2+2

I understand that this is what can happen if you run too much boost and your fuel maps don't correspond so you lean out. What I don't understand is that 951 come with knock sensors that detect knock and retard timing and reduce boost right?!! So when detonation starts to occur, doesnt the 951 knock sensor sense this and computer take corrective measures to prevent detonation? This will prevent damage that can be caused by running lean right? What am I missing here that everyone is so concerned about running lean? <img src="confused.gif" border="0">

tazman

The bigest problem is the KLR is no longer controling the boost with most modified cars. So I imagine it can only retard the timing so much then there is nothing else the computer can do.

slevy951

Tazman is right. When completely stock, the system works VERY well. It's when you start modifications that the system can't keep the safety factor that it has built in.

Danno

Well, retarding ignition in order to compensate for a lean air-fuel mixture is not the correct solution. These are two separate independent issues we're talking about here: air-fuel ratio and ignition-timing.

The optimal air-fuel ratio under full-load for maximum-power is 12.5:1. See the BMEP chart here: <a href="http://forums.rennlist.com/forums/ultimatebb.php?ubb=get_topic&f=18&t=001921" target="_blank">Air Fuel Ratio under boost Question :</a> The problem with increased boost and using aftermarket chips like the APE Stg.2 is that they only increase fuel in the midrange and gets lean up top because 100% duty-cycle is 100% duty-cycle regardless. Attempting to use richer mixtures than 11.5:1 in order to combat detonation is a cover-up, sweep-it-under-the-rug quick-fix for the real issue: too much ignition timing advance for the conditions (too rich of a mixture also washes oil off your cylinder walls and leads to faster wear). That is, if you have a lean air-fuel mixture, you cannot simply dial back ignition-timing to make up for it. The mixture will still burn too hot and you'll end up with with burnt exhaust valves rather than blown headgaskets if you try to retard the timing enough to ward off knock.

Then ignition timing needs to be addressed once you get an optimal air-fuel mixture dialed in. Since a modified car with free-flowing mods and higher-boost is more efficient, not as much ignition advance is needed. The mixture burns faster and hotter, so you can dial back ignition a couple of degrees and using a colder plug is recommended. If you have too much ignition advance, you can't just dump in more fuel either. You'll end up wasting power because the mixture will be fully combusted, ready to do work too early, and it will try to compress the pistons and con-rods instead of rotating the crank (try to set this point at 15-20 degrees-ATDC).

So to answer the original question, both of the two independent factors of air-fuel ratio AND ignition-timing can lead to knock and detonation when set improperly. However using one to try and compensate for the other is not the correct solution.

2+2

Ok, so if I understand this, at higher boost pressures, if you don't have proper mixture/timing, detonation will still occur eventhough 951 has a knock sensor and KLR/DME can control timing, fuel and boost. This is because although detonation may be occuring, the stock safety mechanism does not properly compensate for the detonation due to the higher boost pressures. It may try to compensate by adjusting timing etc but combustion response to such compensation efforts will be insufficient/inaccurate due to the higher boost.
Is my understanding right??

BTW: Danno, I am just amazed at how much you know about these cars. I lurk here trying to learn and your posts are everywhere!! Why does Gururacing have only one "chip" shown for the upgrade? I thought there were 2 chips in most upgrades? <img src="graemlins/beerchug.gif" border="0" alt="[cheers]" />

adrial

Almost...

When you turn up the boost..it is almost always done by bypassing the factory boost control (the cycling valve). The new SFR chips keep the factory boost control 100% in place when they turn up the boost...because they do it via software rather than mechanically controlling the signal to the wastegate.

Without the cycling valve, when the KLR detects knock it can't pull back boost because it isn't controlling it anymore.

2+2

Adrial, thanks for chiming in....about bypassing the cycling valve...yea I think I understand now what you guys are saying. A banjo bolt or LBE prevents signal to the wastegate until a boost level is reached so the safety system is effectively bypassed...even though it is not taken off the system. The safety measures can't operate properly since they dont receive the proper signals.

Thanks. Let me know if I am off my rocker.

2+2

Adrial,
That SFR chips you were referring to which controls boost through programming and maintains the stock boost controls, I went to their site but cound't find them. Where did you get the info?

adrial

It was posted either here or on the e-mail lists...

Maybe Tim can chime in?