Okay thought I'd chuck this one into the mix. I've recently completed and flown a H9 Miss America Mustang and lost count of how many people were surprised I'd fitted a two stroke ( OS 91 FX) instead of a four stroke.  I also had many comments along the line of "well, it shouldn't go too fast as its a war bird" Are we losing the plot here? WWII war birds were the peak of piston engined planes and out performed the early jets. The V12 Merlins and Griffons were using superchargers and intercoolers and were way beyond the power output of any similar car engines at the time. These warbirds are now invaluable classics and so are nursed through modern air displays to conserve the engine and airframe, but I clearly remember the sight and sound of Rolls Royce's own Spitfire being hammered around the sky over Hucknall in the late seventies and early eighties.  That Griffon howled and screamed and was just as chilling as the Vulcan's howl.

So in model terms where does a four stroke stand? In my opinion unless you are building a model of an under powered one cylinder plane, in the bin! A real Merlin makes 12 bangs per revolution. The red line for a WWII Merlin is 2,700 rpm. That equates to 32,400 bangs per minute. My two stroke 91 only does 16,000 rpm which at one bang per minute is 16,000 bangs per minute, half of what a merlin does. So a two stroke weighs less, has more power and is closer to scale than any four stroke. However for a Reno racer they over tune the engines and run at 3,600 rpm which means even a two stroke isn't making enough bangs per min!

 quote - equates to 32,400 bangs per minute. My two stroke 91 only does 16,000 rpm which at one bang per minute is 16,000 bangs per minute, half of what a merlin does. So a two stroke weighs less, has more power and is closer to scale than any four - unquote -

One bang a minute, Paul, thats a SLOW tickover! Or did you mean 16,000 bangs per rev? 

12 bangs per rev? come on Paul its a 4 stroke! As Sir Stanley Hooker commented 1 stroke for power, 3 to wear it out, so its only 6 bangs - not that you could ever call the Merlin exhaust a bang! Merlins sing, Griffons growl and the Bristol Centaurus sleeve valve woofles rather loudly.

Incidentaly RR did make a 2 stroke V12, the Crecy. It was as powerful as the Merlin but only a protoype. Just imagine what that must have sounded like!

16,000 rpm with a 91 four stroke? What size prop is that on? That is 8,000 bangs per minute.

In comparison with 'similar' car engines, in 1938 Mercedes were using engines for racing that were producing 158 bhp. per litre and Auto Union 140, about twice of that ever produced by a Merlin, so comparisons of this sort are difficult. But, use whatever engine you like.

I totally agree, use what you want, but that's my point. Some people think that a WWII fighter should have  a quiet, gentle, torquey four stroke as its a war bird. However mine unlike the real planes is not a 60 yr old invaluable antique and so I want to fly it as they were intended to be flown. A late WWII fighter could "g" a pilot unconscious, catch up with a V1 in level flight  and hit the sound barrier in a dive,  

Yes Eric I meant 1 bang per rev ie 16,000 bangs per min!

Good point Simon, you are right, a real merlin would have only gone bang on each cylinder every four revs, but Richard, there's no passive stroke on a two stroke so its suck squeeze bang blow every revolution as bang, then induction and exhaust all overlap on the down stroke.

So from Simon's comment my two stroke is over reving, bangs per minute wise but I still nearer the full size sound than a four stroke in my view.

I do like the sound and torque of a four stroke, but I don't think its a match for the roar of a supercharged V12. But as many of you have said, fit what you want and enjoy.

 I wonder if I'll be similarly critised for fitting a four stroke in my DH Chipmunk. But the Gypsie Major engine isn't really in the same same league as a Merlin or Griffon so I think a four stroke is ideal in that plane.

Agree entirely - but its still a bang per cylinder every 2 revs (4 strokes) - not 4 revs. Ooooops!

Simon is correct RR did make a 2 stroke V12.....it used superchargers for the induction rather than through the crankcase so it had a 4-stroke bottom end but fired every two strokes of the piston. They reckon it could have produced over 3000 hp from its 22 litres (provided the pistons didn't melt!!!)

I have an article from a magazine on this very engine. Its a scanned copy so not its not brilliant but it is readable...if anyone would like a copy please get in touch & I'll email one over......