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Robertb said:Great write up! I wonder what the increase in engine displacement costs, over and above the standard full-fat rebuild?
Why did they choose 3.9 as opposed to the 'S' spec 3.8? (or indeed 4 as per GT3 4.0...)
Any increase in rev limit?
Steve997 said:Although I trust Hartech engineering if they are boring out the 3.8 this seems to go against common advice perhaps even Hartechs own on bore score prevention advice.
Saying that I'm sure Baz could explain the reasons why it would still be ok. They must engineer in other solutions/ betterments :thumb:
Hope I never need a rebuild but if I do it would be a desirable option.
What are the bhp and torque figures?
alex yates said:Only the top of the deck is solid, ie. The liners are like a top hat and provide more support at the top where all the 'explosion' happens. Lower down is still cooled 'open deck' style. Suppose its a trade off against strength and cooling.
bazhart said:It was good to spend some time with someone that knows their stuff and get out for a drive!
Thanks various contributors for answering most of the questions for me.
The standard engine suffers because they made the engine bigger with a new type of cylinder bore surface that turned out to be less reliable than other previous solutions and then combined it with a softer piston coating - which made things worse. The more powerful the engine and the more that performance is used - the shorter they last.
We machine out the whole cylinder and replace it with a new one from Aerospace Alloy with a Nikasil finish (recognised as the best on the market and as used in GT3, turbo and all air cooled 911's for decades).
The standard cylinders are only connected to the outer casting in the centre (like holding a tube in your hand half way down) leaving the top and bottom free to move and distort.
True closed deck supports top middle and bottom and we do this with our replacement cylinders and yes all in machined interfaces.
The basic block is the same sizes it was when the first 2.5 Boxster engine was built so as the bores increased so the coolant space diminished. By using all alloy new cylinders with ribbed externals we have both increased cooling surface area and heat transfer - so no problems with cooling and we have been making and supplying these for 15 years now.
The cars run very well prior to re-map but we have re-mapped them for oversized engines to be on the safe side.
The bottom ends are reliable enough to take 3.9 cc and we do this (as already stated) with 100mm diameter pistons.
Because the time available to "suck in" air reduces in a straight line as the revs increase most engine designs have a kind of breathing limit at peak revs and create max torque lower down where their is more time to fill the cylinders.
An oversize engine exploits this benefit creating much better torque in the lower and mid range and better throttle response but at the top end max revs it is only a little more powerful because of the breathing limit. This makes a fast reliable engine.
I am preparing a long (and if I say it's long watch out) report on why torque accelerates engines and why a broader power-band (that you get with an oversized engine) will out perform a standard capacity one with typical tuning enhancements like porting, exhausts, re-maps etc,
Probably a month or so away yet but will answer all such questions and explain a few other issues.
We do not provide bhp figures as they are too easily adjusted by others but will eventually provide comparative graphs std to oversize. I prefer comparative road tests and the press will inevitably gets there hands on some to try.
There is a lot of new production to get in stock to handle the anticipated demand so although we are building some at the moment I think there will then be a delay before we can run the 2nd batch (while we work on standard rebuilds that are booming).
Our main effort has been in quality and reliability (as the performance just sorts itself out).
Thanks for coming over Alex - the 4 litre Gen 2 will perhaps be the next test drive!
Baz