Synthetic Leak, Part Two
Q This is in response to Bob Kay's problem with a persistent oil leak on his 305 (Apr. '10 PQA). One thing you didn't mention is crankcase pressure. In all my years of turning wrenches, almost all persistent leaks are due to excess crankcase pressure. No matter how well you have an engine sealed up, and no matter how many times you do it, with all the best sealants, seals, and gaskets, excess crankcase pressure will always rule. I've found over the years (GM and Ford dealers) that if the engine's PCV system is functioning properly and the engine is new, even normal crankcase pressure can-and sometimes will-push oil out through gaskets and seals during hard pulls. As much as I hate to do it (I think they are ugly), in addition to the PCV system, for these persistent leaks my remedy is to put a breather or two on each valve cover. It's a simple and cheap (though ugly) fix. Except for a couple of blocks that were porous, this has fixed all my persistent oil leak complaints from customers over the years-and for two BBCs of mine.
Baltimore, (Snow bound) MD
A Sometimes you can't see the forest for the trees! We're always looking for the cause of the problems and sometimes overlook the common-sense answers. You're right, you cannot beat excessive crankcase pressure, especially if the engine is breathing out the standard, Bow Tie filter in the air cleaner housing. This is on the early cars, and on most of the late-model EFI cars they are breathing through a straw. Most of the crankcase vents are going through a 3/8-inch tube to the inlet ducting or throttle body.
Going with extra breathers on the valve covers won't make the smog police too happy on late-model cars. The crankcase must be sealed with all oil and combustion vapor going back into the induction system. However, it's a great tool to diagnose oil leak problems. It's much easier to add a breather than drop the oil pan for the fourth time! Thanks for the great tip, and keep bending those wrenches.
Dual 4 Pots
Q I recently built a 383 stroker motor with a 9.4:1 compression ratio. It consists of the following: a 3.750-inch stroke cast-steel crank with 5.700-inch CNC forged I-beam rods, forged flat-top pistons, and AFR 195cc heads. Up top, I'm running an RPM Air-Gap manifold with a Holley 750-cfm vacuum secondary carb. The camshaft is a Comp hydraulic roller with a lift of 0.502/0.520-inch on the intake/exhaust. At 0.050-inch tappet lift it has duration of 224/236. This combination on an engine dyno turns out 491 hp at 6,000 rpm and 489 lb-ft of torque at 4,600 rpm. I would really like to use a dual-carb setup just for something different, plus they look way cool-does it run off just one carb at cruise or is it working off both? Edelbrock offers a kit with 600-cfm carbs. I just wonder if this would be too much carburetion for this engine combination? Thanks in advance for your help, Great mag!
A Dual quads would be way cool on your 383. Edelbrock offers two kits for small-block Chevys. The original C-26 Dual Quad manifold is still around after some four decades! Several years ago, Edelbrock stepped up to support the vintage market by producing an RPM Air-Gap Dual Quad version, which features the distinctive Air-Gap design to give you cool, dense intake charges, and larger and taller runners for great airflow. The RPM Dual Quad is 1 5/8-inch taller at the carb flange over the C-26, which could cause hood clearance issues. This manifold and dual quad setup will not perform as well as the single four-barrel RPM Air-Gap, but I don't think that's what you're about. With the new RPM design you should see minimal power losses.
Edelbrock offers a complete RPM Dual Quad manifold and carburetor kit under PN 2025. This package features their Thunder Series AVS 500-cfm carbs, progressive throttle linkage, fuel lines, and intake gaskets. These 500-cfm carburetors will be perfect for your application. The secondaries on these carbs feature an air valve that tailors the airflow to the demand of the engine. The high-tech progressive linkage offered can be set up for either progressive or 1:1 ratios. In the progressive mode, the rear carburetor is for slow-speed driving (up to 20 degrees of throttle opening). Past that point, it brings in the front carb as well, until you reach wide-open throttle. If you're looking for a more aggressive throttle response you can set the linkage to 1:1, which opens the carbs in unison.
Check with Edelbrock for more information. Have fun. Your 383 will have some real authority with dual quads atop it.
Technical questions for Kevin McClelland can be sent to him at firstname.lastname@example.org