Oil clearance needed for drag race motor using thinner oil

[racinvrxse]

I'm rebuilding an engine that I would like to spin to about 11,000 to 11500 rpm. The idle is set at 2000 rpm and the operational range will be from about 7000 to 11500 rpm. The max oil pressure is about 95 psi, and the oil volume flow is 8.4 gpm at 5000 rpm and 16.5 gpm at 9800 rpm with synthetic 20W-50 motorcycle oil (specific gravity of .8800 @60 degrees). There seems to be an oil pump (gerotor) cavitation issue starting at 9800 rpm with this 20W-50 oil, but there may be a new design out next year that will output more volume and not cavitate. I hope to maybe try running a thinner oil (synthetic 5W-20 motorcycle oil with specific gravity of .8650 @60 degrees OR synthetic 5W-30 motorcycle oil with specific gravity of .8597 @60 degrees) which should raise the cavitation point of the pump a bit, but I realize I will have to tighten up my oil clearances on my main, rod, and camshafts as well to prevent oil whip and premature bleeding of the oil. I'm not yet sure what my stock bearing oil clearance was, but I will be checking that soon. The general rule for these motors while running synthetic 20W-50 oil is .003" or .0015" all around the journal's perimeter. I realize that this not a single formula, but how do I calculate a new clearance to tighten this up for running thinner oil?

 

[tbuelna]

racinvrxse,

Journal bearing oil flow requirements are based on heat rejection rates. The oil mass flow rate should be sufficient to keep the bearing and journal surface temperatures within acceptable limits for the materials' fatigue capabilities. The oil mass flow rate thru the bearing is a function of pressure delta, lubricant properties, radial and axial clearance space between the journal and bearing, and geometry/location of the oil feed.

Using an oil with a lower viscosity will typically result in thinner dynamic oil film thickness in the bearing contact. If the oil film thickness becomes too small relative to the mating surface roughness, the bearing can briefly be subject to boundary contact conditions at peak loads, which would greatly increase friction and thermal loads.

So be careful about reducing journal bearing radial clearances. Reducing the bearing radial clearance will definitely reduce oil flow volume thru the bearing. But if there is not enough oil mass flow to ensure adequate heat transfer away from the bearing/journal surfaces, the bearing may overheat and suffer mechanical/scoring failure.

As for your gerotor pressure pump suffering from cavitation at the inlet, you need to check the flow velocity at the inlet port. Gerotors are somewhat sensitive to high inlet flow velocity. With a flow volume of 16.5gpm the inlet port area should large enough to limit flow velocity to 6 ft/sec, or about 0.88 sq.in. Also, check to make sure that your gerotor pump gear's peripheral velocity is within acceptable limits. Nichols' website has some good information on gerotor design:

http://gerotor.net/about_gerotors.asp

Good luck.
Terry

 

[patprimmer]

My first reaction would be to inspect the oil pump pick up system and streamline any sharp corners or restrictions and maybe increase the bore of the pick up system, especially within the pump itself.

Regards
Pat
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[MikeHalloran]

If I couldn't find a particularly small portion of the pump suction to bore out, I'd look into gearing the pump down, by approx 9000/11500, and increasing the pump rotor axial length by the inverse ratio.



Mike Halloran
Pembroke Pines, FL, USA

 

[dicer]

With 5w-30 I think the stated clearances would be okay, there will be more flow than with the higher vis oil, more windage.

 

[racinvrxse]

Thanks to all for your input! I've been looking into a lot of things concerning this subject.

tbuelna, patprimmer, and dicer: I've recently leanred there is someone in my community using 5W-30 safely with .002" oil clearance. I speculate that the thinner viscosity helps to increase the cavitation point a bit, but I'm trying to find by how much. I'm trying to use the Velocity Head or Head Water Column equations, but since I'm dealing with oil and not water I'm assuming that I use numbers for that oil (?)...specific gravity or weight in lb/ft³ of the oil that I'm using (I'm not sure! I'm not as much of a math wiz as I used to be). And YES there seems to be an undersized plumbing issue, but due to case thickness and twist under high stress, I have to be mindful of removing material. I was considering boring a small channel near a case support rib rather than milling the bore out all around on the diameter (where there isn't as much support). Alas, the MAJOR inlet area is actually the one closest to the pump in the cast pump housing...it's a snout whose exit at the gerotor is only .337 in². I may be able to open up the entrance to the gerotor plenum, but the oil velocity in the snout (which I cannot open up) is going to be greater than 6 ft/sec. It seems that the whole housing needs a redesign!

And while the idea of redesign seems to be the best option, I like Mike's idea of gearing it down and increasing the axial length to increase volume. I did hear a rumor that someone is in the process of redesign...to a whipple style. I think it might work if one could control the heat, which I understand is a design side effect of that style...if the oil doesn't boil, it might be the best idea yet.

 

[Tmoose]

".0015" all around the journal's perimeter. "
Between pressure and inertial loads I'd guess a rod or main journal is never centered in it's bore. Still, it's best to spell out or otherwise indicate clearance as diametral, like you did.

In addition to clearance I'd be concerned with rod big end geometry.
High revving mc and race engines have moved toward the thickened rod big end housing, probably to maintain bore roundness.

http://cflcycleparts.com/images/05GSXR750/piston_3.jpg

I have not had a chance to check any installed inserts yet to see if the inserts create more clearance at the parting faces, as advocated by Bill Jenkins, the late great Smokey Yunick, and for some time a standard design feature (Deltawall) in high performance bearings from Clevite (now mahle).

http://www.mahleclevite.com/publications/EB-40-07.pdf

 

[patprimmer]

Bearing shells normally have more clearance at the parting line. You measure at a point perpendicular to the parting line.

Regards
Pat
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[tbuelna]

racinvrxse,

As others point out, a crankshaft main or rod journal bearing bore is not truly cylindrical, either at operating conditions or as installed. Since the direction and magnitude of radial load in both rod and mains is constantly changing with respect to the bearing, the hydrodynamic oil film thickness and its center of pressure as well as the radial position of the journal within the bearing bore, are also constantly changing.

A typical journal orbit plot would look something like this:

Reducing radial bearing clearances slightly will reduce oil flow thru the bearing. It won't improve the bearing efficiency, but there may be some very modest gains from reduced oil windage and reduced oil pump drive power losses. On the other hand, reducing the radial clearance will definitely reduce oil mass flow thru the bearing, reduce the heat transfer rate in the bearing, and quite possibly lead to overheating of the bearing shell and journal surface and/or excessive flash temperatures within the hydrodynamic oil film.

Good luck.
Terry