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Big Block Chevy Engine - Battle Of The Boost

Mini Blower Vs. 8-71 On A Basic Rat

David Freiburger Dec 17, 2006

Want big grunt? Go Roots, man. No other single bolt-on will give you a bigger kick in the torque curve, at least not without having to refill a bottle all the time. A Roots supercharger is the simplest way to get boost, since it doesn't require EFI, trick carbs, complicated fuel systems, or any air-tube plumbing. Not that centrifugal blowers aren't fantastic, but they usually don't deliver the same snap-to-life start-up and right-at-the-toe low-end bark you get with a Roots. The only problem is you may get some flak from today's hipsters who'll guffaw at the '80s-throwback hole in your hood and want to talk adiabatic efficiency instead of good old-fashioned tire frying.

Of course, the solution for the hole in the hood is a mini blower, which will fit under most small cowls. In this story we compare the price and performance of a Weiand 177 mini blower to a traditional and far more imposing Weiand 8-71 fullsize supercharger. Unlike the old days, when magazines made it seem like you needed a full-on race motor to support blown power, we'll run these on a very pedestrian big-block.

The Victim
Look at the parts lists and you'll learn every single component that went into this Rat motor. Make sure you check this out: It's got a Scat cast crank, Scat's most basic connecting rods, and a tiny Crane Saturday Night Special flat-tappet cam (PN 134551) with 236/246-degrees of duration at 0.050, 0.553/0.571 lift with 1.7:1 rockers, and an LDA of 110. It couldn't be much more basic. There was a time when you'd never consider using a cast crank with supercharged power, and while it's still not the first choice, the new Scat Series 9000 cranks seem to be stronger than factory castings. Since we're not going to try to overpower it or snap off the snout with huge boost, it works. We wouldn't recommend it with a transbrake, though, even at the power levels seen here.

The crank itself is just $299 and the rods are $320, which is almost cheaper than reconditioning the stock parts and adding good rod bolts. Since that's true, and because you've got to buy new pistons for an overbore anyway, there's no reason not to stroke a big-block these days. The 4.250-inch stroker crank is the same price as the stock 4.00-inch stroke, and combined with a 0.030-inch overbore our Mark V block from the junkyard now delivers 489 ci. With affordable SRP forged flat-top pistons (because we still can't bring ourselves to recommend hypereutectics with a blower) and 110cc chambers in Holley oval-port heads, the compression is 8.95:1.

It's practically a tow-truck engine. In naturally aspirated trim, using a Professional Products high-rise, dual-plane intake, and an 800-cfm Holley, it made 549 hp at 5,300 rpm and 597 lb-ft at 4,000 rpm. Not earth-shattering, but pretty good for junk that will run on 87-octane. Now let's wake it up.

Weiand 177 Mini Blower
Would you pay $1,660 for an extra 112 hp and 101 lb-ft? That's the difference in Summit Racing prices between the $1,819 blower kit we used and the cheap $159.95 intake manifold it replaced. Since the mini blower only uses one carb, which you'd need for a naturally aspirated engine anyway, there shouldn't be any other expense to adding a mini blower other than perhaps a cowl-induction hood for clearance and maybe some fiddling with the throttle and trans cables. With just 3 psi boost, our 489 made peaks of 661 hp at 6,000 rpm and 698 lb-ft at 4,000 rpm.

Weiand offers two basic mini-blower kits for big Chevys: the Teflon-striped, 174ci former-B&M design that's 2 inches lower overall than the 177ci no-Teflon, Weiand-design unit. Both are available satin or polished, for either long or short water pumps; the 174 uses a 10-rib drive belt, and the 177 has a 6-rib belt. We used the cheapest kit, the satin 177ci model for long water pumps, PN 6521-1. Anyone with the skills to swap an intake manifold can install the blower.

The only drawback is that this blower, while conveniently small, is also a bit undersized for the displacement of a stroked big-block. The out-of-the-box pulleys only made 2 psi boost, gaining 82 hp and 70 lb-ft over zero boost. We ordered a 2.85-inch blower pulley (PN 6790, $71) and used it with the stock 6.00-inch crank pulley to make 3 psi at peak. We later tried a 7.00-inch crank pulley for a blower-drive ratio of 2.46:1 and made a peak of 5 psi, good for 749 lb-ft and 700 hp, but the blower had its tongue hanging out. The belt was starting to slip and the case was getting very hot, causing extreme power dropoff in back-to-back pulls.

So 3 psi was about the limit of the blower on this engine. Not that we're complaining about a simple 660 hp and nearly 700 lb-ft.

Weiand 8-71 Blower
For bigger power and shock value, we bypassed the Weiand 6-71 kit (PN 7483, 11/42-inch-pitch belt, satin finish) that is $2,139 at Summit Racing and went with the larger 8-71 blower kit (PN 7186, satin) Summit sells for $2,589. That's just $320 more than the price of the 177 mini blower kit, but you also need to buy one extra carb-ours was a 750 HP Holley supercharger cab, PN 80576S for $709.95-and a carb linkage kit, PN 7166 for $145.95. Throw in a couple of pulleys for tuning purposes-they run between $100 and $135, depending on size-and you've got a significant investment. Our kit, carbs, linkage, and pulleys added up to $4,390.85. You could knock that down significantly by using regular 750 double-pumpers (PN 0-4779C) rather than the HP-series supercharger carbs; the price would drop by $626 in carbs alone. However, we will say that this is the third big-block we've tested with the blower carbs, and they usually bolt right on with a near-perfect tune-up, so you don't have to mess with them. They also have boost-referenced power valves that can help with street driving.

On the subject of fuel systems, we'll also point out that you don't need to spend a fortune on pumps. The dual quads on the 8-71 actually reduce fuel-system demand since there is always twice as much fuel in reserve in the bowls than with a single carb. At this engine's power level, you could feed the carbs with a Holley PN 12-454-20 mechanical fuel pump ($99.88) to keep it simple.

Been waiting for the punch line? In the first test, in an attempt to match the low boost levels of the mini blower, we gave the 8-71 a 63-tooth blower pulley and a 47-tooth crank pulley for a drive ratio of 0.75:1 (25-percent underdriven) and achieved peak boost of 4.8 psi. That was good for 717 hp at 6,200 rpm and 674 lb-ft at 4,500 rpm. That's an increase of 56 hp, but note that we lost 24 lb-ft, even with extra boost. Since air inlet temps were not increased, the only answer is that it just plain eats more power to spin the larger blower. More evidence of that: We gained 1.8 peak boost, but only picked up 56 hp, or 31 hp per pound of boost. With the mini blower, going from 2 psi to 3 psi picked up 30 hp. The 8-71 doesn't gain a torque advantage until 5,100 rpm, when the smaller blower is probably starting to heat up.

That's interesting stuff, especially considering that the 8-71 displaces 436 ci versus the mini blower's 177 ci, which is why the big blower can spin so much slower yet achieve the same boost. At 6,000 engine rpm, our 8-71 was only spinning 4,500 rpm, where the mini blower was cranking a whopping 12,600 rpm. Perhaps that's why the big blower works better up top. But it makes a really compelling argument for the mini blower at low rpm and low boost.

The Glory Run
You didn't think we'd stop at 4.8 psi boost, did you? With a little guessing and gambling, we figured that the cast-crank, stock-block engine would probably stay reliable to around 750 hp, especially since we didn't think we'd have to lean on the blower too hard to do that. The more you crank up the boost, the greater the load you put on the crank snout. With a casting and a single keyway, we weren't thrilled about trying to stuff 8-plus psi into this thing. But we felt pretty good about 6.4 psi, which was delivered with a 63-tooth blower pulley and a 52-tooth crank pulley for a ratio of 0.83:1 (17-percent underdriven). That gave us a nice 766 hp at 6,200 rpm and 726 lb-ft peaking at 4,600 rpm. And, of course, the 8-71 was now in its glory, killing the mini blower at every point in the curve.

Not bad for a tame little tow-truck motor. Maybe you should go get a blower.

Long-Block Parts List

Federal-Mogul 4400M Main bearings
  8-3190CP Rod bearings
  1404M Cam bearings
Holley 300-554 Oval-port heads, no EGR
Pioneer FRA-321 Flexplate
PowerForce 80005 Balancer
Scat 9-454-4250-6135 Cast 4.250 crank
  2-ICR6385 I-beam, 6.385-inch rods
Speed Pro R9745-35 Sealed power rings
SRP 142979 Pistons
Dougan's   Oil-filter adapter
Milodon 30955 Gen V oil pan
  18760 Oil pump
  18301 Oil driveshaft
B&B   Valve covers
Crane 13975-1 Timing chain
  134551 Saturday Night Special cam
  99277-16 Lifters
  96881-16 Valvesprings
  99954-16 Retainers
  99098-1 Locks
GM 12551544 Gen V front cover
Fel-Pro 1037 (2) Head gaskets
  1212 Intake gaskets
  OS34407R Oil-pan gasket
  2703 R.A.C.E. set
Pioneer 854011 Intake bolts
  859013 Crank bolt
  S1129 Flywheels bolts
  S454 (2) Head bolts
  854002 Oil-pan bolts
  S1107 Cam-bolt kit
  854005 Front-cover bolts Extra long bolts for Holley heads Freeze plugs


Summit Racing
Akron, OH
Simi Valley, CA 93065
P.O. Box 10360, Bo KY



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