Outside of the main jets the...
Outside of the main jets the only adjustment to the Demon was to set the idle mixture via the idle mixture screws. A smooth 650-rpm idle was achieved with no sign of a lope.
And that best proved very well as you can see from the output comparisons on the "WC Holley Vs 850 Demon Carb" graph. From 2,800 to 4,400 the Demon was up on the WC Holley with differences in torque amounting to as much as 18 lb-ft at 2,900 rpm. From 4,500 to 5,300 rpm the Holley won out by up to about 5 lbs-ft and from there on up it was anyone's game. Peak power with the Holley was 501 while the Demon never made 501, but consistently hit 500.5!
From the output aspect the $523 Demon looks like it is every bit as functional as a $1,400 Winston Cup carb. The Demon also returned exceptionally good BSFC figures with numbers running from 0.37 at 2,800 rpm to 0.42 at 5,800. The part throttle cruise test showed even better fuel economy than the Holley's already excellent figure. On this test the Demon showed 0.5 lbs/hp/hr.
One might suppose that the Holley could be at a disadvantage in this test because it was built to be optimal on an engine operating in the 450 to 780 hp range. Certainly from the point of view of airflow capacity there is little to choose between the Holley and the Demon. And yes the Holley could well be at a disadvantage. For instance booster atomization characteristics would probably have been optimized for best output within 50 hp of peak power where as the Demon almost certainly would favour operation in the lower ranges exactly as was found. The extent of such possibilities we could not establish because our test engine only just reached into the Holley's range by 50 hp. However, it says reams for the Holley, in that it produced excellent results even when operating below what its normal range would be and at the same time shows the Demon to be every bit the top-notch carb that it is.
After some more tweaking in...
After some more tweaking in the spark department our RDI powerplant (dark curves) generated a peak of 503.6 hp and 515 lbs-ft. The light colored curves are the average of a number of Ford 392 crate motors. Great value for money though a stock crate motor may be the superiority of a custom crate motor is easy to see.
Final Tuning
Once the 500 barrier was broken, a new goal was set. Could we make 505 hp? An increase in spark intensity might help. With Performance Distributors HEI unit plug gaps up to 0.75-inch can be successfully bridged. The wider the plug gap used the more intense the spark. Our gap was already at 0.050-inch so not a lot was expected here. However some experimentation here proved worthwhile.
By making an already big fat spark a little bigger the motor rewarded us with a new peak of 503.6 hp but try as we might it would not, at least in this round of events, go 504. The final curve is on the graph "RDI 392 Custom Crate Vs Ford Crate Motor". Here I have also put the average of a number of regular, non-custom 392 crate motors that Kenny has tested. Note our Custom 392 beats out the regular crate motor by 55 hp and 55 lb-ft of torque. So how come the big difference? In a sentence it is build quality and component compatibility. The build quality is what you get for the extra cash it costs to do a custom crate motor as apposed to a regular crate motor.
As far as parts compatibility is concerned the biggest influence on the end results would be the heads and the cam. In Part 1 you read how the head port cc/velocity was matched to the peak power rpm. Note that this computation was less than 100 rpm off the projected figure. Next compare Ford's crate motor cam spec with the Motor Machine computed cam spec in the "Cam Comparison" chart. The cam probably accounted for half the difference between the RDI motor and a stock Ford crate motor. Probably the most interesting point here is the cam in our Street Pro 392 is 8 degrees less duration on both the intake and exhaust than the cam in a Ford crate motor. Even though shorter on duration this cam allows a higher peak power at the same rpm and delivers far greater torque.
All this points to one thing--more accurate cams out perform cams that are simply bigger in every respect period. Many pro engine builders are finding having the cam spec computed often delivers as much as twice the increase that is seen by just guessing the cam spec from experience or buying an off-the-shelf cam just because it has worked in the past. In our case it looks like RDI's Preston Miller added some 30 hp to the final result by simply making a phone call and shelling out the $40 fee it cost to get the cam spec computed. Putting this into perspective 30 hp for $40 represents three times the power increase per dollar spent of even the cheapest nitrous kits.