Answers provided by Senior Tech Editor Bob Bolles (unless otherwise noted)PCV PrimerQ I was wondering if there is any information on the use of PCV (positive crankcase ventilation) on circle track engines. I know it's been used on high-horsepower drag engines, but was wondering if anyone has used such a system. What would be the advantages to a 400hp engine? If there is no horsepower gain, would there be any other reason to use such a system?Darrel BauerDenver, CO
Q Do you think it would be worth my time to hook up a PCV valve and seal the oil breather? My reasoning for considering this is that the reciprocating/rotating parts would have less resistance in a vacuum. What thoughts do you have on this? Any potential pitfalls? What percentage of torque increase (if any) might I reasonable expect?John KingCedar Rapids, IA
A Even with the best of ring seal, some amount of combustion "residue" will escape into the crankcase area. One of the original purposes of venting oil pan pressure into the intake track (the so-called "PCV System") was to reduce exhaust emission levels (unburned hydrocarbons) in over-the-highway vehicles. PCV can also reduce fuel economy, but that's an entirely different subject.
From the perspective of a race engine, PCV-vented combustion by-products tend to dilute fresh air/fuel charges, thereby reducing combustion efficiency and power. Further, the higher the crankcase pressure, the greater the tendency for this residue to bypass ring seal and get back into the combustion space, essentially during the intake stroke when cylinder pressure is less than pan pressure.
If you're comparing the effects of pan pressure on crankshaft rotation to that of combustion residue on combustion efficiency, the latter is more critical. Instead, routing pan pressure into an exhaust header collector will reduce windage loss and tend not to reduce combustion efficiency. The amount of improvement largely depends on ring-seal effectiveness. Also, you'll find gains to be greater at lower rpm than higher, primarily because the lower engine speeds allow more time for dilution to occur. With an efficient venting system, I've seen net torque gains of 2 to 4 percent, but this varies with power level and ring seal. Oil attachment to crankshafts can by far cause more torque loss than pan pressure.
A pretty good rule of thumb you can apply to many types of engine modifications is, "What am I doing to combustion efficiency?" The venting of crankcase pressure falls into this category.Jim McFarlandAutotronicsAustin, TX
Housing CheckQ I race a Limited Late Model on a 11/44-mile, low-bank asphalt oval. The car is a Camaro front stub with fabricated frame and rollcage. I run a Ford 9-inch floater rearend with a conventional three-link rear suspension with a full-length, straight panhard bar.
During the last race of the season at my local track, I came out of Turn 4 a little high. The rear of the car came around and tagged the wall pretty good, directly on the right-rear tire. The rim was toast. I put on a good rim and measured the toe with toe plates. It is toed-out 11/48-inch. I did the same test with all my spare rims and came up with 11/48-inch toe-out every time. I went back out on the track for the next race and the car was definitely looser. Tire temperatures on the right-rear also changed. The inside now runs about 30 degrees hotter than the outside. I have done a close inspection of the housing and can't find any bends, kinks, changes in paint, etc. My wheelbase also has not changed.
What is the best way to try to straighten out the housing? I saw a guy on TV do it with a rose bud and a wet rag, but I have no idea where to start. Is my only option to purchase a new housing? Like everyone else, my racing budget is pretty non-existent.Name Withheld by RequestVia e-mail
A If the rear is toed-out, and probably all of that is in the right-rear, as a result of the hit, then that could be a part of the problem. Bent rear-axle tubes are very hard to detect just by looking at them. There is also a possibility that the right-rear axle tube is bent in a way that increased the camber of that wheel.
Heating and cooling one side of the axle tube will shorten that same side. If excess camber is a problem, heating the bottom of the tube will correct the camber problem and heating the front of the axle tube will eliminate the toe-out. Heat the middle of the tube halfway between the hub and the rearend. Stay away from the gears and the wheel bearings. Drain the rearend lube first and replace it with fresh fluid after the tube has cooled.
Port FlowQ On page 52 of the January 2003 issue, there is a chart on "Mass Flow at Intake Valve." It shows a reduction of flow around peak lift. Please elaborate. Is this because of flow separation in the port? I would be interested in hearing if gains at peak lift are that beneficial, such as a 25 to 40hp gain.Gary CunninghamGrace City, ND
A As in the case with exhaust ports, intake port flow is non-linear. Simply stated, it begins with a brief period of back-flow (reversion), continues to a peak value, and then diminishes to a stop shortly after intake valve closing. Pressure excursions continue along the passage toward and away from the valve until the next event begins.
There is a point during the lift cycle (opening and closing) where the valve "resides" the longest. Depending primarily on rpm and rod/stroke ratio, this point (for the intake cycle) is around 65 percent of maximum net lift. In many instances, this coincides with a crank angle of about 65-70 degrees. Experience and data have shown that improvements in intake passage flow beyond this point yield less volumetric efficiency (torque) gains than at or near this "residence" period. So, if you had a choice between improving inlet flow at peak valve lift or within this 65-percent range, the latter is the clear choice.
Interestingly, you might think volumetric efficiency gains would be greater at this point on the opening side of the cycle than closing. This is typically not the case. Kinetic energy gains achieved before and near peak lift will continue into the 65-percent lift range past peak lift, thereby "skewing" the symmetry of an airflow curve superimposed on a lift curve. There is also a reason why inlet closing points (relative to piston position) are sensitive to an engine's overall volumetric efficiency capability.
Finally, the airflow reduction you noted around peak lift (per the January story) is caused by several factors that include combustion chamber wall effects, valve head influence, and the flow separation you suggested, along with other factors. As an interesting experiment you may want to conduct, try overlaying the traces of piston motion, intake track flow, and valve lift, all as a function of crankshaft angle. You're likely to discover some additional areas of interest leading to volumetric efficiency gains. Synchronization of valves' motion-to-piston motion can be a revealing aspect of part selection.Jim McFarlandAutotronicsAustin, TX
Ackermann ApplicationQ I have read numerous articles on the Ackermann principle. I understand the theory and principles presented, but I am having a problem applying this to a stock frame/spindle setup. The basic setup is a Chevy metric chassis. The drag link and tie rods are in front of the crossmember and the steering arms are forward of the pivot point. Correct Ackermann steering geometry should cause the inside (left) wheel to toe-out to a greater angle than the right wheel during a left turn. I understand that this toe-out is not to be confused with static toe-out that is set by adjustment of the tie-rod length. If proper static toe-out exists, how does one achieve Ackermann, the difference in turn radius between the front wheels, with a stock spindle and stock front-end steering linkage?Bob TrykaRochester, NY
A Significant amounts of Ackermann would not be desirable in a stock car. Small amounts can be used for smaller tracks under 31/48-mile in length or tracks that have small radius turns (paper-clip shaped).
We have provided information about Ackermann and defined how it is created. Production cars usually have Ackermann designed into their steering systems for large amounts of steering input. At most racetracks, even the smallest of radii do not require Ackermann, and the steering inputs we use to turn the car are not enough to make us worry about excess Ackermann in a stock steering system.
You should check your toe with the wheels straight ahead and then at 10 degrees and see how much Ackermann your system has. If you gain 11/48-inch of toe or less, I would not worry about it. Toe gains of 11/44-inch or more will really affect your lap times. Excess toe is like putting the brakes on when you steer the car and can cause a severe push that cannot be dialed out with setup changes.
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