There are cheaper ways to make 750 hp, but not on pump gas, and no way are they this easy. Here’s the plan: Take your Mopar Performance 426 crate Hemi out of the box, bolt on a Weiand 8-71, and watch it barf up 753 hp at 6,300 rpm and 669 lb-ft at 4,900 rpm. That’s a gain of 259 hp and 169 lb-ft over our tested baseline—simple!
Many of you will guffaw that the dorks at HOT ROD spent a fortune and called it cool when of course a blown Hemi should make big power. OK. But consider that not one internal modification was required. We even used the stock hydraulic flat-tappet cam (278 degrees advertised duration, 0.490/0.480 lift), so it starts and idles like a bone stocker. There’s no reason this thing shouldn’t be Power Tour reliable. Furthermore, remember that the Hemi is a comparatively tiny 426ci—guys routinely build bigger small-blocks these days. The Weiand-blown Hemi made an impressive 1.77 hp per cubic-inch. We’ve seen 8-71-blown 502 Chevy crate engines make similar power, and that’s just 1.5 hp per cube.
So what’s the secret? Firstly, perhaps the bench racing axiom that “Hemis like blowers” is true. That’s because the big-domed pistons in 4.250-inch bores provide lots of surface area for the boost pressure to shove on, delivering more power to the crank. Secondly, the bone-stock cylinder heads are glorious overkill for such a small engine (forget fitting the Hemi’s 2.25/1.94-inch valves in any 426ci small-block). Finally, the Weiand 8-71 proved a nice match for the 426 cubes. A Roots blower is just a big air compressor that has the potential to overheat the inlet charge if it spins too fast; you reach the point of diminishing return between gaining boost and airflow and losing power due to increased air temperature. This relationship is called adiabatic efficiency, and it can be improved by reducing the blower speed for a given boost target by either decreasing the engine displacement in comparison to the blower displacement or vice versa. A blower of a given size can make the same amount of boost at a lower blower speed on a smaller engine than it can on a bigger engine (and therefore with greater adiabatic efficiency and with lower-octane fuel). So bigger blowers on smaller engines are happier than smaller blowers on bigger engines. Get it?
The Weiand 8-71 displaces 436.29ci of air for each complete rotation of its newly redesigned two-lobe rotors. The rate of blower speed versus engine speed can be altered with the blower drive ratio. We requested that the pulleys supplied in the Weiand kit have 57 teeth on one and 54 on the other. Placing the larger pulley on the blower and the smaller one on the crank creates an underdrive ratio (the blower spins slower than the engine) of 0.947:1.
Therefore, the blower was moving 413.17 ci of air for every complete rotation of the crankshaft. This resulted in a peak boost pressure of 9.1 pounds at 6,300 rpm. At this point, the engine screamed with no detonation on 76 Products 91-octane fuel and 34 degrees of total timing, which proved optimal. Then, for kicks, we swapped the pulleys top to bottom to create an overdrive ratio of 1.06:1 and added 76 Performance Products 114-octane fuel to prevent detonation.
The result was more airflow, more boost, and more power: The peaks were 783 hp at 6,200 and 709 lb-ft at 5,000. That’s not a big percentage of improvement considering that the boost increased to 12.5 pounds, and we suspect that the difference in blower speed heated the air enough to reduce the gains you’d otherwise see from that kind of boost. So the race-gas version wasn’t really worth the aggravation. This one’s content to be a docile street Hemi.
But it looks killer, doesn’t it? One reason for that was our selection of Holley’s monster Dominator carbs and optional top-plate for 4500-series carbs. By conventional thinking, two 1,150-cfm jugs would represent heavy-handed stupidity on a 426, even at this power level. Sure, we wanted the wow factor—but we think we got power as a bonus. A Roots blower hates inlet restriction (yet another factor than can contribute to inefficiency). These carbs provided unlimited air intake, allowing the blower to work even better.
Additionally, the lack of power valves in the blower-series Dominators made for easy tuning. Holley also offers blower-series HP carbs with boost-referenced power valves for those part-throttle conditions when the blower is making boost, but there is still enough vacuum under the carbs to prevent the power valves from opening, thereby causing a lean surge. Holley’s 950 HP blower carbs are probably more driveable during throttle tip-in, but the Dominators seemed responsive under simulated road loads on the dyno, and they never woofed fire.
One thing we didn’t anticipate with the Dominators was a bit of a jetting anomaly. The carbs came with 99s in all four corners, which is typical for race carbs with no power valves (which, when open, contribute fuel at a rate roughly equivalent to 10 Holley jet sizes). However, the Domis ran scary lean right out of the box, and the biggest killer of blown engines is being too lean. It took Max Jet alcohol jets in a 0.128-inch size (the equivalent of a Holley 108 or so) to get the engine safely in the rich zone. This would also seem to be excessive, yet what we’ve found is that jetting sizes are not as consistent as you would think when comparing fuel curves from engine to engine or from carb to carb. Booster design, venturi diameters, engine size, and the air inlet speed all contribute to how hard the engine pulls on the jet (how much of a vacuum drop is present at the booster), thereby changing the jet size requirement. If less vacuum drop is available, the jets need to be bigger to deliver ample fuel. In our case, Holley explained that the 1,150-cfm Dominator model we used was calibrated for race applications on big-inch Chevy Rat motors with 10-71 to 12-71 blowers. Those obviously pull on the carb harder and draw more fuel than our nearly stock 426. Perhaps an indicator that the Hemi overcarburetion wasn’t as slick as we gave ourselves credit for earlier, the jetting needed to be extreme because the inlet airspeed was obviously low. Good thing the blower made up for any soggy throttle response.
So that’s our story, and we like the ending. But don’t think that the moral can only be applied to a Hemi—nothing but a Roots blower can make this kind of power this easily on any engine. They make killer torque right from idle, deliver top-end power, there’s no bottle to refill, and nothing looks like ’em. The hole in the hood may be the drawback, but with this kind of power, we can get over it.
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