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Thread: Determining correct oil viscosity for track use

  1. #1
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    Determining correct oil viscosity for track use

    Car Use: Track + autocross, only driven on the street to get to events
    Motor: M20B25, 10:1, 284/272 cam, VAC springs, intake, 20lb injectors, long tube headers, 2.5" straight exhaust, chip tuned
    Oiling: New oil pump, I-J crank scraper, VAC oil pan baffle, 19 row oil cooler, motor seems to have been built to "tight" tolerances

    Prior to installing the oil cooler I was running Rotella T6 5w40 synthetic just because I had it and it was cheap. My oil would go up to 230* just cruising on the highway (4k RPM with a 4.10), so I knew I'd need an oil cooler. Oil pressures were around 22psi at idle and about 75-80psi at redline with that oil. The motor idles around 900-950 rpm.

    When I installed the oil cooler, I switched over to Redline 15w50 synthetic. Oil pressure went up a bit across the board, but keep in mind also my temps dropped a good 20*+. My idle is now about 28-31psi and at redline I am at about 85-90psi.

    Side note: My oil pressure gauge is an Innovate unit that has a resistance-type thermistor. With the engine OFF, the gauge reads 9-10psi, so keep in mind my pressures may not be that accurate (could be off by 10psi?). I just use it as a point of reference and to make sure the shit is not hitting the fan while on track. Innovate says this is normal behavior, so I have no idea.

    During my track day yesterday, my temps averaged 220-225*F and the max temp I saw was 232*F. Pressures were solid, never dipping 60psi or so, even on sustained left hand corners which are tough on M20's. Water temps were always good, in the 185-190*F range.

    Even if you deduct 10psi from my pressures, I'm still on the strong end of what you normally see in M20's. I've read that thinner oil can shed heat better, and I don't want to be putting unneeded strain on the motor and oil pump if this oil is thicker than it needs to be. On the street the oil only gets up to 180-185*F, and I think it's going to be tough to keep it that hot at autocrosses.

    Based on the data above, would you say this oil is just what the doctor ordered, or should I give Redline 5w-40 (or 10w40 or 15w40) a try on my next fill? I have not been able to find much information on determining if oil is too heavy, just that if oil pressure is low heavier oil can bump it up. I'm comfortable with the oil getting up in the 230* range, but wouldn't want it getting a whole lot hotter than that.

    Any feedback would be appreciated. Thanks in advance.


  2. #2
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    I would say it's too thick. It's just a guess since you didn't post many details like what your bearing clearances actually are, where the oil pressure is being measured and whether the oil pressure relief valve was shimmed.

    Measuring oil pressure as it enters the engine is misleading since you could be simply measuring the difficulty with which the oil is passing through the filter or entering the engine. You may in fact be starving other parts of the engine by increasing viscosity. think of it as switching from coke to a milkshake without changing the straw you're sucking through. It'll take a lot more force to get it through the straw but you end up with a lot less of it on the business end.

    The reason I would guess it's too thick is that you're seeing a pressure bump to 85-90 psi. I'm not sure what the relief valve on the M20 is set to fire at but from what I've seen BMWs aim for 70-75. So you're having to jam an extra 20psi out of the pump to sustain the same pressure in the main oil gallery.

    For what it's worth, having talked to NASCAR guys, they said 70-80psi is about the sweet spot, there is no benefit to going any higher other than heat and power loss (and thicker oils flow slower so you get accelerated piston skirt wear). The old school rules about having all the pressure and viscosity you can cram into the engine is for good ole boys building very loose engines to compensate for the lubricant technology limitation at the time.

    Run the thinnest oil you can which at your operating conditions is able to sustain the 10psi/1k rpm relationship. Again without knowing the actual clearances it's hard to say but if it's truly "tight" (ie 15 tenths or even lower) then even a good 30 or 20 weight would be worth a shot.

    Also if this is a dedicated track car you could also try a racing oil. They have no longevity additives or any considerations besides performance (like emissions) so the film strength is substantially higher than street oils but for the same reason it needs to go in the garbage after every event - when you get home change it out to a cheap detergent filled oil for parking it.

    Regarding your gauge, your pressure is probably not off by 10psi, the sender is just out of range any lower - in other words you can trust it to 10psi and any lower it may be 10 or 0.

  3. #3
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    I bought the motor second hand from a guy who swapped it out to race SpecE30 (stock motors). I don't have many details on the build (just some receipts for parts) so I am really just guessing, even on if the oil pump is new. I do not know if the relief was altered. It seems like a lot of E30's tend to idle much closer to 10psi regardless of the oil, but those are typically motors with 150-250k miles on them. There are others with fresh motors and/or fresh oil pumps that idle in the 20's and maybe even the low 30's like mine does. It's hard to determine if the pressure is higher because of the pump, motor tolerances, the gauge(s), or any other factors. I can just compare to what I've seen with this motor since I first got it up and running in February.

    What I can tell you: oil temperature and oil pressure are both measured from a sandwich plate before the oil filter. The pressure is read from a remotely mounted sensor via a -4AN line, and that line is on the side of the plate that has oil flow BEFORE the oil cooler. The oil temperature thermistor is directly in the flow on the other side of the plate, after the oil cooler and before the filter. The water temperature thermistor is in the flow on the thermostat housing. All the gauges are Innovate MTX-L and are grounded to the same ground as the ECU. When I asked Innovate about the oil pressure reading, this was their response: "Since the gauge's sender has a resistance output it will not read zero, this will not change the unit's accuracy in it's functional range."

    I would call it a dedicated track/autocross car, but the lack of time to get the oil fully up to temp at autocross is a concern of mine, and the car does get driven on the street to and from events (only). I don't mind changing the oil after every event, in fact I plan to anyways. Besides the additives and detergents, I think a concern with a street oil is it sheering down due to heat. That concerns me less when it is changed very frequently.

    Thanks for taking the time to read my long post and reply with a detailed answer!
    Last edited by Digitalwave; 08-16-2017 at 01:16 PM.


  4. #4
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    See if you can get the build sheet out of the previous owner, the clearances will help you make a decision, most of the high perf companies have engineers on stand by who will give you a recommendation based on those specs and your operating conditions.

    The only way to really know if your oil is shearing down is to do an oil analysis but if after beating on the car and having it operate at your normal track temps, you get the 10psi for every 1000rpm it's probably fine. For what it's worth, also from the experts I talked to, the old standards for oil temperature are also not as applicable in the modern world. Good modern synthetics will be fine at 300F. Your bearings may not be but the oil will still be doing its job. So high oil temperature isn't an automatic cause for concern but if you increased viscosity and you got a large jump in measured pressure entering the engine (ie you're measuring the resistance of the motor, not how much pressure is actually in the various passages inside) and your temperatures went up some, the oil may be too thick.

    FWIW you'll also gain a substantial chunk of power back going to a lighter oil.

  5. #5
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    I don't see a problem....other than a gauge you can't trust. Ask around the paddock and see who's lost a "track event" or an "autocross" because their oil was too thick.

    Don

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  6. #6
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    The only way to know what oil is protecting your engine the best is to send your used oil samples to Blackstone. I have done that constantly with every oil I have used and I have found what oils work and which don't, FWIW Mobil One has the highest engine wear of the 3 I have used so far. Internet knowledge and conjecture and stories about what my girlfriend's cousin's boyfriend's father uses in his racecar are useless. As are oil manufacturer claims.
    Last edited by jbrannon7; 08-18-2017 at 11:40 AM.

  7. #7
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    Quote Originally Posted by dcvee View Post
    I don't see a problem....other than a gauge you can't trust. Ask around the paddock and see who's lost a "track event" or an "autocross" because their oil was too thick.

    Don

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    Point taken!

    Quote Originally Posted by jbrannon7 View Post
    The only way to know what oil is protecting your engine the best is to send your used oil samples to Blackstone. I have done that constantly with every oil I have used and I have found what oils work and which don't, FWIW Mobil One has the highest engine wear of the 3 I have used so far. Internet knowledge and conjecture and stories about what my girlfriend's cousin's boyfriend's father uses in his racecar are useless. As are oil manufacturer claims.
    I do use Blackstone for oil analysis.


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