From: ntaib@silver.ucs.indiana.edu (Iskandar Taib)
Subject: Re: FAQ? (questions about engines)
Organization: Indiana University
Date: Sat, 5 Sep 1992 01:54:39 GMT

In article <1992Aug31.111533.2265@inland.com> bloom@inland.com writes:
>In article <1992Aug28.220048.9625@c3177208.ssr.hp.com>, bill@c3177208.ssr.hp.com (Bill Chidester) writes:

>> : 2. Too cold a fuel or bad fuel contaminated with water.  Cold fuel 
>> : is equivalent to low or no nitromethane.
>> 
>>     Actually, adding nitromethane to fuel will make an engine run cooler, not
>>     hotter (allowing lower temperature for a given RPM, or higher RPM for a
>>     given temperature).  From the engine's point of view, "cold" = "good", so
>>     it will run faster and more efficiently.  I agree with the water contami-
>>     nation, especially if you leave fuel sitting in an unsealed container in
>>     your flight box.
>
>I don't think the laws of thermodynamics can be violated!  Nitro gives you 
>more power thru more heat per power stroke.  Have you measured the 
>head temperature of an engine running more nitro???  I'd bet a big dinner 
>on this one.


Actually he's right under some conditions. 

When the C/L speed fliers were told to limit their nitro (they were 
running 50-60% nitro, and the new rules said they had to run somewhat
less - 40%?) they discovered that the engines ran hotter. Why? Well, 
with more nitro, you actually have to feed more fuel in per unit time,
since nitro contains less energy (joules, or btu, or whatever unit 
you'd like to use) both per unit mass and per unit volume, since it 
releases oxygen upon burning, which is why it it used to augment power 
in the first place (it doesn't "contain" more power; gasoline has 
far more btu/cc than methanol or nitromethane does - it releases oxygen 
which would give the same effect as a turbocharger would). 

Now...

Why do sport engines run hotter with more nitro? Well, nitro also has
more or less the same effect as lowering the octane does in a gasoline 
engine. My conjecture is that it would be the same as increasing the 
compression ratio, the same way a turbocharger does (the end result being
more air+fuel in the combustion chamber). So using more nitro can cause 
detonation, or pre-detonation conditions, unless you raise the head 
with shims (yes folx, those aren't gaskets under there, they are shims!).
Us Combat people have known this for a long time. If your engine runs
hot and blows plugs, put extra shims under the head. When an engine is 
new and tight an extra shim or two keeps you from blowing a plug every 
run. As the engine gets older and looser the number of shims can be re-
duced. Reason is blowby decreases effective compression ratio.

Hot plugs vs. cold plugs???

Well, what I've heard is use the warmest plug you can get away with. Use
a cold plug out of necessity (because they are blowing, or causing pre-
detonation, etc.) not because they are "better", because they will actu-
ally cut down on power and idling performance.






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Iskandar Taib                          | The only thing worse than Peach ala
Internet: NTAIB@SILVER.UCS.INDIANA.EDU |    Frog is Frog ala Peach
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