Header question

[Av8ferg]

3 hours ago, S30Driver said:

Can't beat that deal!   If they work out, I can loan you a PLX wideband o2 kit with gauge to see what your afr is when you get the motor in.

Cool, not sure what that is but happy to learn.   I noticed one of the O2 nuts is missing,  Maybe it comes that way.  Wasn’t sure if I needed to install an O2 sensor, not even sure if my 77 had one.  

[S30Driver]

Nope, no O2 sensor on the 77, not until the ZX.   You need a second nut if its missing to seal the exhaust or screw in a sensor.   With a wideband O2 sensor installed, you can monitor the air / fuel ratio of the running motor throughout the rpm range.   Nice to know if running too lean,  14.7 is considered ideal.

Here is the gauge without the O2 plugged in.

Edited May 17, 2020 by S30Driver
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[AK260]

I have a very unhealthy AFR fetish! I put this together based on quite a lot of hours of reading / internetting / speaking with engine builders / tuners and trackside mechanics at classic races.

 

 

Heads up: 14.7 as correctly stated above is the ideal mix to get a full burn and is good as your idle baseline afr. There are of course many situations that you want to be either side of it.

 

With modern fuels, the consensus in research papers / controlled experiments is that optimum power is produced between 12.5 - 12.8. Aim for this across the range at WOT. On a side note, some modern engines prefer just north of 13 due to head designs etc.

 

If you are planning on hooning it at max revs in 4th on the long straight of a race track, racers typically aim for closer to 11.5 as the richer mix has a cooling effect on the engine (but without going into details, it does put a stress on it too). My L28 AFR sits doggedly between 12.2-12.9 from 3.6K to the 6.5K rpm limit and pulls like a train. I have ran it at 11.5-12 and the engine is marginally but noticeably keener on the transition from part throttle to WOT; however the power difference is difficult to discern without a rolling road.

 

I find anything 11 or below loses power / makes the engine bog down.

 

On the cruise you ideally want to be north of 14.7 if you have a working vacuum advance. Ideally 15.5-16ish. This makes for a slightly hotter cruise but will give you way better fuel economy. Modern cars can tolerate HUGE timing (c. 60 degrees) and with super high pressure common rail injection

/ better atomisation run even leaner.

 

Anything north of 17 on the overrun will give you fun pops and crackles on the tail pipe at around 3k-4K rpm. My L26 running a little lean once shot out a 2 foot flame out the back when coming off the power at 6k rpm. Scared the b’jeesus out of me but my friend following thought it was mighty impressive! ;)

 

The other thing you will be surprised by is just how much a mere 0.5 difference on the AFR can affect your fuel economy and richness of the sweet aroma of your exhaust.

 

Whatever you do, don’t run your engine lean on WOT as it does horrible things to your exhaust valves and piston crowns.

 

Welcome to the rabbit hole of the AFR gauge - you just took the red pill ;)