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Fitting a Supercharger, Part 2

Making the blower mounts

by Julian Edgar

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At a glance...

  • Part 2 of a 4-part series
  • Bracket design
  • Bracket materials
  • Making trial brackets
  • Making cutting templates
  • Cutting out the brackets
  • Welding the assembly
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This article was first published in 2005.

Last week (at Fitting a Supercharger, Part 1) we looked at the basics of installing a supercharger of the sort that’s widely available from wreckers. These designs are of the ‘plumbed-in’ type, where the blower is mounted as a standalone, and intake and outlet plumbing is run to it. So far, we’ve thought through the location, making sure that the drive belt can get to and from the blower, that the blower is upright and other stuff like that – and now it’s time to make some brackets.

Bracket Design

To make rigid and strong mounts, heavy gauge steel needs to be used. Unless you use incredibly thick material, aluminium sheet and bar simply doesn’t have the strength (not without fancy ribs, anyway) and even the sort of normal 30 x 5mm flat steel bar that you might make a heavy duty bracket from is likely to flex. Instead, 12, 10 or 8mm flat steel sheet makes a good raw material from which to construct most of the bracket.

The use of sheet allows a bracket to be made that curves in one plane while staying straight in the other. In other words, if the bracket is to be mounted to the engine using (say) a mixture of the water pump bolts and other bolts at the front of the engine, the blower will end up being mounted directly in line with these points. Any required offset forward or rearward (in the case of a north-south engine!) can be made by the use of bushes – and you’re always working from that parallel plane.

In this case we were mounting a small ex-Subaru Vivio supercharger on a Toyota four cylinder engine. The engine is mounted transversely in the car. Positioning the blower by hand in the engine bay showed that it j-u-s-t had sufficient clearance to the plenum chamber and the throttle body when the ribbed supercharger pulley was lined up with the crankshaft pulley. (When lining up these pulleys, use a long straight edge to check – you can easily see misalignments of less than 1mm.)

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However, this trial fit showed that the existing belt tensioner pulley (circled) would have to be removed – it fouled the underside of the supercharger. The bracket that holds this pulley in place is integrated into an iron casting that also forms one of the engine mounts. The engine was supported by a jack from underneath then the engine mount removed and the offending tab cut off with a hacksaw. The engine mount was then temporarily reinstalled and this time the supercharger could be nestled into position by hand.

But what about the blower mounts? It was initially thought that a new lower mount could be made by welding to the engine mount but when it was found that the mount was cast iron and so the weld wouldn’t be particularly strong, it was decided instead to make a whole new engine mount which would incorporate the blower mounts. While this would be a lot of work, it would also give an extremely strong and rigid blower mounting.

Trial Brackets

But with that decision made, where to start? The answer is with a pair of scissors! By using the scissors on thin but rigid plastic sheet (salvaged from an old advertising hoarding), lots and lots of different bracket shapes could be quickly ‘made’ and fitted. By taping bits of plastic to the trial brackets and then doing more cutting, or alternatively cutting away more of the original plastic, any shape could be quickly and easily produced.

Don’t underestimate the importance of this step. I tried more that 50 iterations of bracket before settling on a pair that would work.

So why so many different shapes? Firstly, it’s easy to make brackets that don’t allow other required functions to occur – like leaving sufficient room for the plumbing to get to the blower, or allowing the blower to be removed with just simple unbolting. Secondly, brackets like these need to conform to the existing shapes: being curved to snugly fit into the shape of engine castings, providing clearance to bolts, having plenty of ‘meat’ around the holes that will be used to bolt the brackets in place - and of course, positioning the blower in exactly the right location!

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There’s almost an unlimited number of ways of achieving the same outcome: will the arm of the bracket go here or here? Perhaps if it goes up high and then comes back down there’ll be more clearance for the outlet hose. But then again, that will make the bracket longer and so less rigid. But in order to compensate, what if the bottom bracket is beefed-up to better locate the blower? Then the top bracket will become just the stabilising support. Yes, there’s plenty of clearance for a thicker lower bracket – let’s add some plastic sheet there and take it away from the top bracket. Snip, snip, snip...

When making trial brackets from thin plastic, you must keep in mind that the final brackets will be much thicker than the plastic sheet – because of course this alters clearances. In this case, 9mm plate was being used. (Why the odd size? Well, the brackets were cut from the flanges of 150 x 10mm angle – and the ‘10mm’ in angle is actually 9mm after it’s been through the rolling mill.) For example, will the bracket bolt to the front or rear faces of the blower mounts? Depending which way you go, and on the thickness of the plate and blower mounts, the blower could end up 20mm further forwards or backwards. However, this won’t be so clear when you’re working in the thin sheet.

Templates

With the brackets finalised in the plastic sheet, their shapes were traced onto aluminium sheet. These aluminium templates were cut out with a jigsaw and the mounting holes drilled in the right places. The aluminium templates were then installed on the engine and the blower location again checked. In this case, two templates were used. This was required because the mounting bolts on the engine were in two planes, one 10mm behind the other. By using two overlapping brackets, this ‘step’ could be catered for.

Cutting and Drilling

A variety of methods can be used to cut the brackets from the steel sheet. Laser cutting, plasma cutting – or good ol’ fashioned oxy-acetylene cutting. Despite the fact it gives the worse edge, we choose oxy-acetylene cutting. If the oxy torch is wielded by a careful hand, the accuracy can still be very good and the edge can be cleaned-up with a bench grinder, belt sander or even a file. In this case, the major incentive to use oxy cutting was that it could be completed by a welder who works only 15 minutes away.

If cleaning-up of the cut is needed, use the aluminium sheet templates as the guide to the required edge. Then when the brackets are smoothed to the right shape, use the aluminium template to locate the holes and then drill them. A drill press needs to be used so that the holes are at right angles to the plane of the brackets. Crooked holes won’t make bolting things up very easy....

Trial Fitting

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With the thick brackets cut out, bolt them to the engine and mount the blower. In this case, only the front bottom and top mounts of the blower were supported by these brackets – the rear bottom mount was still to be made. (The rear, upper mount of the supercharger wasn’t to be used.) The next step was to weld together the two overlapping brackets. In addition, the part of the bracket that formed the new engine mount was still to be made and so it was decided to do these operations as one step.

An aluminium template was made of the required shape of the engine mount, two triangular gussets were cut from flat bar – and the whole lot presented to Peter the local welder. He cut out the engine mount (from 12mm sheet – substantially thicker than the 9mm sheet used for the blower brackets) and used an arc welder to tack the assembly together.

If the welding is being done remote from the car, it’s very important to initially only tack weld parts together. A tack weld is just a very short bead – one that can be ‘undone’ with an angle grinder if there proves to be a problem. Even a few tacks will be quite strong, allowing the hanging of the blower off the mount and so letting you assess bracket rigidity, mounted blower position and so on.

Welding

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The tacked-together mount was again fitted to the engine and the blower temporarily bolted into place. This showed that the bracket needed no further adjustment – the welding could be completed.

However – and this is a very important point – if the brackets need to be extensively welded, you must expect some distortion to occur. For example, in this case, where the engine mount and its strengthening gussets were welded to one side of the flat plate, the plate ‘pulled-in’ on that side, slightly curving the bracket. Obviously this isn’t wanted! The bracket can be straightened with either the strategic applications of heat from an oxy torch – or a GBH, a Great Big Hammer.

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A lower rear blower mount was then incorporated. This is formed from 40 x 8mm flat bar, triangulated to the rest of the bracket. Other jobs included the design and fabrication of a mount of the belt tensioner – but we’ll cover that next week.

Costs

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The cost of making the bracket soon adds up. By the time this stage was reached, the welding and cutting labour time was 5 hours. Peter-the-welder charged a modest AUD$50 an hour, so making the mount had already cost more than the secondhand blower! And that’s with nearly all the work – the templates, trial fitting, hole drilling, edge smoothing – carried out by me. The likely bracket costs are something to keep in mind when working out your budget.

Anatomy of a Bracket

So, after sandblasting and zinc coating, how did the bracket look?

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Here’s the final item. (1) engine mount, (2) mount for tensioner pulley, (3) lower blower mounts, (4) upper blower mount.

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Here’s a close-up of the engine mount. Note the heavier 12mm plate used for the mount and the gusset that strengthens it. Not able to be seen is a lower gusset that mirrors the shape and location of the visible gusset.

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This rear view shows the 9mm ‘step’ in the mounting plate that was needed to accommodate the offset in the planes of the mounting bolts on the engine block.

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The steps from (1) plastic template > (2) aluminium template > (3) final bracket can be seen here – the piece is the lower, front part of the final bracket.

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This view shows the bracket orientated as it is in the car – engine mount on left that connects to the bodywork, belt tensioner pulley in the foreground and the lower supercharger mounts at the lower right.

With the blower rigidly mounted in place, the next step was to organise the drive belt. We’ll cover how that was done next week.

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