Hi all,
         just wondering what the effect of using a slightly longer rod in a high timed engine might be for stunt use?

Say for example I have an engine with 160º exhaust timing and I get a rod made that is 1mm longer in order to reduce that timing does it unduly effect the transfer timing?

Thanks.

Hi all,
         just wondering what the effect of using a slightly longer rod in a high timed engine might be for stunt use?

Say for example I have an engine with 160º exhaust timing and I get a rod made that is 1mm longer in order to reduce that timing does it unduly effect the transfer timing?

Thanks.

Hi Chris

That can be a very complicated retro fit that can kill many things,and cause lots of grief... first it will also change the main intake and boost port timing, It can give you sub port induction ( bottom of piston uncovers) It will raise the C/R, it can interfere with the head, causing much damage, it can and most likely will affect the piston/sleeve fit. in motors like new ST and T.T. motors you may not be able to bolt the head on without interference . If the ports are short ,top to bottom, and it is a square or short stroke engine, you can drastically affect scavenging. Others problems are numerous, You will need to look at all of this before doing that type of mod.

Regards
Randy

Chris   Many years ago in the 1980-90's when I was doing a lot of work on the ST/40-46's I built several that had the sleeve dropped .005-.015" . Hemi the head to keep compression down and control flame front. Cut head fins down to keep heat in motor. Most ST motors run to cold. These motors ran very well and had more power with large props in large planes.  I stopped flying those large models and went to 600Sq" and have for years run the much smaller 11/5 prop with a stock timing. These mods were common on the older cross flow motors Also those motors used a lot more fuel.
Ed

Um, ............. ok then.

You got me as the engine in your supplied link is exactly what I have.

A longer rod is a moot point here, thanks Brian.

Longer rods also affect the acceleration and deceleration around TDC. How heavy is the X45....that's something else to consider....like others have said it's a tangled web of sub piston induction, deck heights, thransfer timing (which you could raise back up again), but in the long run a lot of screwing around

Thanks for the rapid replies guys, and I am roughly aware of the rod angularity changes, primary comp ratio change etc - its more the effect on the transfer timing.

Its also the sort of thing that can be reverted easily though.

The TDC fit concerns me too.

The engine in mind is  (or was) a Super Tigre X45.

Thanks again.

Are you talking about an increase in stroke by 1mm or increasing rod length by 1mm (new piston with raised pin)?
The latter will give you more piston dwell at TDC and a more favorable rod angle.

I ran an MVVS 40 on pipe very successfully for a while, it had 160 deg exh. You couldn't run a typical stunt 4/2 switch, rather the rattly 2 clcle in level flight which loaded up and came on song going verticle. It worked great in a heavy model, but was hard to get right in a light model.
The correctly timed engines were vastly more user friendly.

If I remember correctly, the shorter rod improves torque, all else being equal (timing, stroke, etc). Probably better to make a new liner with suitable timing.  Steve

If I remember correctly, the shorter rod improves torque, all else being equal (timing, stroke, etc). Probably better to make a new liner with suitable timing.  Steve

Steve,

A longer conrod gives a THEORETICAL mechanical benefit, a better lever action at power stroke, that is favorable in low RPM/high torque ideology.
The original question was stupid at first glance but when I spent some time pondering it, it suddenly became interesting . If we are only talking about a small increase of rod lenght, like some tenths of a mm or max. 1mm, the cylinder head relocation is a rather simple job to do.
The modification may be beneficial in turning a normal schnuerle engine into a 4-2-4 running engine also because raising the b.d.c. effectively fucks up the scavenging, as you can forget the original port angles. It seems that inefficient scavenging is an important factor in a classical stunt run, together with bad thermal balance. My only worry would be the exhaust port, personally I prefer it so that in b.d.c. the piston edge is clearly below the bottom of exhaust port.
Basically, these things cannot be calculated, trial & error rules!

Lauri

I ran an MVVS 40 on pipe very successfully for a while, it had 160 deg exh. You couldn't run a typical stunt 4/2 switch, rather the rattly 2 clcle in level flight which loaded up and came on song going verticle. It worked great in a heavy model, but was hard to get right in a light model.
The correctly timed engines were vastly more user friendly.

I don't have the engines anymore, but I may possibly have the book I kept all the specs in. Peter Norrie has (did) one of my old models with the 50 that I made a piston & liner for. It worked pretty well. Dunno if he still has it.

Hi Andrew,
 the supplied snippet from a test was for the Akrobat liner  and if you are dabbling with diesel conversions here I have heard that even the high timed liners make for good stunt diesels - they simply don't seem to suffer from runaway at all!

Cheers.

  Using the longer rod and the high timed sleeve will shorten the power stroke....

How does changing the distance between crank pin and piston pin centers ever effect the distance between the crank pin and the crank center?

To change the stroke distance you would need to independently change what determines top dead center and bottom dead center (the crank arm distance) and a longer rod simply displaces the stroke.

Or am I missing something here?

Andrew,
 still looking for the original information about the liner timings but please accept that my memory gives me a 6mm exhaust port height with the RC timed liner.

Now none of my MVVS's have that height so make of it what you will but I assume that the lower timed liners will be  ........ well, lower than that.

Thanks Chris and Brian, now to do some measuring!

Thanks,

Andrew.

ST/46 and 60 did the long rod short rod in the early years. They moved the wrist pin location to keep the timing the same. There was no difference in power.More clearance is needed in the case for a longer rod.