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The Lowdowndry sumps

drysumps

WHAT Is A dry sump?

We quiz the guys at Pace Products tobring you the info on dry sump systems.

hen it comes todry sump systemstheres one name

that instantly springs tomind, Pace Products.Pace has been designingand producing dry sumpsystems or all manor o

ast road and race carapplications or over25 years, so the guysthere know a thing ortwo about how a drysump works.

Dry sumps are oneo the most exclusive

modications made to aroad-going motor, but inthe world o motorsportthey are common place.The advantages dry sumpsystems give makes itar superior in termso perormance and

reliability thanthe conventionalwet sump set-up.

Pace Productstechnical design managerTom Morris, explainswhats involved with drysump systems.

W

Words:Jamie Photos:Jon Hill

In simple terms a dry sumpreplaces the original wet-sump ull o oil with a sumppan and moves the supplyo engine oil elsewhere inthe car. It is then stored in aseparate tank and pumpedto and rom the enginevia a series o oil pumps(assembled into one part),which are usually externallymounted and are belt drivenrom the crankshat.

Moving the oil rom thebottom o the engine maysound like a silly idea as youhave to pump it urther, butit has many advantages.Starting with the most basicadvantages, simply storingthe oil in a tall cylindricaltank helps prevent oilsurge when cornering hard

compared to a

Dry sumpsprovide amore reliablesupply of oil

relatively short but widewet sump design. AsTom Morris puts it: Thisthereore ensures a morereliable supply o oil tothe engine when drivingenthusiastically

Another major beneto a dry sump is that itmeans the crankshatdoesnt have to ght itsway through a sump ullo oil. This reduces thedrag on the crank andactually rees up a bit opower. Also storing theoil away rom any heatsources, namely theengine itsel, helps keepthe oil cool and within itsoperating temperatures.

There are three mainparts to a dry sumpsystem including thetank, the pressure sideo the pump, and thescavenge side o thepump. To get a betterunderstanding we

need to look at eacho those aspects

individually

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prEssurEIn a dry sump systemoil is no longer stored inthe sump at the bottomo the engine. You needsomewhere else to keep it,hence a dry sump tank.

CAPACITY

The tank plays a crucialpart. Its size dictates howmuch oil capacity you have,and can be tailored to suitthe application.

Tom explains: In alightweight hillclimb car likea Westeld, where it is runor short periods at a time,

you can run less oil and asmaller tank. Dependingon the engines spec, youdrun between 3-5 litres,

because you wouldnt wantto carry extra weight. Ina big-engined endurancerace car youd need alarger capacity tank toensure the engine oil iskept cool enough, andthat there is enoughcapacity to take intoaccount the enginesoil consumption overthe long period itis running.

More importantly,the condition o

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OIL TANK

The pressure side othe pump provides oilto the engine at therequired pressure. Itis possible to run adry sump systemwhile retainingthe original oilpump to supplythe pressure anduse an externallymounted pumpor the scavengeoperation.However,this isnt verycommon. Typicallya dry sump pumpwill be belt drivendirectly rom the

crankshat andwill consist o onepressure pump,and a number oscavenge pumps.

Pumping elementscan be o dierentstyles (gerotorand gear are themost common).They produceoil pressure in anumber o ways.(Note pumps dontmake pressure,they move the oilwhich when met withresistance generatespressure.) The mostcommon pump and thetype used in most oPaces dry sump systemsis called a gerotor.

GEROTORSShort or generatedrotor, a gerotor is quitesimple in terms o itsdesign. There is an innerand an outer rotor whichboth rotate on dierentaxes. Usually it isthe inner rotorwhich is drivenbut is possibleto drive the outer

rotor, although morecomplex design workis needed to make thiswork eectively.

The inner rotor willalways have one toothless than the outerrotor, meaning there willalways be one chamberavailable to be lled withoil. As the inner rotorrotates into this space itorces theoil out,creating oilpressure.When theinner rotorcompletelylls the emptychamber andorces all othe oil out, thenext chamber

to 40mm thick using thismethod. The alternative isto use a piece o billet steel,rom which it is possibleto produce gerotors up to100mm thick. However,the wire eroding processneeded is very expensiveand increases the costconsiderably, meaning it isusually only reserved orone-o or custom projects.

PRESSURE

REGULATING VALVE

The PRV (PressureRegulating Valve) within

the pressure pump isvital in controlling anengines oil pressure.

At low enginespeeds the oil

pressure and smoothnesso fow is determined bythe displacement o thepumping elements, but asthe engine speed and pumpspeed increases so does theamount o oil displacement.

Because the oil pumpand crankshat are directlylinked, the relationshipbetween engine speed andoil displacement is linear(up to the point that thepump reaches it limits andcannot physically displaceany more oil). As the enginespeeds increase, so doesthe amount o oil displacedby the pump. However,at high engine speeds we

need something else toregulate the oil pressure.

Tom explains: Therelationship betweenengine speed and amounto oil an engine needsisnt this straight orward.Taking our example o thePinto engine, imagine weare revving that engine to8000rpm. We know thatat 1000rpm the pumpdisplaces 10litres/min andat 4000rpm it displaces

the oil will take longer todeteriorate, because thereis more oil to absorb anycontaminants rom theeects o blow-by. So, theoils lubrication and heatexchanging properties will

stay within a reasonableworking condition or longer.

It sounds strange talkingabout the volume o oil in adry sump system aecting oilcooling and oil condition, but

as Tom pointsout, more oil inthe system willhelp dissipate

heat and absorbcontaminants.

Take the Pintoengine. At 1000rpm

the standard

40litres/min, so it wouldbe sae to assume thatat 8000rpm the pumpwould displace 80litres/min. However, at 8000rpmthe engine wont require80litres/min, it may onlyrequire 40litres/min.

This is where the PRVcomes in. Its a sprungvalve that opens at a

predetermined pressureand allows the pressurisedoil to return back to theinlet side o the pump. Theoil pressure on the outletside o the pump is keptconstant and is regulatedto whatever we

require it to be. ThePRV is also adjustableso we can make neadjustments to theoil pressure.

pump displaces 10litres ooil per minute. Theoretically,(I say this because the PRVplays a part, but well coverthat later) at 4000rpm thepump will displace 40litresper minute. I you have

a system with a 5-litrecapacity, all o the oil will berecycled through the engineevery 7.5secs. With a biggercapacity, say 10litres, atthe same rpm the oil will berecycled every 15secs. Youcan see how this gives the oilin the system a greater timeto cool, reducing the temp.

POSITION & PLUMBING

The position o the tank playsa vital part in the systemsperormance. The urther

away rom the engineit is mounted, the

urther the pumphas to move theoil, and whenthe oil is cold

this can bevery dicult.

Pressure is lost overdistance in oil lines, whichmeans that the pump has towork harder to ensure the oilis delivered to and returnedrom the engine at the correctpressures. This brings us on tothe size o the hoses.

I the oil lines are toosmall the pressure loss willbe large, or worse still theywont be able to fow theamount o oil required.

Typically the return hose willbe a larger diameter thanthat o the tank outlet. This isto allow or the act that thereturn hose will be fowing amixture o air and oil rom thescavenge pump, not just oillike the outlet hose.

BAFFLED

The tank also acts as a bafe,and separates any air rom theoil to ensure a constant supplyto the outlet hose.

As the oil enters the tank itfows in a spiral motion whichhelps separate it rom any airtrapped within. The air thenescapes out o a breather atthe top o the tank. The tankis also bafed to preventoil surge leaving a constantsupply o de-aerated oil at thebottom ready to be ed to thepressure pump.

opens up, lls with oil, andthe process is repeated.

Size, shape and designo the rotor sets will

impact on the pumpscharacteristics. Tomexplains: The biggerthe rotor set, the more

volume there is availableto be lled with oil, sothe more oil will bedisplaced. Its not justdiameter that aectsthis, because athicker rotor set willalso have a greater

volume than a thinnerset. The number anddesign o the teeth on

the rotors will have a

similar eect. The morethere are the morespace there is availableto be lled, so more oilwill be displaced.

Tom also describesthe two manuacturingmethods which are usedto make the gerotors.The rst, and mostcommon, is sinteredsteel. This processinvolves heating andcompressing an ironpowder and graphiteto orm a solid piece o

sintered steel. It is possibleto produce gerotors up

The pumping elementconsists of a housing,

and inner andouter gerotors

The Lowdowndry sumps

The tank size andposition are criticalto the operation ofa dry sump system

Size,shape and design of

the rotor will havea huge impact on the

pumps characteristics.

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sCAVENGE

This side o the system isresponsible or returningthe oil back to the tank,completing the cycle.Unlike the pressurepump the scavengepump(s) dont fow

purely oil. Instead theyscavenge a rothyair/oil mixtureas it exitsthe engine.

Oil iscollected inthe sump pan,which in a dry sump systemis eectively a blankingplate or the bottom othe engine. Unlike a wetsump design, the sump

pan in a dry sump systemhas oil galleries machinedinto it which lead to the pickup points or the scavengepumps, which then pump the

air/oil mixture back to thetank where it is separated.

The scavenge pumps areprincipally the same in design

as the pressurepumps, and are

usually the same gerotortype. However, whereasonly one pressure pump isused, a dry sump system willtypically consist o at leasttwo scavenge pumps and inextreme cases can eature upto ve or six scavenge pumps.Firstly, because the

scavenge pumps are movingan air/oil mixture not just oil,they need a greater volume toensure all the oil is returned

to the tank. In the worst casescenario, ailure to do so maycause the tank to run dry andpotentially destroy the engine.Also, having two scavengepumps helps eliminate anyproblems with the return oilfow during hard cornering.For example on a very astbend it is not unusually tohave just one o the scavengepumps returning all the oil,because the centriugalorces involved mean it isbeing thrown away romthe other scavenge pick-up,Tom explains.

Tom also points out that

it is possible to scavengeoil rom other parts o theengine, not just the sump pan.

In engines which havepoor oil return rom thecylinder head it is not unusualto scavenge rom areas in the

head to prevent it llingwith oil, as this ensures aconstant return back to thetank, rather than waitingor the oil to make its wayback down through therest o the engine until itreaches the sump pan,says Tom.

The oil is then returnedto the top o the tank,where the whole cyclestarts over again. So, giventhe importance o theoil system coupled withthe act that an enginecan recycle the entireoil capacity as quickly as

every 7.5secs, you canbegin to see why providinga reliable oil supply bytting a dry sump is a verygood idea.

The dry sumppump is belt drivenfor ease of accessand reliability

The Lowdowndry sumps

Scavenge pumps areprincipally thesame in designas the pressurepumps.