RDI Street 392 Crate Motor - Wicked Windsor In A Box, Part 2

At first it may seem an unlikely piece of go faster equipment, but an effective crank damper cuts the amount of torsional vibration transmitted to the camshaft. In so doing it allows the cam to much more nearly deliver the intended lift/duration characteristics to the valvetrain. Forget light when it comes to dampers. Even for a drag racer, where acceleration is all that matters, a functional damper will out perform a light, non-functional one every time.

So how much power can a damper be worth? Forget about hearsay and some of the opinions of even well-known racers. Testing on my own dyno on a nominally 400hp motor showed a good damper was worth 5 hp over a typical factory damper (which was designed for the stock bottom end not a modified one) and over 11 hp over that delivered by a lightweight hub.

As far as measuring and refining the damper action was concerned, the plan was to have ATI build what, based on their huge data bank, they felt would be required for the longer than stock stroke (3.5 vs. 3.85) of the 392 Street Pro. Once the engine was on the dyno and broken in JC would set up ATI's torsional measuring gear, measure the crank's vibrations, then build the appropriate damper specifically for the Scat crank and rods, the Probe pistons, or rods and pistons of comparable weight.

Going into this test Beattie commented he did not expect to be far off the mark with the damper spec we started with. How right that statement was. We never got to see the selection process in action. Without a damper most cranks will suffer torsional vibrations up to a degree or more of double amplitude (+/- 1/2 a degree). Steel, as opposed to aluminum, can withstand a certain amount of deflection indefinitely. However, after a certain point the hours and minutes begin to count against it.

Within the industry it is generally accepted that a typical V-8 can, at what appears to be the most important third and fourth order harmonics, tolerate about 0.4 of a Degree Double Amplitude (+/- 0.2 of a degree) almost indefinitely. These third and fourth order harmonics also seem to be the ones that have the greatest adverse effect on power. The object of the exercise then is to damp crank vibrations to something below this 0.4 DDA threshold. As can be seen from the graph at the top of the page the ATI Super Damper did just that. Below 4,750 rpm the torsionals exhibited by the Scat cast steel crank were very low. This was not unexpected as cast steel, like cast iron, has higher self-damping properties than are seen with forged or billet cranks.

The bottom line here is, so long as the engine spec does not allow excessive rpm and/or out-power the crank, a cast steel piece is actually better than a forged or billet piece and a heck of a lot cheaper. The numbers at the top end right up to our self imposed 5,800-rpm limit show the ATI Super Damper was more than up to the job of keeping torsionals under control with a maximum of 0.14 and 0.30 DDA for the third and fourth order harmonics respectively. Now we were secure in the knowledge our crank was likely to stay in one piece for a long time we moved on to our next phase of dyno test--namely carburetion.

Mega-Buck Winston Cup (now Nextel Cup) vs. Demon
Now for the carb test promised last month. As mentioned the plan was something a little different to the normal like-for-like shootout between a Holley and a Barry Grant Demon carb. The plan here was to pit the Demon against Kenny' Troutman's dyno shop Winston Cup 830 Holley. This carb was initially built by one of the country's leading WC carb builders and has since been progressively refined by Kenny during who knows how many circle track motor dyno pulls.

The first task was to ring out all the power from the WC Holley. This meant not only calibrating the carb spot on, but also the ignition timing. The speed of combustion changes as the mixture is changed. The fastest rate of burn usually occurs around about 14 to 14.5:1 air/fuel ratios. This means to see the exact nature of a jet change the timing also needs to be re-optimized. Timing then, should be considered a secondary part of any jetting exercise.

This being the case a few words on our Performance Distributors GM HEI distributor adaptation for SB Fords. First the custom mechanical advance curve built into it proved to be right on the money. Second the micro advance adjuster allowed for very small adjustments to tune for the absolute optimum timing. The procedure here was we jetted, timed and tested and slowly the power came up.

Being familiar with the idiosyncrasies of the Ford Racing/Victor Jr. intakes, Kenny used his experience to stagger jet the combination. Kenny's stagger jetting utilized three sizes of jets for the four corners of the carb. There is little point in my giving you those jet sizes, as they will be a little different for each motor. This aspect of the motor tune is just one reason why setting up on the dyno is a good idea. Granted at $750 a full eight-hour day (plus fuel) it's not cheap, but when you have an expert like Kenny on the job the dyno set up finds every ounce of power the motor is capable of.

But back to the plot: After dialing in the WC Holley's fuel curve to the best possible and following this along with exploratory timing changes in 1-degree increments we had a motor that would hit 499 point something hp on every pull--but never the magic 500. At 32 and 33 degrees of total advance just about the same results were delivered from this highly repeatable engine.