What is the correct way to vent a Chevy big-block? Are two push-in breathers, one on each valve cover, enough? Is using a PCV on one and a breather on the other valve cover better? Do you have other ideas on this? What happens if the engine is not properly vented? This is not a smog engine; it’s a race-only car. I enjoy reading your column every month—keep up the great work.
Dual push-in breathers will vent a big-block just fine. If you’re using this as a racing application, we wouldn’t use a PCV valve. With larger camshafts with low vacuum, a PCV valve can cause idle stability issues that will cause you to chase your tail. You’ll think it’s a carburetor or timing issue, and it’s just that the valve won’t seat and it’s constantly causing a vacuum leak through the crankcase out the other breather.
As a reader brought to our attention not too long ago, inadequate ventilation can cause oil leaks from many places on our engines. You may think that the engine is sealed up, but if the ring seal isn’t happy, the crankcase will build pressure at full-throttle very quickly. This can push oil out of seals and gaskets that you think should be sealed up well. Happy racing!
Kickin’ It Sideways
I have a ’72 Chevelle with a big-block Chevy 469ci (0.070-inch overbore, making approximately 550 hp and 580 lb-ft at the crank). My problem is it goes sideways when I hit Second gear. It heads for the ditch and does not want to straighten out. The drivetrain and suspension consist of the following: solid motor mounts, an Art Carr 200-R4 transmission with super servo, a 12-bolt posi with 373:1 gears, Hotchkis 1-inch lowering springs with Bilstein shocks all around, a Hotchkis hollow-tube front sway bar, Hotchkis lower rear trailing arms, Air Lift airbags with 10 pounds in the right side and 5 in the left, an NOS rear sway bar, pinion angle set to -2 degrees, and Mickey Thompson drag radials 275/60R15. The front tires are 215/75R15. I have purchased the adjustable upper rear trailing arms but have not installed them yet. I hope I listed enough information. Any help would be appreciated.
OK, first the obvious: You’re doing this on the street?! There is road crown, and with you mentioning that it’s tossing you into the ditch we’ll assume the car is turning right. The road crown is going to pitch the car to the right, as will the torque applied to the left rear tire. When you apply large amounts of torque to the rear suspension and the rotation of the engine and driveshaft imparts a twisting force that plants the left rear tire. When you’re at speed, the car doesn’t weight-transfer to the right rear to compensate for this added load to the left rear tire. This is another reason that it’s tossing you in the ditch. You can add more air pressure to the right rear airbag to artificially load the right rear tire to compensate. We run only one bag in our Malibu wagon and it’s in the right with 30 psi of air to launch the car somewhat straight. We’re right on the edge of needing to add a rear antiroll bar to keep the car square on the launch.
Finally, get off the street if you’re going to do WOT Second gearshifts. We don’t want to see you getting hurt or hurting someone else. The rest of your package sounds like it should take the power you’re working with. Keep the upper bars equal length. We’ve tried putting preload in the upper bars in our wagon with no success. Stick with the preload in the airbag. Be safe, please! CHP
We love letters, especially technical questions. Submit your tech questions to Kevin McClelland at firstname.lastname@example.org. Regular shout-outs and good tidings are also always welcome.