Engine Loading Question
 

A friend of mine just had a pair of 350's put together for his 1978 30' scarab. The boat originally had 330's with TRS and TRII trans. The motors are 9.5:1 compression with a nice cam (not sure of specs.) spinning 23 pitch bravo props for engine break in.

Oil pressure at idle was approx. 60-65psi cold with coolers.

He went out and ran it. Easy up on plane and never over 3400rpm for about 5 minutes. Then the oil pressure started to drop off. Now he is getting appox. 40-42 psi at idle and pressure drops to 15-20 under load.

I suspect a clearance issue or oil system problem. THe engine builder is trying to tell him that he is over loading the engines with the 23 pitch props causing the problem.

What do you guys think?????

The oil pres drop after running and getting hot is normal. My concern would be pressure falling under load never good. Do you have any oil coolers or remote filters that had to fill up with oil maybe the oil level is low now? As for over loading the engine thats BS trust me. Give me some rpm numbers with the oil pressure numbers.

Budz is right, as the engine breaks in, pressures will decrease,
lower at idle, 20-30psi hot, maybe even a litle lower, 5-10 at idle is all that motor needs,40-60psi off idle hot, oil pressure is in direct relation to rpm, not load, even if it was loaded so heavy it would not come up on plane, if the rpm's are there, so should the oil pressure.
Larry

Load causes heat which thins oil. If it thins it so much that he's losing pressure, that's bad. Since you don't cite an oil temp, I'll assume there's no gauge- that's VERY bad. Gauges cost lots less than engines.

First, change the oil in those motors- if you got it hot, it needs to be replaced. Install oil temp gauges. Then, start looking for causes. If there's no external oil cooler, there needs to be. If they're too small, that's way worse than no cooler at all. Too small of lines and sharp-angle fittings are engine killers too. If you still have the same pressures you cite, you have a serious internal problem. A big block Chevy needs more than 40 PSI at running speeds- over 50 at least.

There should be very little break-in with a modern, properly machined engine. The bores should have virtually no wear-in. That cross-hatch should be visible in the bores for some time. Modern rings are made-to-size. The don't wear to the bore anymore. At least not in any significant way that resembles how engines used to break in rings. Besides, rings have nothing to do with oil pressure. The bearing clearances should not be opening up. If they do, it's because the crank is touching the bearing inserts and wearing the surfaces- that's not only bad, it's fatal.

The boat does have 12" coolers. No oil temp gauge. I've seen these boat run with small blocks with no problem before. Just trying to figure out what my buddy is up against. The motors are coming out this weekend so I can look at them and see what is happening. Thanks for the input.

If by 12" coolers you mean the black Merc one's that have the two fittings on the side and a spiral tube inside the cooler, those are OK for transmissions. They are way too restrictive for an engine. Keep in mind, your entire engine flow has to go through there. The internal diameters are about 1/4". Then take into account the restrictions you add in an adapter, fittings, elbows and lines and you can cut oil flow by half just simply adding a cooler.

none of this makes any sense.

first... the boat ran before in this configuration so it should run now. the coolers , the lines, the angles, the fittings, while maybe not world class spec, all worked before as a system and should work now.

second... let assume he's running 300 degree oil... fine. the pressures went from good to garbage because it got thin . so why would it idle at 45 ? if its thin it would make 0.0 at idle.

third, as mentioned above, because the oil pump is a positive displacement device, the volume and as such the pressure is a strict function of rpm. more revs = more volume = more pressure UNLESS :

a) the relief valve is broken
b) the oil is not getting back to the pan
c) the motors just don't have enough oil in them
d) the pickup has broken off the pump.

no scenario i know will give a hot motor MORE oil pressure at idle then at higher revs...

unless of course its some bizzare electrical issue w/ the sending unit... but of course you have confirmed all this with a mechanical gage...

rite ?

Cut the oil filter open before you even attempt to identify/diagnose the problem. Looking for the flakes of death of course between the pleats. Go from there.

what oil filter is on there????

I ran 23 inch props on my 78 scarab with 454 420HP's. I would guess that boat needs 19's or 21's at most. Still seems unlikely that it would hurt oil pressure that much. Start with the simple things first. Change the oil and filter.Cut open the filter and look for gold. Put an oil temp guage on it, all perf. boats should have them. All of my boats have had them right from the factory. Make sure the plumbing and coolers have enough capacity. If all else fails the engines will have to come out.

Anther idea, make sure they aren't overfilled with oil. Possible oil foaming. Does the pressure come back after sitting and cooling? Do the engines have windage trays? Air bubbles get beat into the oil while on plane and then slowly come out at idle. The problem is worse with high oil temps.

Well he had the engine builder down there last night. They started the port motor and it idled at 45. Started the starboard and idled at 45. Idled to mouth of river about 3/4 mile and got on plane. With a 1/4 mile the port engine was a 0pressure and the starboard was at 25 at 2800rpm.

As Steve said the thing ran with the big blocks and the same cooler configuration. My thought is something is wrong with the mains (bad line bore) or the oiling system. The motors are coming out this weekend for as autopsy. We'll see what happens.

Here's one to think about. I bought a Trans Am with a Pontiac 400 in it. It idled badly but oil pressure was normal at cold idle, about 65 lbs. I went for a short drive and it didn't run great, pretty rough. I went ahead an bought it because I wanted the car, WS6, and I figured I could fix it. The price was right. After I got it home I did all the normal tuneup stuff but I couldn't get rid of the miss. Also if I drove it very far and got it real warmed up the oil pressure would drop to about 20 psi. Continuing to dig, I decided to check the cam, see if it had a flat lobe. When I took off the valve covers, I could see a bent pushrod and looking lower I could see there was no lifter in the bore. It was sitting loose in the valley. The engine still had pretty good oil pressure with that big of an internal oil leak! It's a good possibility you'll find something like that if you tear your engines down. And since it's both engines, whatever mistake was made, it was made to both.

not a bad guess... but i don't think so... there is a guy on here named smitty who discovered an oil pressure problem that was related to where the oil groove in the lifter was relative to the gallery... a wonderful piece of deduction and effort... one of those horses not zebras things that really WAS zebras...

but i don't think this is one of those. this is going to be something really basic. the indicator is that it is both motors. thats is simply beyond the realm of statistical probability.

someone has done something wrong and its going to be the same on both motors.

and its may well be something weird... like not putting a plug back in the oil galley after cleaning which would leak like hell internally but w/ cold oil be ok... or a lifter deal like smitty had...

in a lot of years of doing this i can say one thing for certain... when this happened to me, i simply sat down and said to myself..." what is the LAST thing i did that was different from what i have done before...? " and thats where i would start looking. nothing happens for no reason. these motors ran before, they should run now. what specification has changed ?

Well just a shot in the dark but it sounds like the oil plug under the rear main cap.

Well this might make it easier for you. In the back of the block next to the dist. there is a oil fitting take it out and get a hanger or welding rod, measure 8 1/4 in. up and mark a line on it. Drop it in and it should stop at the line flush with the block. If it goes further then the line on the rod then the plug is not set in place!

that is very very good.

I'll vote for the pickup either being bottomed out on the oil pan or it has fallen off... the symptoms are dead on for an oil pickup that's off... when the pan is full the pump can suck the oil up - at speed the level in the pan starts dropping and the pressure drops as the pump sucks air - to do a quick and dirty test - add 1.5-2 more quarts of oil - take it out for a short run - if it holds better pressure - that's it... just DON'T run it a long time with this setup as it may cause oil foaming - of course foamed oil is better than air...