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Original Message
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| RE: 390 reliability |
| By cdailey - 03/21/2005 9:16:11 PM; IP 216.170.25.233 |
Any bore scoring (overheated piston) or bearing damage (hammering)? Bearing discolor (blue / black)? Any time the same part fails over and over, it's environment must be checked.
If there is bore scoring, the piston is likely overheating. I'll assume your piston and ring clearances were checked. We'll also assume that oiling is OK (unless the bearing is discolored), because the opposing cylinder would be splash oiled by the same journal and would fail as well (or show the same signs). This points to a lean condition or preignition / detonation causing the overheating. Make sure the intake manifold gasket surfaces are flat, machined to match the cylinder head angle, and well sealed. Don't use the Felpro gasket with the blue sealer ring, they crack when heat cycled. Seal with correct gasket sealer. If preignition or detonation is the culprit, it will show on the plug. Black specks on an otherwise white insulator is a good indicator. If you can run an 02 sensor, do it. Cheap insurance. Make sure fuel pressure is maintained during the run, and that the needle / seat flows sufficient fuel. Otherwise, fiddling with carb jets may be the only answer, but if you have a true fuel distribution problem, either pick a new intake/carb, or solicit the advice of a race engine builder to select another. Porting the intake will likely cost you more than just getting a better unit, but it can be worthwhile if done properly.
If there is bearing discoloring, the journal is overheating. Then you have to ask why... no oil, or too little clearance? Assuming the builder paid attention to the rod bearing clearance, it's probably oiling. Make sure you aren't pumping all the oil to the top of the engine (restrict the valvetrain oiling) and make sure that what's getting pumped up there is getting back (will the engine idle without valve covers and not overflow --it should). Make sure you have proper clearance between the bottom of the pan and the pickup. If you're reusing the same crank, make sure the oil passages are correctly drilled. I tried a trick I read in an article written by David Reher about stablizing oil pressure in wet sump drag engines: Replace diamond scraper windage (or no) tray with solid tray, stops air pulses into the sump, makes for less aerated oil. Before I tried this, my oil pressure was ok, but the gauge was jumpy, and high rpm made it worse. After: Rock solid oil pressure, and the gauge moves very smoothly, and pressure steadily increases with RPM, like I always thought it should.
My two pence. Good luck.
Chad |
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