I just installed my new engine in my truck and i set the idle where i like the way it sounds and when i put in in gear it of course idles lower. I was wondering what is the minimum safe oil pressure reading so that the engine is still getting sufficient oil distribution while sitting still idling and in gear?
I've heard a few old-timers say that as a rule of thumb, for every 1,000 rpm's, you should have 10 psi. Runnin' at 2500 rpm... needs 25 psi oil pressure. Don't quote me... that's what "they" say.
You dont have to be Old But that is the rule 10psi per 1000rpms at speed Idle should still be at least20-25 with Hyd cam so lifters dont clatter Tony
holy ****,, I use that as a rule of thumb and I'm only thirty........... if you tell us the engine we can get you close to what it should be but there is no magic formula, two engines with the same parts will both be different, it's not tapping or knockin is it?
Actually, the indicated pump pressure isn't what supports the crank at all. The pressure floating the crank and rods is only developed by rotating the crank (50 psi at the main bearing would last perhaps 1 second); the "pressure" is only a rough indicator of how fast the pump refreshes the oil going through. Low pressure at speed means that the leak (radial clearance) is losing oil faster than the pump can supply it - that's all it shows. The pressure developed by rotation is a function of viscosity and temp (duh), but not pump pressure (as long as it's enough), and depends on the diameter of the journal and the speed: P ~ OD × RPM. Remember that main journals always rotate at constant speed at constant RPM, but the rod bearings partially reverse direction during a cycle so the speed varies. That's why truck engines have low idling speeds - the pressure is good because the feet per second of rotation is high even at 400 rpm. The 10 psi per 1,000 RPM is fine, but not because the actual pressure is 10 psi etc. but because the end result for most engines is close enough. The actual pressure of a 3.25" main journal (Cadillac) vs. a 2.30" (early SBC) is much higher (but not the exact proportion since the leak area of the cadillac is also higher).
I always went with the 10psi X 1000rpm now with Panic's great explanation, I'll be able to impress my friends next time the subject arises
Thats what we use (actually 1 LB for every 100 RPM...same difference). But, once it gets up int he RPM's that rule can change. Say at 6,000 RPM...that does not mean it would need 60 pounds. Engine would survive with lower than that
the 10x1000 is always what i herd but i have had several motors last till i sold the cars with 5lbs or less at idle!what you need to worry abought is if it doesnt go up!!i have ran my car{35 p***es}at 7200rpm with only 40-50lbs of pressure.cant say its good but it will still be at the track the 12th.
What motor? I've had old worn motors, 429 in Mercury Mauquis where the oil pressure light would blink on at idle when I put it in gear, didn't sit around much with it in gear at idle, put a gauge on it and the needle went down to 0 at idle. it ran like hell and never failed until I sold it. I increased the idle speed some, just so it didn't lurch when I put it in gear. Similar situation on chivvvy six's. My personal rule is if it comes up when you are driving it it's likely to be okay.
In the early 80's GM issued a spec of 5 lbs at idle on some of the newer emission engines. Come off idle the engine picked it up to 20 or so right away.
The oil film required to prevent metal-to-metal contact at idle, is extremely minute. In the old days at country fairs, there used to be guys who drained an engine of all the oil and you could bet how long the engine would last at higher rpm without any oil pressure. Engines ran for hours and sometimes days without load....
sorry guys i forgot to mention what engine it is. It is a brand new 350. There isn't anything wrong with the sound but i was really just curious and wanted to make sure with the idle sitting so low i wasn't hurting anything sitting still and the pressure reads low. Thank you guys for the replies and help.
i have a 36 ford pickup it has the orignal flathead 21 stud motor it only has about 7 lb while going 35 mph
i looked in a gm shop manual for a 98 chevy pickup with a 5.7 (350) and what gm called normal is 6 pounds at 1000. rpm 18 pounds at 2000 rpm and 24 pounds at 4000 rpm . i once had a oval track engine that came out of a craftsman truck that i ran in a latemodel and it had a 5 stage barnes drysump setup on it it idled at 20 pounds and racing at 7500 rpm it carried 50 pounds , i talked to the engine builders because i always thought the 10 for a thousand was the needed pressure and was told that that additional 25 pounds took away from horsepower and created more heat and that the 50 was all i needed and apparrantly it was true because after racing it 3 years i tore it down to check bearings and everything was perfect . they had set the relief valve to 50 pounds.and this was on a 650 hp 9 to 1 ford with the yates heads
Sounds good to me! 10 lb at idle in a 350 is fine, I like to use a High Vol. pump with a low pressure spring in SBCs this gives you good idle pressure and 40 lbs off idle on up. Volume increaces with RPM as needed but pressure stays the same (some what) so as not to take more power form the engine and wear out the dizzy gear. (both on the cam and the dizzy) I don't agree with Panic about the rod juornal "changing direction" the piston changes direction but the juornal is always rotating one direction and the bearing knows this.
In that case, you just need to remove the gauge and not think about it. She will probably go another 50K miles. Kevin Lee's flatty never went over 10psi the whole time he had it... Just as a side note, remember that pressure is resistance to flow. The flow comes out of the pump (regulated by the relief valve) and teh resistance varys by how much the oil bleeds off all the places it can. That is why closer tolerances make more pressure than looser ones, the looser ones have less resistance to flow. Good luck, -Abone.
the rod juornal "changing direction" the piston changes direction but the juornal is always rotating one direction My fault - poor choice of phrase. The journal speed is constant, but the rod and its bearings rotate backward at times on every stroke, which reduces the speed differential between them (it's pretty small). High vol + low press: True that the pump drive won't be stressed as much as high pressure, but the volume going through the pump is higher at all times, therefore the load is higher (although without the instant strain on the drive), th oil temperature is raised slightly and the oil is slightly more aerated. The byp*** prevents the maximum press from going any higher by venting the excess - but it still all goes through the pump gears first. To lose as little as possible from the pump you have to slow down the capacity (smallest gears if using the OEM style pump, slower drive ratio if using external and/or dry sump) and use the lowest maximum pressure (weakest spring) that is safe for the engine - 10 psi per 1K is only a known-to-be-safe for most engines as a starting point, and can be reduced in many cases - carefully!
Not that I would recommend this, but back in the mid 60's, I built a 354 hemi, with grooved mains and extra clearance, I increased the volume of the oil pump, but failed to change the valve that allows oil to the lifters. It had roller lifters, I ran the engine Carbuerated,With probably 650 HP, for 2 years in a flatbottom Sanger at up to 6500 rpms. It had a constant 20 PSI oil pressure. I believe the hydrodynamic wedge was working well. When I took it down, there was no unusual wear.
Interesting, I had the same thing with my 67 xl 428, put a gage on it because the idiot light would come on at idle in gear. Literally had no pressure, but I drove the snot out of it and never a problem, smoked the c6 but not the mill... back in my younger day,,,
The Jalopy Journal
