I've done a bunch of work on the HPI .15FE engine in my HPI RS4 2 -- replacing the single-needle carb with a two-needle carb, cleaning out obstructions in the transfer channels, smoothing airflow through the transfer ports, rebalancing the crankshaft, raising the compression by lowering the shim stack from 0.5mm to 0.3mm, etc. I've managed to wring-out a significant increase in performance from all these mods. I tried a 0.2mm shim-stack and the top speed decreased a little, so that tells me 0.3mm is optimal.

After all this work, the engine now produces enough torque on takeoff that it's actually a little difficult to control, yet the top-end still isn't quite as fast as I'd hoped for. One thing I haven't done yet is raising the sleeve by inserting a shim underneath the lip of the sleeve. As I understand it, this will increase breathing at high RPM and decrease torque at low RPM, which is pretty much exactly what I'm looking for. Is my understanding correct? Should I raise the sleeve by only 0.1mm, or should I go crazy and raise it by 0.2mm? Obviously I can try one then the other, but at-present I only have one spare shim of each size, and I'd rather not cut-up both of them without having some reasonable expectation that there would be a noticeable benefit.

Thoughts?

0.3mm head clearance can be too low when the engine will gain rpm, due play and flex on all internal components the piston can hit the head at high rpm.

If you lower the head you will shift the ignition, it will be sooner. That can cause a lower top rpm so you have to correct that with a colder plug. If you are running a 3 or 4 you should try a 5 or 6.

Raising the sleeve will raise the port timing, a higher exhaust will give more rpm and a higher intake can give more overall power.

Okay, so raising the sleeve is worth trying. Do you have a recommendation for raising the sleeve 0.1mm or 0.2mm? I assume if the sleeve is raised too much, the engine performance starts to decrease again.

Regarding your other points:

I checked for interference between the piston and cylinder head. There was no evidence of impact with a 0.3mm shim-stack; I haven't opened the engine to check the result of the 0.2mm shim-stack, but at least there was no interference when the engine was cold, and I didn't hear any unusual sounds from the engine when it was running.

Why would advancing the ignition timing lower the max RPM? I thought high-RPM engines need advanced ignition timing to ensure the combustion completes before the piston starts to descend. Every time I've tried to get more power from an old engine, a hot glowplug improved the performance, and then I knew I could increase the compression to get the same effect with a medium glowplug. I've ever actually used a cold glowplug in any of my engines. Keep in mind, my fastest engine has about a 38000rpm max, which is still about 10,000rpm lower than race-grade engines.

Take a kid on a swing, help to push the kid on the swing. When he comes back to you you try to get the hands on the swing but you always have to push the swing after the dead point to speed up. If you push it directly at the dead point you need a lot of force to speed up the swing but if you start to push it before the dead point you can actually slow it down.

In our engines it is the same. There is a small delay beween ignition (get the hands on the swing) and combustion (the push of the swing) That delay is caused by several things:
- cool down of the plug wire (the colder the wire gets after the combustion the next ignition will be less stong)
- flex on the internal parts (more rpm -> higher moving piston -> more compression)
- the fuel itself (the combustion speed is depending the oil)

The first 2 are rpm related and is indeed a wanted factor to get an advanced timing. The last one is just something to take care off when switching fuels and you still want the max performance.

The advanced timing needs to shift in a same amount of the rpm but sadly that is not always controlable so with a high compression setting you can get more motor performance on the low and mid range but on the high rpm range the ignition is to soon slowing down the piston and with that giving huge forces on the crank pin and rod.

Maybe time for a book regarding some basic engine know-how...
There has been several written about 2 strokes that I believe could help sort the questions here and also give a better overall understanding.

I've already read the Two-Stroke Tuner's Handbook cover-to-cover. I especially enjoyed the part about port modification, wherein the author said "It would be nice if an engine's performance could be reduced below a certain minimum by even the worst butchery of the ports, but unfortunately that is not the case." I appreciated the snark.

- - -

So...should I raise the sleeve by 0.1mm or 0.2mm? Is there likely to be a significant improvement from raising it 0.2mm vs. 0.1mm, or is 0.2mm likely to be too much?

All those handbooks are written arround larger motors with an electric ignition. And yes, the basics of ports, exhaust, intake, bore/stroke, crankcase volume etc can and may be directly transferred to our engines.

But our engines have a self combustion concept where some factors have influence on the ignition timing and the shifting of it. Those are not hard to understand but very important to understand our engines and why some changes are not doing as expected or doing half as expected.

Because it is a smaller engine with low placed ports a 0.1mm could give arround 2 degrees extra timing. I would make a 0.1 and a 0.2mm shim so you can experiment 3 positions.

Yes, I understand that nitro engines have thermal ignition instead of spark ignition, and I understand the basic factors that control the timing of thermal ignition. I have read about 2-stroke diesel engines too, because they also use thermal ignition, and I have experimented with adjusting the ignition timing on my nitro engines. I just haven't experimented with raising the sleeve, that's why I'm asking for more information about it.

Okay. How much extra timing is likely to be too much? Has anyone ever taken this modification to the extreme to see what the limit is?

I tried raising the sleeve by 0.2mm and saw no beneficial effect after re-tuning the carburetor. It ran hotter, didn't idle as well, and had a slightly lower top-speed. (yes, I remembered to remove shims above the sleeve to compensate for adding shims under the sleeve, to maintain the same compression ratio.) So I guess that's the end of it -- a 0.3mm shim-stack with no raising of the sleeve is the best configuration for this engine. If I used a cold glowplug, maybe I could run a 0.2mm shim-stack, or a 0.1mm shim-stack with the sleeve raised 0.1mm, and maybe there would be a benefit, or maybe there wouldn't be. I'm not sure it's worth further experimentation at this point.

It's also a matter of understanding, not only reading and memorizing, and that is a big difference. If you understand, you can most of the time implement the knowledge to other similar circumstances, or as a next stage reform it to suite other circumstances.
And as you implement, it's not a single point solution.

Anyhow, carry on...

A while ago on a Dutch forum I had a huge arguement about the flex in the rod and crankshaft that it is acting as an advanced timing. The guy was convinced those parts are solid, no way that it will flex while we all know a too low head clearance will cause the piston touching the head at a certain rpm. The higher the revs the higer the piston goes, the higher the compression becomes the sooner the ignition goes.

I'm sure some have done a FEA on these engines.
As I haven't won the lottery yet, there is no plan at this moment to get that kind of software here...

Generally speaking I comprehend new information as fast as I can read it, or as fast as someone can explain it to me. As I get older I sometimes have to think for a while, but only for really complicated concepts. I have an engineering degree despite graduating from high school with a 2.4 GPA -- my GPA was so low because I never did my homework, and I never did my homework because I never needed to. I aced tests with little to no effort. So yes, I comprehended what I read in the Two-Stroke Tuner's Handbook.

Of course the conrod stretches when the engine is running. However, the amount is infinitesimal -- if it actually stretched by 0.1mm at max RPM, it would shatter almost instantly, because that much stretching repeated 30,000x per minute would require the same energy released by a stick of dynamite, and the conrod would be heated to its melting point in seconds. In the real world, the more significant change in length comes from thermal expansion, not from stretching. Additionally, at very high RPM the upward momentum of the piston near TDC may be enough to pull the conrod upwards despite the intense air pressure above the piston, so if there's any play in the conrod bushings the piston will be able to travel further upwards than expected *without* the conrod stretching.

- - -

Anyway, I have decided to be content with the engine's performance as of right now. This car has a 17-year-old repurposed airplane engine, but nonetheless it has a top speed of 48mph (77km/h) and it produces enough torque from a standstill that I can easily make it drift on pavement despite having HPI Pro-Compound tires. That seems like a good enough accomplishment.

Scaling down a rod for a large engine, a stretch of 0.15mm (each cycle) is nothing special.
The thermal expansion doesn't even come close.
But that's just the beginning, then there is crank flex, crank case flex, radial clearances etc. 0.3mm @ peak rpm isn't anyway out of the ballpark.
Add a misfire to the equation and things get really interesting.

So you aced every test but didn't get the grades... That makes about as much sense as asking basic questions when you say that you already comprehend the subject...
But is well within the norm of many engineers I've met, so please carry on...

It's called "getting a second opinion". Good engineers do it to make sure they haven't forgotten or misunderstood anything when implementing theory. Engine modification is *not* basic and it's entirely reasonable to ask questions about a specific implementation even if you understand the theory and have implemented it before.

You can't "scale down" a large engine like that. The materials are different. Conrods in automotive engines are usually made of steel, which is much more robust than the aluminum used to make conrods for nitro RC engines. Steel has a minimum repetitive-stress threshold, below which it can endure repetitive stress forever; aluminum does not. An aluminum conrod measuring 23.5mm center-to-center will expand .06mm in length when heated from 21°C(70°F) to 127°C(260°F); if the stress-elongation from repetitive loading comes close to that, to say nothing of exceeding it, the conrod will not last long.