Brake BiasQ Would you please explain the use and adjustment of proportioning valves? Are you adjusting for less braking to the front or the back? Are you attaching it to the front and back brake line? Which do you want to lock up first? Please help! I have all the pieces, but not sure about this one thing.Doug AdkinsDurham, NC
A You need to adjust the braking bias because as the brakes are applied, the weight of the vehicle is shifted to the front wheels. When this happens, approximately 70 percent of the vehicle's weight is on the front wheels and 30 percent is on the rears. Generally, braking systems compensate for most of this weight shift in the brakes themselves. As you have noticed, the front brake rotors are usually a few inches larger in diameter than the rears. The front calipers are usually multipiston design, and the rears have much smaller friction pads and calipers. Fine-tuning the brake balance is where the adjustable proportioning valve comes into play. The valve is installed in the brake line going to the rear wheels. You want to apply as much pressure to the front brakes to get maximum braking work. To adjust the valve you want to go out on a very safe road where you can do multiple aggressive stops (50-0 mph). You will want to do braking runs that are called threshold braking. This is just shy of locking up the brakes. The valve is adjusted properly when the rear brakes won't lock under maximum threshold braking until you get to about 10 mph. When I say lock, I mean that you want the right rear wheel to just lightly skid. Obviously, you will need to do this in dry conditions, on clean asphalt, and again, in a safe place.
There is only one situation in which you would want to install an adjustable prop valve in the front brakes. This is in a drag-racing application in which you are running very small (24x4.5x15-inch skinny) tires up front and very large rear slicks. Most real drag cars have more weight on the rear wheels to aid in traction. In my super-gas roadster I've installed an adjustable prop valve in the front brakes to prevent the front wheels from locking up at the finish line. When you're in the heat of battle, and you bomb the brakes to dump someone, locking the front wheels at 150 mph isn't much fun.
Missing LashQ I know you have said never to lose your cam-spec card. But sorry, I did and was hoping CHP could help me out with my valve settings. I'm running a 350 small-block with Scat cranks and rods, TRW forged pistons, Sportsman heads that are bowl ported, a '70 LT-1 manifold, a Holley 750 vacuum carb, and a Mallory ignition. Everything is blueprinted and balanced, except for the cam valve settings. The cam is a Chet Herbert solid-roller with 0.501 inch max lift, 224/232 degrees duration at 0.050 inch tappet lift, and ground on a 107-degree lobe-separation angle. I am hoping it makes about 425 hp and 450 lb-ft of torque.
The little Mouse is stuffed into a '62 Corvette with a four-speed and 3.70:1 gears. It runs great and pulls hard, but I would really like to get it dialed in right. Currently I set the valves at 0.022 inch of lash, because I didn't know what else to do. Chet Herbert doesn't keep spec cards on special grinds. Any help you can offer would be greatly appreciated.Joe FrinkRedlands, CA
A It's tough to guess a valve-lash setting for any camshaft. It's predicated on the clearance ramp, flank, and ramp of the camshaft profile. With roller camshafts you will find lash designs that come in as tight as 0.012 inch, and I've seen rollers with lash points as loose as 0.035 inch. Your first cut at 0.022 is a pretty good guess. We would loosen the exhaust another couple of thousandths. This is for exhaust-valve expansion. Also, most manufacturers will give you a range of +/-0.004 inch on tight lash rollers, and +/-0.010 inch on wide lash designs to adjust for testing. You can tighten up the lash, or loosen, to find the best performance on the dyno. This will give you a direction on a better camshaft profile selection.