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Date: Tue, 10 Oct 2000 23:10:35
Subject: Re: Tomos Moped RPM limiter
From: Marko
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I haven't been clear enough in my explanation. Apparently Steve's thinks
that this limiter works on speed kits also. It does, but in a different
way.. I thought that Mike did not replace the cylinder as he said he only
mounted the tuned pipe, that why I explained he'll have problems going over
38. The limiter is (as previously said) of a non-conventional
type and is limiting the A5 to 8000 with STOCK cylinder and head
ONLY. Due to the way it works there is a point in which the
preignition it controls, cylinder port time-area values and head
compression just can't function properly and the engine boggs
down. This happens at 8000 on stock. However, due to the fact
that the 70 cc kit has different port time-area values and
compression, the preignition point is satisfactory up to (as you
have heard from Steve's) 9500+, but the limiter still limits the
RPM, just at a different value. If someone was lead to believe
otherwise, I'm sorry (the kit power issue is still topical). I
personally have information that these ignitions make no problems
on heavily modified manual speed Tomos mopeds even at 11000-12000
RPMs, but at these revs, the stator plate needs to be rotated
(which I would strongly warn against doing on stock, because it
won't idle afterwards). The more interesting thing is the nature
of the A5's tampering protection. If you look at the "usual" US
tuning procedure, you go for the tuned pipe in the begining.
When you realise you can't go over 38 (see above), you buy a
complete speed kit. Now comes that problem with transfer ports
(if we count out the base sealing problem resulting from the
cylinder being designed and built for Puch rather than Tomos).
So you file down the transfer channels as far as possible and get
real 50 mph. The revs are now at 10000, and the crankshaft is
starting to complain. So you are thinking of further increasing
the final transmission ratio. But there's no room neither on the
rim or the engine. The A5 is always a step ahead :-)

P.S. Here's the formulae for calculating RPM value from speed:

RPM [1/s](Speed [m/s]*Total transmission ratio)/(PI*Tire radius [m])

The "relaxed" version:

RPM [1/min](Speed [km/h]*Total transmission ratio*60)/(PI*Tire radius

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