Installing a Weiand 142 “Underhood” Roots Blower

I’ve recently installed my third Weiand 142 blower kit. This one happens to be on my 78 Malibu but almost everything in this article applies to a carbureted 3rd gen F-Body- even a computer control carbureted one. In fact, one of my previous installs was on a carbureted 3rd gen. I’d like to answer the most common questions up front:

What’s it cost? About $1500 for a non-emissions kit, a few hundred more for an "emissions legal" one. Yep, the whole kit is only $1500 and includes almost everything you need to complete the install minus a few odds-n-ends.

Will it clear my hood? No. The word "underhood" is misleading. On my Malibu it required a 2 1/2" Cowl hood and a really low profile air cleaner to make it fit- and my ‘Bu has a bunch more clearance to start with than a 3rd Gen. For a 3rd gen you better figure on at least a 4" hood if you want to be able to fit any decent sized air cleaner. Figure the cost of this hood into your overall budget! It might not be a bad idea to install the blower and THEN decide how tall a hood to get- my idea of “tall enough” might not agree with yours once you see the final carb height and the air cleaner you want to use.

How difficult to install? Ever install an intake manifold? Not much worse than that. I did it in 6 hours by myself, going slow and listening to the game on the radio most of the time. I would go broke if I turned wrenches for a living.

How much power will it make? On a mild (~250HP) 350ci motor you’ll pick up about 100HP and about as much torque, pushing about 6 PSI. For wilder combos I’d say the ultimate upper limit for the little 142 is about 500-550 HP (can’t move enough air beyond that), but making 350-450HP with it is like falling off a log.

Now for the nuts-n-bolts details…

I did the install of this Weiand 142 on my non-emissions 78 Malibu. It has a very very mild 383 in it that was built from day one to take a "power adder." Usually I do nitrous but I wanted something different. I run ~9:1 compression and a mild Crane blower cam (under 220* duration @ .050) and Dart Iron Eagle heads along with the usual bolt on stuff. I recommend you don’t bolt ANY blower on a high compression N/A motor. It won’t work well and you’ll quickly break the motor. A stock carbureted low-output 305 or 350 (LG-4 or L-69) will easily take this blower kit. If you’ve got a mild "universal replacement" or "targetmaster" 350 in your ride it’ll work great with that, too. Of course, the best situation is an engine built specifically for a blower from the start, but don’t be too shy about tying it out on other engine combos. My ‘Bu ran a 13.4 @ 107 in n/a trim (blower cam, low compression, street tires) with Qjet carb and aluminum 2-plane intake before I put the blower on.

When choosing a kit you’ll usually want a standard "long nose" kit to fit any small block built after about 1968 with a long-style water pump. The short nose kit is for older motors, some ‘Vettes, other oddball combos. If you’re into emissions compliance they actually make a kit for 3rd gens that allows retention of the smog pump, the EGR, computer controlled carb, etc, etc. This kit uses an "extra long" nose blower not used in any other kit and they say it’s for 1986 models ONLY, although it can be used on other years with some exceptions. If you don’t need to pass a visual test DON’T buy this kit- it’s more money and doesn’t really make any difference to what comes out the tailpipe itself. You can run either a square bore or spread bore carb (Qjet, Holley, Edelbrock, etc) on top of the blower- it’s dual-patterned for both types of carbs. Holley 144 and Weiand 142 blowers are very similar kits- Holley bought Weiand a few years ago so I’m not sure why they still sell both kits but they do.

The kit comes with the blower, lower crank pulley, blower lower intake manifold, serpentine belt, various bolts, instructions and that’s about it. It almost looks like they forgot to pack everything if you’re used to seeing a centrifugal kit come out of the box!

Bolt on the lower crank pulley- easy. Just take off the stock crank pulley, put the blower drive pulley inside it like stacked cups and bolt it back on with supplied longer bolts. Monkey work. Simple. Long nose blower kits will work with 3-belt lower pulleys, short nose will only clear 2-belt pulleys in general, FYI.

Up top, install the blower lower intake manifold in place of your stock intake. This intake will ONLY fit an 86-down intake manifold bolt pattern. It could probably be modified to work with 87-up bolt pattern but NO WAY with Vortecs (sorry). The Holley version of this blower can be had with an 87-up intake bolt pattern, but still not for Vortec heads.

Drop in your distributor (yes, a big cap HEI or computer controlled HEI will clear the blower, no problem- that’s what I use). Install the thermostat housing- but you MUST use a relatively low profile one or it will hit the blower snout later. ANY housing that has sensors sticking out the top, or screw-in plugs of any kind will NOT clear. A cheap chrome replacement housing with a smooth low-profile top is the cheap and easy way to go here.

Now put the blower-to-manifold gasket in place. I HIGHLY recommend you lightly coat this gasket with baby powder so you can reuse it if you ever have to take the blower off again. Put on the blower and drop the 4 HUGE bolts through the blower case holes and screw them into the lower intake to retain the blower. DO NOT OVERTIGHTEN THESE BOLTS! They are HUGE bolts but they require VERY VERY LITTLE torque- 10-15 ft/lbs. Think "Spark plug tight." Not anywhere near "Gorilla tight." Overtightening will warp the blower case and cause catastrophic damage to the blower in only a few miles. Don’t forget to check for lobe-to-case clearance with supplied feeler gauges! Also, MAKE SURE THERE IS ENOUGH LUBE IN THE GEAR CASE!! Although it comes “pre-filled” with lube it never seems to be quite enough. The level should be up to the bottom of the sight glass or a little higher on the front of the blower with it installed on the engine, car on level ground. Better to overfill SLIGHTLY and lose a few HP than to run a little low and destroy the blower. Add slowly in small amounts and wait for it to trickle down through the case- the level won’t come up the instant you add it. It just uses regular old GL-5 like you would use in a rear end. Upgrading to synthetic would probably be a good idea just like it is everywhere else on your car.

Install the front end accessories. Your stock 3-point alternator upper bracket won’t work. It won’t line up with the bolt hole in the front of the intake manifold. This kit was designed a while back and GM moved stuff around quite a bit since then. DON’T drill the intake. DON’T follow their recommendation to call GM and order a bracket (it isn’t available and you’ll go bonkers trying to locate the right bracket). Just go down and pick up a cheap chrome 2-point alternator bracket that just ties the alternator to the water pump with NO bolt into the intake. Here’s a neat little bonus with these kits- YOU CAN KEEP YOUR A/C! Yeah, if you’ve got a driver-side-mounted radial compressor setup like on my Malibu and many early 3rd gens there is the necessary bolt hole on the lower intake to install the support bracket that goes there and the blower belt will clear the A/C belt! I’m reinstalling mine next summer!

You can put on the blower belt now. Amazingly, it clears the stock waterpump-driven clutch fan on my 78 by about 1/4" with NO changes (I have the stock serpentine 6 rib belt drive system, but they are available in 10 rib drive and 16 for higher performance applications). It probably won’t work that well on a 3rd gen but you can buy fan shims to space the fan forward until you gain some clearance. Or you can, of course, convert to electric fans if you’ve been looking for an excuse to do so. No kit is every truly a “bolt-on” as we all know, but this is a fairly minor issue.

The only other hassle left is the throttle and trans kickdown cable. Since you’ve just moved them up about 4 1/2" from their original location, you’ll have bracket and cable problems. This is the only moderately difficult part of the project. I took my intake manifold-mounted cable bracket and chose to modify and reuse it by cutting the bracket, spacing it up the ~4 1/2" I needed and then welding it back together. Holley guys can buy brackets that simply bolt to the back of the carb, but Qjet guys will find these brackets difficult to find (impossible?). The throttle cable hooked right up but the trans kickdown cable is now too short to reach all the way up there from the Trans. Not to worry, many companies make adjustable length kickdown/TV cables for various transmissions. I bought one from B&M for my TH-350. You don’t need to dig into the trans too far on a TH350 to replace it. Manual trans guys are spared the kickdown cable problem but still have to make the throttle cable fit!

So now we’re down to tuning issues…

Do I need to rejet my carb? No. Generally not in any major way. This isn’t a blow-through setup like a centrifugal. There is no boost above the lobes- only below them in the intake manifold. If you’re the nervous type then jet up a couple of sizes to stay a little on the rich side. QJet guys I’d recommend you install a set of "DR" rods and a "B" hanger for starters. If you are the nervous type or have more than 350 cubes to feed then go with a set of "DA" rods and a "B" hanger. This applies even if you are using the computer controlled QJet for emissions compliance or just to be weird Yes, you can maintain the computer controlled carb & distributor and run it with the stock computer with this blower installed!! In fact, it works beautifully, although you’ll have to relocates some sensors/vacuum switches up on the front of the intake!. Remember, they make an emissions legal version of this kit, too. Might be a good idea to talk with Holley/Weiand first to decide on a specific kit before launching into a project like this.

What size carb? Well, a big one, actually. For a 350 with this blower I couldn’t imagine using anything smaller than a 750. Don’t even THINK of trying to reuse an old 600 CFM Holley even on a mild application- it’s way too small. Remember, you’re feeding and engine that’s going to make an extra 100+ HP with the blower. BUT the carb never sees that boost- the blower isn’t shoving extra air past the carb by force. It’s DRAWING it through the carb normally and THEN compressing it. So you want to make sure the carb itself doesn’t become a bottleneck or you’ll lose boost, efficiency and of course, power. A 750-800 CFM Qjet will work just fine- that’s what I used. Holley, Edlebrock, they all make high CFM carbs that will work well, too.

Do I need to back my timing down? Depends on a lot of stuff. Generally, on a near-stock low compression motor, no. This thing doesn’t make that much boost or intake heat. Again, if you’re the nervous type why not back it down 4* and work up from there gradually. I run 10* initial timing, 30* total, all in by about 2800 and have no detonation on pump gas at 4 PSI. Yes, I still run vacuum advance (12* max) from the same port on the carb as always since the carb never sees boost- only vacuum, just like on a N/A motor. Certainly, it’s better to stay conservative than to run a blown motor on the ragged edge. I run an MSD setup on my HEI and run about .042" plug gap (on a slightly colder plug) but a stock HEI can still get the job done if you back the gap down to around .035" or so.

What kind of RPM range is this blower good for? Holley/Weaind recommends a redline of about 5500 RPMs, so it’s not advisable to plan on spinning the motor to the moon. Select your cam and heads appropriately. Can you spin it higher? Sure. I go to 6000 all the time myself. It’s just that the blower gets more inefficient the higher you spin it and there are limitations on the blower’s maximum safe RPMs. In reality, it’s not NECESSARY to spin it all that high. This is a difference between a roots blower and a centrifugal one. With a roots blower you get all the boost from the instant you go wide open and it stays there all the way through the gear. A centrifugal builds more boost the higher you spin it. So think about this 142 blower for big torque increases across a broad, usable, real-world RPM range, not for a high RPM screamer motor.

How much boost am I making? Again, depends. There is a port on the lower intake where you can attach a boost gauge and measure it for yourself. My 383 with decent heads and cam outruns the blower so that with the supplied pulleys I only get about 4 PSI of boost. There are different sets of pulleys available to raise or lower your boost. Those using this kit on a larger cube small block- 383 or 400 and/or with a large cam will find the stock pulleys are not adequate to build meaningful boost. You’ll need to raise the drive ratio from the stock 1.95:1 to something higher- although 2.46:1 is as high as you can go, so don’t expect to build a motor over about 500-550HP with one of these blowers- they’re just too small. Those putting this kit on a little 305 might have the opposite problem- too much boost. Like any non-intercooled supercharger things get dicey when you get up over ~6 PSI. Especially a roots blower that makes full boost from the instant your pedal hits the carpet! Fortunately, there’s plenty of pulleys available to lower the boost for you. Here is a list of all the possible pulley ratios:

Lower Pulley Upper Pulley Ratio
6.00” 4.25” 1.41:1
6.00” 4.00” 1.50:1
6.00” 3.75” 1.60:1
6.00” 3.50” 1.71:1
6.00” 3.25” 1.85:1
6.00” 3.07” 1.95:1 Stock pulleys supplied with kit
6.00” 2.85" 2.11:1
6.50” 4.25” 1.53:1
6.50” 4.00” 1.63:1
6.50” 3.75” 1.73:1
6.50” 3.50” 1.86:1
6.50” 3.25” 2.00:1
6.50” 3.07” 2.12:1
6.50” 2.85” 2.28:1
7.00” 4.25” 1.65:1
7.00” 4.00” 1.75:1
7.00” 3.75” 1.87:1
7.00” 3.50” 2.00:1
7.00” 3.25” 2.15:1
7.00” 3.07” 2.28:1
7.00” 2.85” 2.46:1
Please note that not all size combinations
of upper and lower pulleys are recommendedby Holley/Weiand.
Check application catalog.

So now the big questions…..

What’s it drive like? Well, like it’s not even there most of the time. There is almost no blower whine- a little bit when the blower is cold- the synchronizer gears in the front of the case sing a little but are dead-silent after they warm up. You only get noise when you open it all the way up. An unearthly sound. Like "…zzzzzzzzzZZZZZZZZZZZ!!!!" on up through each gear, clearly audible over my Flowmasters and headers. And, of course, it gets from A to B a lot faster than before.

How much faster? I haven’t had mine to the track since the blower install but by running against other known-quantity cars I’d estimate I’m running about 12.5-12.6 and 114-115 MPH now with only 4 PSI, on street radials. I can rip up near-stock LS-1s pretty easy. Likewise, laying down 50 ft of rubber on a downshift at 25 MPH and then all the way back up into 2nd gear is pretty easy.

How is it on gas? Not much worse than before the blower. I might have lost about 2 MPG or so due to the extra drag from the blower and it’s drive but I would say about half of that is from jetting the carb a little richer (to help stay safe) and backing the timing down several degrees from it’s N/A setting. Plus it’s awful tough to keep yourself from laying into that boost! With some careful part throttle tuning I should be able to get some of that mileage back.