Vacuum Pump Sizing Issues
Vacuum pumps can be rated by their ability to flow air, the more air a vacuum pump flows the more vacuum it will make on a given engine. A "small" vacuum pump would indicate a less airflow capacity than a "big" vacuum pump. Airflow is measured in CFM (cubic feet per minute), vacuum is measured in "inches of Mercury"
All engines create a certain amount of "blow by" (leakage of compressed fuel and air past the rings into the pan area). The "blow by" airflow created creates pressure in the crankcase, the vacuum pump "sucks" air out of the crankcase with its "negative" airflow. The net difference between the air low being "sucked" out and the airflow being "blown" in by blow by yields vacuum. If the airflow being "sucked" out is less than, or not much more than the air flow being blown in, the vacuum pump is not creating enough "sucked" airflow to eliminate pressure in the crankcase or to create adequate vacuum to not only stop leaks, but to create horsepower.
Typically smaller engines generate less "blow by" and don’t require as "big" of a vacuum pump as large displacement engines. However the addition of low tension ring packages, power adders such as Nitrous Oxide, Alcohol, Blowers or Turbochargers (and the ring and piston types, clearances and modifications that go with those power adders) will increase crankcase pressures requiring either a "larger" pump or to spin a "smaller" pump slightly faster. The following guide is based on our experience of testing vacuum pump airflow rates and typical crankcase pressures found in racing motors.
A "small" vacuum pump, that is making inadequate but almost adequate vacuum on an engine may be made to act like a "larger" vacuum pump by increasing the rotational speed, or RPM, of the pump. Of course maximum engine RPM needs to be considered as well, as vacuum pump RPM should be considered. Although we have vacuum pumps being successfully operated at a maximum RPM of 7000 or so, we highly recommend keeping the vacuum pump RPM below 5000 RPM. Alternatively, a "larger" vacuum pump could be used instead. However, our experience has shown that using a vacuum pump that is to "big" may eliminate horse power gains altogether because of the parasitic horsepower it takes to run the pump. Keep in mind that it takes horsepower to pump air, so one should choose a vacuum pump based on your specific application, bigger isn’t always better.
How much vacuum should you use? You should check with your engine builder as there may be some special issues or firm opinions that they may have. In general GZMS recommends no more than 15" of vacuum in a drag race wet sump application without pressure feeding oil to the wrist pins. In our own in house testing, anything over 12" seemed to offer relatively nominal increases in horsepower (maybe 15% between 10" and 15").
Why limit vacuum? Engine builders appear to believe that the lack of oil to wrist pins caused by removal of too much oil mist from the crankcase causes wrist pin damage. Some engine builders report fluctuations in oil pressure above 12", we have not noticed that on engines we have observed during testing. However in a recent article, it was suggested that the air velocity passing through the block to heads at the oil return locations causes resistance to oil flowing back to the pan, which could indeed reduce oil pressure. An air line from the fuel block off on a Chevrolet to the valve cover helps mitigate this problem as well as possibly helping to balance the vacuum in the crankcase to that in the valve cover.
Why use a Vacuum Control Valve? Naturally to limit the maximum vacuum. Also, it is more advantageous to achieve maximum vacuum early in the power band, so if you wish to do that it makes sense to use a vacuum pump with enough air flow to require the use of a vacuum control valve. If you do not want to install a vacuum control valve use a smaller pump than recommended, check your vacuum and determine if you really need one.
Vacuum Pump Application Guide
427ci and Smaller…
427ci and Smaller, Naturally Aspirated, wet sump For engines smaller than 427 we recommend our VP101 Sportsman pump driven at 64% of crankshaft speed
427ci and smaller, with Low Tension Rings or Small Power Adder
For engines smaller than 427 that have low tension racing rings or a smaller power adder, like a 150 hp Nitrous Kit, we advise using our VP101 Sportsman Pump running at 75% of crankshaft speed.
427ci and smaller, with Low Tension Rings and/or Large Power Adder
For engines smaller than 427 that have low tension racing rings and a large power adder, like a 300 hp and up Nitrous Kit, we advise using our VP103 Pro Pump running at 64% of crankshaft speed.
427ci and smaller, Dry Sump Applications
For engines smaller than 427 with a Dry Sump system (which includes a pressure feed of oil to the wrist pins) we advise using our VP101 Sportsman Pump running at 75% of crankshaft speed, or our VP103 Pro Pump running at 64% or slower.
427ci to 522ci
427ci to 522ci, Naturally Aspirated For engines smaller than 522 we recommend our VP101 Sportsman pump driven at 64% or 75% of crankshaft speed
427ci to 522ci, with Low Tension Rings or Small Power Adder
For engines smaller than 522 that have low tension racing rings or a smaller power adder, like a 150 hp Nitrous Kit, we advise using our VP103 Pro Pump running at 64% of crankshaft speed.
427ci to 522ci, with Low Tension Rings and/or Large Power Adder
For engines smaller than 522 that have low tension racing rings and a large power adder, like a 300 hp and up Nitrous Kit, we advise using our VP104 Super Pro Pump running at 64% of crankshaft speed.
427ci to 522ci, Dry Sump Applications
For engines smaller than 522 with a Dry Sump system (which includes a pressure feed of oil to the wrist pins) we advise using our VP103 Pro Pump running at 64% of crankshaft speed or the VP104 Super Pro pump running at 64% crankshaft speed..522ci to 555ci
522ci to 555ci, Naturally Aspirated For engines smaller than 555 we recommend our VP103 Pro pump driven at 64% of crankshaft speed
522ci to 555ci, with Low Tension Rings or Small Power Adder
For engines smaller than 555 that have low tension racing rings or a smaller power adder, like a 150 hp Nitrous Kit, we advise using our VP103 Pro Pump running at 75% of crankshaft speed.
522ci to 555ci, with Low Tension Rings and/or Large Power Adder
For engines smaller than 555 that have low tension racing rings and a large power adder, like a 300 hp and up Nitrous Kit, we advise using our VP104 Super Pro Pump running at 64% of crankshaft speed.
522ci to 555ci, Dry Sump Applications
For engines smaller than 555 with a Dry Sump system (which includes a pressure feed of oil to the wrist pins) we advise using our VP104 Super Pro Pump running at 64% of crankshaft speed.555ci and larger
555ci and Larger, Naturally Aspirated For engines larger than 555 we recommend our VP103 Pro pump driven at 64% of crankshaft speed
555ci and Larger, with Low Tension Rings or Small Power Adder
For engines larger than 555 that have low tension racing rings or a smaller power adder, like a 150 hp Nitrous Kit, we advise using our VP104 Super Pro Pump running at 64% crankshaft speed.
555ci and Larger, with Low Tension Rings and/or Large Power Adder
For engines larger than 555 that have low tension racing rings and a large power adder, like a 300 hp and up Nitrous Kit, we advise using our VP104 Super Pro Pump running at 64% of crankshaft speed.
555ci and Larger, Dry Sump Applications
For engines larger than 555 with a Dry Sump system we (which includes a pressure feed of oil to the wrist pins) advise using our VP104 Super Pro Pump running at 64% of crankshaft speed.Vacuum Pump Application Notes
- Pulley ratio is typically 64% of crankshaft speed, this is generated by using a 3.5" Crankshaft Pulley and a 5.5" Pump Pulley unless noted otherwise.
- Ring Packages such as low tension rings typically generate an increase in ring blowby requiring a larger, or faster spinning pump
- Power Adders such as Nitrous Oxide, Superchargers, Turbochargers all increase cylinder pressure generating increased ring blowby requiring a larger or faster spinning pump. The amount of Nitrous you are spraying, or the amount of boost you are generating increases the needs for a larger or faster spinning pump.
- Large Displacement engines move more air and generate proportionately larger amounts of blowby
- The use of alcohol, especially in a "rich" condition generally reduces net vacuum and could require a slightly larger pump than a gasoline engine.
- Vacuum leaks in the crankcase, valve covers, distributor base, timing chain cover, oil dipstick, etc. reduce the amount of vacuum you will generate. You can easily check your engine by pressurizing it with air, be careful though, you don’t need much to find leaks and you could damage gaskets and seals.
- All engines are different depending on how fresh they are, how much ring end gap is used, cylinder prep and ring design. This is a general guide based on typical engines GZMS has fitted over the years as well as customer reports, some situations may be different, if in doubt email or call our tech support line.
Pump Sizing Chart
Adjust pump size to desired net crankcase vacuum desired. The following chart shows the net vacuum you can expect in a naturally aspirated motor with standard or low tension rings and no vacuum control valve. These values assume the rings are in good shape (leak down is not excessive) and there are no vacuum leaks in the engine. Note that net vacuum measured will tend to increase with RPM unless "blow by" into the crankcase increases enough to reduce the negative net airflow out of the crank case, thus reducing vacuum at higher RPM. It is common to see the vacuum increase to a maximum and then reduce some at max RPM if the vacuum pump is not rated for enough airflow to maintain a net vacuum throughout the RPM range. Of course to achieve a constant net vacuum you need to have a vacuum control valve installed in order to let air into the engine when the maximum desired vacuum level is achieved.
Engine Size | VP101 Pump | VP103 Pump | VP104 Pump |
400ci and smaller | 15-20" | 20"+ | 20"+ |
400ci - 468ci | 12-16" | 14-20"+ | 16-20"+ |
468ci – 522ci | 10-14" | 12-20"+ | 14-20"+ |
522ci – 555ci | 8-12" | 10-17" | 15-20" |
555ci and larger | 6-10"+ | 10-15"+ | 14-20" |
The larger the pump you use, the sooner the vacuum will be generated, and the quicker you will reach the limit of a vacuum control valve.
The larger the pump, the more horsepower required to turn it. For example, a VP103 Pro or VP104 Super Pro vacuum pump may not generate a horsepower gain on a small, naturally aspirated engine with standard rings that inherently will not create much "blow by".
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