oil pressure for 427 aluminum block

[blykins]

Never heard those kinds of numbers....

Of course a Modular Ford doesn't have cam bearings, but if clearances grew that much in an aluminum block, there would be big issues there.

[olddog]

I decided to calculate the gap changes myself. I got the thermal coefficient of linear expansion from this website.
http://www.engineeringtoolbox.com/li...ents-d_95.html
I expect these numbers to be in the ball park, however alloys and forging can impact these numbers by quite a bit.

Thermal coefficient of expansion in/in F
aluminum is 12.3 x 10^-6
Steel is 7.3 x 10^-6
cast iron gray is 6.0 x 10^-6

Let's say from 70 F room temp, when you assemble, to 220 F for engine running is a 150 F change in temp.

Consider an aluminum block with steel main caps with a 2.75 inch main bearing with a Gap of 0.0015 inches. 2.750 Bearing ID – 2 x 0.0015 gap = 2.747 crank journal OD. Note I used the term "gap" rather than "clearance" --- less typing.

Block - Aluminum grows by 0.0051" from 2.7500 to 2.7551
Main cap - Steel grows by 0.0030" from 2.7500 to 2.7530
Crank shaft - cast iron grows by 0.0025" from 2.7470 to 2.7495

First note that the nice round main bearing is no longer round and has an average ID of 2.754 inches. Now 2.754 – 2.7495 crank = 0.0045 / 2 = 0.0023 gap. So the bearing gap grows by 0.0008 from 0.0015 to 0.0023 inches. On a -5 morning the gap would close to 0.0011 inches.

If the main cap had been aluminum, the gap would have grown by 0.0013 to 0.0028 inches.

If the crank was steel rather than cast iron:
Crank shaft - steel grows by 0.0030" from 2.7470 to 2.7500
So the bearing gap grows by 0.0005 from 0.0015 to 0.0020 inches.
A steel crank reduces the change in bearing gap.

Note that a steel crank in a cast iron block will cause the bearing gap to get tighter not looser, when the engine warms up!

[vector1]

those are nice calculations olddog, consider the oil coming off the bearings might be more indicative of the temp in that area. or the temp might be higher as the heat crawls down the cylinder walls into the main webbing area.

[Barry_R]

Thats a bunch of math.

Kinda fits in with my strategy of running .001 per inch of bearing diameter on iron (very much standard old school), and scary good with my "normal" .002 on all aluminum FE stuff. Since it agrees with me I am calling it "correct"

[Caprimaniac]

This thread got me thinking.... As i have assembled my aluminum SBF (302 mains, chevy rods) with forged crank and checking "gaps" according to factory (iron block) specs, there might be a 0.001 undersize set of main bearings to get the clearances correct in respect to the alu block?

Oil pressure is 25 psi at warm idle (1250 rpm). 50- 60 at cold, 40 at room temp.

although, I have seen people saying idle oil pressure down to 10PSI is ok, I am concerned...40 at hot idle? never seen it that high, even in iron blocks... Should I be concerned?

[olddog]

For reference I have a cast iron 5.0 - 347 stroker. Idle is 800 rpm.

With 10-40 oil cold idle is 65 psi - prv is open and hot 45 psi.

I didn't like the high oil pressure when cold so i switched to 10-30 oil. The distributor gear stories always leaves me a bit nervous.

With 10-30 oil, carries 60 cold idle and 40 hot. It still pegs 65 psi by 5000 rpm (not sure where it actually hits 65).

On a 90+ F day and running hard the water temp was about 205 (normally 190), and it idled at about 37 psi with 10-30 oil.

[mike harter]

Hi oil pressure does not mean better lubrication 40-45 lbs. is fine

[mike harter]

High oil pressure does not mean better lubrcation 40-45 lbs is fine