[diyengineer]

Anyone have any pictures, links, stories, etc?

[Geof]

Your post is a bit cryptic. Do you mean converting a V8 engine into a hulking great vacuum pump. I have never dome that although I did convert a V2 air compressor into a vacuum pump. I think a V8 would make a great high volume moderate vacuum pump.

[diyengineer]

Yes, use a v8 engine as a vacuum pump. Or other DIY devices.

[Geof]

Do you have an engine and are you planning on doing the conversion. I am interested to find out if it would work as well as I suspect. I think it should be possible to get a bit better than 25 inches of mercury vacuum with a very large volume capacity.

[UUU]

You could use four of the cylinders to drive the beast and the other four as the pump.

[Geof]

And if you had natural gas available and ran it on that the cost would be minimal.

[diyengineer]

And if you had natural gas available and ran it on that the cost would be minimal.

I do have natural gas available as well as propane if thats a possibility. I would rather not pipe/plumb it into my house supply. I would probably go with a bottled route of some kind.

I don't have the V8 yet. I was trying to find someone who has gone this route before finding one. I could get just about any old V8 and hone the cylinders and re-ring it and should work well enough for a pump. How do i convert it to run on propane/natural gas? What other modifications do i need to do to it? Would adding a large 60gallon compressor tank help or do you think it would produce enough suction and flow it wouldn't even need a tank?

[UUU]

There are conversion kits to run gasoline engines on LPG. There's not too much to it, they mostly relate to the storage requirements. But running off gaseous fuel, I'm not so sure about, as the existing carburator or fuel injection system would not be suitable.

[packrat]

Piston style vacuum pumps are not very efficient. A piston make a good compressor but not a very good vacuum pump. Most vacuum pumps are vane or scroll type.

[Geof]

I am wondering why you are asking about converting a V8 when you do not have one already available. Do you just want a cheap, high volume medium vacuum, vacuum pump? As mentioned most vacuum pumps are vane type although scroll and lobe vacuum pumps exist. I believe the vane type can develop the best vacuum but they are not efficient because there is a lot of operating friction from the vanes so they turn a large amount of the elelctrical energy into heat whether or not they are pulling a vacuum. They are also notoriously sensitive to contaminants such as dust. Lobe pumps are more efficient in that they do not have the same operating friction so they don't turn as much eletricity into heat. They are also more tolerant of dust but they cannot develop a vacuum much better than 27"Hg.

However, if your definition of efficiency is the best vacuum available per dollar I think it is very difficult to beat converting a clapped out compressor into a vacuum pump simply by re-routing the inlet and outlet piping. I did this and got a vacuum pump that can move about 10cfm and develop 25"Hg and all it cost was a few dollars for copper pipe fittings and a couple of hours work.

[diyengineer]

I am asking because my large gantry cnc table originally came with a vacuum table top. It still has it but i do not use it because i don't have a beast of a vacuum pump. The table is 15 long and about 7' usable wide. The entire table is plummed for vacuum. A compressor may get you somewhere on a smaller table or fixture but i need something huge. It has 6 electric valves so im guessing it has 6 controllable zones? I don't have a v8 laying around, but i could easily acquire one and convert it. A v8 5.7L liter running on liquid propane, that has water cooled jackets, oil lubricated, and 4 cylinders dedicated to vacuum operation sounds fairly effective for a diy type pump. All the vane and rotary pumps are $$$$$$$. Even if they are junk, too small, or garbage.

That would give me roughly 175 cubic inches of air volume (4 cylinders) and 3-4 times higher revolutions per minute then any compressor could go (assuming it could run decent on half the cylinders). You could put the whole setup on a small cart and small tank.

Just exploring what options i have.

How could i methodically go about checking to see what kind of CFM requirements would be needed? count the number of valves and diameter? Also count the number of holes in the table top?

I am wondering why you are asking about converting a V8 when you do not have one already available. Do you just want a cheap, high volume medium vacuum, vacuum pump? As mentioned most vacuum pumps are vane type although scroll and lobe vacuum pumps exist. I believe the vane type can develop the best vacuum but they are not efficient because there is a lot of operating friction from the vanes so they turn a large amount of the elelctrical energy into heat whether or not they are pulling a vacuum. They are also notoriously sensitive to contaminants such as dust. Lobe pumps are more efficient in that they do not have the same operating friction so they don't turn as much eletricity into heat. They are also more tolerant of dust but they cannot develop a vacuum much better than 27"Hg.

However, if your definition of efficiency is the best vacuum available per dollar I think it is very difficult to beat converting a clapped out compressor into a vacuum pump simply by re-routing the inlet and outlet piping. I did this and got a vacuum pump that can move about 10cfm and develop 25"Hg and all it cost was a few dollars for copper pipe fittings and a couple of hours work.

[diyengineer]

Propane Conversion: How to Make LPG Cars

Here is an article on it. Seems like its easy to do with a carb motor.

[Geof]

......How could i methodically go about checking to see what kind of CFM requirements would be needed? count the number of valves and diameter? Also count the number of holes in the table top?

I don't think you can because there are too many 'unknown unknowns". (To use a phrase which has been ridiculed but which is entirely valid.)

There are many ways to use vacuum fixturing but you can more or less represent them on a triangle: One apex would be small area pods holding totally non-porous material with good gasketing and virtually zero leakage. Another would be a (porous) spoilboard which would hold indeterminate shapes over their whole area with (probably) serious leakage. The third would be a gasketed table holding shapes over most of their area with minimal leakage.

Commercial machine manufacturers have to configure their machines for many eventualities. When using pods it may be adequate to include a small vacuum pump, low flow rate but very good vacuum, because with no leakage even a small pump will develop full holding power. A medium pump with moderate pumping capacity, able to reach a moderate vacuum, will probably be adequate for gasketed fixturing on areas larger than a couple of square feet because the larger area can provide adequate holding power without full vacuum and the gaskets minimize leakage. Way out on the final apex is the pump needed for spoil board fixturing. Because the vacuum is applied through a porous surface from a plenum chamber below there is a vacuum drop between the plenum and the top surface so the plenum needs to be pumped down as low as possible. In addition, because there are no sealing methods used, other than tape or similar on the exposed areas of the porous surface, a (very) large volume flow is needed. So it becomes obvious why commercial machines often have a huge, expensive, noisy, electricity gobbling vane pump. This provides the high vacuum need for the pod option, the high flow and high vacuum needed for the spoil board option and everything in between.

A converted piston device, whether it was originally an air compressor or a V8 engine, is not likely to be adequate for spoil board work because while it may have a high volume capacity it will not develop adequate vaccuum. Similarly it will (probably) not develop adequate vacuum for pod fixturing with the volume capacity being irrelevant in this case. Gasketed fixtuing over moderate areas, or larger, is practical with a piston device and because this can be configured with minimal leakage, and an initial 'surge' of good vacuum to snap the part down onto the gaskets, can be obtained simply by having a large reservoir tank.

Converting a V8 to run on propane using 4 cylinders with the other 4 pumping would certainly provide a portable, albiet noisy and smelly, moderate vacuum pump. Taking the revs up would increase the cfm pumped but would not affect the maximum vacuum available so there would not be much utility in that.

[diyengineer]

I don't think you can because there are too many 'unknown unknowns". (To use a phrase which has been ridiculed but which is entirely valid.)

There are many ways to use vacuum fixturing but you can more or less represent them on a triangle: One apex would be small area pods holding totally non-porous material with good gasketing and virtually zero leakage. Another would be a (porous) spoilboard which would hold indeterminate shapes over their whole area with (probably) serious leakage. The third would be a gasketed table holding shapes over most of their area with minimal leakage.

Commercial machine manufacturers have to configure their machines for many eventualities. When using pods it may be adequate to include a small vacuum pump, low flow rate but very good vacuum, because with no leakage even a small pump will develop full holding power. A medium pump with moderate pumping capacity, able to reach a moderate vacuum, will probably be adequate for gasketed fixturing on areas larger than a couple of square feet because the larger area can provide adequate holding power without full vacuum and the gaskets minimize leakage. Way out on the final apex is the pump needed for spoil board fixturing. Because the vacuum is applied through a porous surface from a plenum chamber below there is a vacuum drop between the plenum and the top surface so the plenum needs to be pumped down as low as possible. In addition, because there are no sealing methods used, other than tape or similar on the exposed areas of the porous surface, a (very) large volume flow is needed. So it becomes obvious why commercial machines often have a huge, expensive, noisy, electricity gobbling vane pump. This provides the high vacuum need for the pod option, the high flow and high vacuum needed for the spoil board option and everything in between.

A converted piston device, whether it was originally an air compressor or a V8 engine, is not likely to be adequate for spoil board work because while it may have a high volume capacity it will not develop adequate vaccuum. Similarly it will (probably) not develop adequate vacuum for pod fixturing with the volume capacity being irrelevant in this case. Gasketed fixtuing over moderate areas, or larger, is practical with a piston device and because this can be configured with minimal leakage, and an initial 'surge' of good vacuum to snap the part down onto the gaskets, can be obtained simply by having a large reservoir tank.

Converting a V8 to run on propane using 4 cylinders with the other 4 pumping would certainly provide a portable, albiet noisy and smelly, moderate vacuum pump. Taking the revs up would increase the cfm pumped but would not affect the maximum vacuum available so there would not be much utility in that.

My personal plan is cutting out .063" aluminum in 5'x12' sheets. Maybe i wont cut through all the way leaving tabs. The sections i do go through on will only be by a hair but will still cut into the sacrificial material. I wont be removing panels and if i do i can always use ceran wrap to cover up the empty holes. I will also have zone control so that should help. As long as i can hold down a sheet of aluminum i will be happy.

I will get some pictures posted up later this evening of the table and how it is setup. Maybe a small video would work better? We shall see!

I would be all for buying and fixing up an old clunker vaccum pump if the price was right, but i just cant seem to find one that is in my price range that will work for my application ($1000?).

[diyengineer]

What would be the limiting vacuum factor in a naturally aspirated carb v8? Say the average compression in a cylinder is 150psi, how does that relate to vacuum? Also, if you used it as a compressor would 150psi be your max no matter how big of tank (if the cylinder compression was 150psi). You seem to know your stuff haha! I read your entire compressor thread very very cool!



Thanks Geof!

I don't think you can because there are too many 'unknown unknowns". (To use a phrase which has been ridiculed but which is entirely valid.)

There are many ways to use vacuum fixturing but you can more or less represent them on a triangle: One apex would be small area pods holding totally non-porous material with good gasketing and virtually zero leakage. Another would be a (porous) spoilboard which would hold indeterminate shapes over their whole area with (probably) serious leakage. The third would be a gasketed table holding shapes over most of their area with minimal leakage.

Commercial machine manufacturers have to configure their machines for many eventualities. When using pods it may be adequate to include a small vacuum pump, low flow rate but very good vacuum, because with no leakage even a small pump will develop full holding power. A medium pump with moderate pumping capacity, able to reach a moderate vacuum, will probably be adequate for gasketed fixturing on areas larger than a couple of square feet because the larger area can provide adequate holding power without full vacuum and the gaskets minimize leakage. Way out on the final apex is the pump needed for spoil board fixturing. Because the vacuum is applied through a porous surface from a plenum chamber below there is a vacuum drop between the plenum and the top surface so the plenum needs to be pumped down as low as possible. In addition, because there are no sealing methods used, other than tape or similar on the exposed areas of the porous surface, a (very) large volume flow is needed. So it becomes obvious why commercial machines often have a huge, expensive, noisy, electricity gobbling vane pump. This provides the high vacuum need for the pod option, the high flow and high vacuum needed for the spoil board option and everything in between.

A converted piston device, whether it was originally an air compressor or a V8 engine, is not likely to be adequate for spoil board work because while it may have a high volume capacity it will not develop adequate vaccuum. Similarly it will (probably) not develop adequate vacuum for pod fixturing with the volume capacity being irrelevant in this case. Gasketed fixtuing over moderate areas, or larger, is practical with a piston device and because this can be configured with minimal leakage, and an initial 'surge' of good vacuum to snap the part down onto the gaskets, can be obtained simply by having a large reservoir tank.

Converting a V8 to run on propane using 4 cylinders with the other 4 pumping would certainly provide a portable, albiet noisy and smelly, moderate vacuum pump. Taking the revs up would increase the cfm pumped but would not affect the maximum vacuum available so there would not be much utility in that.

[Geof]

What would be the limiting vacuum factor in a naturally aspirated carb v8? Say the average compression in a cylinder is 150psi, how does that relate to vacuum?....

The compression ratio equates to the vacuum that the piston can develop more as an expansion ratio. Imagine the piston at top dead center with the valves closed; there is a certain trapped volume in the space above the piston. Now move the piston to bottom dead center and the volume is increased by whatever the compression ratio of the engine is. This might be 8:1 so now the pressure inside the cylinder (ignoring any leakage) will be 1/8 of an atmosphere and that will be the maximum theoretical vacuum you can generate with an unmodified engine is around 26"Hg. This more or less matches what you will see on a carburated engine with a vacuum gauge when you close the throttle at high rpm.

(Theoretically) this is okay but you will not reach this maximum because leakage exists. Also on an unmodified engine you are not developing vacuum on every stroke and your valves close at the wrong time and far too slowly to be effective.

For any conversion to be effective it would be necessary to replace the pistons (which are probably crowned) with flat topped pistons that just came flush with the top of the cylinder and replace the heads with a flat head fabricated out of aluminum plate. This would take away the combustion chamber volume and give a much larger expansion ratio. Replacing the heads also takes away the valve mechanism and reed valves would be installed in the fabricated heads (similar to the valves in a piston compressor).

[packrat]

Would this work? Surplus Center - 5 HP 190 CFM 5" HG VACUUM PUMP

Using half an engine to drive and the other half as a pump or compressor is not as simple as it seems. To do it right you would need to get a special camshaft ground. Engines don't run good with just half of the spark plugs removed, try it sometime

Then comes the problem of oil control. The rings are made to work with the pumping force in one direction. A vacuum pump does just the opposite.

Then comes valve leakage. The valve in a engine are again made to work in the opposite direction than a vacuum pump. When the combustion in the engine takes place the force against the valve head is forcing it into the seat. Vacuum will pull it away from the seat.

[Geof]

If it develops only 5"Hg it hardly qualifies as a vacuum pump it is more like a dust extractor or vacuum cleaner.

[diyengineer]

I have abandoned the massive vacuum pump idea. As you suggest its not feasible. I have moved onto a dedicated gasketed work fixture.

All Star Adhesives - gasketing products for CNC routers and v-groove tape for v-grooving and coving applications

I'll be making a new thread.

[j r teichman]

i did own a vac trailer many years ago for liquid waste w/ a chevy 6 cly. motor driven by hydraulic motor i think the name tompson rings a bell they sold the kit or complete pump it worked very well hi vac more than 20 in.but slow on the cfm side for tanker truck.