Bore and Stroke, there is no substitute for horsepower! Part 1.

I have owned my 1989 928 S4 since 2004 and always love driving it, back then it was my daily driver. The flat six society may argue not a real Porsche, but the 928 for me is a well balanced drivers GT, one of those cars you just want to get back into and drive again.

Hamish McKendrick introduced me to track days in 2005 and I was hooked. Since then, I have driven this 30 year old 5 litre V8 hard, once a month on a Sunday. I think the odometer displays over 200,000 miles and in true Porsche style, it is check the oil and add petrol, ready for next month’s track day. Certainly, a bucket list item for me was to run around the Bathurst race circuit, which we have managed 5 times so far thanks to PCNSW Regularity and Sprint events.

Easter 2019 having the time of my life https://youtu.be/ k0UlPikUsic or google (Porsche 928 at mount panorama) the evening that destroyed the bearings.

The more you enjoy the sport the more you push the limits of the car, and with additional braking, tyre grip, exhaust mods, suspension mods and a lot of track time, I finally found the weak link in the 30-year-old 5litre, quad cam V8, 4 speed auto GT classic.

The weak link is oil starvation, aeration after several hard laps at full rev’s for 15mins, combined with hard lefthand cornering, there goes the big end bearings. A very nasty rattle heading up mountain straight the next morning. Found some metal in the sump!

I knew what I had done, and sadly had to put my beautiful car in the back corner of the shed waiting for inspiration and confidence to pull the motor and rebuild it.

With so many hot hard miles under her belt, I expected some badly worn internals so I planned to do a full crank, rods, piston rebuild at least, so the sourcing and financial deliberation began. So……… if you must buy a crank, pistons and conrods then why not live the teenage years of bigger is better? Bored and stroked with increased compression, sounds a treat.

There are a lot of induction kits to increase power in the 928 and other similar marques but I wanted to maintain the makeup of the car with a quad cam normally aspirated set up.

In today’s money, 310hp from a quad cam 5 litre, German V8 is disappointing, but this is a 30-year-old car. And we are told, good for 1 million miles!! So, the investment in major items was made and the removal began. This motor only just fits in this car…….. no space each side in fact you must manipulate it on the way out, but wow what a piece of engineering when it comes out, a beautiful big chunk of motor.

Thank goodness for digital photography, a careful library of all connections, hoses, wiring, etc, etc, etc. 18 months before this baby goes back in.

So strip me down and get ready for some surprises!

An awesome piece of engineering, just got to remember how it all goes back together.

A shame the bores had to go for new pistons as they were still in a very serviceable condition, pistons and valves were a bit crusty though.

Bore out the block and fit the new cylinder sleeves

As I mentioned 30 years old, and a few years of hot hard work is not what she was expecting. Not everything was in good serviceable condition, at some points it felt like the end of the blues brother’s movie when the Caddy falls to pieces. Undo a few bolts and see what falls out.

A couple of examples.

1. The drive plate, not sure how many more track days this had in it, but it fell to the floor in pieces when I unbolted it.

2. The cam chain tensioners, some of the pieces were blocking the oil return channels.

Constructing the short motor was relatively simple, with several dry runs at assembly and the inclusion of crank scrapers and oil management baffles required the biggest effort with trimming, fitting, removal, trimming, fitting 4 or 5 times to ensure clearances were suitable and managed the best possible crank scrape to reduce oil misting, and aeration.

So, the Block assembly aimed for a 6.5litre capacity, strokedhollow crank with knife edge counterbalance, Oliver race conrods, connected to Wiseco ceramic topped race pistons. Custom crank scraper and baffle system and dropped sump. To achieve this the original cylinders had to be completely removed and replaced with steel liners. I hate to mess with German engineering, but I wanted the cubes.

So now what to do with the cylinder heads?

Well, there is not much room to increase valve sizes, but you can squeeze in 2 x 968 intake valves, port and flow the heads and skim a little off the surface to support at least the best air flow possible with the original casting and manifold set up and just north of 11:1 compression ratio. Happy days. Oh, and did I mention camshafts are expensive, but one of the best engineering shops in Australia took care of that for me. Tighe Engineering, manufacture new cams or re-engineer existing. Adding higher lift and more duration to really extend the opportunity to consume lots of fuel!

So with the bottom end sorted and 4 new wild cams, nut much left to do or spend….ha,ha,ha.

And then the shopping list really starts, replacing those 30-yearold accessories, belts, chains, sprockets, valves x 32, cam followers x 32 and heads to strip and machine Adelaide Cylinder Heads did the machining and testing, definitely the best job in town. Awesome work and know their stuff. Now comes the scary job of assembly, line everything up and bolt it together.

The engine design is one that gives an interference condition if operating out of sync, ie. The valves will hit the pistons if this is not correct. If you are one tooth out on the chains or belts, everything will seem fine but won’t run perfectly, if at all. So first prep all the parts including the intake plenum and cam covers below and spend a lot of time on the internet watching and reading all the documented experiences.

Crinkle paint gives up to 2 times more surface area for heat exchange, something I’m going to need :) Experience, something you get immediately after you really needed it!