Tuning AFR on 1.8L w/DJET Options

[Spoke]

I bought a Zeitronix ZR-2 Wideband AFR to tune my 911 and decided to use it first to tune the 914.

I am datalogging to my laptop since I didn't get the gauges.

The ZR-2 is temporarily connected to the engine until it's tuned properly then it will end up installed in the 911.

I picked up the 1.8L engine a while back and mounted the original DJET FI from this car. The engine seemed to run lean so I am "fooling" the FI by adding resistance in series with the CHT sender. Right now I have about 400 ohms in series with the CHT sender. The goal is to get the AFR right by either tuning the MPS or changing the CHT sender if it is low in resistance or both.

I need to check the fuel pressure before making any changes to the MPS or CHT sender.

The ZR-2 is just sitting in the trunk for now. I have the data cable running out of the trunk and into the passenger window with the PC in the passenger seat.



Since I'm not permanently mounting the ZR-2, I didn't want to cut into any wires to get 12V. So I used the unswitched 12V from the 25A fuse and just connect and disconnect power to the ZR-2 when testing. I was able to get power from the relay connector for the rear window defroster and held it in with a tiny screw.



The throttle position sender was a little tricky to figure out. I mounted a potentiometer to the throttle body then rigged an idler arm to rotate the pot when the throttle is opened. Everything fits around the return spring and intake boot. The rotation of the pot is only about 30% of its throw so WOT is at 30% of potentiometer rotation.



The AFR sensor is mounted on the exhaust tip.

[Spoke]

I'm not really sure what I should be tuning to on the AFR. During WOT below, the AFR drops to about 12.0-12.7.

What is the optimum reading for AFR?

WOT for the datalogging is around 30% of the potentiometer.

At idle, the AFR is around 16.

Here's a run going through the gears with WOT.



This run is with 1/2 throttle and the AFR is slightly higher but still less than 13.

[Mike Bellis]

12.5 is rich but that is what you want at WOT. You could play with the mixture there up/down slightly to find max hp. Where is it at cruising speed on the freeway? It should be around 14.4-15..

[brant]

12.5 to 13.3 at wide open throttle at redline

[Bartlett 914]

12.5 is rich but that is what you want at WOT. You could play with the mixture there up/down slightly to find max hp. Where is it at cruising speed on the freeway? It should be around 14.4-15..

That Lean? I was having problems with running hot. All recommendations were in mid 13's I was running hot at 14.1. I am currently lower and running pretty good.

[brant]

I'm more familiar with race motors and dyno runs at WOT
so I can't say I absolutely know what cruising/partial throttle should be
there was a great post fairly recently about the stock MPS settings

but flat fours do not like the same theoretical AF ratio's as water cooled cars. These air cooled motors like a richer mixture than other cars. And I have a theory that the "factory" settings were too lean
I believe that the factory was going for mileage and emissions at the expense of too much heat. My theory is that this is why djet cars drop valve seats, and carb cars don't as often.

so I would personally go for a richer cruising mixture than the factory did.

only my 2cents... I have more experience at wide open throttle and am really only an amateur at partial throttle numbers.

brant

[Bleyseng]

Where is the AFR at going down the freeway at 70mph not coasting?
Should be 13.5-14 to one.......
Idle is too lean at 16 to one

[Drums66]

.......Correct me if I'm wrong but.........is'nt that Ljet(you said Djet)??? (IMG:style_emoticons/default/idea.gif)
(IMG:style_emoticons/default/bye1.gif)

[Spoke]

It is DJET from my 71 motor. The engine block must be from a 73+ car.

Thanks for the responses. I haven't tried cruising yet. I'll get that info tonight or tomorrow.

If the idle is lean and the 1/2 and WOT throttle are rich, how do I balance this out?

[Bartlett 914]

It is DJET from my 71 motor. The engine block must be from a 73+ car.

Thanks for the responses. I haven't tried cruising yet. I'll get that info tonight or tomorrow.

If the idle is lean and the 1/2 and WOT throttle are rich, how do I balance this out?

Some ECU's have an idle adjustment knob on the top. This only affects the Idle. I was able to richen up using higher fuel pressure and disconnecting the Temperature sensor in the air distributor. I suspect my actual fault is in the MPS and what I have done is patch work. I will get to it but for now, that is what worked.

[brant]

Geoff probably has the most hands on MPS tuning experience of ANYONE!

Geoff is that cruising range of 13..5-14 the factorys stock setting or one that you use as an improvement over the factory?

brant

[Bleyseng]

Stock settings...of course the AFR goes all over the place depending on driving conditions but at 2500-3000rpms, part throttle, up a slight hill for load thats where is should be. This is where you would be driving on the freeway at 60+ cruising along all day long on with part throttle..
WOT should start at 11.5-12 and end up at 5500rpms at 13.5 to one

[Spoke]

Here is the AFR at 70MPH on a relatively flat stretch of highway. Seems to hold steady at 15.



Here's idle. The blips towards lean are accompanied by a slight tumble of the engine. The RPM doesn't change but the engine shakes a little bit.

[brant]

I'd try to fatten that up a little bit
Geoff mentioned in the previous thread, and this one that he does his constant state highway cruising at a very slight load...

did you take that measurement on a very slight incline or load?
or was it 15to1 when slightly off throttle
(make sure you are slightly on throttle to get it accurate)

I'd adjust the MPS if it was me.
these motors make much better power and run better when slightly rich in comparison to modern watercooled engines.

remember there is a reason that air cooled engines are phased out in the world.
they can't compete with the lean conditions that water cooled temperature regulation allows.....

brant

[Bleyseng]

yes, richen it up a bit.

[Spoke]

did you take that measurement on a very slight incline or load?

The measurement was on flat highway, sustaining speed.

So how do I adjust the MPS for running richer?

I got the following info from Paul Anders website.

[brant]

I think you just answered your own question.
you have to be very careful when picking the epoxy out of the end plug. I've ruined one not being careful enough.

but a little heat and an hour of picking will expose the adjustment screws.

I've set a few up for myself and its not impossible. It just takes lots of time and trial.

brant

[Spoke]

So if I want to richen up throughout the band, should I only adjust inner screw alone, or inner screw alone and full-load stop?

[Bleyseng]

inner screw only, hold the outer screw in place.
The WOT is set only after the inner screw is perfect as you again have to test drive it and set it by screwing it in until its correct-11.5-12 to one at WOT at first and finishes at 13.5 to one at 5500rpms. Go for the later number at 5500 and see where it starts at 2000rpms when to stab the throttle wide open in say 3rd gear.