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Many of you know that I built a 4.2L project a few years ago. The short story is the clutch we spec'd for it failed and caused the crank to be pulled into the journal bearing killing ring seal and a few of the bearings. So what to do?
Do it bigger, better and the most bad *** that has ever been done.
The specs we're aiming for:
500whp with a 9000rpm redline (gotta keep up with the new kids on the block). Including full Motec M1 plug and play integration for engine management.
To make this happen a lot of design work was required and changes to the bottom end, pistons, rods, and major changes to the stock valve train. Removing mass in the right places from the rotating assembly is obvious, another key to achieve our goal was to pull mass out of the valve train to support cams with more aggressive lift and ramp rates and to be able to use valve springs with less preload to not beat up the softer BeCu valve seats/guides. Lots of engine **** pics to put up as we go and happy to answer questions along the way.
I'll add some pics a bit of background as we go. For now a picture of her before going under the knife...
The 997.1 GT3 stock valve train has 4 valves/cylinder, 2 cams/bank with hydraulic lifters/lash adjusters. We took a long look at this and found some areas of improvement for us to reach our goals.
To get more air into and out of the cylinder we need cams that are more aggressive, more lift, higher valve acceleration (opens faster) and more duration. Unfortunately you can't just cut a more aggressive cam and put it into the existing system. A few areas we knew we had to address were the mass of the valvetrain, and the geometry of the cam/lifter interface.
As the valves move at higher rpms and faster off of the seat, we need stiffer valve springs to get them to close as quickly. If you reduce the mass of the valves, there is less momentum to control and you can use a less stiff valve spring. A less stiff valve spring will make it so the Beryllium Cu valve seats we're using will survive longer. So we went to :
Ti valves
Dundon 42R Ti Valves, BeCu valve guide, double wound valve spring
To further remove mass from the system we went to a solid lash cap, this has a radius on the top cam mating surface and is DLC coated for longevity. This removes a large amount of mass from the system which allows things to move quickly without requiring super stiff springs to control it all
The radius follower increases the speed the valve is put into motion off the seat and "fakes" out the cam lobe to think the follower is larger in diameter which allows greater lift geometry. Other wise if the follower was flat, the cam lobe would run off the edge of the follower and things have a habit of breaking when this happens.
The purpose of the rod is to transfer the force of combustion happening above the piston to the crank shaft to create torque. Requirements on the rod are to be strong enough to handle the cylinder pressure and light enough to not put undue load on the bearings as the piston changes directions.
Dundon 42R 4.2L Stroker Ti Rods
Dundon 42R 4.2L Stroker Ti Rods, all the areas of Ti removed to reduce weight and keep strength.
Dundon 42R 4.2L Stroker Ti Rod pin oiling hole, and bolts.
Dundon 42R 4.2L Stroker Ti Rods
We've put a lot of engineering time into the rods to gain as much length as possible (this also blurs into the piston design, wrist pin location and size, rod bearing size etc.) and to keep them as light as possible.
You gain mechanical advantage by increasing the stroke of the crank. the rod/stroke ratio needs to be as high as possible to reduce side loads on the cylinder walls or you will prematurely wear the cylinder bore, scuff the sides of the pistons, etc... We went through a few iterations to get to the rod you see here. Adds length, keeps the wrist pin oiled, is super light and will allow the entire rotating assembly to rev to 9000rpm!
Next up the 2 ring 105.4mm custom Mahle Pistons...
So 105.4mm pistons from Mahle are already available and have been used by others in their builds, why go through the trouble to make different custom pistons, also from Mahle...
Well first off, let's back up. The 105.4mm piston commonly available is for a stock stroke 996/997 GT3 engine looking to go to 4.0L without changing the crank or rods. This is all fine and they work great. PJS has a 4.0L engine built by Dundon Motorsports built exactly this way and it's been awesome, so nothing wrong with them at all!
If you're looking to get the most out of a stroked big bore engine then you need to get the pistons as light as possible, reduce as much friction as possible and make the rods as long as possible (covered under the Ti Rods discussion above).
The pistons below have been reworked to be a 2 ring piston similar to what's in the 991GT3. In every engine, the amount of energy released when the combustion even happens is not fully transferred to the wheels. There are losses for pumping the air into and out of the engine, that is one reason intake and exhaust modifications can free up power to the wheels. Another area that power is lost is to friction, this is why OEM's have been going to thinner and thinner oils, if less power is lost to friction, they can make an engine that produces less over all power but has the same power to the wheels and consumer less fuel, but I digress. So if we're looking for an engine to produce more power one way to do it is to reduce frictional losses and going to a race style piston with 2 rings is a significant frictional savings..
The wrist pin is another area not to be ignored. It is thicker and of smaller diameter so we can push the rod closer to the crown of the piston, this allows us to get the rod as long as possible improving the rod/stroke ratio and minimizing the side loads on the cylinder.
Dundon 42R Mahle Pistons and Wrist pins
Dundon 42R Piston Top
There are the typical gas expansion ports by the rings and bridging and gusseting from the crown to the wrist pin.
I'm sure I've forgotten to include some stuff, but this is the general gist of it.
Engine is being assembled as we speak, won't be long now until it's done and we break her in to get started on tuning!
Dundon Motorsports 997.1 GT3RS 42R Build
