[Diy_efi] Re: diy_efi Digest, Vol 16, Issue 14

[Michael Richards]

> A richer mixture burns faster requiring less advance.
> Correct?

Not necessarily. The rate of burn will have a lot to do with actual 
compression ratio, combustion chamber design and octane. In general you 
want to cruise lean for fuel economy and when you want to make power 
richen it up. 

> This is on an air cooled 1386 cc normally aspirated
> V-Twin with a 3.66" bore.
> 
> Let's say I was running 14.7:1 (closed loop) at 40%
> engine load with 35 degrees of advance and let's
> assume the onset of knock occurred at 40 degrees.

Under cruise you really need to dyno tune as the minimum timing for 
best torque is seldom right next to the onset of knock. For example I 
did a mazda engine that could go up as high as 47 degrees before 
beginning to knock. This was at 8.4:1 and about 60kpa MAP. The most 
power in this case occurred at 37 degrees. You do not want to run too 
much advance here. Even though it may not be knocking it isn't doing 
your rotating assembly any favours as you're compressing a burning 
mixture quite a lot.

> If I switch to an open loop map with an A/F of 13.7:1.
> How much should timing be retarded as a starting point
> for tuning?

Your timing should be modified based on engine load. The load is really 
a representation of cylinder filling. More air and fuel in means higher 
cylinder pressures and this is what will cause the mixture to burn 
quicker. EGT is a good way to figure out how lean you can go and a dyno 
will tell you what mixture to use to make the most power. I suspect at 
WOT you will find it around 13.0:1.

> I built up a little microphone amp circuit and glued a
> small condenser mic to a spring clamp. I plan on
> clamping it to the fin closest to the bottom of the
> piston skirt at TDC. Anybody care to describe what the
> onset of knock will sound like.

Location is not that important as the metal will do a great job of 
transmitting the sound. Personally I find a cheap GM knock sensor works 
just fine. Find something to thread it in and you've got something 
cheap and effective. As for your audio amp you should really use some 
filters as the raw signals are much more difficult to pick up the 
sounds of knock. My brother is a sound engineer and he did some work on 
it for me. I believe it's an 8khz bandpass filter that can be adjusted 
by 1-2khz depending on the engine. With bore and stroke you can 
calculate the expected noise frequency from detonation. The sounds of 
knock fall in the same range.

-Michael
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