Aftermarket Injection & Cali Emmision

[jdbubb at verizon.net (Dan Bubb)]

Check out this graph:
http://i9.photobucket.com/albums/a95/jdbubb/Misc/cylinder_pressure1.jpg

It shows a graph of cylinder pressure vs. crank angle. Light the fire too late and maximum cylinder pressure is low because the piston is already moving down the bore sacrificing compression and the intake valve can open while the charge is still burning if the timing is late enough.
Light the fire too early and maximum cylinder pressure is much greater, which can cause pinging, but maximum pressure occurrs too early and pressures are high while the piston is still coming up the bore. This creates a lot of negative work in that the pressure is trying to stop the piston so the engine looses torque.
You can see the curve of relative torque vs. ignition timing.
The optimum is to light the fire so that maximum cylinder pressure occurrs somewhere around 20?ATDC.
To get the maximum to occurr at ~20? requires different ignition timing for varying load and RPM. Obviously as RPM increases the ignition needs to advance because the power cycle simply happens quicker.
Also, the mixture will burn at different speeds depending on its density. Low load, low density burns slower so needs more ignition advance. High load, high density burns faster and needs less ignition advance. This is the reason for vacuum advance or its equivalent in modern mapped ignition systems. An engine may make best torque at WOT with 32? advance but at part throttle while cruising down the highway it'll make best torque, considering the throttle position, with 45? advance.
Bear in mind that if your CR is high or you have poor octane gas the engine might ping before the ignition can be advanced enough to bring the maximum cylinder pressure to the optimum ~20? angle in which case your torque will suffer.
OTOH, an engine with a low CR or a very knock resistant combustion chamber design and high octane gas could potentially be advanced to the point that while it's still not pinging it is loosing torque due to the maximum cylinder pressure occurring too soon in the engine cycle.
Dan

From: <fahrvergnugen@cox.net>
To: <scirocco-l@scirocco.org>; "Dan Bubb" <dan.bubb@gmail.com>
Sent: Monday, February 26, 2007 9:37 AM
Subject: Re: Aftermarket Injection & Cali Emmision


> 
> ---- Dan Bubb <dan.bubb@gmail.com> wrote: 
> Keep in mind that setting the
>> ignition just short of ping is not necessarily best for either
>> performance or emissions though.
> 
> Dan,
> 
>  Can you expound on this a little bit?  It may be hard to discuss this independent of fueling, so let us say that you run as rich as needed for optimum performance.  
> 
> I plan to go with MS eventually on my 2.0 build-up, and I would no doubt benefit from your reasoning...
> 
> Cheers,
> 
> David
> 
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