Monday, 7 November 2011

Virtual Cobra Build

With another batch of engine parts delivered, I have been progressing the block build up although missing a couple of vital items off the order means I am not as far as I could have been! Still, it gives me the chance to prepare the parts properly while I wait so I have colour matched the timing cover and water pump to the block.

In addition, I have been working on the installation of a vital part of the fuel injection system – the crank position sensor. Since the stock version of this part is prohibitively expensive to buy new and there aren’t many Mustangs in scrap yards around here, I have bought a more readily available standard component and designed a combined bracket and timing pointer along with a suitable trigger wheel.

Other ‘virtual’ progress has been around the design of the instruments and I have decided that an illumination test is required before committing to a proper PCB. I therefore need to mock up the instrument face and back lighting to ensure I have sufficient brightness to meet the Individual Vehicle Approval (IVA) test requirements. To this end I have designed a first pass circuit (which I should be able to build at work thanks to the prototype PCB facility here at work) and a ‘mask’ to direct the light. This is designed to be water jet cut along with the timing pointer components and some other instrument parts so I am aiming to group these together and procure them in the near future. Once these are in, I can build and test a number of aspects of the design before finalising the board design and getting it manufactured properly.

Monday, 17 October 2011

Engine build up - part 1

With a recent bout of reasonable weather, I have been able to start assembling the bare block back into a short engine. First job was to install the cam bearings which required a quick measurement of the bores before using my home made tooling to wind them in.

It was a little tricky at first, but by the time I got to the last bearing, I had it down to a fine art! With these installed, I could lubricate the cam and slide it home as this is an easier operation without the crank in position.

Next up was the crank itself starting with the main bearings and I checked the actual oil clearance using plastigauge (a first for me!). With all well and the crank spinning freely, I could move onto the pistons. A couple of the new rings needed filing to get the correct end gap, but for the most part they dropped in without hassle, forming the short block.

With the amount of machining carried out on the block, along with aftermarket heads and tappets (‘lifters’ for the subscribers on the other side of the Atlantic) I am unable to rely on the stock pushrod length being correct. I have therefore procured an adjustable pushrod which I used to check the required length. This involved marking the tops of two valves and mocking up the top end then turning the engine through one complete cycle. This leaves a witness mark on the end of the valves which needs to be centred across the tip. The adjustable pushrod is wound in or out to centre the mark then the process is repeated and checked for a pair of valves (intake and exhaust) on each cylinder head. Now I have determined the required pushrod length, I can go ahead with purchasing them and the full engine build can continue.

In addition to this, I have been creating a CAD model of the engine as accurately as I can which will aid me in the design of the intake and exhaust manifolds. I currently have a virtual engine in a similar state to the real one as the two need to be built in parallel. The key part missing at the moment is a rocker cover model as this will determine the location of the intake manifold plenums and routing of the corresponding tracts. I will be procuring this part along with the pushrods to help progress this part of the build.

Wednesday, 7 September 2011


With a recent bout of extra hours at work have been a double edged sword – on one hand I have not had the time or energy to work on the Cobra much, but on the other it has boosted the funding! I have therefore been able to procure a significant set of parts from the US that are otherwise difficult or impossible to source in the UK. These include:
  •  A pair of aftermarket aluminium cylinder heads and head bolts
  • Roller cam shaft and lifters suitable for a fuel injected engine
  • A set of roller rocker arms
  • Cam position sensor housing (which normally replaces the distributor, but I will use only as an oil pump drive)
  • Appropriate head and port gaskets
  • Pushrod length checking tool
  • Billet piston ring compressor 
The main job with most of these parts is to digitise them so I can build up the engine CAD model in preparation for the manifold design – I’d better get out the measuring stick then!

Other progress has finally seen the completion of the Engine Control Unit customisation. I built the basic Megasquirt kit a while ago and combined all of the modifications that I wanted onto a custom designed auxiliary Printed Circuit Board. I have now finished incorporating this extra PCB into the enclosure and have successfully bench tested the system using an electronic ‘stimulator’ to mimic the engine signals and loads. My additions over the basic assembly are:

  • 4 coil drivers for wasted spark ignition
  • 5V square wave output tachometer signal
  • Second wide-band Exhaust Gas Oxygen (EGO) sensor input
  • Open collector ‘engine running’ signal to disable starter motor
  • Relay driver to control radiator fan

With these additional functions operational (well, flashing a bunch of LEDs on the stimulator board!), I can now move on to building up a hardware test bench to run the actual engine components (coils, injectors etc.) before attempting to control an actual engine.

Friday, 12 August 2011

End of the strip down - its all build up from here.

Having managed to arrive at a bare engine block, I finished the strip down by removing the gudgeon (wrist) pins from the original pistons in order to recover the original connecting rods. The pins are a press fit, but fortunately I have access to a 50 tonne press at work which made light work of the job requiring a measly 5 tonne to push them out. The next job was to get the parts over to the machinist, so with a borrowed van, I hauled the block, crank shaft and con rods from the original engine along with new pistons, piston rings, bearings (main and connecting rod), flywheel, clutch, damper and timing set to Rob Walker Engineering near Oxford. This company comes highly recommended from a few guys in the office, so I knew it would be worth the effort. The machining list was quite comprehensive:
  • Deck the block to zero clearance (increases the compression ratio to improve power output)
  • Bore out and hone the cylinders to the new (oversize) pistons
  • Grind the crank journals to the next under size diameter
  • Weight match the pistons
  • Weight match the connecting rods
  • Dynamically balance the rotating assembly
  • Install the con-rods onto the new pistons
Two weeks later I was on my way back to collect everything and the quality of the machining work was excellent - the recommendations were well placed. Since it was a bare (and now substantially oil-free) iron block, I proceeded to de-grease the outer raw surfaces then treat them with ‘Metal Ready’ as used on the suspension parts. This serves to both neutralise any remaining rust and provide a Zinc Phosphate coat to which the top coat can adhere well. The last stage was to apply the Ford Blue engine enamel - this is not an original colour for this engine nor the installation in the Cobra, but I liked it!

From here, I need to continue to collect together the parts to rebuild the engine. Matching the camshaft, rocker ratio and cylinder heads will be key to get the most out of the package and I believe I am homing in on a combination that should work well together. Only time (and the dyno) will tell!

Tuesday, 9 August 2011

Bearing Up (or out)

Despite the lack of Cobra updates, I have not been totally idle. Since my last update, I have been slowly completing the engine disassembly, to the point where all that remained were the cam bearings. These need to be removed for the machining work, the fact that they are worn out is incidental at this stage! A cam bearing is typically removed or installed using an expanding mandrel to hammer it in or out. I have heard some stories of bearings being damaged by this process, so I decided to design a tool to wind them out and in on a thread. I then turned the required parts (5 different bearing sizes so 5 individual mandrels!) and gave them a test. Success! The 5 bearings came out without a hitch.

The other area of progress has been on the gauges. I have never been keen of the normal kit car approach of a set of aftermarket warning lamps festooning the dash, so I thought I would take a more contemporary approach. To this end I have decided to attempt to manufacture my own gauges so I can integrate the lamps within the dial faces. This has taken quite a lot of research, but I am now at the stage where I have a schematic and board layout for the minor gauges (fuel level, water temperature), along with a plan for the major gauges (speedo, tacho). Oil monitoring (pressure and possibly temperature) will be achieved with the use of an LCD which will also provide trip and mileage information. There is still a long way to go on these, but it seems to be in the right direction so far.

Monday, 9 May 2011

Oily and Greasy part 3

The lower rear wishbones represent the last of the donor suspension parts that I needed to clean and protect, so it was out with the wire brushes, and a fresh tub of elbow grease at the ready. These took some effort, mostly from their sheer size and weight, however they are fully cleaned and painted now.

Some replicas shorten the rear drive shafts and wishbones to give a more 'original' look to the rear wheel offset (deep dish on the rear wheels) however, most Gardner Douglas kits do not do this and mine will not be an exception. I still need to procure most of the bearings, spacers, shims, shields, washers and the other 101 parts that make up the working elements of the suspension, but for now I am happy that what I have is good to go into storage until needed.

The other source of grease and oil (the engine) has also received some attention. I purchased a cut-out short block which needed final disassembly and assessment for machining requirements. All was going well until I removed piston number 8 (actually, it fell out of the block once I had freed the big end – this was not a good sign!) Upon closer inspection, It appears that this piston had made a bid for freedom from the engine through the exhaust valve in the form of aluminium vapour. Apparently, it is a known fault with engines of this age in certain vehicle installations where part of the induction system becomes blocked, forcing cylinder 8 to run lean. If it is left unchecked, it gets to the point were detonation occurs which eventually melts the piston – this is what appears to have happened in my engine. The upshot of it all is that the remains of the oil ring scratching the nicely honed surface along with aluminium being smeared up and down the bore means a re-bore and new pistons all around. At least I had budgeted for this!

I now need to put together a shopping list of all the parts required to turn this into a fire-breathing Cobra power plant, along with a list of banks I might raid to pay for it all!

Tuesday, 19 April 2011


It was a big day yesterday with the collection of my Cobra's powerplant core - the engine. To call it an engine is a bit of a stretch since it is a short block (block, camshaft, crankshaft, pistons and con rods) however, since everything missing (and even some of the things fitted) will be replaced with new, it saves me the job of stripping down and scrapping the rest of the engine. The all important casting numbers on the side of the block indicate it was cast week 26 of 1993 which is just what I was looking for:

  • This age of engine has a one-piece rear crank seal which is better leak free solution
  • The crank has 50oz external unbalance which makes finding a flywheel for it marginally easier
  • It has a roller lifter camshaft of which there are more fuel injection friendly cam profiles available
  • The emissions test it will eventually have to pass during its IVA (individual Vehicle Approval) test has non-catalytic converter emissions limits. I therefore do not need to purchase and install cats in the exhaust system saving time, effort and cash!

From here, I need to finish stripping it down and assess just how much machining is required. At the very least I will be likely zero decking it (to raise the compression ratio), and boring it to remove the current wear marks. A decision on crank grinding will have to wait until I can measure the journals, but the general condition of the working parts suggest this may not be necessary.

Oily and Greasy take 2

More rust stripping, cleaning and painting has occurred with the completion of the vast majority of the donor suspension parts.

One particularly stubborn output yoke proved a little difficult in separating it from its seal track, but access to more 'industrial' tools at work and they soon parted company. This along with the two lower rear wishbones represent the last steel suspension parts requiring the POR15 treatment. The rear uprights still need a good going over, but as they are aluminium they will require a different approach

Tuesday, 29 March 2011

Oily and Greasy

To balance my work on the nice and clean circuit design and software writing, I decided to get some muck work under way. To this end I have started reconditioning the donor parts. The first task is to strip it all down and clean up, then protect with a suitable finish. The donor parts came from the scrap yard in major chunks which required final disassembly.

Front upright assembly as supplied from the scrap yard
After a flurry of activity with hammers, sockets and a handy press, I reduced it to its component parts:

Some people choose to grit blast and powder coat, but many use the POR15 system so I thought I would give it a try. The process requires the loose rust removing, then a good clean with 'Marine Clean' followed by a coat of 'Metal Ready' to neutralise the rust and provide a zinc phoshpate coat. Finally, two coats of POR15 and you are finished. If the parts are to be exposed to direct sunlight, a UV barrier is recommended. Since these are suspension parts, I don't need to worry about that.

I set about cleaning the rust and grease off using wire wheels and paint srtipping wheels in my drill. Once reasonably clean, I could move onto applying the POR15 system. The paint covers well and the scond coat was applied when the first was 'dry with a slight finger drag' as required by the instructions. I trialled the system on the two front lower wishbones and two front uprights:

It seems to have gone reasonably well, so I need to press on with the remaining donor parts

Oil and Grease Free

Despite my lack of updates, I have not been idle on my Cobra project. I have mostly been working on the 'cleaner' aspects of the job, concentrating on some of the electronics I will be using. First off, I finished the Megasquirt auxiliary board and since it was a two layer (top and bottom copper) layout, I have had it made by one of the many PCB prototyping houses. This was recently delivered:

A quick check in the enclosure with the vehicle connector in place:

Now all I need to do is solder it up and give it a test!

In addition, I have been developing the vehicle electrical system and have decided on at least trying to build my own electrical modules. These will cover the instruments, direction indicators and heating controls. So far I have identified the use of a Picaxe Programmable Integrated Chip (PIC) as supplied by Revolution Education. These have a simple BASIC programming laguage and are used for a whole host of projects, mostly by hobbyists. This system will allow me to control the blower and heater solenoid valve, flash the indicators and drive instrument stepper motors, and display information such as miles covered and ambient temperature. A lot of work will be required to get these moduiles up and running, but I think they will be worth it in the end.

Wednesday, 5 January 2011

Single Plane Manifold

A new year and a new set of results...

As with the dual plane manifold, I have created a CAD model that lets me simulate and analyse the air flow down individual runners of the single plane manifold:

The model was run under the same conditions as the dual plane manifold i.e. each inlet valve is opened in sequence and a pressure drop applied to approximate the piston velocity. The simulation is then interrogated for the mass flow into each cylinder. The mass flow variation against the average for each cylinder is shown below:

The average mass flow was 18% higher in the single plane manifold when compared to the dual plane design indicating that the engine would be able to breathe more easily and hence produce more power with this type of manifold (unfortunately it does not represent an 18% gain in power due to the complex flow conditions present when the engine is drawing air in sequence with the firing order, not the simple steady state conditions of the simulation). However, it can be seen that the cylinder-to-cylinder variation is still in the order of the dual plane design (-20% / +10% variation) indicating that this manifold is also not ideal for a port fuel injection application. Either manifold is suited to a carburettor or throttle body injection since the air-fuel ratio is established at the throttle plate rather than the valve.

Given the above information and a distinct lack of a readily available stock port fuel injected manifolds on this side of the pond (I didn't like the look of it much anyway!), I have decided to look at the possibility of fabricating my own. The CAD work I have done to date suggests a twin-plenum design would be the best for packaging around the known hard points. Watch this space...