well mannered .40 has more than enough performance in a plane why would you want to put a 45, 46 or 50 in there. Conversely, maybe the larger engine is the better choice and is the better engine .... you get the idea here, choose the engine for the application not by its size.

Then there is the matter of vibration. Larger displacement engines almost always have more vibration than smaller ones and I have seen some large displacement engines in small cases get the reputation for being "shakers"! Again, not always but its still something to be aware of. Maybe the larger engine is smoother .... it happens from time to time.

Because this is a "gas engine" forum, an example of two gas engines that share a very similar size case size and weight (not identical) are the Zenoah G45 and G62. The 45 weighs almost the same as the 62 and the physical size is very similar. Why would anyone want a G45 if they are so similar in all respects including price? Well for one, even though the 62 is a great engine and far more powerful, it is a bit of a shaker compared to the 45. Add to this the 45 is an extremely well mannered engine, a real peach of an engine in my book. If the 45 has enough power for the application, I would choose it every time over the 62 because it is so well mannered and smooth running...... and yes, the 62 is a great engine any way you rate it. Again pick the engine size for the application not because it has more power.

captinjohn

My SuperTiger .75 would kick butt on my friends SuperTiger .90...be both had the same airplane. Capt,n

Truckracer

My Feedback: (19)

I had the same experience ... and the old round head ST 75 would run away and hide from the newer versions. All pretty good engines though.

w8ye

My Feedback: (16)

The "G" series super Tigers have smaller port passages than the old "S" series