XR7 Dave Could you help me out

Bleed_Ford_Blue

Registered User
You seem like one of the more knowledgeable guys here so I thought I might ask one of smartest of the smarties to explain something that I dont quite grasp. I understand that a rich condition is too much fuel in the air/fuel mix and a lean condition is not enough fuel in the air/fuel mix. I do not however understand how these conditions happen. And I do not know why or how a lean condition can be so detrimental to an engine, or if a rich condition can be as problematic or even how to diagnose either in this engine. I would greatly appreciate it if you could explain this to me in detail ( I am in no hurry). And please dont be afraid to put it in laymans terms or explain it like you would to a guy who only knows where the gas goes, I wont be offended. I hope Im not asking for too much. I just think it is probably pertinent information that every car guy should know. Thanks alot in advance.
Dan Smith
 
Bleed_Ford_Blue said:
I understand that a rich condition is too much fuel in the air/fuel mix and a lean condition is not enough fuel in the air/fuel mix. I do not however understand how these conditions happen. And I do not know why or how a lean condition can be so detrimental to an engine, or if a rich condition can be as problematic or even how to diagnose either in this engine. I would greatly appreciate it if you could explain this to me in detail ( I am in no hurry). And please dont be afraid to put it in laymans terms or explain it like you would to a guy who only knows where the gas goes, I wont be offended. I hope Im not asking for too much. I just think it is probably pertinent information that every car guy should know. Thanks alot in advance.
Dan Smith
That's a tall order for just a simple post. Most of us take years to really learn all that stuff.

First off, the computer has limited ability to control the mixture. If you "adjust" things too much (different injectors for example) then the computer is unable to compensate. Furthermore, the computer only compensates at part throttle and doesn't really compensate at full throttle. Therefore a car may run fine at part throttle cruise yet go way off target under boost.

One of the challenges to diagnosing a rich/lean condition on this car is that reading plugs is often not useful. If the car is running fine at part throttle, the plugs will stay clean and look fine, but it may be going out of range only under boost. You may never see evidence of this on the plugs.

Also, unlike a carburator which commands a set AF ratio vrs airflow speed through the venturi, the EEC relies to an electronic representation of airflow measured in the MAF. This works great as long as the MAF is reading correctly. However, because it is not actually reading airflow, it is very possible, and actually quite common with modified SC motors, to have the MAF reading innacurately. Garbage in - garbage out.

Finally, regarding the results of said AF mixtures, a lean condition under boost will result in a very hot flame and an unstable mixture. This increases the tendancy and severity of a dual flame front occurring in the chamber. When the two flame fronts meet we call this phenomenon "detonation" and what it really amounts to is an explosion. Contrary to common belief, gasoline does not normally "explode" in the cylinder but rather it burns very smoothly and quickly. When gasoline explodes it releases energy like TNT (actually more) and results in damaged headgaskets, spark plugs, pistons and connecting rods. If your SC ever sounds like it has a tin can of marbles in it under boost, let off imediately and do not go into boost again until you have corrected the problem. :)

Rich mixtures result in poor mileage, low power, smelly exhaust, overheated converters, and fouled plugs. In severe cases it can result in washing down the cylinder walls to the point of loss of compression and contamination of the oil (and consequently damage to bearings).

Once you have modified your SC hoping to achieve more power, it is good practice to get the car to a chassis dyno and have the car at least checked for AF ratio. If it is out of line, then additional tuning and most likely a chip for the EEC will be required. Anytime you modify the car, a dyno tune and chip are a GREAT idea. Many people have had great success modifying their SC's without the aid of dyno tuning or chips (myself included) however, it takes a thorough understanding of what is going on "in there" to avoid some of the common pitfalls that have destroyed more than one SC motor.

Help any?
 
turbospeed said:
well mine sounds like a can of marbles only at low throtle and like a 1000-1100rpm........any ideas

Pull the SC belt to check your supercharger for excessive play in the snout coupler if the sound is coming from up top. The pulley should not have any play, I've seen some noisy superchargers with 1/4" of rotational play.

If the sound is coming from underneath the car, then a broken up converter will sound like that also. Check those things first as they are the easiest to check and are common.
 
I knew if I asked a smart guy I would get a good answer. I appreciate it. My car is currently fine, however When I was bringing it home through Arizona it gave me a lean code when I was at high elevation. But at lower elevation it never does. Would the atmospheric pressure have anything to do with leaning out the mix? And how can the engine run lean on only one cyl. bank?

Also you said that many people have had good results without the dyno tune and chip. I would personally not attmpt this I just want to understand it better. So how do you manually control the mix ( I am guessing that is what you meant) or how do you do it without changing the program code of the EEC. I understand that is is a complex subject and appreciate your time and expertise.
 
turbospeed said:
well mine sounds like a can of marbles only at low throtle and like a 1000-1100rpm........any ideas

I read a while back that detonation can occur from having to big of a load at low rpm's. (from not downshifting)
 
Back
Top