Re: [Amc-list] TBI fuel injection tuning.detonation

[Frank Swygert <farna@xxxxxxx>]

First my thoughts on the pinging issue. You CAN get a 196 to ping, but only under a heavy load at low rpm. You'll have to be lugging the engine to do it. It's happened with me, but it was usually in a situation where I really needed to down-shift from OD or third, and just didn't... or hadn't yet. Arm around girlfriend in high school and it was "inconvenient" to move, and only a little ways up the hill, and the 196 WILL lug a good bit without terribly straining it... something like that! Could be that I always tended to run the valves slightly loose rather than as tight as possible, as most tend to do. I'd rather have 0.001" extra clearance than risk burning a valve by being right on the money or 0.001" over. Except for a very slight increase in valve noise 0.001" isn't noticeable as far as performance is concerned. 

I think it's resistant to ping because of the long stroke and relatively slow flame travel. The offset dome of the piston may have a lot to do with it too. You don't hear detonation because of the heavy casting. It may be occurring, but you don't hear it. 

I'm not sure it does ping easily though. If you read up on quench theory you'll find that the edge of the piston doesn't really matter. That 1/8" or so land on the edge of a dished piston is what needs to be closest to the top of the bore, and the inside edge should be rounded. The main flame front is in the CENTER of the piston, where any sharp edges will get hot and can stay hot enough to preignite the incoming mixture (detonation). If you look at the offset dome of a 196 piston you'll note that it has well rounded edges. The dome also concentrates the flame front on the right 2/3 of the piston surface, away from the top of the dome. So I think the piston design as well as the long slow stroke makes the engine very ping resistant. That "dead space" on the "back" side of the dome shouldn't affect anything but total compression, and that's taken into account by the designers. 

Now figuring out what the compression would be on a highly modified 196 would be a PITA -- you'd have to figure out just how much space that dome takes up! A start would be to reverse the math, assuming the factory compression ratio is correct. Measure everything that's easily measured (cc of combustion chamber, thickness of compressed head gasket, bore, and piston distance to top of bore) then solve for the unknown space of the dome. That could be off by a cc or two though. 



-----------------
Date: Sat, 22 Nov 2008 13:51:36 -0800
From: Tom Jennings <tomj@xxxxxxx>

> > What about a knock sensor?  I'm not familiar with the technology
> > used, but you could mount one in an appropriate port on the engine
> > and just connect it to a light on the dash.  Perhaps just JB Weld a
> > knock sensor bung to the block somewhere.
>   

I guess I should try one out it can't hurt. It's weird that this engine 
will not ping -- see if yours does! I wonder if the knock sensor would 
work on an engine that doesn't make that loud noise.


> > I'm planning to play with the timing on my 196 too and to use the
> > wide band oxygen sensor to tune the carb.  And now that I think about
> > it, it would have been a good idea to smooth the rough spots on the
> > pistons before I assembled my 196.  I'll try to remember to do that
> > if I ever have to open it back up.
>   

I wonder if it's worth it, the whole piston is a sharp edge  :-)  When 
its' up at the top, there's a 1/16" - 1/8" space between the popup crown 
the the cyl walls. That just screams 'dead spot'. Somehow, it works.

-- 
Frank Swygert
Publisher, "American Motors Cars" 
Magazine (AMC)
For all AMC enthusiasts
http://farna.home.att.net/AMC.html
(free download available!)

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