OS.91 SZ-H shims...?

[ferincr]

Hi I've been reading an old thread (from last year) where everybody talks about shimming it around .024 (for 30% nitro).
Is that still the way to go??? and are the shims included with the engine??? (enought to get to .024)???
I just ordered the non pumped version and I'd like to know if everythingI need will be inside the box or should I order some extra shims??
Thanks,

[dogg5306]

You're getting enough shims to go to .020...
That said for 30% nitro fuel... you should go for 0.016 at the minimum but I would choose 0.020 :wink:

[ferincr]

Thanks dogg, I decided to gwt an extra set of shims anyway, I think for 5 extra bucks it's worth having them.
Since I fly at 4000ft ASL and humidity around 90% with temps around 80F I don't know how that would affect.
Thanks,

[ferincr]

Ok, I got my new toys :mrgreen:
Should I go with just 0.020 on my OS 91 SZ-H (non pump)?
Or is there any benefit on keep adding more head shims, if so how many?
By the way, I'll be running it with a MP5 (just in case it matters)
Thanks for chipping in,

[BarracudaHockey]

Try it and see. If you are getting predetonation add another one so you lower the compression ratio.

[ferincr]

Ok, here I go...
Let me wear my stupid hat first, there you go.
How does pre detonation sounds like? I mean is it something really recognizable???
The only experience I have is my OS 70SZ-H and what I noticed was that when idling after the engine is warmed up didn't sound smooth and I opened (richened) the mixture control Screw (the one between the two needles) and it worked out but that was just for the idling part of the engine.
I read some people talking about 4stroking also can someone jump in and explain (if possible) the differences between all these?
Thanks,

[cdrking]

PREIGNITION OR DETONATION

Two different things!

Jeff

[ferincr]

PREIGNITION OR DETONATION

Two different things!

Jeff
Thanks for the correction Jeff, now can you elaborate a bit more on those?
Like causes, how they look/sound like, and solution?
Thanks,

[cdrking]

http://www.misterfixit.com/deton.htm This is a great link.

Jeff

[ferincr]

thanks, I'll give it a good read.

[BarracudaHockey]

I'm sure one of the engine experts (harkey) will correct me if I'm wrong but here goes.

With preignition, if the compression ratio is too high (or the nitro or both) as the piston is traveling upwards the heat of compression fires the mixture prematurely resulting in a downward pressure on the connecting rod as its still traveling upwards so its bad for the wrist pin, rod and crank pin AND its wasting energy by having to over come pushing the piston over the hump as opposed to using that energy efficently after the piston passes TDC and all the force is pushing downwards.