4 stroke to 2 stroke

[smartie49]

Is the a ratio between the two i.e.2.5 cc to 3cc 2stroke.What size 4 stroke would be matched?

[Plummet]

The answer is not so simple. The 4 stroke engine will be bigger and heavier than the 2 stroke equivalent - so just looking for the same horsepower is not quite right. But then, a 4 stroke is more economical than the 2 stroke, so a smaller tank might be adequate.

That said, I would suggest that a trainer using a 40 or a 46 2 stroke might find a 52 4 stroke adequate.

And they sound a lot better.

Plummet

[Bob Cotsford]

In the past I worked on the basis that a 4 stroke needed to be half as big again as the suggested 2 stroke, ie a .60 4 str to replace a .40 2 str or a .90 4str for a .60 2 str.

[Braddock, VC]

It all depends on which 2 stroke you're talking about. Contrary to what's been said before 4 stroke equivalents are normally lighter than the two stroke as they don't have to tote around a silencer, so the average 46 2 s weighs about 500 or so grammes and the average 52 4 stroke is about 50 grammes lighter when each has their muffler fitted.

This is comparing an asp 46 2s with an asp 52 4s, which are just about equal power-wise when kitted out with a suitable prop that is quiet enough not to lose sites. This is generally because the 4s will turn a larger, more efficient prop than the two stroke and develops more thrust, as i said in general.

Later 2s like the OS AX series are ported such that the difference isn't so marked ie the 46 can turn a similar sized prop to the 4s so they only weigh more and use a bit more fuel but up against the newer 4 stroke models from OS the gap has once again widened so the newer 4 s turn more revs on siimilar sized props like 12x7 and 13x6.

2 strokes tend to be cheaper and when fitted with a prop that allows them to rev can produce more thrust than a 4 stroke but then use considerably more fuel and are markedly noisier.

In practice the only place where you will see a difference is in vertical climb. My wot4 with an OS 46 fx would definitely outclimb itself compared to when fitted with an OS 52 fs but when fitted with a saito 62 the performance was similar. When fitted with a saito 82 the performance greatly surpassed the 46 Fx.

My acrowot when fitted with a YS 45 2 stroke with throttle pipe greatly out performed the same bird when fitted with a saito 82. The ys was very much louder though and about 115 grammes heavier

Both the two strokes were fitted with either 12x6 or 12x5 graupners and the os 52 and saito 62 had graupner 13x6 the 82s had JXF 14x6 props.

Going down to the size mentioned in the OP, my OS 26 FS surpass is out performed, in the same airframe, by an OS 25 FP. Similar props too, 9x5. Plane is a junior 60.

Going up a size, my OS 91 FX 2 s turns a JXF 16x6 prop at about 600 rpm more than my Saito 125 4s, surprisingly the 125 has better vertical performance than the 91 and I can only guess that it's because the torque output of the 4 s is less peaky than the two stroke so it maintains those revs better as the load increases. Also judging how far up the planes have flown is purely a subjective estimate anyway. Plane was a H9 Pulse xt 60.

All the engines are operated on the same 10% nitro fuel.

Don't know if that helps.......

Edited By John Gibbs on 19/02/2012 23:22:33

Edited By John Gibbs on 19/02/2012 23:25:34

[Steve Hargreaves - Moderator]

I use Bob C's method....half as much again....replace a 60 2 stroke with a 90 4 stroke.....

But as the other guys say there is no simple answer....4 strokes will usually swing a bigger prop than the equivalent 2 stroke.

Personally I try & avoid the issue by always fitting a 4 stroke.......

[Biggles' Elder Brother - Moderator]

100% agree with Steve on this one - on all points!

BEB

[..]

I am looking to maybe fit a 4 stroke into my 46-55 size seagull Edge 540 . Reading on here and other forums it appears to fly well with a 60 size 2 stroke.

What would be the rite 4 stroke to fit in this airframe?

Any thoughts ?

Edited By Justin K on 17/03/2012 19:06:36

[Steve Hargreaves - Moderator]

Justin, Seagull do have a habit of underspeccing the engine requirements for their models. Given that its a 46 size model but flies well on a 60 then maybe a 90 4stroke would be good if you have room to fit a 15" prop. Might be a bit overkill though....if you do fit a 90 I might be tempted to beef up the fuselage to firewall joint slightly with some triangular section fillets

If it helps I have a CMpro Yak 54 which should take a 46 2 stroke & this flies very nicely on an ASP70....certainly not overpowered...sort of scale power if you follow me!!

[..]

Thanks Steve, I have an OS 91 fs that was destined for another project and a 70 fs. The 91 is not to much bigger than the 70 .
I have read that people who have fitted a 70 have found it underpowered in this airframe so maybe I will wait till its assembled and offer the 91 up to see whas happens balance wise.
Or then again I could just fit the two stroke I bought to go with it in the first place.

[john melia 1]

Reviving this thread a little ... I have just purchased an ultimate .40 bipe from HK , and the recommended engine is a .40 . The wingspan is 43 inches and I thought with it being a biplane it would produce more drag , and would require a bigger engine . I'm looking for vertical performance , and the ability to prop hang with this model , has anyone got any recommendations regarding engine size , both 2 stroke and 4 stroke for this model , or would the .40 be enough

[cymaz]

I would put a 46ax in the bipe....here's why.......

[john melia 1]

Thanks cymaz

[john melia 1]

Was also thinking of an asp/sc 52 ?

[cymaz]

Don't forget the usual artf improvement, beefing up the firewall plate with epoxy and tri-stock in the corners and possibly an extra u/c plate and bracing.

[cymaz]

Good choice too, got one in a wot4, forgot about that one

[Glyn R]

I recently replaced my Irvine 53 2 stroke by an ASP 61FS. 4 stroke, in a Wot4. The Irvine is much more powerful. I would now reckon to use a 91 four stroke for equivalent power. However there is the weight problem, almost 50% more. The ASP is fine and with the right prop would come close, starts first time and idles forever, in another model it will be perfect. I think prop pitch is an issue I need to look at.

[J D 8]

Agree with Steve about seagull being conservative with engine sizes,my 40 size SNJ much better with a 60 4 stroke.

[iqon]

Posted by Justin K on 20/03/2012 13:43:18:
Thanks Steve, I have an OS 91 fs that was destined for another project and a 70 fs. The 91 is not to much bigger than the 70 .
I have read that people who have fitted a 70 have found it underpowered in this airframe so maybe I will wait till its assembled and offer the 91 up to see whas happens balance wise.
Or then again I could just fit the two stroke I bought to go with it in the first place.

70fs should be plenty big enough to have some fun.

[..]

Posted by iqon on 18/03/2014 15:54:59:

Posted by Justin K on 20/03/2012 13:43:18:
Thanks Steve, I have an OS 91 fs that was destined for another project and a 70 fs. The 91 is not to much bigger than the 70 .
I have read that people who have fitted a 70 have found it underpowered in this airframe so maybe I will wait till its assembled and offer the 91 up to see whas happens balance wise.
Or then again I could just fit the two stroke I bought to go with it in the first place.

70fs should be plenty big enough to have some fun.

Its almost 2 years to the day the day since I posted that that question and after many flights with it I'm afraid to say a 70fs wouldn't be enough. I have flown it on an Irvine 53 and had some fun but the airframe really would be best suited to a 60 size two stroke or an 80 four stroke .

[Shaunie]

Justin,

I have the 68" Seagull Edge 540 with an RCV91 in it, 9lbs AUW dry. Definitely needs more power, even after I fettled the inlet manifold which gained 200rpm. I know the RCV is not the most powerful in class but basically I end up with the throttle at the top of the box all the time.

Shaunie.

[..]

Posted by Shaunie on 18/03/2014 18:53:57:

Justin,

I have the 68" Seagull Edge 540 with an RCV91 in it, 9lbs AUW dry. Definitely needs more power, even after I fettled the inlet manifold which gained 200rpm. I know the RCV is not the most powerful in class but basically I end up with the throttle at the top of the box all the time.

Shaunie.

Its the smaller one I have Shaunie, but I also have theSeagull Extra 300s and that goes well will an OS91 fs.

Your the only bloke Iv'e come across who's got one of them RCV's running

[Steve Hargreaves - Moderator]

I have a 42" Pitts Special & that flies very nicely on an ASP61 4 stroke (12x6 prop)......it certainly won't prop hang nor go unlimited vertical but the power is adequate for "Scale" aerobatics......worth remembering that the ASP61 4 stroke is actually a bored out 52 so the crankcase is "52" sized.

An ASP52 would make it pretty lively I reckon John......try an 11x7 on it & remember you CAN close the throttle if it gets too exciting...

[john melia 1]

Thanks steve , I like to ride on the wild side , pedal to the metal lol , nah I do close the throttle a bit when I'm landing still would have preferred a fourstroke , but I've just been on hobbyking looking for a .70 and theres none to be had , and I'm not paying the price thats been asked for the same engine in model shops around the uk . I bought a .91 fourstroke off HK cheaper than the uk shops are selling a .70 .

No wonder these shops are closing down .

[Steve Hargreaves - Moderator]

Well I know what you mean John but its not really a level playing field in that the shops have to pay UK rates & taxes so they are bound to be more expensive than a huge warehouse in Hong Kong....

Kings Lynn models sell the SC70 (same as ASP) for £120 so not a bad price. I do think the 70 might be a bit...."chunky"...for that airframe though......they are quite a large engine...not much smaller than a 90. I'd certainly look at beefing up the firewall if you fitted one......its a big chunk of metal!!!

[john melia 1]

The .61 fourstroke asp on HK works out at £75 pity it wouldnt be powerful enough . And I think you may be right steve , the airframe is VERY light , so I think its gonna have to be the 52 two stroke , yet another screamer , oh well wont be as bad as that crazy delta I have with the prop running in a slot