Oil Cooler Restrictive Only On Start Up!!!

I need some help from the experts. Some background, I am running a brand new bridgeport 4 port motor in my sorta like a WC RX7. I like to think that if the WC still allowed 1991 RX7's in the series, my car would be the type. I run mid-pack WC times. We are running a Turbo II oil pump with an FD regulator and Aeroquip #10 lines. Valvoline 20 - 50 Racing Dino oil until we finish break-in. The oil cooler is a brand new stock Mazda FC cooler with the thermostat removed by the Mazda dealer. It was run in my previous engine and never really showed any issues other than a sleepy gauge (slow to react when cold). The oil pressure gauge is plumbed using the stock pressure pick-up point. We ran a different cooler before (a Long style) and the pressures were better but the temps were higher.

Now the issue. When the oil is hot the car runs 60-70 psi at idle and 110 at 2500 rpm and the car never has oil temps above 200 F (just ran a 3 Hour enduro yesterday and the car was making these rates after the race as well). When I start from cold the oil pressure takes about 10 seconds to read above 15 lbs and slowly crawls up as the oil temps go up. Once the oil temps reach 140 no issues.

The second tell tale is that if you start the engine from cold then shut it off right away (5 - 10 seconds), then you check the oil it has tons of little bubbles in it. The bubbles go away after you let the oil sit for 5 minutes or there are none if the oil is hot.

My crew chief and I have 2 theories. First, the oil is very thick and is cavitating as it passes through the cooler when cool and as it warms the viscosity drops and it flows more freely reducing the cavitation. Or secondly the cooler is very restrictive and the cavitation happens in the oil pump as it tries to force the oil through the cooler and is unable to.

We believe it is the cooler as the cause, but what is the cure, should we re-install the thermostat into the cooler to allow free flow of oil when cold? I don't understand why this is a problem with the new Mazda cooler when cold and not the ancient corroded oil cooler I had on the car previously?

Eric

Eric,
What type of gauge are you using, mech or electric sender

Have you tried bypassing the cooler to see what kind of cold start behavior you get? Seems like an easy way to confirm/eliminate a theory.

Actually Marty my crew chief and I just decided to add a manual bypass that we will open up until the oil is warm. After the oil is warm then we will close it and pass the oil through the coolers.

Eric

Michael, I am using an Autometer SportComp mechanical oil pressure gauge and everything work well when the oil is hot.

Thanks for the help

Eric

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Actually Marty my crew chief and I just decided to add a manual bypass that we will open up until the oil is warm. After the oil is warm then we will close it and pass the oil through the coolers.

Eric

Michael, I am using an Autometer SportComp mechanical oil pressure gauge and everything work well when the oil is hot.

Thanks for the help

Eric
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How did you bypass or remove the thermostat in the cooler? When we install a factory cooler we insert a slug that seals off the thermostat bypass ports in the cooler. This forces the oil to circulate through the cooler at all times. A prod racer friend of mine turns the bypass slugs on his lathe

I run two 1st gen coolers in series and initial gauge response is slower with two coolers than it was with one. However, once pressure is up it stays up and the gauge reacts normally when I rev the motor - cold or hot. I have 78psi at anything over 2500 regardless of temperature. The car runs cool now even in 100 degree weather. I am also using an Autometer gauge plumbed with -3 braided lines.

If you are concerned about a "dry" start up you could add an Accu-sump and use it to prelube the motor. I used one in my old Solo II Mustang and it worked well for this - lots of engine starting in Solo II. It also solved oil starvation problems that are common in a 302 Ford.

Scott, will you be at the Runoffs & hanging with which car.

Have Fun ;)
David

Steve, great question on how was the cooler bypassed. I bought the cooler from my team mate and he used it for 1 race from new. He bypassed the cooler thermopellet and it worked for him, so I just put it on my car. I am going out to the shop tonight to take a look.

I also used to run an Accusump (3 quart model) but removed it as I needed the weight savings. May be a good idea to put it back in the system.

Great clues, thanks :023: ! This is why I always read this forum.

Eric

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Scott, will you be at the Runoffs & hanging with which car.

Have Fun ;)
David
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Yes I will! I am crewing for David Long in EP 82. We will be paddocked with EP 18 and EP 6 if everything works out.

It will be nice to meet you David

Just wanted to give you guys an update. We installed a second stock oil cooler in parallel with the original.

We started the car and bang, the oil pressure shot up to 110+ (over 60 lbs cranking). After the car heated up we had 50 lbs at idle (around 900 rpm), 90 lbs at 1500 rpm and 110+ at all rpms above that. My crew chief who is an incredibly anal kind of guy did the calcs and figured out the oil resistance rates by viscosity, pressure and temp. What is really cool is that our actual results mirrored his calculations.

That stock cooler is very restrictive with 20-50 dino oil but substantially less restrictive with 15W-50 synthetic. Very interesting effects. I never would have thought that a 5 lb difference in viscosity could have this much effect on pressure drop through a stock Mazda cooler

:happy204: