TheSamba.com :: View topic - L-Jet tweaking for MPG?

Tue Jan 19, 2010 12:10 pm

My L-jet is working just has it was designed, right down to the VW published 20mpg with the automatic. Can the L-Jet be tweaked a bit to get a couple extra MPG without hurting engine life longevity? On an exhaust stiffer, I'm running right where it says to be at idle, but can it be leaned out a touch without killing the engine? I would just love to get 23-24 on the freeway. thanks!

Tue Jan 19, 2010 3:09 pm

The 20 mpg should be city. You should get about 25-28 highway. Waht is your C.O. at idle?

The biggest problem with L-jet is not really the sensitivity of the flap, its the sensitivity...or over-sensitivity of the rising rate fuel pressure regulator. Most of your mpg losses are due to excessive fuel usage in the mid-range part throttle area....after taking off the line and before shift points. You can prove this to yourself in a very simple way by driving the car for a few days....and studiously using the shifter to keep rpms up in traffic and throttle opening small. In short...stay in second and first a lot. It keeps a smaller throttle opening and higherrpm...which keeps vacuum high and fuel pressure low.

In a more studied method...get an accurate fuel pressure gauge and mount it where you can see it with as short of a hose as possible. While working through the gears...wathc the fuel pressure. You will note that the book...and common sense says...your C.O. should be low at idle.
VW and the EPA only checked CO at idle for the 1970's spec. So having perfect low CO...might have you at maybe 16-17:1 air to fuel ratio at idle.
Taking off the line...cracking the throttle....pumps fuel pressure up....so you end up quickly at between 12 and 13:1 if all is well. The problem is...is that say...2300 rpm up to shift point you should be tapering back toward 14-15:1 AF......but it won't be. The pressure will still be higher than it needs to be.
A rising rate fuel pressure regulator can help this. In many instances...that corrects low end and part throttle but may leave you too lean in the upper end before shift point. That is the point where you then might make a slight adjustment to the cog-wheel.

The problem with L-jet was not the system. It was the miserable level of tune dictated by the EPA. All they cared about was excellent readings at idle. They never checked part throttle performance. 95% of all L-jet systems back in the day...even while being reliable and excellent starters and runners...were either overheating dogs....or gas guzzling pigs...because of how the emmisions nit wits required your vehicle to be tuned to pass. I have never gotten a VW 412 with L-jet to EVER run in a worthwhile fashion when running the ridiculously low CO #'s the book required by 1974. They ran well ...but were hot running and gutless.
Pump the CO back up to say.....1972/73 411/412 numbers...and they ran much better but would not pass emmissions. Ray

Tue Jan 19, 2010 3:16 pm

okay... i'd have to recheck the idle CO as that was tested right before it got parked.

i do know that my kick down switch is inop and that the shift points are all over the place. maybe that is a bigger issue than anything else. the engine always has that 'too warm' smell, i've not got a CHT on this engine. I can hold the dip stick in my hand (the end in oil) for a few seconds after a full run.

my MPG is almost all highway. so, something is amiss

Wed Jan 20, 2010 12:25 am

A bus can get 20mpg on the highway with a 1800 and L-jet so I would think that a T4 should be capable of much better. I have kind of wondered whether the rear ends on these cars were too long to get good mileage at normal road speeds. In day to day driving my 411 almost never gets above the speed limit (55 mph on most local roads) so the longer gears don't offer much at all IMHO. I need a gear set that works best for climbing hills and accelerating out of sharp curves, not one designed to cruise at 80 mph.

Wed Jan 20, 2010 10:31 am

No...actually the gear ratios of both auto and manual were quite good for the torque band of a type 4 engine for climbing hills. The type 4 puts out better torque at a lower rpm than a type 1....so the 4.12:1 gearing standard in a bug ...while being in third a lot... would not do well for a type 4 engine.
In a well tuned and not heavily loaded 411 or 412.... speed an fairly heavy grades can always be mainatined without downshifting. I have driven the I-35 between Oklahoma city and Dallas. The two mile hump is a 10%+ grade. I can maintain 70-75 mph without a sweat and still stay in the 3200 rpm range.

Even in the 55mph range....what the high gear ratios (most notably the final drive get you) is that you can get good torque and accleration from 55-70 when you speed up without always having to downshift (with the type 4 engine coupled to it). However....if your auto had a 4.12:1 final drive ratio....you would almost always have to downshift to get acceptable passing accleration.
Along the same vein....a type 1/3 engine does not have the guts to drive the 3.73 or 3.91:1 final drive without downshifting and bring rpm up. Its odd that the type 3 does not have a slightly higher final drive for its torque band in its automatic. It would drive better for passing in highways and require less downshifts. Ray

Wed Jan 20, 2010 10:59 am

The auto box T4's were certainly noted for being pigs because of the combination of too high a rear end and too low of a torque converter stall speed. Buses ran pretty much the same tranny but with a lower rear end and didn't smoke the trannies anywhere near as easily. I think the poorly geared automatic was one of the things that really killed VW's sales of these cars.

Wed Jan 20, 2010 11:08 am

so... if i'm not currently running a 205 distributor (stuck with an 009 for the time being) how much max timing should i be running? also, just to clarify...Ray, you are advising running a bit more rich on idle to gain MPG at freeway speed?

Wed Jan 20, 2010 11:16 am

vwfye wrote: so... if i'm not currently running a 205 distributor (stuck with an 009 for the time being) how much max timing should i be running? also, just to clarify...Ray, you are advising running a bit more rich on idle to gain MPG at freeway speed?

You will run the coolest head temperature somewhere the the 27-28 BTDC full mechanical advance range but will probably have an off idle bog. A little more initial timing will help with the bog and maybe give a little better gas mileage as well at the cost of higher head temps.

I am not sure what Ray's point was with the idle mixture as it can be adjusted independent of running mixture???

Wed Jan 20, 2010 12:27 pm

Wildthings wrote: The auto box T4's were certainly noted for being pigs because of the combination of too high a rear end and too low of a torque converter stall speed. Buses ran pretty much the same tranny but with a lower rear end and didn't smoke the trannies anywhere near as easily. I think the poorly geared automatic was one of the things that really killed VW's sales of these cars.

You are spot on about the torque converter stall speed issue.
But also....bear in mind they were famous for being pigs off the line....not in passing speeds or in 2nd to 3rd shift point. Also...unless most people are over 60 years old on this forum....I have only met about 10 people in my life that have driven a 411/412 ..with a clean, fresh, properly tuned engine and a clean properly adjusted automatic. I'd also wager......and probably accuratley....that there might be one person...if that many....(drivers of 411/412) on this forum whose auto box is properly adjusted for shift point at the vacuum modulater/main pressure.

In all of my years....I have NEVER found a properly adjusted autobox in either type 3 or 4....that belonged to anyone. Why should you? There is probably not one person on this forum who has ever even purchased a 411/4412 that is less than 25 years old or with less than 70k miles.
That being said....I got my first 411 in 1979. It was 7 years old and had 55k miles on it. It was quite quick when tuned well...and nothing like most of the much higher milage 411/412's I found later....when the main pressure was way off and the bands worn...etc etc.
I drove these cars for 15 years before actually discovering that myself....after having a box rebuilt for me...then having to properly adjust it myself.
With a properly adjusted transmission and engine...these vehicles were actually not dogs at all. They were quite quick.

Of course..with the sloppy converter...if you do a lot of short light to light and never get out of second, they will eat gas. 17-18 mpgcity in this situation is common.
Ray

Wed Jan 20, 2010 2:40 pm

Wildthings wrote: vwfye wrote: so... if i'm not currently running a 205 distributor (stuck with an 009 for the time being) how much max timing should i be running? also, just to clarify...Ray, you are advising running a bit more rich on idle to gain MPG at freeway speed?

You will run the coolest head temperature somewhere the the 27-28 BTDC full mechanical advance range but will probably have an off idle bog. A little more initial timing will help with the bog and maybe give a little better gas mileage as well at the cost of higher head temps.

I am not sure what Ray's point was with the idle mixture as it can be adjusted independent of running mixture???

The ability to adjust fuel mixture independent of idle mixture is not actually a function of L-jet. The mixture adjustment screw on the AFM comes in two forms for different variants of L-jet, Digijet and digifant.
(1) there is the air bypass version of the bleed screw. That bypasses air which changes the sensitivity of the flap at low airflow (idle) and also bleed unmetered air into the mixture. However...being upstream of the TB...it has no effect whatsoever on vacuum for the fuel pressure regulator......which supplies most of the part throttle and transitional enrichment.
(2) On units that have a potentiometer on the AFM instead of a bleed screw....you get varying functions of when the input from that screw is active (a) there is the only at idle function.When the flap moves more than a certain distance...the small 5-10% variance the pot gives is bypassed. All enrichment form there is a combination of flap movement and fuel pressure rise. (b) the version where the 5-10% the pot gives simply moves the scale of enrichment across the board so you can have 5-10% enrichment from idle and across the board.

My point....was that L-jet has totally uncontrolled mid-range/part throttle enrichment. Its controlled by vaccum....which the ECU has no control over.
In the real world its kind of a worthless endeavor to set...or even care about setting....what the CO mixture is at idle. You don't drive at idle.
In general...most L-jet...and especially the low compression 412 version of L-jet....could run perfect idle CO to pass emmisions....but because of over enrichment by the fuel pressure regulator...would be gas eating dogs in the rpm range between 1700 and about 2300 rpm or so while upshifting.
The adjustment screw whether potentiometer or bleed screw ...on the AFM might only be able to lean you out a few points at idle...and many times could not seem to get you lean enough to pass late model 74/75 emissions CO standards.
So....even if you can get 5-10% leaner by using the pot or bleed screw....at idle....for baseline....that is no guarantee that the second you blip the throttle ...that the vacuum calibration of the fuel pressure regulator will not make you grossly rich or lean. They are just not very uniform from regulator to regulator....or very accurate.

To further this point.....if over rich or over lean is your problem at part throttle.....how would screwing with the flap cog wheel help you in this range....without hurting you elsewhere? Hint....it can't.

If you are excessively lean right off the line no matter what you do....adjusting the cog wheel looser can help make the flap movement more sensitive to smaller amounts of air. You gain dynamic enrichment off the line (via injector pulsewidth)....added to whatever enrichment the fuel pressure regulator gives. But.....its a progressive spring. Loosening the cog wheel will help to make the off the line flap movement more senstive but it also makes the midrange and upper end more sensitive and the flap can max out too quickly. This is not saying you will run lean....you just run out of additional ability to increase rpm.

Likwise if everything is running too rich at idle and off the line......you can tighten the cog wheel making it less sensitive to airflow......but again...that makes the entire range less sensitive now.

The cog wheel is the LAST adjustment you do. baseline fuel mixture (via temp sensors first, fuel pressure second and flap sensitivity last.

If I could put it simply.....L-jet is very adjustable....but its not readily or simply adjustable. There is no simple adjuster knob on it.
It was designed to run a generally correct baseline fuel mixture by uniform fuel pressure and properly calibrated temperature sensors. The flap is simply a method to multiply the baseline pulsewidth to reflect dynamic load. Actually L-jet was functionally obsolete by 1975 emmissions standards. L-jet basic came out in 1974 IIRC....by 1975....they EPA was asking for idle CO standards that a crude system like L-jet could not hope to maintain and still be correctly driveable. Ray

Wed Jan 20, 2010 2:52 pm

Ray, do you know what the fuel pressure should be at 19-2500 rpm for best fuel economy in this system?

Wed Jan 20, 2010 3:32 pm

vwfye wrote: Ray, do you know what the fuel pressure should be at 19-2500 rpm for best fuel economy in this system?

The fuel pressure is not related to RPM's, it is a function of inlet manifold pressure. The pressures are well spelled out in the manual.

Wed Jan 20, 2010 4:11 pm

Wildthings wrote: vwfye wrote: Ray, do you know what the fuel pressure should be at 19-2500 rpm for best fuel economy in this system?

The fuel pressure is not related to RPM's, it is a function of inlet manifold pressure. The pressures are well spelled out in the manual.

you are spot on about the relationship of vacuum to fuel pressure....but...

Which manual list this relationship in #'s? I have every available manual for 411/412....and nowhere have I ever seen what fuel pressure is supposed to be relating to what vacuum pressure. Thats also kind of the problem with this system. That could only loosely work.

If for example you were able to have say 35 psi at 7"hg of vacuum (part throttle....just a pure for instance.....and the book says that is correct.....it must also assume that every engine is operating to the same efficiency to use those pressure#'s. One engine may be running rich at this set of parameters ...another will run lean or right on the money.

Also....are you in part throttle accleration or part throttle deceleration? Both conditions will produce the same fuel pressure but one condition is not correct and will run rich because of that.
This is what later L-jet with the 0-2 sensor...corrects.

But yes..the factory did a good enough job making most of these engines uniform that a general baseline of pressure to vacuum relation should work well save for the problem I listed just above. The second ugly part of this problem....is that no two regulators these days are exactly alike ...linearly...across the entire pressure range. Some is brand to brand...some is build quality....and some is the fact that any spring device changes with age. Ray

Thu Jan 28, 2010 10:44 pm

my 009 is jumping around, so i'm looking for a good used distributor. is the l-jet a system that uses the 205 series distributor?

Fri Jan 29, 2010 12:17 am

vwfye wrote: my 009 is jumping around, so i'm looking for a good used distributor. is the l-jet a system that uses the 205 series distributor?

http://www.oldvolkshome.com/ignition.htm

Fri Jan 29, 2010 12:38 am

thanks