I have a SH 15 that has a total of .020" when I use a single head gasket. This
clearance seems Huge! for the engine size.

Are they worried about the engine ingesting dirt or something? Is it "Goon Clearance."

Thanks


Bill Mazz

0.5mm is very normal....

There are some factors to calculate with it: play and flex.

Both can make it possible that in high rpm the clearance goes down to 0.2 or even 0.1mm

Even if you account for the con-rod's main bearing clearance, and distortion, the
length the rod expands, the piston pin's clearance, and distortion, much of that is
off-set by the expansion of the cylinder block.


Do expensive and / or modified engines use the approximately the same squish clearance?

If they all use .5mm or greater squish clearance, why don't they use a hemispherical
(homogenous) combustion chamber. At some point, any gains in reliability by using a
stratified combustion chamber is lost when the squish clearance is too great.

Why lower the compression (by increasing squish clearance) for nitro content, if you
exceed the ability of the stratified combustion chamber, to keep the piston crown
cool?


Bill Mazz

Yes. Novarossi so far I have seen standard 0.55 up to 0.7 at sea level, Picco is arround 0.5 but in several ocasions adding an extra shim is better.

All is about igmition.
Compression, nitrocontent, plug number and the weather is making the ignition timing because it is based on a self combustion. For a part the flex in the system making the gap smaller at higer revs is acting like a shifting ignition as a lot of 1:1 racing engines also use. A higher compression does shift the ignition to an earlier moment.

Thank you for your answer, isn't it about 6am where you live?

Sincerely


Bill Mazz

if the squish is too tight it will cause pre detonation (knocking noise, ping) and will destroy a piston...

There are some books that support that point of view, mainly by G.P. Blair. He is
responsible for the bulk of 2-stroke exhaust system research. But his books are
geared towards design of a product to be manufactured, and not peak performance.
(except for one paragraph)

When repairing an engine that has failed, and was caused by detonation, and is often
followed by pre-igntion, I identify rpm ranges that may be running lean, and I decrease
the squish clearance to prevent the problem from reoccurring.


Bill Mazz

Pre detonation can be seen, the piston and combustion chamber will look like as it was sandblasted. It will look like a rough gray surface.

Here you can find a lot of another great person about 2-stroke:
http://edj.net/2stroke/jennings/

My understanding is that given a constant compression ratio a motor with a tight squish band will make more power, but it may not be as smooth as a motor with a looser band. When I last raced RC boats around 2003 I was setting my .21s to ~.004" and my .90s to .008".

Actually, it's not the tight squish that causes the detonation it's the raised C/R. If you could lower the squish and open up the chamber the motor would actually run cooler. This is what the high end tuners of the late 90s were doing to boat and ducted fan motors.

We were running really tight squish bands (.003"-.008") on up to 65% nitro. You just have to open the bowl of the head to compensate.

This all applies to hemi head motors. I haven't looked at a RC car motor in a long time.

Exactly right, Chris!

Jennings copied the work of Blair. But I did buy a copy of
Jennings book back in 1977.

Normally detonation precedes pre-ignition, and it causes carbon to crackle and pop. It can
also leave a nearly black stain on the piston crown, and a feather edge on the top of the
piston crown.

Roelof: Thank you for posting the engine timings on your web site. It exemplifies the best
of the web, "to share information."

With that information and my experience with exhaust and head temps of similar nitro glow
engines, I was able to calculate the proper speed of sound, and the tuned lengths for tuned
mufflers and tuned pipes.

Thanks


Bill Mazz

Because of forum rules, cut and paste these url into Google:

books.sae.org/author/1073416002/
profblairandassociates.com/GPB_Tribute.html

So what is your point of posting here? Are you going to make something? Do you even race r/c cars?

You are kinda gruff.

I don't mind sharing, but who am I sharing with?


Yes I also agree with you.

But what I don't understand is why a good 2.5cc airplane engine will run .005" - .006" and run up to 35% nitro
on the same set-up, why doesn't the car engine manufactures do the same? Is this something they save for there
own team engines, or is it goon clearance, or something else?

It seems counter productive for the manufactures to sell 30% nitro, and promote such large squish clearances.
But the same myths were relevant in the 70's. I found an article a while back written by Henry Nelson complaining
about engines that were sent back to him, and this very issue.


Bill Mazz

Roelof: If you don't already know how to calculate the speed of sound for a given engine set-up, and tuned length
for mufflers or pipes, I will share. BM

RPM & Oil Content are the two main things I can think of, off the top of my head. The oil being much more resistant to detonation than Nitro or Methanol, and the engines only spin 50% the RPM that car engines do.