Jerry Feather

Thanks Greg for that. However, I would expect that the 968 rods are the same length as either the 5L or the 5.4L engines. Which one it is I have to assume is the 5.4, since I think the 968 engine is a 3.0L. That tends to suggest that the deck height of the 968 piston is the same as the GTS pistons, and for that matter the 5L pistons. Am I right? If so, and the stroker cranks are made for the 5.85 chevy rods, why does it sound like there is so much complexity about the rods?

There may be a lot of choice in style of rods, but not much in terms of dimensions. Again, I am sure you know a lot more about this than I do; and I am eager to learn. Thanks. Jerry Feather

Vilhuer

Compression height in 968 pistons is different than S4 and GTS pistons, which are also different between each other. Using 968 pistons with GTS crank require 153.4mm rods IIRR. All 928 stock rods are 150mm. Also 944/968 rods are same 150mm except '89 944 2.7 which has shorter rods as it uses 3L long stroke crank with 100mm bore pistons. None of the factory rods are useful in stroker applications.

Vilhuer

I can be wrong about 2.7L 944 engine parts and it has 104mm bore. In any case there aren't rods which are useful in any "normal" stroker setup.

Cheburator

You are sceptical about what precisely? If you used the Rennlist tools and looked at previous posts on the subject you would have found the following:

Unknown block - yes, it is very much unknown because:

- the letter in the engine number does not tally up with the model year, moreover it has the marking NB at the front, which has been done by Porsche when reusing old blocks

- yet the numbers on the block/girdle around the oil cooler outlet indicate it is one of the last blocks cast in 1995

- all cylinders are sleeved with what appears to be alusil material, but no one actually knows the proportions of aluminium/silica. It is most definitely different to the one in the block since the liners are bluish grey in colour, yet the liners pass the screw driver test for alusil vs. nikasil/grey cast iron

- Kolbenschmidt pistons with no part number on them and 4 out of the 8 oil drain holes drilled by the factory. No other GTS engine ever has been opened and Kolbenschmidt pistons found inside. Mahle has been the usual supplier https://rennlist.com/forums/928-foru...-pictures.html

- it appears that the Kolbenschmidt pistons were not coated. Pics in the link above. The engine ran fine for 28,000 miles until I destroyed it through lack of oil in the tank

The engine was supplied by Porsche as a warranty engine on a crate in 2003 to its previous owner. I have all the paper work related to the claim and even the warranty receipt from Porsche with the bodged engine number. The engine was installed by Lancaster Porsche into a blue 5spd GTS, which went on to appear on a Jeremy Clarkson DVD. The clip can be found on YouTube. Then the car burned completely in a fire. Since then the engine stood under a tarp in a garage. Thus there has been no chance of a back street garage mechanic slipping in a few liners and dodgy pistons while we were looking the other way.

Does that clear the confusion enough for you?

P.S. Greg rightly says that I have limited mechanical knowledge, but I am learing. One of the things that I have learned here is that a GTS block is not a good candidate for boring to 104mm... Neither is an S4 block

Jerry Feather

Thanks. That certainly is a lot of history for your engine. Actually I was not confused, but rather just skeptical. When you said to "remember" something about this block that appeared to me that you were referring to remembering some general information about GTS blocks being sleeved, not remembering something you might have posted before. It did not occur to me to search for something else you might have posted before that I should be remembering.

I think the limitations on boring the S4 and later blocks has to do mostly with the risk of core shift that might have allowed the cylinders to have been cast off-center slightly--not enough to make 100 mm bores bad, but perhaps not good for 104mm which would leave a thin cylinder wall on one side. I am sure that when boring an S4 block one should first verify that the 100mm bores are very well centered in the cylinder casting.

I think there are plenty of S4 and later blocks making the foundation for 6.5L strokers with 104mm bores to bear this out. If not, I wonder what kind of block is otherwise commonly used. If I am right about the risk of core shift, I wonder why that would not be a problem with the earlier blocks? Then, I have to wonder now if there is something else about the S4 and later blocks that might make them a poor candidate for a stroker with 104 bores. What do you think is the limitation? Too, if as S4 block is not a good candidate, what is a good candidate block for a 6.5L stroker?

In the end, in regard to your original concern, my vote would be to bore a substitute block, of whatever heritage it might need to be, out to 104 and put in a good used set of 104mm pistons for the 968. I can't figure out why that would not be the most cost effective solution for you. Actually I suppose that an S4 block with your GTS innards and perhaps a good used set of GTS pistons might even be less expensive, unless you are going to have to have the used block bored anyway. As soon as you have to bore it, going to 104 makes the most sense to me.

I think this suggestion has already been made, so if you thiunk I am being redundant, please disregard.

Jerry Feather

Jerry Feather

Vilhuer

Core shift is not only problem in late MY blocks. Most of them simply have too little material in cylinder towers. I haven't found single block which has over 117mm tower diameter. (116 - 104) / 2 = 6mm. Minimum wall thickness which is ok is said to be 6.35mm. This means practically all '87+ blocks are useless in 968 piston stroker build. '85 MY 32V is said to be better but I haven't found these mythical 118mm blocks anywhere. Finding really good used 968 pistons isn't easy either. Easiest way is to buy complete engines and sell extra parts.

Jerry Feather

P.S. Please disregard the redundant signature, also. JF

Jerry Feather

Thanks. I wonder what the diameter is of the cylinders cast into the 968 engine? If the later blocks are 116mm and the 118mm in earlier blocks is mythical, I am still wondering what block is commonly used for the 6.5 stroker?

Actually, I seem to recall something (in a thread here?) some time ago where someone had taken the cylinders completely out of the block and then worked the block over to replace the cylinders with something new that simply slipped into place. Anyone remember that?

Jerry Feather

Darklands

That was M. Simard with his project car.

Jim Devine

Any chance of using custom sleeves from Darton?
Then you could use custom pistons/ rings from JE
or other sources.

http://www.darton-international.com/tech_ctr.htm


might be worth a call.

Vilhuer

120+ and towers are connected to each other. Also water jacket is about inch lower than in all 928's. All these together mean 3L block is much better suited for 104mm than any 928 block.

Darton style wet sleeves have been used. Just don't ask what Greg thinks about their survaibility in 928 block or more precisely 928 blocks survaibility when they are installed. Block will crack around main bearing area or something similar. Darton do not do ready to fit wet sleeves to 928 but they can do them as custom order. 944 setup will not work as 928 deck height is taller than 944/968.

Jerry Feather

Hi Erkka. You certainly have a lot of information about these engines, so I am going to try to hope that I can learn a lot from you and put it to good use. When I ask questions about something you are telling us, I hope you do not take it as argumentative, but rather questions from what I hope is a good student.

The question I seem to have now is where did the minimum wall thickness said to be 6.35mm come from? That really sounds scientific and perhaps well engineered to be so precise. However, when that is converted to inches it comes out to just about exactly a quarter of an inch. That sounds more to me like someone, perhaps who even is very knowledgable, simply said " you need the keep the wall thickness about a quarter of an inch, then it got converted and became a sort of gospel at precisely 6.35mm.. I have to wonder what the data is that might actually support that limit.

And that leads me back to an original question about which of the 928 blocks is actually used for the 6.5 stroker conversions, if not the S4 and later ones? If actually the S4 blocks, what is the failure history based on wall thickness? Any ideas on these questions? Thanks, Professor.

Jerry Feather

Jerry Feather

Hi again Erkka. When you get up again, here is another one.

I can't tell from the above information what I will be faced with, or what the OP might be faced with if it were to be feasible to put the 968 pistons in a bored out S4 block.

For me, the question is, what is wrong with using stock 968 pistons in a bored out 928 block (any one that is appropriate) with a stroker crank that is designed to use the chevy rods, and using the 5.85 CTC rods?

For him the question is which rods would he use with the GTS crank and bored out S4 block to 104mm and 968 pistons?

In other words, for the OP, how does the compression height of the GTS pistons compare with the 968 pistons?

Thanks again. Jerry Feather

mark kibort

as far as thickness, we bored out a 97mm 4.7 block and got 100mm and the walls were 5mm thick when done. I would think that is the limit, because it is working in scots motor.

Rob Edwards

Just an n of 1 here, but my stroker started as a '91 GT block and ended up at 104 mm. I don't recall discussing or measuring cylinder wall thickness at any point. So far so good, AFAIK.

Maybe it's the Nikasil holding it all together?