What is the advantage to using a large amount of crankcase vacuum in a race engine?
HORSEPOWER; WHAT ELSE?
And it is the cheapest HP you can buy. If you can achieve a crankcase vacuum level of at least 8 inches HG, you will very likely realize an immediate power gain of at least 15 HP.
If you run a dry sump system with a three stage pump (one pressure stage, two scavenge stages), in most cases you cannot achieve a sufficient level (8 "HG) of crankcase vacuum to achieve that power gain. The extra cost of a four stage pump will net you around 15 HP in most cases. At NRC, it costs less than $100 for that extra stage. How can you beat that price for an extra 15 HP?
HOW IT WORKS
The reduced pressure ("vacuum") in the crankcase is generated by having a substantial excess of scavenging capacity with respect to the engine's oil flow rate. The "vacuum" increases the pressure differential across the ring package, producing an improved ring seal. The improved ring seal allows the use of a low-tension (reduced friction) ring package, yielding a power increase as well. Further, the reduced crankcase pressure dramatically reduces windage losses at high RPM.
Here are a few observations we have made over the years of developing winning race engines. First of all, in most engines, the expected power gains will occur with 8 to 10 inches HG crankcase vacuum. Beyond that point, more vacuum does not generally produce any measurable power gain until (a) you get more than 20 inches HG of vacuum AND (b) you are operating in excess of approximately 8300 RPM.
However, we generally size the systems on our engines to produce around 14 "HG when the engine is fresh. That provides sufficient capacity so that as the engine wears and blowby increases, there will still be sufficient scavenging capacity to achieve the 8"HG minimum, and power does not drop off noticeably.
If you want to run a high level of crankcase vacuum (18 inches HG or more), there must be provisions in the engine to supplement the lubrication that used to occur when oil was being thrashed about by the moving parts ("windage"). There will likely be problems with at least wristpin and cam follower lubrication. The best solution will be the addition of piston oilers and, if your engine has a flat tappet cam, provisions for extra lubrication of the cam lobe-to-lifter interface will certainly be required. If you are trying to achieve over 18 "HG, you will need to install special crankshaft seals (front and rear) which have the sealing lips reversed to hold that higher level.
In order to achieve 8 "HG or more, the engine must be well sealed. In order to check for leaks, you should pressurize the assembled engine. You will need an adjustable pressure regulator with a low range (like 0 - 10 psi) air pressure gauge. With the engine completely assembled, cap off the fitting that feeds the oil into the main oil gallery, and cap off the scavenge exit fittings from the pan. Install a pressurizing fitting into one of those caps.
We use 6 to 8 PSI (which is equal to 12 - 16 inches HG) of air pressure to test our 8 - 14" engines. Start with the regulator set to ZERO and slowly add pressure, up to the max test pressure you decide to use, and listen for air leaks. If you hear any and can't pinpoint the source, spray some windshield foamy cleaner in the area with a hand spray-bottle. NOTE: You will get better sealing with the cork rocker cover gasket than with the rubber steel lined rocker gaskets.
You should be aware of a few potential glitches. Some silicone sealers cure differently than others, and most take weeks to cure if it is 1/8” or thicker. An uncured bead of silicone sealant will tend to be pushed out when pressure is applied or sucked in if not cured long enough. At NRC we think the best silicone sealer is the OEM stuff, and the Permatex Ultra-Gray is also a good product. The OEM and Permatex Ultra-Gray tend to cure harder on the exposed areas which makes it a little harder to be pushed out or sucked in. However, both these products take a considerable amount of time to fully cure when the bead is thick.
We have found that the softer curing stuff tends to develop leaks after the engine has been placed in service, because it moves around during the race. Recently, we have been experimenting with a two-part silicone, which is similar to two-part epoxy, but looks and feels like silicone and cures for limited use in about 30 minutes, and completely cures in 24 hours. So far, the results are encouraging.
DRY SUMP OIL PANS
The pickups in most of the aftermarket pans are horrible. The pickup fittings which are usually found are rectangular boxes with sharp, square corners. Those square corners play havoc with the orderly pickup of scavenge oil and add to the turbulence and aeration which occurs at that end of the system. A good hint about how the pickups should be formed can be seen by examining the interior shape of the pickup attachments on your wife's vacuum cleaner.
Next, almost all the aftermarket pans use dash-12 scavenge fittings. Common sense says there is more volume inside a given length of a dash-12 hose than in the same length of a dash-10 hose. Using the smaller dash-10 hose causes no meaningful increase in flow losses, but it causes a larger percentage of the hose volume to be filled with oil instead of air. Reducing down to dash-10 scavenge lines will help achieve a higher level of vacuum. You can buy the reducers from the common suppliers such as Moroso.
As far as pan design is concerned, the wider and deeper the pan is, the easier it is to control the thrashing of oil, and the easier it is to scavenge the pan well. We also found that the better the pan design (wider, deeper, with scrapers, louvers, one-way-mesh, etc) the expected gains from a high vacuum will be less. The high-vacuum system will produce the best power increases on engines with shallow pans, which are often required as a result of engine placement restrictions or from chassis construction.
