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Oil cooler lines

Christopher Murphy

Well Known Member
Ive run -8 aeroquip lines to and from oil coolerf for 20 years. Was thinking about switching to - 6 lines.

Any knowledgable insight about either size?
Thanks
Cm
 
We've only plumbed 1 RV with -6 cooler hoses--against our advise because thats with the client wanted. He changed to the standard -8 pretty quick after the temps started getting out of hand. (yeah is was during the summer in southern SC!)

Tom
 
I have -6 cooler lines on the 6 with IO-320. Don't think I have ever seen oil temps above 190 in 800 hours, except when the vernatherm failed. It is not universal that -6 lines cause high oil temps. That said, if I did it again, I would use -8.

Larry
 
Parasitic losses through pipes/tubes are proportional to the fifth power of the diameter. A 1/2" ID tube or pipe would have 32X the losses of a 1" so some systems are very sensitive to ID changes (all other things being equal). The related system is a little more forgiving versus one where the DP was generated by a centrifugal pump.

The heat exchanged through the oil cooler is NOT linear but a function of the logarithmic mean temperature difference between the two fluids. The wider the LMTD, the more heat can be removed from the oil (again, all other parameters being constant which they are here).

In summary, keep heat transfer available up -> by keeping the LMTD up -> by keeping the flow up -> by NOT unnecessarily increasing system backpressure ->. You're also loosing an unmeasurable fraction of a fraction of a percent of engine shaft HP by decreasing the ID/making the pump work harder.

Would the 6 line be OK in all flight conditions? Decent chance if the overall system already was. Worth the time and added expense of a rework if not? Up to you. I wouldn't. The small weight saving of the smaller lines would probably be mostly lost with the added, required adapters.
 
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I run #6 lines on my Rocket (IO-540, 330+ HP, WITH PISTON SQUIRTERS). Despite living in one of the hottest places you will ever hope to fly, I have a hard time getting my temps above 185. Initially, I was very careful to avoid any 90 degree fittings for the flow loss, but I found that I had so much excess cooling capacity that I could accept the loss. Now I have a single 90 coming off the back of the engine which buys me some clearance. The lines are very short however - again, to minimize hose losses.

I used to battle high temps (even with #8 hose) but then dumped the vernatherm in favor of the viscosity valve. Made a HUGE difference in temps. IMHO, Vernatherms are evil things, even when functioning properly.
 

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There are exceptions and there are crutches. I think "most" RV's have less than ideal baffle sealing, less than ideal oil cooler sealing, vernatherms that blow a large portion of the oil flow right past the seat, and cooler hoses that are way longer than needed, using flow choking 90 degree fittings. Add all that up and one might get the sense that #8 hose is required, but I think I have shown that is clearly not the case "IF" you fix the other stuff.
 
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There are exceptions and there are crutches. I think "most" RV's have less than ideal baffle sealing, less than ideal oil cooler sealing, vernatherms that blow a large portion of the oil flow right past the seat, and cooler hoses that are way longer than needed, using flow choking 90 degree fittings. Add all that up and one might get the sense that #8 hose is required, but I think I have shown that is clearly not the case "IF" you fix the other stuff.

What size oil cooler?
 
What size oil cooler?


Airflow 2007x. Same unit some are using on their 4 banger 390's.

Readers need to understand that my comments are meant to inspire critical thought - they are not direct guidance. My point is simply that if your oil cooler line size drives oil temperature, then you are bumping up against the thermal limits of the whole system. Time to dig a bit deeper and find the root issues.
 
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