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 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.

For smaller engines and low power adder ouputs we recommend our VP101 or VP102 Sportsman pump driven at 54% of crankshaft speed with a single input line.

For large cubic inch small blocks with power adders moving towards the 600 hp range, we we advise using our VP101 Sportsman Pump running at 64% of crankshaft speed and #12 inlet line.

For engines around the 600 hp range, we we advise using our VP101 Sportsman Pump running at 64% of crankshaft speed and #12 inlet line. Pump speed will need to be increased to 75% in most cases for best results as your power levels near 750 HP for the VP101/VP102 Pumps. The 75% pump speed should not be used for engines turning over 8000 rpm to avoid overspeeding the pump.

If it's likely you'll be making more power down the road, you might want to use our our VP104 Super Pro Pump with single #10 intake line running at 54% of crank speed for a 750 HP Engine. This larger pump can be spun faster if needed, and you can increase the inlet line size to flow even more air. #12 line is a good upgrade to more more air at lesser drag to the pump, however it is more expensive option.

On the low end of this range we suggust using our VP104 Super Pro Pump with single #10 intake line running at 54% of engine speed for optimum results. At the upper range of 900 hp, we recommend increasing the pulley ratio to 64% of crank speed and increasing the inlet line size to #12 to one valve cover, or two #10 lines to both valve covers.

For large cubic inch engines with power adders the VP104 Super Pro Pump is needed at 64% on the lower HP end of this range to 75% of crank speed at approximately 1500 HP and above, we also recommend #12 inlet line to one valve cover, or #10 lines to two valve covers for maximum air flow.

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.

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 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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gzms@gzmotorsports.com