Piston-to-head clearance affects "quench distance" in an engine. It's a fine line to walk
So Many Questions
Q: I build some restricted 500 4412 carb engines for some local racers, and I have some questions. First, how tight can you run the piston-to-head clearance on an engine like this? I have run it as tight as .030. Is there a point that it gets too tight and adversely affects burn rate or hurts power in some way? We are restricted to a solid cam, but we can run any exhaust we want. We can also run alcohol. We must run a production head with no porting. Would you put oversize valves in a Vortec head? As far as cams go, would a single pattern benefit me?
A: Consider that alcohol tends to burn slower than gasoline. Because its latent heat of combustion is roughly half that of gasoline, an increase in volume is necessary to approach equal or higher power levels. You already know this, but it sets a stage for comments that follow.
While the intake-to-exhaust flow relationship for the Vortec heads is pretty good for gasoline and rules limitations, you might want to consider two possible changes to the intake side. One is to increase valve size and the other is to use a dual-pattern cam that favors a later intake valve closing point. This can allow an increase in volumetric efficiency and help compensate for the alcohol's lower heat of combustion (compared to gasoline) by the potential for increased fuel volume. Larger intake-valve size may help net airflow at higher lift values, but it can lead to reduced flow velocity in the under-4,000rpm range if that's important to you. The camshaft might be an easier, more effective and less expensive area to investigate.
The head-to-piston clearance issue may be a little less defined. Since you didn't reference compressed gasket thickness, I'll assume you're not talking about deck height but total quench distance. Generally, you'd like sufficient "quench" to keep mixtures active (for improved combustion activity or "mixture motion") but not so tight that it reduces net burn. Opinions will vary on this subject, but we've seen clearances in the 0.024 to 0.028-inch range run successfully. Combustion chamber shape can influence this minimum especially with alcohol.
You may also discover that quench height reduction can have an effect on optimum ignition spark timing. For example, if you're "over-quenching" the burn, then you may find an additional one to two degrees of spark timing is required (short of detonation) as compared to optimum spark if the burn is progressing normally. While you may consider this nit-picking the issue, it's simply to suggest there's an explorable relationship between flame rate and spark timing. Rather than look for all the reasons, this is only to suggest that changes in quench height should be accompanied by spark timing experimentation in small increments.
Hopefully, this will stimulate some thoughts.Jim Mcfarland
Q: I run a '79 Camaro stock front-clip sportsman car on a high-banked 31/48-mile asphalt oval. I purchased a Howe drag link and the 511/44 steering arm and six-inch idler they said works with it. With this configuration, the drag link is angled back on the idler end and so is the tie rod. We have to maintain stock lower control arms and spindles. Is my geometry OK?
Crew Chief Strategist
NASCAR on FOX
Tex Racing Enterprises
Another question I have is about roll center and camber gain. I notched the top of the frame for the upper control arms about 31/44 of an inch and laid in a flat plate. My control arms are heims at each end with screw-in ball joints. The problem is I can only get about 16 degrees on them and fear my roll center will be a little high. I have a five-inch ride height and the lower control-arm points are at equal heights. Should I concentrate a little more on camber gain or rework what I have done and get shorter front springs?
Name Withheld Upon
A: I would suggest the angle of the drag link and the angle of the right-side tie rod in the top view will have an effect on your ackerman. I assume you have bump-steered the front end after the changes, and if you haven't, you should do that first. Then do a test with the front end on steer plates. If you zero them out before you start, record the steer angle on the left wheel while steering the right wheel in two-degree increments. You can then look at what kind of ackerman you have. Plotting the angles on graph paper with the left-steer angle on the Y-axis and the right- steer angle on the X-axis will show you if you have parallel steer, ackerman steer or anti-ackerman steer geometry. You want to have something close to parallel steer from zero to 10 degrees. Another factor to watch is the overall steering ratio. The idler arm is a different length than the steer arm. This means the arcs they follow will be different for a given input from the pitman arm (the length of the pitman arm was not given). The steering system has an overall ratio between the steering-wheel angle and the right-front wheel steer angle. The ratio comes from the steering box ratio as well as the linkage ratio due to pitman arm, idler arm and steer arm lengths. You should measure the overall ratio by recording the steering-wheel angle to get five degrees of right-front wheel steer. If it took 45 degrees at the steering wheel, you would have a 9:1 ratio. If it took 50 degrees at the steering wheel it would be 10:1. Normal short-track ratios will be around 12:1, which would be 60 degrees at the steering wheel.
Regarding the upper control arm, I am assuming this means the upper-arm angle is in the front view. You said you could get only 16 degrees. If you were able to get more angle, the roll center would actually be higher, while the camber change rate would be quicker. As for the question, about whether you should rework what you have done or get shorter front springs, there's really no way to answer that because you didn't say anything about how it performs. How do you know that what you have will not work? If you need more camber gain, use a shorter upper-control arm. The shorter front spring would lower the ride height and change the upper-arm angle, giving more camber gain, but we don't know if you have enough ground clearance for this or if the rules let you lower your car any further. However, if you raise the spring rate by cutting off the spring, you might make the car push (because of the stiffer spring) more than you gain in front grip from the increased camber.
Front-end geometry is a complex problem. There are many interactions of the geometric properties, so it is very difficult to change just one factor. The best method is to measure all of the front-end components and then analyze the geometry in a good three-dimensional computer program. Lowering the inner pivots of the upper-control arm will change the camber gain, the bumpsteer, the roll-center height, the roll-center lateral position, the tire scrub and the anti-dive. Which one of these factors that changed can the driver attribute to a different feel in the car? The answer is not necessarily obvious. A method for evaluating these factors is difficult, which is why even the top Winston Cup teams continue to work on front-end geometry to get the right combination of these many factors working together.
Knowing your front-end geometry is an exercise in patience and mathematics. Even Winston C
Regarding Reverse Rotation
Q: I have heard of reverse-rotation engines, but haven't seen any articles on the subject. I would like to know the process for converting an engine to reverse rotation (cam, ignition, valvetrain, oil pump, starter modifications, etc.). What kinetic/gyro effects would result?Guy LavertyVia E-Mail
A: Years ago, I did some work with Smokey Yunick on reverse-rotation engines. My recollection is that Smokey said they were "a hell of a lot more trouble than they're worth when you consider all the changes required." In fact, while you're likely to see some reduction in chassis torque that favors less left-front corner lift, there are some related suspension tuning issues that also require rethinking. So even if you get past the mechanical problems involved with changing crankshaft and camshaft rotation and solve technical requirements of the ignition and oiling systems, there are on-track situations that require attention. Maybe Smokey was right. He often was.Jim McfarlandAutocomAustin, TX
Editor's Note: If you wish to do further research on reverse rotation, reference Smokey Yunick's report on it in the May 1991 edition of Circle Track. While it is too large to reprint here, the article covers all your questions in Smokey's own inimitable style.
Needs More Vacuum!
Q: I need some help! I run a 406 small-block Chevy. The specs are fresh block, scat crank, Eagle six-inch rods, flat-top forged pistons, Edelbrock RPM heads, cast-iron factory four-barrel, high-rise intake and open three-inch pipes from stock exhaust manifold.
What I need is two more inches of vacuum. Will mufflers help? Will a restrictor plate under a factory Quadrajet? I tried moving timing, and I can get 18 inches, but I need 20. The cam is a .410 lift Crane Saturday-night spec.
A: Scott, it's hard to tell exactly what you need to do without seeing your engine, but I can give you a couple of ideas to check out. You asked about mufflers, and there's a possibility that might help a little, but there are two bigger things that probably will help you more.
I don't know where you have the cam in the engine, but it might help to advance the cam just a shade, two or three degrees. That should help your vacuum some. The other option is likely to make the biggest difference. You don't say, but I have to assume from the other specs you gave that you are running a hydraulic cam and lifters. Competition Cams makes a lifter called a Hi-Tech Hydraulic lifter that actually bleeds down. That should definitely get you the two more inches of vacuum you need. It doesn't hurt your engine at all and it helps to get the vacuum down when your engine is being checked, but once the rpms get up there it all straightens out and does just what it's supposed to do.Jay DickensJay Dickens EnginesAberdeen, MS
Formulas in Mark Chevalier's story "Wrapped Up in Traction Control" (Nov. '01) were misprinted. The formulas appeared correctly, without the exponents. The formulas with exponents are as follows:
To calculate wrapup stiffness with a pull bar:
S = 4.77 * K * h2 * (1 - .005 * t)
To calculate wrapup stiffness with a lift bar:
S = 4.77 * K * L2
h-distance from the axle centerline to the pullbar mount
t-angle of the pullbar
K-distance from the axle centerline to the line of action of the spring
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