X-Mozilla-Keys:                                                                                 
Brian D. Cain wrote:

Subject: Oil Temperatures

Tracy Crook wrote:

These are the same  numbers I see on my RV-4 on a hot day during climb
(195 - 210) and cruise (175-180).  I use a stock Mazda (76 Cosmo) oil
cooler.

Tracy
 http://www.rotaryaviation.com

I am currently still using the Series 4/5 (88 Rx7) factory oil cooler.  I've
been told that it was over-designed for the needs of the engine and I'm
beginning to agree.  Last night, on my way home from a friend's house, I had
a chance to get some numbers.  During your usual 'cruise down the highway'
bit, I was going right around 80mph with an engine speed of around 3200 to
3400rpm or so in 5th gear.  Ambient air temps were in the high 70F's and my
water temps were just below 180F.  Before I got on the highway, my oil temps
were in the high 190F's (stop 'n go lights/city driving).  After about 5
minutes on the highway the oil temperatures dropped by almost 20F degrees to
right at 180F.  I was putting more load on the engine for a longer period of
time.

I would say the thing is doing its job quite well.  I'm almost tempted to
hit the junkyards, find all of the spare oil coolers from the wrecked 2nd
gens, and sell them as 'race oil coolers'.  =)

B

Brian, a car cruising on the highway is not generating very much HP
unless it is near top speed or climbing a steep hill.
I would say an RX7 perhaps 30 HP max.

I was never able to drive flat out in West Texas for more
than about 15 minutes. I would then arrive at the next
town and be forced to slow down.

The formula is: FA X Cd X V^2 X .0026  = aero drag.

FA is frontal area and is roughly 0.8 X height X width. 
(Crude rule of thumb for cars.)

Cd is the drag factor and for most cars these days it is about 0.3.

V is the velocity in MPH. 

Rolling resistance is about the same as aero drag at 50 MPH and is
directly
proportional to speed. (yet another crude rule of automotive thumb).

HP required is total drag X FPS / 550

FPS = feet per second.

When climbing, HP required over and above aero and rolling drag for 
cars and airplanes is vertical feet per second times gross weight 
divided by 550.  In the case of Jim Mosur's 1400 feet per minute
and 1690 pounds gross weight he needs 72 HP over and above the 
level flight aero drag at 95 MPH. 

This is indeed most impressive oil cooler performance and would
indicate the Mazda cooler is way over designed for car use. 

Paul Lamar
 
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