Bolt Power?
Just how much difference can one bolt make to the power putput?
Text, Photos and Drawings,
By
Jon ‘Dusty’ Kennett
After spending four hours grinding on a set of iron Chevy heads, believe me, lunch time is a very welcome break. It was over one such lunch hour that I got to talking with DV about unusual or unexpected sources of increased power. I had supposed, that with over 40 years of prolific dyno testing, DV might have one or two stories to tell here. As it turned out one or two was not even close. Some examples were the 25 hp airline trick for restricted intake turbo motors, the 4 hp donut deal for street stockers, the 14 hp knife edged jet, and the list goes on. But one just happened to hit close to home and that is the crank bolt scenario we are going to look at here.
Over the last couple of years I have accumulated time in several engine shops. These shops are pushing engines out of the door every day and the way they stay in business is to have a streamlined assembly process. One aspect of assembly that always takes time and involves a lot of carefully applied brute force is installing the crank damper. Over the years this job has involved increasingly higher install and removal forces. Although the cranks and dampers are, in the main, made to GM or Fords original tolerances the materials that the cranks and dampers are made of has become better. The harder and tougher the steels involved the more force it takes to install/remove a damper. To expedite installation and removal it seems that it has become relatively common practice to hone out the damper so that it has less press fit interference. While this might make assembly quicker and easier it needs to be understood that a damper only works as it should when it is bonded to the crank to a degree where absolutely no motion, however small, can take place between the crank and damper.
An extensively modified small block Chevy comes into DV shop and is loaded on the dyno. After calibration of the carb and timing a graph of the output shows two fairly large dips in the output curves. These occurred at 5800 and 7000 rpm. At this point Tech Ed Vizard told the engines owner that he seemed to remember being told by one of GM engineers that the crank in a small block Chevy went through torsional peaks at 5800 and again at 7000 or so. That was just where these troughs in the curves were occurring. The power loss here was thought to occur by un-damped torsionals in the crank being transmitted to the cam. From here it was theorized the cams dynamics got fouled up and drove the valve train into a spring surge or seat bounce situation.
After evaluating the valve springs for sufficient poundage they were provisionally pronounced probably OK for the job. From here attention was turned to the damper. This came off far too easy for something that is supposed to have a really tight press for. Inspection with an eye glass indicated some fretting of the keyway and the cranks damper diameter. To fix this would normally mean a new (and expensive) damper and that would take about a week to get. As it happens the damper was used together with a stock Chevy crank bolt. It was suggested to the engines owner that a higher strength (and hence a little more torque) ARP crank bolt torqued to the limit might just tie the damper in sufficiently to at least temporarily solve the problem. Being a simple move it was worth a try. Based on an average of three runs the results of a before and after installation of the ARP crank bolt are shown in the following chart.
As interesting as looking at the raw numbers of the chart is, the situation can be seen far more clearly when shown in graphical form. Take a look at graph Fig 2 and the dips in the output curves where the suspected crank torsionals are at there supposed peaks can clearly be seen.
When a crank damper has been honed, a practice conducted far more than the engine building fraternity might admit to, it might be a good idea to simply specify you want an ARP damper bolt not a stock one. For the small price difference it brings about on an overall engine cost it looks like cheap insurance to me.
AS with any ARP product the quality of workmanship is simply top notch. With this in mind I would have to award the ARP bolt a GFN 5 star product rating.
08-22-2007, 03:28 PM
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