Turbo Oiling and Cooling, Part 1

When a turbo is being fitted to a previously naturally aspirated engine, oil and water supplies for the turbo need to be organised. Normally, oil is supplied via the standard engine oil pump and is then returned to the sump. But where do you pick up the supply from – and how exactly do you get it back to the engine? And what about cooling water? How do you tap into the engine’s cooling system without cooking the engine?

In this story we background what you need to know and then next week, we show the step-by-step installation process.

Turbo Oil Needs

Turbos rotate at very high speeds and their centre bearings require a continuous supply of pressurised, clean and cool oil. Sleeve-type bearings need considerably more oil than ball-bearing turbos but in either case, if the oil supply is ineffective, the turbo will soon fail. The same importance also applies to the oil exit: if the oil cannot freely escape, it will force itself through the seals, causing smoke.

The specific oil needs vary from turbo to turbo, however, as an example, the manufacturer’s data sheet for the small Garrett GT12 shows the turbo requires a supply of engine oil:

• Filtered at a 20 micron level
• Fed to the turbo through a minimum internal diameter line of 3.2mm
• At a minimum pressure of 2 Bar (ie 29 psi) at peak engine torque
• Not less than 0.7 Bar (10 psi) at idle
• At a temperature below 130 degrees C

Ball-bearing turbos use a restrictor (normally positioned on the oil inlet) so that the amount of oil that flows through the turbo bearing is less than in a conventional turbo.

The oil drain from a turbo operates via gravity. That is, the pressure drop across the turbo bearing is nearly complete and it is only gravity that causes the oil to flow back to the sump. In fact, it cannot really be termed ‘oil’ as after it has passed through the turbo, the oil becomes an aerated foam – one reference suggests it looks like ‘dirty whipped cream’. It’s therefore important that:

• The drain line is much larger in diameter than the pressure feed line
• The connection to the sump is made above the oil level
• The drain pipe is kept as close to vertical as possible

If the sump drain joins at a level below the oil, the much lower density of the aerated oil will cause it to sit on top of the sump oil, gradually backing-up to the turbo.

In applications where the turbo is positioned too low relative to the engine for normal drain-back to occur, an electric or mechanical scavenge pump can be fitted. (This approach is adopted in many piston aircraft engines.) In this situation a small sump must be installed beneath the turbo allowing the oil to accumulate within it, with the pump then drawing from this sump. Turbos mounted remote from the engine (eg at the back of the car) also use a scavenge pump to return the oil to the engine.

The oil requirement of the turbo will have an affect on the engine oiling system. System oil pressure will drop a little, a major reason why factory turbo versions of naturally aspirated engines often use a larger oil pump. In aftermarket applications, the fact that the engine oil pump is normally a little oversized is taken advantage of to allow for the extra supply to the turbo. However, in applications where a turbo hasn’t previously been fitted to the engine, system oil pressure measurements should be taken after fitment of the turbo to ensure that adequate oil pressure is maintained at the engine. Manufacturer’s oil pressure specs can be used to ascertain if this is the case.

The Pressure Feed

Engine oil pressure can be derived from any fitting on the pressure side of the pump. This diagram, of a Toyota 1NZ-FXE engine, shows the flows of oil through the engine. In most cases, a T-piece fitted to the oil pressure sensor (or switch) can be used to obtain the required oil feed. The oil pressure sensor (or switch) is unscrewed, an appropriately threaded T-piece is inserted, and then the turbo oil feed hose and oil pressure sensor attached to the two vacant arms of the T-piece.

The easiest way of sourcing an appropriate hose is to visit a hose specialist (such as Enzed) and take with you the oil pressure sensor and the turbo oil pressure inlet connection. Carefully measure the length of the required hose run, taking into account that you will not want any sharp bends in the hose. It also makes sense to request that a swivel fitting be installed at the oil pressure sensor end of the hose, as this will allow the fitting to be tightened without having to rotate the entire length of hose.

Expect to pay about AUD$100 for a typical oil pressure feed hose made from stainless steel braided Teflon hose and complete with T-piece and fittings.

Oil Return

As indicated earlier, the oil return line from the turbo should be a large diameter, steeply sloping line that joins the sump above the engine oil level. In nearly all cases, this requires the attachment of a hose fitting to the sump. (The exceptions are where the oil return can be directed into a tappet cover - eg in a flat-configuration engine.)

Many turbo kit instructions suggest that it is fine to attach this fitting with the sump in situ. That is, a hole is drilled in the sump and the wall tapped. The oil is then drained, hopefully taking all the metal particles with it. A fitting is screwed-in and a hose attached. Job done! But we think that is a bloody awful way of doing it.

Firstly, it’s extremely unlikely that all metal particles will be removed. While these won’t pass through the engine (the oil filter will catch them), larger ones may well block the pick-up for the pump. Secondly, the wall thickness of the sump (even those cast in alloy) is unlikely to be thick enough to provide the requisite number of threads needed to adequately hold the fitting in place. Finally, the sealing of such a fitting can be done from only one side.

We’re sure that many turbo kits have been successfully fitted in this way – but it’s not for us. Instead, we suggest that a bulkhead fitting is purchased, the sump removed and a hole drilled through the wall. (Some cars use sumps formed in two parts. In those cases, just the lower part of the sump can be removed, giving access to the interior of the upper part of the sump.) Sealing compound is used around both the inside and outside of the hole, the fitting is inserted and then the nut adequately torqued. The sump is then thoroughly cleaned before being replaced on the engine.

The difficulty with this approach is that many modern sumps are very hard to remove. Chassis cross-members and sub-frames may need to be removed or loosened, and the engine may need to be jacked a little off its mounts. So it’s more work – but then, that’s often the difference between good and bad engineering!

The connection between the turbo’s oil outlet and the sump should incorporate some flexible hose to allow for the expansion and contraction of the exhaust manifold. This hose should be able to cope with the high temperature oil it may at times flow – up to 120 degrees C will not be uncommon on hard-worked engines.

Cooling Water

All turbos of the last 20 years use water cooling of the centre bearing. Engine coolant flows around the bearing, being especially effective in cooling it once the engine has been switched off and the oil flow has stopped. Despite the cooling water flow no longer flowing, the greater volume of water in the bearing helps to prevent a major temperature rise which could cause the oil to coke. (However, idle-down procedures should still be followed before switching off the engine after a hard drive.) The easiest way of deriving this coolant flow from the engine is to plumb the turbo in series with the water supply to the throttle body.

Connections

When mounting a turbo in a non-standard application, you should very carefully check that the oil feed, oil drain, water inlet and water outlet pipes don’t foul the turbo mounting bolts or the exhaust manifold. In some cases, modifications to the lines will need to be made – for example, the coolant pipes separated from the unions and then silver-soldered back onto them with a slightly different orientation. The same company that made the oil supply hose will be able to do this.

Next week: doing it all on a car