04-22-2012, 03:05 PM
as pretty much everybody knows, i have been working on trying to create the ultimate 968 for about 9 years now. i have gone through countless iterations of the car, and have found a number of things that work and a number that don't. the 968 is a great platform for a wonderful GT car. after bracing it up, it has just the right amount of flex to absorb the road anomalies, but is stiff enough to handle the twisties. the suspension has enough flexibility to allow installation of enough aftermarket components to really take advantage of the balance of the car and the nature of the unibody design. power was another story though.
i went through every kind of bolt on mod out there, and some that weren't. none of them satisfied me, and all of them meant compromise. i am not the compromising sort. so, i came up with the centrifugal supercharger kit. this provided me with the torque i was looking for in the midrange, and without compromising drivability, emissions compliance, or any other complications.
in developing the kit, i had to push the envelope to find the boundaries of what the 968 engine could take. i found them all right. i bent 3 rods in the process.
so, it came time to go through the engine and build it up. not wanting to revisit the rod bending nightmare, and knowing i might ultimately want to push the envelope again, i decided to really beef things up. i skipped right over the 2R porsche rods, as the place that my 1R rods bent was not at the area where they reinforced the rod. instead i went to carrillo rods. these are a bit lighter than the OEM rod, which should make for a bit snappier revving, but no so light as to cause a problem in balance, as the crankshaft is an opposed crank, putting the rods opposite each other in the stroke. were this an offset crank, it would have been a problem.
because my bent rods had caused the pistons to move off axis, they caused the pistons to scrape the walls, and wore them enough to require a rebore. this created a very real complication, as our block is alusil. this limits what you can do in the way of pistons, rings, et all. i chose not to sleeve it, as was my initial thought, due to the known failures of both wet and dry sleeving. dry sleeving doesn't work well, as the sleeves tend to move, and knowing that i was boosting, this was not a good idea. wet sleeving meant a lot of block flex, as the 968 is an open deck design. that left me only with over-bore, and there is now only 1 size of OEM piston out there for that. so, i am going from 104 to 104.5. that should make for a bit more torque (darn) and a bit snappier throttle response (darn again). it's a known commodity, so there should be no complications there.
i had to rework the head again, as the last build did not change all of the valve guides, and they were too loose for comfort knowing what we wanted the engine to do. also, only the intake seals were changed, so we wanted to do all of those anyway. between all of that, it meant yet another valve job. oh well. while we were in there we made a couple of other changes that should result in a lot better upper end operation.
both the head and the block needed to be "kissed" which will raise the compression a bit, as will the stiffer rods (the porsche rods collaspe a bit under load). the factory compression is NOT 11:1. that is something that porsche published that is misrepresentative. it seems they do that all the time. they show what the "static" ratio would be, but not what the actual ratio is. if you go through the trouble to do the math though, you end up with more like 10.5. at any rate, kissing the block and head should bump that up a bit, which for a centrifugal unit is ok and can even be a good thing. with a turbo or a roots blower, this would be a huge problem. with a centrifugal unit though, not so much. this should result in a bit more torque and a bit snappier throttle response. i'll probably have to do a bit of retuning, but no worries.
the rest of the motor we are leaving stock, but everything has been balanced and blueprinted, ported and polished. nearly every bolt has been changed. quite a few things have been "tweaked" for reliability. we are leaving nothing to chance. this should turn out to be as tight and solid as any 968 engine can be.
i'm really looking forward to this. i should end up with a very punchy and very reliable little motor. i expect it all to be together in 2-3 weeks, and following the break in, i should be romping around soon.
i went through every kind of bolt on mod out there, and some that weren't. none of them satisfied me, and all of them meant compromise. i am not the compromising sort. so, i came up with the centrifugal supercharger kit. this provided me with the torque i was looking for in the midrange, and without compromising drivability, emissions compliance, or any other complications.
in developing the kit, i had to push the envelope to find the boundaries of what the 968 engine could take. i found them all right. i bent 3 rods in the process.
so, it came time to go through the engine and build it up. not wanting to revisit the rod bending nightmare, and knowing i might ultimately want to push the envelope again, i decided to really beef things up. i skipped right over the 2R porsche rods, as the place that my 1R rods bent was not at the area where they reinforced the rod. instead i went to carrillo rods. these are a bit lighter than the OEM rod, which should make for a bit snappier revving, but no so light as to cause a problem in balance, as the crankshaft is an opposed crank, putting the rods opposite each other in the stroke. were this an offset crank, it would have been a problem.
because my bent rods had caused the pistons to move off axis, they caused the pistons to scrape the walls, and wore them enough to require a rebore. this created a very real complication, as our block is alusil. this limits what you can do in the way of pistons, rings, et all. i chose not to sleeve it, as was my initial thought, due to the known failures of both wet and dry sleeving. dry sleeving doesn't work well, as the sleeves tend to move, and knowing that i was boosting, this was not a good idea. wet sleeving meant a lot of block flex, as the 968 is an open deck design. that left me only with over-bore, and there is now only 1 size of OEM piston out there for that. so, i am going from 104 to 104.5. that should make for a bit more torque (darn) and a bit snappier throttle response (darn again). it's a known commodity, so there should be no complications there.
i had to rework the head again, as the last build did not change all of the valve guides, and they were too loose for comfort knowing what we wanted the engine to do. also, only the intake seals were changed, so we wanted to do all of those anyway. between all of that, it meant yet another valve job. oh well. while we were in there we made a couple of other changes that should result in a lot better upper end operation.
both the head and the block needed to be "kissed" which will raise the compression a bit, as will the stiffer rods (the porsche rods collaspe a bit under load). the factory compression is NOT 11:1. that is something that porsche published that is misrepresentative. it seems they do that all the time. they show what the "static" ratio would be, but not what the actual ratio is. if you go through the trouble to do the math though, you end up with more like 10.5. at any rate, kissing the block and head should bump that up a bit, which for a centrifugal unit is ok and can even be a good thing. with a turbo or a roots blower, this would be a huge problem. with a centrifugal unit though, not so much. this should result in a bit more torque and a bit snappier throttle response. i'll probably have to do a bit of retuning, but no worries.
the rest of the motor we are leaving stock, but everything has been balanced and blueprinted, ported and polished. nearly every bolt has been changed. quite a few things have been "tweaked" for reliability. we are leaving nothing to chance. this should turn out to be as tight and solid as any 968 engine can be.
i'm really looking forward to this. i should end up with a very punchy and very reliable little motor. i expect it all to be together in 2-3 weeks, and following the break in, i should be romping around soon.
94 Midnight Metallic Blue Cab Porsche 968 w/deviating cashmere/black interior and WAY too many mods to list - thanks to eric for creating www.968forums.com
"It isn't nearly as expensive to do it right as it is to do it wrong."
(This post was last modified: 04-22-2012, 03:06 PM by flash.)
