Fuel System - Part 3 - Deep Breathing

Next up on my job list was installing the air intake and air filter.

I purchased the stainless steel air intake pipe from AK; another bit of engine bay bling! This includes a couple of spigots to connect to the crankcase breathers so that any oil-contaminated air from the engine is recirculated back into the air intake.  It also has a hole underneath into which the Intake Air Temperature (IAT) sensor is installed; this is one of the sensors monitored by the Canems ECU.

Shiny intake pipe!

I ordered a standard GM IAT sensor from Billy's Speedshop via eBay.  I needed to clean up the hole in the inlet pipe slightly with a hole drill to get the sensor to fit.  This will be held in place with some polyurethane sealant.

Intake Air Temperature Sensor...

...installed into air intake pipe.

For the air filter, I went with a K&N RD-1450 unit.  This was basically the largest diameter but shortest height unit I could find with a 4-inch inlet flange diameter to match up with the AK air intake pipe (the short height is important as there is not a lot of room to fit the filter in between the intake pipe and the radiator). As well as providing increased airflow, the K&N units are washable and will last pretty much forever.

K&N Air Filter with oiled cotton filter element

The air filter was a simple slip-fit over the end of the air intake pipe and was secured in place with the supplied jubilee clip.

To attach the air intake pipe to the throttle body on the engine required a 102mm ID silicon Straight Coupling Hose (from Merlin Motorsport) and a couple of suitably sized jubilee clips.  These were torqued up to 6Nm as per the Samco recommendations and then checked again after at least 30mins to allow for any relaxation of the silicon pipe.

Filter and intake pipe installed - I even made sure the logo was level...

For the connections to the crankcase breaker pipes on the rocker covers, I used some 200 Series -8 braided steel line from Merlin Motorsports and some neat -8 hose finishers (which incorporate a jubilee clip) from Torques UK.  

Offside breather hose completed

The nearside hose was a bit more of a faff as the outlet breather pipe on the rocker cover faces towards the rear of the car.  This needed the breather pipe to turn 180 degrees; unfortunately the bending radius of -8 braided hose is rather large so an alternative solution was needed.

I used a couple of -8 straight female fittings and a -8 180-degree adaptor from Torques UK to come up with a suitable arrangement.  Unfortunately I couldn't quite finish off the install as I managed to misplace the final -8 hose finisher somewhere; no doubt it will surface once I've ordered a replacement...

My solution to redirect the breather pipe through 180-degrees

Somewhere in the garage, there is a stray hose finisher...

Fuel System - Part 2 - Regulating the Service

With the engine in, I could start connecting up the fuel lines in the engine bay and routing them down to join up with the supply and return hardlines.

As I'm running fuel injection I needed a fuel pressure regulator to supply a constant 58psi to the injectors.  I bought an Aeromotive A1000 High Flow regulator with -6 inlet ports to match my supply line from Summit Racing.  This is a 3-port regulator and so is installed into the fuel supply line ahead of the fuel rail; there is then one port that supplies pressurised fuel to the fuel rail and the bottom port returns excess fuel back to the tank.

Aeromotive A1000 Fuel Regulator...shiny...

The regulator ports needed some -6AN to ORB-6 adaptors fitting to allow connections to the braided fuel lines to be made.  I removed the brass plug on the front and installed a fuel pressure gauge into the port using some PTFE tape to help with the sealing.

The regulator comes with a mount suitable for fixing to a flat bulkhead.  However, I decided to fit mine as close to the engine as possible and mount it onto the tubular chassis bracing.  This meant I needed to make up my own bracket.  I designed a bracket in CAD and printed out a "flat sheet metal" template which I stuck to some 2mm sheet steel and used to cut, drill and bend bracket into shape.

Paper template used to cut blank from 2mm steel sheet

Using a 25mm diameter aluminium bar as a former for the curved section

The final 90-degree bend formed

2No. M5 Rivnets fixed into bracing tube...

...and mount fixed in place with 2No. M5 button head screws

I could then make up the fuel line between the regulator and the fuel rail; this was a short hose with a 90-degree -6AN fitting at the regulator end and a straight -6AN fitting to connect to the fuel rail connector.  The fuel rail connector I bought was a 90-degree item, with the usual 3/8" GM push-fit connector to the fuel rail and a -6AN female fitting on the other end.

-6AN Fuel Rail connector purchased from Torques UK...

...is a simple push-fit onto the fuel rail

Completed fuel line from the regulator to the fuel rail

I made up two oversize lengths of braided line; one with a 45-degree -6AN fitting for the line which will come up from the supply hardline and one with a 90-degree fitting from the bottom of the regulator which will then run across the engine bay and down to the return hardline.  I will finalise these connections once I know exactly how I am going to route the braided lines down to the hardline fixed to the chassis rails.

Fuel regulator and lines - sorted!

Fuel System - Part 1 - Hard Lines

The common approach when it comes to running the fuel lines for the AK builds is to use -6 or -8AN stainless steel braided hose from the tank, along the inner chassis rail to the engine and, in the case of fuel injected engines, similarly run a return line down the opposite chassis rail and back to the tank.  The fuel pump and filter are typically fitted on the supply pipe run and also fixed onto the inner chassis rail.

Although the AK fuel tank for a fuel injection set-up has a small sump to prevent fuel starvation under hard driving, a conversation with an AK owner at Stoneleigh a couple of years ago about his experience of fuel starvation, convinced me that I was going to go belt and braces on this and install a surge tank in my fuel system.

A surge tank is a small volume tank fed by a low-pressure fuel pump from the fuel tank; any excess fuel is fed back into the fuel tank.  The engine is then fed via a high-pressure pump from the surge tank.  This means that even if the fuel pick-up in the tank is uncovered, there is sufficient fuel reserve in the surge tank to provide an uninterrupted fuel supply to the engine.  An added advantage is that since the fuel between the surge tank and the fuel tank is constantly being recirculated this keeps the fuel supply to the engine cooler which is a big advantage, especially with the increasing ethanol content in fuels.

The surge tank will need to be installed in the boot of the Cobra and it made sense to me to also install the fuel pumps and filters in the boot rather than fixing them under the car.  Since I would need to run the fuel pipes from the fuel tank up into the boot and then back out again I decided to use run hardlines for the fuel supply and return lines along the chassis rails.  

I went with Goodridge HL836 lightweight aluminium hardline which I purchased from Merlin Motorsports.  This is made from Hycot aluminium; a precision-drawn aluminium tube with a black polyamide coating.

-6 Size Aluminium Hardline

Hardline Fittings 

I used my plywood brakeline template/ guide to help bend the tube to the correct profile.  The hardline diameter is 10mm which worked fine in a mini pipebender.  I made the pipes slightly longer than I thought I would need; these will be trimmed back when I know exactly where I will want to create the transitions to the braided fuel lines.

Hardlines bent to shape

These were then fixed into place on the chassis rails using some of my bespoke mounting blocks; the fixings on the offside chassis rail also being used to hold the rear brake pipe in place.

Supply and return lines fixed in place

Supply line will connect to a braided line at this point 

At the rear, two braided lines will run up into the boot

Everything seemed to fit into place quite nicely and I was very pleased with the final result.

You may spot from the pictures that this blog post is a little outdated given that the engine and gearbox are now in place but for some reason, I just never got around to writing up this instalment.  In hindsight, I was bloody lucky that the locations I chose for the brake/fuel line mounts didn't clash with the transmission as there was no prior consideration of this in my planning!! (Although I did crush the nearside fuel line when installing the gearbox which required the purchase of some additional hardline and a re-bending exercise - luckily I had kept the piece of plywood with all the bend guidelines on!)

Fuel Tank Mounting - Part 1

According to the AK Build Manual, the AK-supplied stainless steel fuel tank is fixed to the rear chassis subframe using M8 bolts; the fuel tank is hung under the subframe and bolted into the chassis through the mounting tabs welded onto the fuel tank.

Now petrol has a specific gravity of 0.720 (compared to 1.0 for water) and the AK fuel tank has a capacity of just over 65 litres.  The empty tank itself weighs approximately 15.2kg. So with when full, the tank and the fuel will weigh approximately 15.2 + 46.8 = 62 Kg.

There are 5 tabs on the AK fuel tank, each held up by an M8 bolt, so each bolt needs to take 12.4 kg.  The proof load of a single standard Grade 8.8 M8 bolt is 2120kg so there is a hefty factor of safety on the bolt even accounting for any dynamic factors.

However...

The rear box sections of the AK chassis that the tank is bolted to are only 2mm thick and an M8 bolt has a thread pitch of 1.25mm, so at best only two threads of the bolt are going to be engaged.  

When a tensile load is applied to a bolt it stretches slightly and the loading on each thread will be different; the first thread at the point of connection sees the highest load and decreases for each subsequent thread.  It is widely accepted that a minimum of six threads is required to establish a full strength connection (the first thread takes approximately a third of the load, the first three threads take approximately three-quarters of the load and the first six threads take essentially the whole load and beyond the first six threads the remaining threads are basically under no load at all).

There is some complicated theory behind all this...

No, I don't understand it either...
(Extract courtesy of Journal of Multidisciplinary Engineering Science and Technology (JMEST)

Basically, the outcome of all this waffle is that in order to ensure a full-strength bolted connection there needs to be a minimum bolt engagement of six threads and bolting the fuel tank to the AK chassis will only result in two threads being engaged (ignoring the fact that each bolt is actually only taking around 0.6% of its ultimate load capacity).  

My proposed solution to this is to create some bosses from 20mm diameter steel bar which will be tapped for a 20mm M8 bolt.  These will be installed into the rear chassis subframe around the fuel tank by drilling a 20mm diameter hole into the top of the rail, an 8.5mm hole in the bottom of the rail, inserting the bosses, which will be held in place by an 8mm bolt to maintain the position, while welding the boss into the subframe on the top surface.  See the diagram below

Section through rear subframe - fuel tank mounting boss

The fuel tank has sat in place on top of the rear subframe since the chassis arrived serving as a temporary shelf for various bits and pieces (I had put some duct tape over all the openings in the tank to prevent the entry of any detritus).

The first job was to remove all the crap off the top of the tank and even up its position within the rear subframe.

Fuel tank in position on top of the rear subframe

The rear subframe rails are 40mm wide steel box. I set some callipers to 20mm and scribed a line across each fuel tank mounting tab; even though the tab might not be sitting centrally on the subframe rail, this operation should ensure the resulting holes are all in the middle of the frame rails.

Scribing centreline of rear subframe rails on mounting tabs

I scribed the centreline of the mounting tab in a similar way and centre punched each of the tabs at the intersection point.  I then drilled a 4mm diameter pilot hole through each of the mounting tabs and through the top of the rear subframe rails. Stainless steel can work harden if it gets too hot while drilling, which can severely reduce the working life of drill bits, so I used plenty of cutting fluid to try and keep the drill bit cool.

Drilling pilot holes...

After removing the fuel tank, the next task was to attempt to drill a 4mm pilot hole in the bottom of the subframe rails perpendicular to the holes in the top of the rail.  I made a simple jig using an offcut of 50 by 25 rectangular steel section.  Using a pillar drill I drilled a 4mm hole through both sides of the steel section from the top; so in theory, these two holes should be exactly aligned and perpendicular to the face of the section.

Using the 4mm drill bit as a dowel I aligned this jig above the pilot holes in the top of the subframe rail and clamped the section in place.  I then drilled through the jig, the pilot hole and through the bottom of the rail.  Hopefully, all these holes will be fully aligned and the bottom hole will be directly under the top hole.

"Verticality" jig from off-cut of 50x25RHS

Next step was to open out the top holes to 20mm diameter using a step drill.  Step drills actually result in a proper "round' hole whereas normal drill bits produce a slightly triangular hole. I did check the length of the bit to make sure that I didn't inadvertently open out the bottom hole to too big a diameter; as if I had planned it perfectly, as the top hole opened out to 20mm, the 4mm step in the drill bit would be sat in the bottom hole!

2-20mm Step Drill Bit

Drilling out top holes - almost there...

...and done!

Unsurprisingly drilling out a 20mm hole creates a large amount of swarf and metal chips.  I used a magnetic probe to try and fish as much as I could out of the rails and then deburred the holes on the inside and out.  

20mm hole in the top of the rail with 4mm pilot (hopefully) dead centre below in the bottom of the rail.

The final step of this operation was to open the bottom pilot holes out to 8.5mm.

To make the mounting bosses I used a section of 20mm steel bar.  In theory, each boss needed to be 38mm long to sit flush with the top of the chassis rail.  However, I noticed that on one of the rails, the welded seam of the box section was on the bottom face. So to be sure I measured each boss individually by inserting the bar stock into each hole until it bottomed out and scribing the bar level with the top of the chassis rail.  I then cut all of the blanks for the mounting bosses (numbering them so I could tell which hole they were for).

Cutting 20mm Bar Stock using a Bandsaw

Five blanks cut to length and numbered

Now each blank needed to be drilled and tapped to take an M8 x 20 bolt.  I marked the centre point of each blank by setting my vernier callipers to 10mm and scribing three lines at roughly 1/3 intervals around the bar.  The intersection point of these three lines should be bang on in the centre of the bar.  This point was then centre-punched.

Centre point scribed and punched

I then set the blank into a drill press vice, checking to ensure it was perfectly square to the drill bit.  With a 4mm diameter bit, I then drilled a 25mm deep pilot hole into the end of each blank in turn.  Again I made sure to use plenty of cutting fluid and removed drill bit and cleaned out swarf every 4-5mm of advance.

The holes will be tapped for an M8 x 1.25 thread so the holes need to be opened up to 6.3mm diameter before tapping.  Again lots of cutting fluid and cleaning out every 4-5mm of advance.  As a final touch, I countersank each of the holes.

Opening Pilot Holes out to 6.3mm Diameter

Ready for Tapping

I then tapped the holes using an M8 x 1.25 tap.  As with the drilling I used plenty of cutting fluid and the recommended technique of 2 turns forward and 1 back to snap off the 'chip' caused by the tapping operation.  I also removed the tap occasionally to clean out the flutes.

Tapping in progress!

Final bosses - all tapped and ready to go!

I did a trial fit of the bosses into the subframe rails.  I was quite pleased to see that they all fitted flush with the top of the frame rail and that the hole in the bottom of the rail matched with the hole in the boss so that I could install an M8 bolt!

All fits perfectly!

I need to weld the bosses into the subframe rails to complete this part of the build.  I will address that operation at some point in the future. 

This is all probably massively over-engineered but I can sleep easy that no matter how much fuel is sloshing around the tank during hard driving conditions, the fuel tank is going to remain securely attached to the car!