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Old 07-30-2005, 12:15 PM
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sbc 400 low oil psi at idle

ok fellas im stumped. i just rebuilt my 73 sbc 400. put new rods mains and cam bearings in it along with flat tops. i put mobil 1 10w30 synthetic in it and at first the oil pressure held a nice 20-30 psi at idle and worked up to 40 when reved or going down the road. then after it got warmed up the psi dropped to about 2 at idle. we tried 3 different gauges and all gave the same reading. we got rid of the synthetic and tried 10w40 havoline like i had run before the rebuild. same problem still occurs at idle. we pulled the filter housing and couldnt see any obstructions. the oil also seems to be thin when we looked at it in the filter but doesnt look dirty and we couldnt see any crap in the oil. all the bearings were supposed to be standard bore so i doubt and hope clearance isnt the prob. anyone got any ideas or know what could cause this?
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Old 07-30-2005, 12:26 PM
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Did you check the bearing clearance? If not it`s asking for it. A rule of thumb on clearences is anything .001 and under is not enough and anything over .003 is too much, so if the clearences were boderline .003 it`s no wonder it has low oil pressure at idle. I`d pull the pan and check the clearences. If it still has good oil pressure off idle then I`d doubt if any of the block plugs worked there way loose or came out, but this is something else to check. Any oil guage is not accurate under 10 psi, have you tried a direct gauge to the rear oil output? Also remember the oil pump is being driven off the cam, and the cam rotates half the speed of the crank, so oil pressure being below 10 psi at idle is normal in most cases since the car idles below 1000 rpm, for example, if the car idles at 600 rpm the oil pump is turning 300 rpm. If the bearing clearences are excessive, you can get bearings in .001 increasements to make up for the clearence, I know this isn`t what you wanted to hear but if it has to be done it has to be. My car reads zero psi oil pressure at idle, and I don`t worry about it any, as long as I don`t hear any rattles It`ll live, but that`s just me.
Also one more thing, Don`t run sythetic oil during the break in period, or the rings will take twice as long to seat.
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Old 07-30-2005, 12:44 PM
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DV is on the money. If you did not check the clearances it is a crap shoot trying to find the problem. You need to first know if the clearances are okay. NEVER build an engine without checking them.

One of my engines idles around 20-25psi hot with 50 wt oil due to the loose bearing clearances. It a race only application so I did it on purpose, but with 30wt I cant imagine it holding hardly any pressure. My clearances are 2.5 rod and 3-3.5 mains. It is loose, but like I said, on purpose.

Chris
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Old 07-31-2005, 05:58 PM
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i sent the crank to a machine before i installed everything and they said the journals were standard bore, so i used standard sized bearings. would a high volume pump help? the only time the psi is low is at idle
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Old 07-31-2005, 07:36 PM
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Quote:
Originally Posted by camino_man
i sent the crank to a machine before i installed everything and they said the journals were standard bore, so i used standard sized bearings. would a high volume pump help? the only time the psi is low is at idle
That doesn't mean your clearances are right. Don't forget the caps have to be the right size too.

I use a high volume/pressure pump on my 400. 400s seem to have lower oil pressure anyway. I have a 7 quart pan so I the higher volume is a little more worth it.
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Old 07-31-2005, 07:49 PM
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A high pressure or higher volume pump will only mask a problem that needs to be corrected. 400s and 350s have the exact same oiling system so pressures should not vary that much from one engine to the next IF they have correct clearances. Oil pressure will always be higher cold because the oil is thicker cold than hot. You really need to find the problem before something goes really wrong.

If you sent the crank to a machine shop and they told you it was stock, thats fine, but was it worn any? It could be stock but worn to .005 or more undersize and actually needed to be turned to get back to a good spec. You really need to pull the pan and plastiguage every rod and every main and see where they are. When rods and mains are too loose, oil pressure will suffer the worst as those 2 have the biggest affect on pressure or lack of it. Cam bearings are another but hopefully the cam is new and so are the bearings.

Mark

Last edited by Jmark; 07-31-2005 at 07:57 PM.
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Old 08-01-2005, 04:34 PM
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the cam isnt new its had about a years run time on it but the bearings are new and the same machine shop installed them for me.
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Old 08-01-2005, 05:08 PM
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Well, new, used or whatever, you still need to find the problem. I still say you need to check the mains and rods for proper clearances.
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