So, where does this leave those of us building our engines at a home workshop with less-than-professional shop tools? Well, there is Plastigage. This stuff is about as cheap as it comes, but it is looked down upon by almost every Cup engine builder I've ever mentioned it to. Why? Because it is not as accurate as the more correct (and expensive) tools for the job. However, I think there may be an element of techno-social climbing going on here. Having used all three of the methods mentioned, here is my take on it. First, Plastigage accuracy is not as bad as some pros would have you believe. Testing this for yourself is easy. All you need is two machine parallels and two 0.002-inch feeler gauges. Just place the two feeler gauges side-by-side on one of the parallels and place a strip of the Plastigage between the feelers. Next, place the other parallel on top and squeeze the pair in a press or vice. When you measure, the now-spread Plastigage will have a reading that is really close to the 0.002 inch that it should read. The not-so-good news is that when applied to a curved bearing and journal, things are not quite that close. Generally, the results are within about +/-0.0002 inch (i.e., 2 ten-thousandths of an inch). Maybe this is not perfect, but it is good enough for most of us if the bearings are about middle limit. Now we come to the best part of using Plastigage. If the bearing being measured is out of tolerance, it will be obvious. If the bearing clearance is wrong, especially if it is on the tight side, it will instantly show up, thus preventing serious engine damage.
Now we know how to measure bearing clearance, but how much of it should we have? For most V-8s, a good working figure is 0.002 inch (2 thousandths) for the rod journals and 0.0025 inch (211/42 thousandths) for the mains. Going up half a thousandth on this is no big disaster. In fact, I have used as much as 0.004 inch (4 thousandths) on the mains when it has been a case of "use the crank or don't race." If you are building an engine that has good components, then these nominal figures can drop by half a thousandth (0.0005 inch). If you are building for a small-engined four-cylinder class, the clearances can also stand to be about half a thousandth (0.0005 inch) less than V-8 clearances.
The amount of crank-to-bearing clearance used also influences the weight of oil needed. The closer the clearances, the lighter (thinner) the oil needs to be. This is good news if you are reducing windage and crankcase scavenging losses are a priority.
Conclusions We have seen the advantage gained from coated aero cranks with reduced MOI in wet-sump engines, but there isn't necessarily a direct carryover to dry-sump engines pulling a lot of crankcase vacuum. The more vacuum pulled on the crankcase, the greater the tendency for oil to drop out of suspension. Sure, coated aero cranks are still an advantage for dry-sump engines, but not necessarily by such a margin. One direct carryover from wet sump to dry is the reduction in MOI. This is good, regardless of what type of engine you are running.
|Scat 331/44-inch Stroke Crank Test |
Stock Square-Face Counterweights vs. Aero Counterweights.
|RPM||TQ1||HP1||TQ2||HP2||TQ diff.||HP diff.|
|Figure 1: Shown here are the results of some very carefully run dyno tests. These numbers are the average of a substantial number of runs, with the best and worst of each discarded. By using this technique, the effects of the scatter of 2 or 3 lb-ft on each dyno run can be minimized. As the results indicate, there is a trend for the aero crank to show (as expected) a bigger increase with increasing rpm.|
|RPM||TQ1||HP1||TQ2||HP2||TQ diff.||HP diff.|
|Figure 2: Using the same test procedure as per Figure 1, these results were obtained from a fairly high-output, flat-tappet cammed, 383-inch small-block Chevy engine. The coatings appeared to be worth a reasonable amount of power in this wet-sump test engine, but not until engine speed had exceeded about 5,000 rpm.|
When choosing rods, don't...
When choosing rods, don't use any more rod than is necessary to manage the engine's output and rpm. These Lunati rods are an excellent compromise between weight and strength and are good for 750 hp in the 8,000-rpm range.
Reduced piston and rod weight,...
Reduced piston and rod weight, along with a hollow journal with a smaller diameter, allows the counterweight to be cut away much more to form a lighter pendulum counterbalance mass.
Because it concentrates the...
Because it concentrates the mass in a more favorable position, it is possible to use heavy metal slugs (arrow) to make a crank with less overall weight and a lower MOI.
Coated bearings are making...
Coated bearings are making a positive contribution toward longer life from smaller bearings.
A micrometer that has sufficient...
A micrometer that has sufficient accuracy to measure crank journals is easily within the budget of the average racer.
Acquiring a $600-plus internal...
Acquiring a $600-plus internal micrometer is usually well beyond the average racer's budget, but it is the way to go if you can afford it.
The strip of squashed Plastigage...
The strip of squashed Plastigage (arrow) is measured for width against the chart to establish the bearing clearance. Although not as accurate as expensive micrometers, Plastigage will reveal a potential failure situation due to incorrect clearances virtually every time.