OK I got a look at the dyno sheet from just before Nats last year on my 924s.

Air Fuel was upper 12s till 5,000 RPM then got fatter as RPM went up. It richened up to 12.0 to 1 at 6,300 RPM.

Peak HP was 143.8 at 6,000 and peak TQ 143.7 at 3,800. The TQ curve is pretty flat from 3,800 to 5,000.

This is with the head that check exactly at 10.5 to 1 compression at Nationals.

It has since been deemed illegal due to the head thickness rule. It was swaped out for a fresh head after Nats.

So I was facing some issues with my motor at TWS earlier this year. It was getting hot midway through the session and seemed like it wasn't making the power I expected.

For reference I spent all day Saturday and the first session on Sunday carrying 4th gear down the entire straight, never hitting the rev limiter.

I have a narrow band O2 on my car. It is simply tied into the stock O2 sensor. I knew from watching the gauge that my car was running lean on track. I decided to do a little track side tuning. Here's what I did:

• Unplugged the O2 sensor from the DME at the back of the engine compartment
• Let the motor get up to operating temps
• Hopped in and slowly rev'd through the RPM range and noted AF on the gauge
• Began moving through the range quicker, still watching results
• Shut off the car and popped the back cover off the AFM
• Marked the origional location of the spring in the AFM thoothed wheel.
• Adjusted the AFM one tooth (couldn't remember which way was leaner and which was richer)
• Tested reving and watched the gauge readout.
• Continued adjusting and testing until the gauge read aproxamately in the 13's over the majority of the range, paying particular attention to the upper RPMs.
• Plugged the O2 sensor back in

Once I figured out which direction lean vs. rich was it only took one notch to get the AF in the correct range.

Note: the DME has a different map for WOT vs. partial throttle. I figured that measuring under partial throttle was better than nothing.

So my result? The next session I dropped 1 second on my best lap vs. my previous sessions and ended up on Pole. I also was hitting the rev limiter in 4th half way down the front straight, now. The car also stayed cool through the entire session.

My advice, invest $30 - $50 bucks in a narrow band gauge. It isn't as good as dyno tuning, but it will give you some ballpark information about what is going on with you AF. People will tell you that a narrow band won't work for tuning, but remember you are reading the same O2 sensor that your computer uses to adjust the maps. It isn't very granular, but neither are the adjustments we can make.

-bj

Sterling Doc wrote:


If our cars utilized two 02 sensors, I could see why this might matter - but we only have one 02, and its pre-cat anyways. Am I missing something here? I don't see the connection between catalytic converter and A/F in this application.

At WOT the car goes into open loop mode. So it does not even read signals form the single O2 sensor. This why you can unplug it and have the still work fine, albeit with a funkly idle.

Anyway when we pull the cat it tends to increase exhaust flow a bit making the car run lean at WOT.

This why it is good tune the AFM a bit.

GT944 wrote:


I agree with Joe, removing the cat increase exhaust velocities. But, cat removal isn't the only reason to bother with this. Our cars a mess of 25 year old, thrown together parts. Do you know if you have early or late injectors installed? Do you still have the original AFM? There are a lot of reasons why it is good to check you A/F ratio.

-bj

Interesting that freeing up the flow will contribute to running lean - I suppose that makes sense though - better airflow than stock = (hopefully) more air intake than the system was originally capable of, leading to the stock *max* required fuel injection to be not quite enough. Have I got that right?

@loftygoals -- well I don't know about all of YOUR guys' cars, but mine was "all original, never raced, babied since day 1, and had a complete motor rebuild" so *of course* its got all of the correct parts It was very... interesting to find all of the "improvements" that had been made to my car before it came to be in my possession.