Go Ahead, Abuse Your Engine!

[stlrj]

There seems to be some support in general aviation to the idea of running leaner in order to run cooler, maintain performance and save your engine at the same time:

http://www.avweb.com/articles/pelperch/pelp0008.html

This artical might help explain how I managed to lower engine temps and increase mpg with my cockpit mixture control valve that I installed in my 86 Carrera.

Regards,

Joe


[This message has been edited by stlrj (edited 06-28-2001).]

[Pillow]

I have seen your setup for the Motronic injection and think it is a great idea.

But can this setup be done to a CIS car?

This is really a shift in thought for me because I was always told and read that the richer a car runs the cooler it will be. Note that this has nothing to do with efficiency.

Thanks

------------------
Adrian Pillow
1979 911 SC
1966 VW Microbus
PCA - Peachstate Region

[Jim T]

Running leaner than what? Its hard to tell from that article, there is some discussion about how airplane engines are set to run unusually rich under some circumstances.

The setup and baselines seem a lot different in the airplane examples, I'm not sure how that would necessarily translate to rigging an O2 equipped CIS or Motronic car to trick it into running leaner than it is set up or wants to run.

Also, my understanding is that in the 80's, emission requirements were pretty tough for Motronic to meet, and the way most car mfrs got it to comply was by setting it up to run lean. I know at idle, they run very lean, which is why a lot of mid-80s Motronic cars (like BMWs in particular) idle kind of rough. Its also why a lot of people feel that Motronic cars run better with the O2 disconnected, where it defaults into a richer mode (I don't personally agree with that, though).


[This message has been edited by Jim T (edited 06-28-2001).]

[wckrause]

From the article:

"Note: A word of caution. While, from considerable personal experience, and from the experience of a number of others, this works very well
on the big bore normally aspirated TCM engines (IO-520 and IO-550) , there are some engines that this should not be tried on."

I would be a little wary of trying to apply one guy's slightly off-the wall leaning process for specific aircraft engines to your 911. Especially without an EGT and a CHT gauge.

Lean = hotter more mpg
Rich = cooler more power



------------------
Bill Krause
'79 911SC Euro

[Superman]

Don't aircraft engines have dual ignition systems (twin plug)? If so, this would help fight the detonation problem you risk with lean mixtures.

------------------
'83 SC

[Pillow]

I agree a CHT would be a definate addition with this type of change.

[adamnitti]

hey, joe-

(there's a song in there somewhere...)

is it safe to say then that in order to use a system like yours a car would have to be setup to run rich by default since you can only move in the leaner direction?



------------------
Adam Nitti
ajnitti@mindspring.com
www.adamnitti.com
'85 911 Carrera Coupe
PCA member, Peachstate region

[pwd72s]

Didn't Porsche have a little problem with the '69 911S models when MFI first arrived?
I remember hearing that some cars ran TOO LEAN, and extended higher speed driving could put holes in pistons? Evidently, it didn't take Porsche long to issue a "set 'em richer" service bulletin...Anybody else care to weigh in on this one?

[stlrj]

Adam,

Setting my system up to rich by default is exactly what I do in order to have the greatest amount of adjustment in either direction of rich or lean so you can set it to whatever you feel is most comfortable.

Regards,

Joe

[911pcars]

For the best power, the proper air/fuel mixture is the correct mixture, neither rich nor lean. Power is lost on either side of this ideal ratio. However, there is greater safety margin on the rich side and better fuel economy on the lean side.

Sherwood Lee
http://members.rennlist.org/911pcars

[stlrj]

To be more specific, best power is at 12.6:1(Rich), ideal stoichiometric ratio(smog)is at 14.7:1 and maximun thermal efficiency- occurs around air-fuel 16-18:1(Lean).

In the real world, the DME operates at 14.7:1 at idle and no load part throttle cruise in closed loop. But get on the throttle and it goes into open loop for "on throttle" and "wide open throttle" enrichment which will take your mixture as rich as 11:1 which is too rich for max power.

So if your base idle mixture is set richer than 14.7:1 which is very common in order to have a very smooth idle, then you will automatically loose power by default as your on throttle enrichment will drive your mixture well past the maximum power ratio of 12.6:1.

The basic false assumption is that fuel mixtures set at idle are static and do not change due to load factors.

Joe

[911pcars]

Joe,
I should have added that the terms Rich and Lean are relative terms. E.g. Stoichiometric is richer than 16:1.

Sherwood

[stlrj]

Since all these Rich/Lean gymnastics are already programmed into the DME, the only area that we can influence is the base mixture setting which would also influence "on throttle" enrichment to the point where, hopefully, it does not go beyond 12.6:1 in either direction if we are lucky.

That's why being able to trim your mixture easily from the drivers seat while you drive can be beneficial

Meanwhile, during most of our low load cruising conditions, the DME is operating in the lean area in closed loop where it would be highly unlikely to cause any damage due to detonation.


Joe

[This message has been edited by stlrj (edited 06-29-2001).]

[This message has been edited by stlrj (edited 07-02-2001).]

[stlrj]

Interesting to see how the subject of mixture is also a very hot topic in general aviation circles:

http://madaket.netwizards.net/vtail/archive/1998/msg04274.html

Look at the charts in Test Results to see how mixtures affect EGT temp.

http://www.gami.com/frames.htm

Joe

[This message has been edited by stlrj (edited 06-29-2001).]

[Wayne 962]

Airplane engines are very different than automotive engines, though. They have to compensate for atmospheric changes, so the mixture changes. The early MFI cars had a barometric compensator on them to help adjust for changes in altitude. Running lean on the ground for an airplane engine may shift to running rich as the air thins out at altitude...

-Wayne

[stlrj]

Wayne,

Don't forget the altitude compensation switch next to my DME box under my drivers seat which activates at about 2600 above sea level...and I cruise above 6000' more often than you would think.

Joe

[This message has been edited by stlrj (edited 06-29-2001).]