Vacuum pump alternative

Dealing with all subsystems specific to the diesel powered Datsun-Nissan 720 pickup trucks.

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bacho
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#16

Post by bacho »

But what if we pulled the turbo intake through a venturi? Could something like that be possible to create enough vaccum?
1992 nissan pathfinder 4x4
1985 KC 720 4x4
1982 KC 720
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asavage
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#17

Post by asavage »

Mike:
The SD's throttle plate does not close nearly as much as in the typical gasser. For that reason, insufficient vacuum is developed in the intake to provide adequate brake booster operation, and running down the road under load, there is effectively almost no intake manifold vacuum (which is the same situation as many gasser FI vehicles, at least my old '73 Volvo 145 B20F had nearly zero vacuum at highway load). It's not the volume, it's the amplitude of the pressure. Is that clearer?

Philip:
Though only 6" will move the pneumatic governor full sweep, that 6" is developed by the venturi port, not the intake manifold. I know you know this, I am merely amplifying your statement for the benefit of those who don't.

Bacho:
No. If you had a venturi small enough (necked down enough) to create sufficient vacuum to operate the brake booster, you may as well have not put the turbo on at all!
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ecomike
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#18

Post by ecomike »

Al, thanks, yes that is clearer, but:
Quoting from this web site:

http://web.ncf.ca/ag384/vacuum.txt

I see the following statements regarding gasoline car engines and their vacuum at the intake manifold.

"Haynes emissions control manual
a. testing at various speeds
- engine starting vacuum should be 1 to 4 in Hg. To test
disable ignition (ground wire from coil), hold throttle
wide open, crank engine slowly with starting motor.
- healthy engine at idle should read steady 15 to 20 in Hg.
- healthy engine at 2000 rpm should read steady 19 to 21 in
Hg.
- healthy engine at open throttle should read close to 0 in
Hg.
- healthy decelerating engine reading should jump to 21 to
27 in Hg as open throttle released."

So even a gasoline operated engine reaches 0" of vacuum at WOT. The gasoline operated engines use a vacuum reservoir or tank along with a one way valve that allows the engine to pull a vacuum on the vacuum bottle and vacuum booster, but does not allow the vacuum to leak back out of the bottle and vacuum lines during low engine vacuum operating conditions.

You say that insufficient vacuum is developed during idle. What is the engine vacuum (at the intake manifold, not the venturi port) at idle on the SD22?

Correct me if I am wrong on this, but doesn't the pneumatic governor operate on two vacuum signals, one from the intake manifold, the other from the connection just above the throttle plate, the one you called the venturi port. And thus the governor operates on the vacuum DIFFERENTIAL pressure between the two vacuum locations? My thinking is that Nissan did not want an intermittent air supply, for example multiple vacuum lines occasionally feeding non throttle body air to the engine from the brake booster and other vacuum operated lines, because it would interfere with the engines pneumatic governor.

What I am saying is that perhaps Nissan had other reasons, like the pneumatic governor vacuum control across the throttle plate for using a separate vacuum pump.
Do all these small diesel engines use a separate vacuum pump, or just the ones with the pneumatic governors?

How much vacuum does the SD22 develop in the intake at idle?

How much vacuum is really needed to operate a brake booster? How much vacuum is the SD22 vacuum pump generating?


asavage wrote:Mike:
The SD's throttle plate does not close nearly as much as in the typical gasser. For that reason, insufficient vacuum is developed in the intake to provide adequate brake booster operation, and running down the road under load, there is effectively almost no intake manifold vacuum (which is the same situation as many gasser FI vehicles, at least my old '73 Volvo 145 B20F had nearly zero vacuum at highway load). It's not the volume, it's the amplitude of the pressure. Is that clearer?

Philip:
Though only 6" will move the pneumatic governor full sweep, that 6" is developed by the venturi port, not the intake manifold. I know you know this, I am merely amplifying your statement for the benefit of those who don't.

Bacho:
No. If you had a venturi small enough (necked down enough) to create sufficient vacuum to operate the brake booster, you may as well have not put the turbo on at all!
Regards,

Mike

1985 Jeep Cherokee Pioneer, 2WD, retrofitted with SD-22 & 5 spd manual trans, a 4X4 Gas Wagoneer ltd. (XJ) Jeep, 4.0 L w/ AW4 auto, and now 2 spare 2wd Jeeps, 87 & 89.
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asavage
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#19

Post by asavage »

The pneumatic governor only uses a the venturi port for its signal; there is no manifold connection. There is a compensation line to the air intake plenum to give a differential reading of a sort in case of air filter restriction.

Many non-pneumatic-governor diesels use a mechanical vacuum pump: MB, Toyota, and Isuzu come to mind as examples off the top of my head.

Vacuum-operated power brake boosters universally utilize a vacuum reservoir and check valve arrangement, since the 40's. But there is no peak vacuum condition in the SD to "replenish" the reservoir.

Philip can quote you SD the intake manifold vacuum, as he's measured it IIRC. Or you can measure yours, if you have the time. I don't have a running SD here at the moment. Because the thottle plate never closes down (in the same manner as a gasser, anyway), there is insufficient restriction to generate substantial manifold vacuum. Didn't member Zen measure his with his boost gauge in another thread? You might search.

Making up a number, power brakes are designed to operate at > 18" vacuum and ideally >20". Six inches isn't going to cut it with the booster diaphragm size in place in our Nissans.
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ecomike
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#20

Post by ecomike »

Well I just learned something interesting while trying to find a reference for the minimum vacuum required for typical brake boosters.

"On cars with a vacuum booster, the brake pedal pushes a rod that passes through the booster into the master cylinder, actuating the master-cylinder piston. The engine creates a partial vacuum inside the vacuum booster on both sides of the diaphragm. When you hit the brake pedal, the rod cracks open a valve, allowing air to enter the booster on one side of the diaphragm while sealing off the vacuum. This increases pressure on that side of the diaphragm so that it helps to push the rod, which in turn pushes the piston in the master cylinder.

As the brake pedal is released, the valve seals off the outside air supply while reopening the vacuum valve. This restores vacuum to both sides of the diaphram, allowing everything to return to its original position. "

From:
http://auto.howstuffworks.com/power-brake2.htm

It also says that diesel engines do not produce a vacuum and must use a separate vacuum pump. While this is not entirely correct the way it is worded, I now understand what you are getting at, at least in regards to typical diesel engines that do not have a throttle plate on the intake like the SD22. However, the NA SD22 is not typical. I suspect that Nissan simply followed typical diesel engine design and continued using a separate vacuum pump, or they ran into control problems with the pneumatic governor design if they had a non-throttle plate air supply from a brake booster vacuum line bypassing the throttle plate, or as Al already pointed out, at heavy loads and WOT the vacuum is too low, and that may be a safety concern if the vacuum line has a leak!

I suspect Nissan had pneumatic governor control problems if they tried it, or just did not try it and used the standard diesel engine industry standard of a separate vacuum pump. One of you experimenters out there might try testing the engine intake vacuum of a NA SD22 engine at various loads and engines speeds and then try a slow, or rapid vacuum leak to simulate a vacuum leak or air flow from brake booster operation to see how it affects the IP governor and engine operation.

Here is a final thought. After reading how the air pressure is sucked back out of one side of the brake booster right after using the brakes,

"As the brake pedal is released, the valve seals off the outside air supply while reopening the vacuum valve. This restores vacuum to both sides of the diaphragm, allowing everything to return to its original position"

It seems to me that such a rush of air into the intake vacuum line right after releasing the brakes would cause a pulse signal to the IP pneumatic governor that would negatively affect engine speed for a brief moment, causing it to suddenly accelerate or maybe stall (not sure which yet). Once again a control problem.

By the way, if you search for small lab vacuum pumps at a place like LabX.com you might find a good used, small, low power, 12 volt vacuum pump for about $50. Thomas and Gast also make small 12 volt vacuum pumps for under $200. Graingers stocks several of their models.
Regards,

Mike

1985 Jeep Cherokee Pioneer, 2WD, retrofitted with SD-22 & 5 spd manual trans, a 4X4 Gas Wagoneer ltd. (XJ) Jeep, 4.0 L w/ AW4 auto, and now 2 spare 2wd Jeeps, 87 & 89.
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asavage
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#21

Post by asavage »

I wouldn't think a lab pump would have sufficient volume for the task.

I thought I searched for a retailer of Gast pumps last year and struck out.
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ecomike
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#22

Post by ecomike »

asavage wrote:I wouldn't think a lab pump would have sufficient volume for the task.

I thought I searched for a retailer of Gast pumps last year and struck out.
Depends on the lab pump. I have several that are 2 & 5 hp! They are designed for lab vacuum ovens and R & D work. The ones I was thinking of are used for things like OSHA air sampling, stack gas sampling, continuous duty.

So the question is what flow rate, and maximum vacuum does the brake booster need to function properly. I have no idea. I suspect it is minimal when used with a vacuum reservoir.

Southwestern Controls in Dallas and Houston are Gast distributors, and Graingers stocks certain Gast and Thomas models. I use a lot of the vacuum pumps and air compressors in my daily job.

The Thomas model #907CDC18, Graingers #4XL33 is 1/10th hp, with 2.05 CFM flow rate at atmospheric pressure, and 0.48 CFM at 15" of Hg vacuum. Maximum vacuum is 22.5" of Hg. It was listed at $170 in 2006 catalog. It pulls 10.8 amps at 12 volts. No doubt the price has increased a wee bit since last year. They are online at http://www.grainger.com
Regards,

Mike

1985 Jeep Cherokee Pioneer, 2WD, retrofitted with SD-22 & 5 spd manual trans, a 4X4 Gas Wagoneer ltd. (XJ) Jeep, 4.0 L w/ AW4 auto, and now 2 spare 2wd Jeeps, 87 & 89.
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ecomike
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#23

Post by ecomike »

bacho wrote:But what if we pulled the turbo intake through a venturi? Could something like that be possible to create enough vacuum?
In addition to Al's comment, a venturi will not generate a low enough vacuum to run the brake booster. You need a positive displacement vacuum pump to get there.
Regards,

Mike

1985 Jeep Cherokee Pioneer, 2WD, retrofitted with SD-22 & 5 spd manual trans, a 4X4 Gas Wagoneer ltd. (XJ) Jeep, 4.0 L w/ AW4 auto, and now 2 spare 2wd Jeeps, 87 & 89.
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philip
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#24

Post by philip »

ecomike wrote:In addition to Al's comment, a venturi will not generate a low enough vacuum to run the brake booster. You need a positive displacement vacuum pump to get there.
Of course, there is this exception rule: :wink:

Air Driven: There's no oil or moving parts to replace. Specifications Drive: Compressed Air Free Air Displacement: 1 CFM Number of Stages: Venturi Input Pressure: 75 psi Vacuum Rating: 29.7 in./Hg Intake Fitting: 1/4" FPT and 1/2" Acme Dimensions: 4-1/2"H x 4"W x 7-1/2"D Weight: 2lbs/1kg

This Unit Is Maintenance Free With No Moving Parts!
-Philip
Passed 08May2008
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1982 Datsun 720KC SD-22

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ecomike
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#25

Post by ecomike »

philip wrote:
ecomike wrote:In addition to Al's comment, a venturi will not generate a low enough vacuum to run the brake booster. You need a positive displacement vacuum pump to get there.
Of course, there is this exception rule: :wink:

Air Driven: There's no oil or moving parts to replace. Specifications Drive: Compressed Air Free Air Displacement: 1 CFM Number of Stages: Venturi Input Pressure: 75 psi Vacuum Rating: 29.7 in./Hg Intake Fitting: 1/4" FPT and 1/2" Acme Dimensions: 4-1/2"H x 4"W x 7-1/2"D Weight: 2lbs/1kg

This Unit Is Maintenance Free With No Moving Parts!
There is always an exception to the rule, or so I am told.

I have seen similar ones used to pull a vacuum on A/C systems after A/C repairs, but they typically only achieve about 22 to 25" of vacuum tops, which is why I am not a fan of them. I have never seen one, or should I say noticed one that could pull 29.7" of vacuum using compressed air at 75 psi.

As Philip well knows, the moving parts he failed to mention (probably on purpose) are in the air compressor and motor needed to produce the 75 PSIG at 1 SCFM needed to produce the vacuum with the venturi. There are still no free lunches. I must admit I will need to look at that web site to see if they really can achieve that much vacuum with 75 psi air, or if they are overstating its capability.

:wink:
Regards,

Mike

1985 Jeep Cherokee Pioneer, 2WD, retrofitted with SD-22 & 5 spd manual trans, a 4X4 Gas Wagoneer ltd. (XJ) Jeep, 4.0 L w/ AW4 auto, and now 2 spare 2wd Jeeps, 87 & 89.
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philip
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#26

Post by philip »

ecomike wrote:SNIP- I must admit I will need to look at that web site to see if they really can achieve that much vacuum with 75 psi air, or if they are overstating its capability.

:wink:
Agreed, their system probably sucks. :wink:
-Philip
Passed 08May2008
My friend, you are missed . . .

1982 Datsun 720KC SD-22

"Im slow and I'm ahead of you"
davehoos
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#27

Post by davehoos »

ive been looking at vac/water/powersteer/brake pumps on late model cars with a veiw to using when these get scrapped.

the new astra [GM-opel] VW,saab,bmw and alfa has an elctric pump T peiced to manifold vacum and some have a venturi in the intake.
i think these are to make certian that there is brakes at all times without effecting emisions.

these look good.ive been looking at powersteer oil pressure controlled brake booster and radiator fans.the brake units off a big BMW are tiny near the master cyl but the accumilator and electric pump is large.

landrovers from 70's in australia have a throttle plate that closes for idle to provide vacum in a tank.the plate opens above idle so you have to get of the throttle to stop.
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Carimbo
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#28

Post by Carimbo »

In case anyone is wanting to replace their alternator/vacuum pump w/ a higher-rated alternator (GM cs130?) and a separate electric vacuum pump, these GM cars had electric vacuum pumps (used mostly as an auxiliary brake booster vacuum source), so search away at your favorite JY:

Buick
82 Skyhawk-J
82- 85 Skylark-X

Pontiac
85-86 6000
82 J2000
82-84 Phoenix-X
Chevrolet
84-86 Celebrity-A
82 Caprice-B
82 Cavalier-J
82-85 Citation-X
Cadillac
82 Cimmaron

Oldsmobile
86 Cierra-A
82 Firenza-J
82-84 Omega-X
Others
93-95 Z-28 and some 96 BOP
Caddy's in 90-96

You'll find these pumps in front of the driver's side front fender well. Look for the vacuum lines running along the fender from the brake booster. Get the electrical plug that goes to the pump and the rubber mounting plate.
The pump is also available new from GM. Various GM part numbers for vacuum pumps are 22062562, 22034995, 24505066, 10090521. Ford part numbers E4FZ-2A451-A
Power connections are easy. Black is ground. Red wire goes to a hot source. Black with white stripe goes to switched power. Either the accessory side of the ignition switch or to a manual switch so that the pump can be run during cool down in the pits. The pump is powered via the A connection, but will only run if it receives power from the ignition switch on the B connection.

Connection A (Red wire) battery positive, 12 volts+ through a fuse.
Connection B (Black with white stripe) ignition positive, 12 volts+
Connection C (plugged) not used (not used in factory system either)
Connection D (Black wire) ground, 12 volts-

The pump should pull 16 inches of vacuum (now I'm wondering if this is sufficient) when run with 13.7 volts and 19 inches when run with 16 volts.

Also these pumps show up in Ford Escort & Mercury Lynx diesel cars.

Source: http://toyotadiesel.com/forums/showthre ... L-T&page=3
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asavage
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#29

Post by asavage »

I saw that info over there too, and I have my doubts about those little pumps getting to 16". I've replaced a couple, and they are, as stated, booster pumps, plumbed to provide additional vacuum on gas engines that don't make much.

Has anyone put a vacuum gauge on a Nissan diesel and determined what is the normal reading?
Regards,
Al S.

1982 Maxima diesel wagon, 2nd & 4th owner, 165k miles, rusty & burgundy/grey. Purchased 1996, SOLD 16Feb10
1983 Maxima diesel wagon, 199k miles, rusty, light yellow/light brown. SOLD 14Jul07
1981 720 SD22 (scrapped 04Sep07)
1983 Sentra CD17, 255k, bought 06Jul08, gave it away 22Jun10.
Carimbo
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#30

Post by Carimbo »

That's what I was starting to wonder about also. Would a reservoir tank help?

This is evidently the source info for the source I cited above:

http://users.sfo.com/~eagle/figs/vacpump/vac.html
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