NissanDiesel Forums

So I've had my '82 KC for about 6 months now and I'm averaging about 27 mpg. Even though my fuel is free I want to get better mileage.
With all the reading I've done I'm still a little confused about what actually controls how much fuel goes into the cylinder.
I understand carbs, the more you step down on the peddal the more fuel is drawn through the jets, plus an extra squirt from the accellerator pump, etc. More air flow = more fuel.
But our throttle body, as best I can tell, only regulates air flow. The fuel is controlled strictly by the IP?
So here's my question... At any given RPM, regardless of gear/speed, the engine always consumes the same amount of fuel because the IP delivers a set amount with each pulse? So at 1000 RPM in 1st or 1000 RPM in 5th (significanlt different speeds) the same amount of fuel is being used?
If this is true then stuffing more air into the cylinder will only gain power, not fuel ecomony, correct? MPG gains would require different gearing or tire size. At least for highway driving.
Would a power gain, in particular at lower RPM, allow for shifting sooner around town so you could cruise around in a higher gear, at lower RPM, thus possibly improving in city mileage?
Any thoughts?

So, along that line, what differential options do I have to increase speed for the same RPM?

I'm not going to address all of your Qs, but on the diesel IP front . . .

All injection pumps can vary the amount of fuel they send to the injectors per pulse. Internal to the IP, the way this is accomplished varies quite a bit over different IP designs. There is some external input into the amount-per-stroke injected (via your foot feed), and there are internal bits inside the IP that also limit the maximum fuel that will be squirted out to the injectors; generally, the maximum fuel that can be squirted per stroke is less at lower RPM, because the cylinder cannot efficiently use as much fuel at low RPM.

On most diesels (but not the SD22, nor the MB 180/190/200D), the foot feed (accelerator pedal) is connected mechanically to the IP. Push your foot down, and the IP senses that as a request to send more fuel-per-stroke to the injectors. The IP might -- or might not -- respond to that request. For example, if the IP senses that the max fuel-per-stroke is already being injected while your foot is only halfway to the floor, if you press further, the IP will not inject more fuel. Another situation is when you are at or near the maximum governed RPM -- AFAIK, every diesel engine's IP has a max RPM governor.

As you've notice, the SD22 and old MB diesels don't have a mechanical connection between the accelerator pedal and the IP. Instead, the accel pedal moves that intake butterfly in the throttle body, and that in turn creates a weak vacuum signal (venturi vacuum, not manifold vacuum!) to vary. That weak vacuum signal goes to the IP, and acts upon a diaphragm that is counterbalanced with a couple of springs.

Simplifying a bit: with throttle near-closed (at idle) = high venturi vacuum signal to IP diaphragm = IP rack closed down to minimum fuel delivery. As you open the butterfly, venturi vacuum drops off, the spring in the IP can move the diaphragm, and the IP rack increases fuel delivery.

When the engine speeds up (with the same butterfly opening angle), more air is drawn around that partially-open butterfly, venturi vacuum rises, diaphragm gets pulled back, IP reduces fuel delivery. Excellent part-throttle speed-governing.

This system is called a pneumatic governor. It is not the only governor on this IP, but it's the one most likely to need maintenance.

There are excellent posts on the pneumatic governor elsewhere on this site.

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27 MPG is less than most 720 SD owners are achieving. Is your commute almost exclusively in-town and stop-n-go? Or is it mostly highway and above 65 ? For the former, I don't know what could help; for the latter, taller gearing (larger diameter tires) would help considerably, as the SD is really not at all happy up above 3600 RPM, and you are pushing that at those high speeds. The truck was sold at a time when the US speed limit -- by Federal law -- was 55 MPH, which was in turn a response to the 1973 OPEC/Arab Oil Embargo and the 1979 Energy Crisis. The little SD, at an unhappy 61 HP at high revs, is not ideally suited for extended high-speed motoring.

If the engine condition is good (reasonable oil consumption and good compression (easy to start in cold weather)), don't look at fiddling with the IP. It cannot really be "tuned". The major maintenance you do on the old pneumatic governor Inline IP is to make certain that someone hasn't screwed with the injection timing (see your FSM), and to check the integrity of the diaphragm (use a Mity-Vac or similar to perform a leak check). There is a lube hole in the pneumatic governor housing to add a few drops of oil to the leather diaphragm to keep it supple, but don't go overboard on the lube (see your FSM).

If your intake tract is unrestricted (clean air filter, no collapsed plenum hose, no EGR sooting up of the intake runners -- if your SD has EGR), then you are doing as best as can be expected of the little SD22.

HTH

Al,
Once again, thanks for all the info. There is so much to learn about diesels.
My little SD22 has 181000+ hard miles, but it still gets twice the mileage my 620 does...
So the little hole inside the throttle body is where the vacuum signal for the IP comes from, or is it deeper in, below the butterfly?
Does stuffing more air through the system (ram-air/turbo) mess with this set-up?
I'll read the pneumatic governor posts and ensure that system is fully functional. Is the lube-hole the one near the front?

I'm still looking for an FSM.

I do have highway driving for my commute, 65~70 mph. I think I need to swap-out the rear-end. I want to go with a lower number, correct?
If anyone's swapped out rear-ends, please let me know what fits. I'll search the forum as well.

I'm getting a tach, is 3600 the red-line? 4000?

Thanks.

Zeyemurgy wrote: So the little hole inside the throttle body is where the vacuum signal for the IP comes from, or is it deeper in, below the butterfly?

There are two holes, fore and aft of the butterfly, each one leading to its own hose, each of which go to opposite sides of the pneumatic governor diaphragm. They provide a varying difference of pressure on the two sides of the diaphragm. That pressure varies with butterfly position and air velocity through the throttle body.

Does stuffing more air through the system (ram-air/turbo) mess with this set-up?

It can, depending on the pressure and turbulence in the airmass as it passes through the throttle body. There are some archived postings under the subject of turbocharging.

I do have highway driving for my commute, 65~70 mph. I think I need to swap-out the rear-end. I want to go with a lower number, correct?
If anyone's swapped out rear-ends, please let me know what fits. I'll search the forum as well.

The power and torque curve developed in these small engines leave little room for modding. Lowering the rear end ratio will also decrease torque available for powering an already modest acceleration rate. The Ford Ranger which houses my SD22 weighs 3675 pounds and top end speed on the level, no head wind, is about 70 MPH. It takes a while to get there and I can't increase on that without getting into my smoke point. Lowering the ratio may well put your vehicle below the point where it can maintain road speed without constant shifting between 4th and 5th. Things like minimizing your vehicle load, maximizing tire pressures, setting toe-in to the minimum allowable for your vehicle, tire type, and good engine maintenance will give more reliable and cost efficient results than changing rear ends.

I'm getting a tach, is 3600 the red-line? 4000?

Thanks.

The figures below come from: http://www.jescoweb.com/SD22.htm#PERFORMANCE
While I believe that the HP being quoted may not be the same as our engines, it does show that the maximum torque is nowhere near the maximum rated 4000 RPM. It does suggest that redline can be set to 4000. I personally wouldn't want to run my engine that fast. It's hard on moving parts and not very efficient. I don't have a tach on my engine, but it just doesn't sound as happy at 65 MPH as it does at 55...

AUTOMOTIVE USE
Max. Output: 65 PS (66HP) at 4000rpm JIS (SAE gross)

Max. Torque: 14.5kg-m (104 lb-ft) at 1800 rpm JIS (SAE gross)

HTH, YMMV

N_R

Just going to throw in my 2 cents worth here. I have the SD25 in my truck. The HP rating on it was 77 @4000 RPM. The transmission and gear ratios are the same as near as I can tell, and the truck has to be close to the same weight. ( 720 VS D21). I also live at 4000 ft too in a very dry AZ type climate (just the dryness not the heat). The fuel mileage when I got it was around 8.7L/100km (27 MPG). The most noticeable improvements I made to it was,,,, Red Line synthetic oils in the transmission and differential, Stanadyne fuel additive, and the recent change to a new fan clutch. I keep the speed around 90-95 KPH, but I agree with N_R in that it is way happier around the 80 - 85 KPH mark, its just all the "drama" that happens behind you at that speed, so I bump it up a bit. The last 4 or 5 tanks of fuel I have received around 7.4 - 7.5 L/100Km ( 31-32 MPG). The absolute fastest that I ever got this little guy was 130 KPH indicated (80 MPH). I did not have my GPS at that time but I believe that it was actually around the 75 MPH. (flat ground, no wind approx 40 F) Like I said I am at 4000 ft here! gasp gasp I got the truck from my dad who bought it new and he says that the milage that I am getting now is about all it ever did when new. FWIW
Paul