bd0nalds0n

I recognize that stoich (14.7:1 or thereabouts, right?) is the optimal point at which everything is burned, and that maximum power is produced at richer mixtures (12.5:1 or thereabouts, right) but my question is this:

Is it reasonable that anything richer than Stoich is non-damaging, even though it may be providing sub-optimal power?

Jim bailey - 928 International

Correct if you must err have it be rich ....Porsche when the O2 sensor fails opted for default to rich.

Carl Fausett

There is a point of diminishing returns, where it is just plain too rich.

It seems that the "black-smoke" level begins at about 11.0 to 1 air/fuel ratio on most cars (that is where you will blow black smoke out the exhaust pipe during acceleration. This will carbon up spark plugs too. HP output begins to diminish at this level also.

More serious are air/fuel ratios below 10:1, where now you have so much fuel that the cylinder walls are actually getting washed down and cleansed of the oil they need for lubrication. Premature engine wear results and crankcase oil contamination.

Remember that your O2 sensor in your collector is running on an AVERAGE of all cylinders on that bank. You can have a too-lean cylinder, a too-rich cylinder, and two average cylinders and the O2 sensor would see that as all one OK number. That's why infra-red temperatures are taken at the exhaust ports per cylinder - to make sure they are within 5 or 10% of each other.

UKKid35

When the MAF ages, which it does gradually from day one, the mixture gets progressively weaker (if you don't have a closed loop system with an O2 sensor)

Scott M.

Brian;

Think of air/fuel ratios this way...

9.0 black smoke/low power
11.5 Approximate rich best torque at WOT
12.2 Safest best power at WOT
13.3 Approxiamte lean best torque
14.6 'chemically' ideal
15.5 lean best power in cruise
16.5 Usually best economy

As Carl points out, too rich can be damaging to cylinder walls, catalytic converters, fouled plugs, etc...

Jim bailey - 928 International

And somewhere in the scale of lean ....the compression ratio,fuel quality, heat , intake temperature,blowby contamination, engine speed ,load ,and camshaft profile ,(boost)all come together and you have detonation pre-ignition which destroys engines.

CWO4Mann

As one ignorant of alternative fuels, how does the propane alternatives sold in Germany fit into the scheme of things? Our detachment in Berlin had a Mercedes 200 with the propane modified engine and seemed to run just fine for a Mercedes 200. I have seen stationary generators with duel fuel carbs where you could literally switch back and forth on the fly between gasoline and propane (Onan 5.0CCK GenSet for example). Could some kind soul point me towards a tutorial? Googling didn't seem to come up with much although I did find Ford's natural gas powered cars. I am curious as to the idea of placing a common barbeque cylinder in the trunk, temporarily, and switchng over before the drive to the emissions test station. While I recognize that the 928 engine properly tuned and with appropriate catalytic converters, etc., will be fine at emissions time, I am still interested in this alternative as an exercise in thinking outside the box.

Thanks!

Cheers,

Dave

bd0nalds0n

I think my throttle position switch may be on the fritz, or the wires, due to the orientation of the TB and the TPS connector bumping against the hood, may have cracked. Yesterday, when I was experimenting, under reasonably heavy throttle the AFM stopped hunting and went to just a little rich and stayed there--I assume the TPS was showing WO. Today it doesn't. I used the 85-86 TB and did not substitute the TPS from the S4.

What I don't understand is this: Unless the TPS is showing WOT, won't the brain seek to keep the mixture around stoich? What I find is that I go a little lean (not necessarily above 16) with modest throttle input, and a little rich (not below 12) as I let off the throttle. But I don't go rich at WOT. What is the benefit of ratcheting up the fuel pressure so high at idle as to cause car to run rich (which I assume is overpowering the brain such that it can't achieve stoich?)? Right now, it seems like the brain will recognize a lean or rich condition and compensate, but not antiicpate the change based on what I would assume would happen if the TPS was providing input. Does the TPS provide anything other than something equal to 0 (non-WOT condition) and 1 (WOT)?

Also, I have an issue with my fuel pressure: Either the sending unit, the gauge, the BEGI, or Fuel pump is not functioning properly. At idle, my electric gauge almost always reads 46lbs. The highest it goes is around 52lbs, under modest throttle to WOT. Even if I crank the RRFPR many turns tighter, with the vacuum/boost line off and capped, the pressure doesn't change from 46. But the idle gets rougher, and the car starts to run richer per the LM1. It also seems like the more I ratchet up the RRFPR (the allen bolt, not the needle valve), the more pronounced the lean condition becomes under modest acceleration, and the richer it runs under deceleration. Once I get the 2nd connecter from Quick Carl, I'll hook up a mechanical gauge. My electric sending unit has been loctite Red'd into the connector Andy gave me, and I don't want to try to muscle it apart in the interim.

Finally, based on previous posts, it seems like as long as I'm driving around conservatively, if the AF ratio is between 16 and 12, then I'm not likely to be doing damage, which is my primary concern until I can troubleshoot the various problems away...

Thanks to all who have commmented and please feel free to keep the suggestions coming.

GoRideSno

I am confused. Did you use an 85-86 throttle switch or did you use the one from your S4? If you used the S4 throttleswitch then do not drive the car again untill you have installed an 85-86 throttle switch.
Andy

GoRideSno

I can't think of any benefit. If you follow the procedure I gave you in your other thread then your fuel pressure at idle won't change unless you have the vac line off of the 2025. Once you reconnect the vac line to the 2025 the fuel pressure at idle will remain at stock.
HTH and check your messages
Andy K

SharkSkin

Brian, make sure you have the right vac line hooked to the RRFPR.

bd0nalds0n

I used the TPS from the 85-86 TB. I didn't take the TPS off the S4 TB. I think I'm getting the car closer to dialed in, even though the FP reads screwy. It idles around 12:1, and under normal driving conditions ranges from 12:1 - 15:1. I may switch out the o2 sensor and as I said I need to get a mechanical guage on the fuel lines. The vacuum line to the RRFPR comes from the manifold, right next to where the boost-line fitting goes in.

Maybe I'm just being paranoid.

Adam C

Here's a dumb one:

How is the air/fuel ratio measured?

bd0nalds0n

http://auto.howstuffworks.com/question257.htm

Adam C

Hi brian,

thanks for the link - this isn't quite what I was asking - What I am asking is how are you getting the actual reading of what your car is doing? How can one establish what his air/fuel ratio is on the car - not how does the car measure it.