i have a rebuilt 350 by a good machine shop about 3 months ago. the motor runs great but it hasnt been test driven yet since my truck doesnt have any licence plates. it has been bored .030 over 9.5 compression , comp cams 268 (PN-12-210-2) WORLD SR TORQUER HEADS EDELBROCK PERFORMER INTAKE, , AND HEADERS. the problem is when i turn on the motor the oil pressur is fine at 60psi but when it gets to operating temperature at aroun 200'F  the oil pressure almost drops to 0!! I AM USING VALVOLINE RACING 50 WT OIL. i do have an oil leak behind the block but i have checked and it is not the valve cover, intake, sending unit or the oil pan. it seems to be between the tranny and motor. do you think this could be causin the low pressure??  also i have 2.73 rear cogs with advance all by in 2200 rpm.  i would do it all by in 1800rpm but i want to lower the operating temp. down to 180'F. and a   TH-350a . what E.T. would you expext in my 86 chevy c-10 shortbed.  THANK YOU FOR TAKIN TIME IN READING THIS  

Your experience of high oil pressure when the engine is cold, becoming low to none when it’s hot is the classic symptom for excessive bearing clearance. You could also rule in an internal leak at one or more of the oil galley plugs.

Excessive bearing clearance can be the crankshaft’s mains and/or rods or the cam bearings, but somewhere the clearances are way too loose.  In the case of the cam bearings, they may have been left out entirely when the block was assembled or, parish the thought, if the block was hot tanked with them in it, the soft bearing surface has been etched away and only the steel backing remains, talk about a loose clearance. One way or another there's massive internal oil leak back to the pan. I say internal because if this was an external leak, you'd have an absolute flood of oil coming out of the engine and it would quickly run out of oil. Oil running out the back side of the block can be main crank seal leakage, not uncommon on a 2 piece seal engine, or it could be coming from a rear oil galley plugs (3) or the rear cam bore plug. You’re just going to have to avoid screwing with this thing and yank it out to take a look at all these possibilities, that’s all there is to it.

From a configuration stand point, you don't have enough rear gears to support the heads, intake, headers, etc. with 2.73 gears the engine just turns too slow to get any port velocity up to where you can fill the cylinders at much of anything short of red line. But don't worry that, you've got to pull the motor and disassemble it and go back over the internals looking for excessive crankshaft bearing clearance, the cam bearings for damage or even being there, and lastly the possibility of leaking oil galley plugs. 

Bogie

THANK YOU. WHAT DO YOU THINK CAUSED THE EXCESSIVE BEARING CLEARANCE. AT FIRST ABOUT 3 MONTHS AGO I DID NOT HAVE ANY OF THESE SYMPTOMS UNTIL RECENTLY THAT I SWAPPED OUT THE DISTRIBUTOR WITH A NEW ONE.  THE NEW ONE DOES HAVE A BOSS WITH A GROOVE LIKE THE OLD ONE. DO YOU THINK IT MIGHT BE THE OIL PUMP SINCE SOMETIMES I HAVE TO GIVE A QUICK HIT TO THE THROTTLE TO PICK UP THE PRESSURE TO 60PSI?? ALSO I SHOULD ALSO MENTION THAT 3 OF MY PUSHRODS WERE RUBBING TOO CLOSE TO GUIDEPLATES AND HAVE "GRIND" SOME OF THE MATERIAL AWAY FROM THE PUSHROD.  DO YOU THINK THE SHAVINGS  COULD OF DAMAGED SOMETHING IN THE MOTOR.  I REALLY DONT WANT TO HAVE TO YANK THAT MOTOR OUT AGAIN SINCE  IM ONLY 17 AND IN A REAL TIGHT BUDGET.  

Metal shavings going thru the motor could do quite a lot of damage. The first place they went was the pan, they'd be too small for the pick up screen to stop them and they would get into the pump. Once in the pump they could damage the gears or housing, this would let the pump leak internally which would reduce pressure. Once thru the gears, they either went thru the pressure relief valve or on to the main pressure system.

Let's take the pressure relief first; passing into the pressure relief they could become lodged in the valve assembly preventing it from fully opening or (in your case) perhaps not closing so when the oil warms and the valve would typically close to stop by-passing the previously cold high pressure oil, instead the valve stays partially open always by-passing some oil. Or the metal shavings could have gouged the valve to valve body surfaces and while the shavings passed on thru the grooves left behind continue to leak oil or prevent the valve from fully closing when hot.

Moving into the main pressure system, the shavings will circulate maybe the filter will get them and maybe not. This is because of the miss-nomer of a "Full Flow" oil filter. Many people believe this means that all oil is filtered, what this really means is that the filter has a bypass valve and if the oil is too cold and thick to flow thru the filter, or if the engine is turning at high RPM and it needs more oil than the filter can supply, or if the filter is plugged with trash what happens is the filter's internal by-pass valve opens to insure that the engine receives its "full flow" of oil. In other words the requirement is to insure the engine always receives the amount of oil it requires and whether that oil is filtered is of secondary concern. So it's as likely as not that these shavings have circulated and done damage to bearings or lifters, or ??? what have you.

I don't know what your idle RPMs are, some oil pumps can be quirky and just running the engine a little faster on idle will pick up the pressure. I sure wouldn't be running 50 wt oil. Thats the kind of stuff you put in old air cooled Harleys and big radial airplane engines because of their loose clearances. Modern liquid cooled engines work just fine on multi-grade oils like 10W-30, or 20W-40 for mineral oils and with synthetics you can drop the low side to 5W-xx if you wish. Heavy oils put a lot of strain on the pump drive, distributor gear, cam shaft and its drive since in the end all that power comes from the driveshaft and the load is carried by everything from the crank gear to the oil pump's internal gears. If your distributor and oil pump uses a plastic mating sleeve, this should be replaced with metal. If your camshaft is a roller, the distributor gear needs to be a factory melonized steel gear or a bronze aftermarket piece, be advised that bronze gears don't wear long enough for street usage and the same can be said for the newer plastic gear. The new distributor's housing needs to fit both down and tightly where the right side lifter oil passage goes around the distributor's grooved area, otherwise it could either form an oil leak or block the passage. The drive tang on the bottom of the distributor needs to sit inside the groove of the oil pump drive shaft with about .05 inch clearance on the bottom. If you don't have this clearance the distributor could sit too high and not close off the lifter passage letting oil leak back into the engine. At the intake, the distributor has a flange the anchoring hold down rides on, there should be no open space between the bottom of this flange and the top of the hile where the distributor drops into the manifold, it there is the distributor is not properly mated to the oil pump drive. There shuld be a thin gasket under this flange to the manifold.   Unfortunately this probably wasn't checked at assembly and is hard to do with the engine installed, what you can try is a piece of Plastigage tacked to the tang with a touch of super glue, install the distributor tightly and then remove it and measure the the Plastigage for the clearance. A little Plastigage and super glue that gets inside won't hurt anything.

Unfortunatly the only way to see the oil pump is to pull the pan which is a real PIA, but your next place to look after examining the distributor's drive is the pump looking for damage done by the metal slivers.

I'm still concerned about the cam bearings, this would be the first engine hot tanked with those bearings still in it and then not have them replaced.

Bogie

thank you very much bogie im sure i can use some of ur info and find out what the problem is. thank you for takin some of ur time in helpin me tryin to figure this thing out....  

hey bogie. well i came to the conclusion that its my oil pump that is causin my low oil pressure. because when the motor is at operating temp, oil pressure is at 30 psi in parking at 800 RPM and once i put it on drive the oil pressure lowers to 15-20 psi at about 700 RPM. i am going to install an oil cooler in the future. whgat type of pump do you suggest, a standard or high volume pump? i also dont think its the cam bearing because there is no engine noise whatso ever and i can rev it up freely to 6 grand easily. well thank you for takin ur time in reading this.  

Pressure of 15 to 20 psi at idle in gear is right on the factory spec. Coming up to 30 psi with only an additional 100 RPMs sounds pretty good, then the pressure should come up quickly to 45-60 psi by 2000 RPM.

The max pressure will be governed by the pressure relief valve. Somewhere in the range of 45-60 psi it will start to open and vent excess oil back to the pan. As the oil warms pressure will drop off and the amount being bypassed will reduce while the pressure remains around the 45 to 60 psi area.

I'll admit that I like to see more idle pressure but with a gear type pump getting more at idle requires a much larger pump which then spends more time with the relief valve open as RPMs climb. Ford and Chrysler use a gerotor pump which produces a higher idle pressure, but as revs come up these pumps have to divert more oil through the pressure relief than a gear style pump. The advantage they have is that the engine has more pressure at low RPM so there's better bearing lubrication for stop and go driving. Their dissadvantage is at high RPM where more oil is excessively heated by the pumping action and then most of it is dumped back to the pan by the relief valve. The gear pump tends to be iffy on the low end of operation but doesn't thrash the oil to death at higher speeds.

An oil cooler is always a good idea, but you must include a thermostatically controlled by-pass valve, otherwise when the engine, its oil and weather are all cold; the could oil circulates thru the cooler and never gets up to engine temperature. This keeps the engine too cold which reduces available lubrication since most of the cold, stiff oil is just being by-passed to the pan instead of circulating thru the engine.

Contrary to popular recommendation, I prefer to run the 12 tooth big block pump instead of the 7 tooth small block pump in the small block whenever I want a high output oil pump. The thought here is for each rotation of the pump it's delivering 12 smaller pulses of oil rather than 7 big pulses of oil. Among other things the pulse rate is further from the natural frequency of the engine and the pulses smaller, therefore there should be less coupling back into the distributor that causes it to wander back and forth between the limits of the its gear teeth clearance, hence resulting in a bit of timing wander.

Bogie

oh good. thank you bogie. you just made my day. im glad you could help me out that way i dont have to work and spend money without cause.

thank you

hey bogie. now that i got that oil pressure problem out of the way. i need your opinion on my next modification. i am plannin on swapin out my th350 with a th200 overdrive that i can get for $200.00 and swapping out the 2.73 rear gears with 3.55. wouldnt this be good for launching yet still get around 1800 rpm at 65mph with 28 inch tall tires. io dont know if you remember my motor specs so here you go: 355, 9.5 compression, comp cams 268 duration PN 12-210-2, 175 cc intake  aftermarket heads, edelbrock performer intake, edelbrock 600 cfm carb, 1 5/8 headers with 2 1/4 dual exauhst pipes. i am planning on swithing to performer rpm air gap and 2 1/2  pipes. what do you about the current overall combination and my future upgrades.  

The TH200R4 just isn't up to the task. Save these for the race guys who are willing to replace them every few runs just to be able to shave some weight. You can replace the TH350 with a 700R4 or 4L60. This gives a deep low for a hard launch, you'll probably want to come up to at least a 3.08 rear gear for cruising in OD, otherwise the engine is just turning too slowly with this combo and 2.73 gears. Even 3.55s would give a comfortable cruise Note the 4L60 is not the same as 4L60E; the "E" is fully computer controlled and the internal valving is run by electric servos not hydraulically balanced valves. So the E must have a computer and specific electronic sensors on the engine and driveline. The 700R4/4L60 offers the same performance but it's done the old fashion way of balancing hydraulic valves against one another. It does use a computer logic to operate the torque converter lock up but this is a stand alone system that can be had over the counter or you can operate the lock up manually, but most of us couldn't remember to operate chokes manually so that should give you some idea of your chances.

I think creeping into 2 and 1/2 inch exhaust pipes is a bit on the too big side unless you're running catalytic converters which restore some heat in the exhaust stream. But a 355 with a Comp 268 cam just can't move enough air thru the engine to justify 2 and half inch pipes. CHP did an article some time back on header effects to a 383. Actually it did really good with Flow Master 1 and a half inch medium length pipes. Using 1 and 5/8s Hooker 36 inch pipes it picked up a little power over 4000 RPM, but in 2 to 3000 RPM range they dropped nearly 20 foot pounds and horsepower, that's a mighty expensive power loss for a tiny top end gain. For some reason it took Hookers of 1 and 7/8s before the power under 4000 again equaled the inch and a half Flow Masters. So you've got to be careful with the exhaust.

Bogie