Fuel system sizing

So, I’ve finally got a break since starting work at 06:30, so I thought I’d jot down the fuel system sizing I need. I have taken lots of sizing information from this zetecinside.com article and injector specifications and ratings from witchhunter (weird name).

I have 4 injectors (Weber IW-31) and desire 215bhp minimum with an option to size up to 250bhp later when I supercharge it (ahem). According to the Weber website, I need to flow 269 cc/min and my IW-31s are rated at 274 cc/min. This isn’t good, because it implies a 98% duty cycle. As such, my injectors will be flowing nearly full flow (and therefor full duration) whereas most people aim for a 80% duty cycle. This is a bummer because it means (potentially) 4 new injectors at £80 each. I will need to sell mine on though which will offset the cost.

Next I need a fuel pump. Now I know I’m flowing 1076cc at full chat, I need a fuel pump (internal) that can deliver the goods. The Walbro GSS 342 can flow 255 litres per hour, which translates to 4250 cc/minute. It seems adequately over spec for my needs.

There’s a reason why roll-cages are expensive, and how not to do it

I’m only part of the way there in the build, and not ready to put the roll hoop in place, nor the placement of the cross members. (stick with me – this is leading somewhere).

I’ve got all the steel together for mine in T45 (the proper homologated stuff) which has cost me £50 per metre before bending, and this is at trade prices.

I know I’m doing it right when I see something like this. Click the picture to take you to the website. It’s made from plumbing pipe and has been screwed to the floor with self-tappers.

Burnt my neck welding

So, this is what it looks like when you give yourself a partial depth third-degree burn to the neck, whilst welding.

This has been two weeks of healing, using a special one-way membrane dressing to aid healing. For the first week Harrogate hospital weren’t sure if it would heal and if I needed to go to wakefield for skin grafts.

As it stands, it’s going to fully heal and will be red for a while. There was a chance I’d then need a graft for cosmetic reasons, but right now I don’t think I will.


look at my lovely spider

So, as you can see from the picture on the right, this is the chassis before I have added the spider in – if you like this is the before shot. I’m adding the spider in place to provide a virtual cross member through the engine to increase the structural rigidity of the engine bay and also increase the tortional rigidity.


TP for my bunghole

I had several requirements putting this in place. Firstly it needed to be demountable, and secondly it needed to be easy to manufacture and locate. To this end, I went to McGill Motorsports and bought some left hand and right hand 1/2″ rose joints, threaded bungs and 1″ wishbone tube (4m of it just to have lots of spare). All in all, I got 10 rose joints (5 left hand, 5 right), 11 bungs, lock nuts and 4m of tube for about £110 all in, which I think was great value. This kit form approach is way easier than trying to get really accurate location on machined parts. The picture shows the bearing-bung-tube arrangement, just in case you were wondering what I was talking about.

I also needed to manufacture some brackets to attach it all to the chassis, which were cut from 1.5mm mild steel, with the holes reinforced with 3.5mm mild steel plates to be sure the bolt (actually a cap screw) shaft had enough support, so the reciprocation of the application doesn’t wear oval holes in the bracket plate, rendering it useless. These were easy enough to make – weld one to the other, and make 6 brackets the same. If I made 8 the same then I wouldn’t be able to demount the spider. I say 8 brackets in total because each rose-joint will be supported above and below. Remember the golden rule – always support a rose joint in double-shear.

I first of all made a turn-buckle to go from bottom left to top right, as shown in the picture. A turn-buckle is a tube with opposing threaded ends attached. If you turn the tube one way, the whole arrangement expands, and the other causes it to contract. This made it easy-peasy to locate and get the length correct for the first cross-member. The brackets are bolted to the turn-buckle first and then welded into place, ensuring a good fit. Next, on went the tube from bottom right to the existing cross-member. I used hole-cutting software to get this right, and the adjustability in the rose-joint to be sure of location.

Next came the tricky design part because it was becoming clear to me now that I may be welding myself into a corner when it came to making it demountable. as it stands in the picture above, you can see how there’s no bracketry for the top left tube, so the whole arrangement has enough pivot room (pivoting from the top right rose joint) to come loose. If I welded in the fourth bracket then there would be no room to get it out.

I hummed and hahed, and thought of making the top left and bottom right bracket-pairs vertical, but that would mean firstly cutting them out, and secondly losing the extra gusseting support they give. It was only when I was yanking it out to show to a friend did I realise that all it needed was a different bracket-shape that didn’t span the entire angle if a simple gusset were used. It still transfers the load into the focus-node of the space frame in a straight line, so I think I’ve ended up with an elegant, if asymmetric solution.

This whole arrangement is strong – really strong. I’ve tested it by standing on it where all four tubes join and there was no give whatsoever. And that’s loading it in a dimension it’s totally not designed for. I think it’s now strong enough for the job in hand, namely keeping the front of the car rigid enough for the suspension to work well, as well as keeping the engine bay strong enough to cope with the torque reaction of 215bhp.

Happy with that.

Look at my shiny new p-clip brackets, fresh from the laser cutter

So, now that I’ve almost finished all the surgery on the chassis, I need to do a quick rebuild of the car to understand where all the cables, pipes, etc need to run.

Rather than drill holes in the chassis and use self tappers, I thought I’d make some weld-in brackets to hang the p-clips off instead.

For this test, I’ve used m5 socket-headed cap screws, secured in m5 nutserts. The holes themselves are cut to 6mm, and to use the nutserts I just ream them out to 7mm. I don’t fully trust nutserts, so I’m going to experiment a little to secure each one with a tiny weld tack. If I get concerned about vibration loosening the cap-screws I’ll go for locktite.


Of course, I could use m6 cap screws and nylock bolts, but sometimes getting back there to tighten a nut is a faff, and better if I can prepare all of this on the bench before welding in. The three tabs on the top are for welding to. My plan is to prevent the heat from the weld getting too close to the holes which would increase the risk of warping or mis-shaping the holes.

Each bracket is 2mm mild steel, and weighs 19g naked and 64g fully configured with the three p-clips in the pictures.

As ever when getting things laser-cut, the cost of a larger run isn’t that much more than a minimal run, so I had 60 made. The cost ended up being 70p each in runs of 60. I’ll be welding them here, there and everywhere to make good use of their options to hang stuff off. I think my new fuel tank for instance may hang nicely off them.

and finally, the back end view: