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Old 10-20-2004, 06:32 PM
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hacker-pschorr
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Default 928 engine combo question

I did search the archives - didn't find this.

What would the compression ratio be with a 928S4 lower end with a 16V US valvetrain? I understand this has been done before with the Euro heads, could not find specs on this combo with the US heads.

What has the longer stroke - 4.5 & 4.7 crank or 5.0 crank?

Any difference in rods between the 4.5 - 4.7 - 5.0?
Old 10-20-2004, 08:37 PM
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Jim bailey - 928 International
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Rumor has it the S-4 piston with 2 valve head makes around 7.5 to one and the engine will barely run (guy in the U S ). The 85-86 5 liter pistons get you around 10 -1 with 2 valve heads. Using 85-86 pistons and 87> heads builds an engine that will not run on pump gas too much compression (guy in Australia). Stroke is the same 1978-1991 all 78.9 mm the bores are 95mm,97mm,100 mm . The GTS porsche stroked it but used the same 100 mm bore (with short pistons) 85.9 mm stroke.
Old 10-20-2004, 08:42 PM
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did the euro 2 valve 5 liter with 85 pistons and it ended up right at about 10:1. ran pump gas, put down 293 hp at the wheels, and was very dependable.
mk
Old 10-22-2004, 09:31 AM
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Jim,
Thanks as always - so all of the connecting rods are the same then? Reason why I ask is I know a few people up here that have aquired various parts they are no longer using. I'm trying to see what I'd be able to use for my next motor. Do you know how the 944 Turbo pistons compair to S4 and S3 pistons?

Do the US heads cause a lower compression ratio? If so the US heads on a 5.0 lower end would be a touch lower than the 10:1 Mark is claiming on his motor.

Mark,
I'm 99% sure the euro head/cam combo will not happen this winter. I figure a 4.7 or 5.0 with US heads and 8+ psi will gain more than euro heads/cams on a N/A motor.

I need to find those threads where a few people claimed 11:1 or more compression with 16V motors with the Euro head / 5.0 combo.
Old 10-22-2004, 12:59 PM
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mark kibort
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your not going to get 11:1 unless you shave the heads dramtically, or dont notch the pistons . (ha ha)
the 2 valve euro on the 5 liter bottomend yields 10;1. if you go US heads, it will be more like 9:1. (euro combustion chamber is smaller)

so, what about the 6.5 liter guys running 968 pistons? are they shorter and then we could use just special rods on a stroker or GTS crank?
mk
Old 10-22-2004, 01:32 PM
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John..
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Hacker, just build what you need. Bore it, plate it and install a good set of pistons and set your CR where you want it. I'm building a NA race 944 motor this winter and that is what we will be doing to it.
Old 10-22-2004, 01:54 PM
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Mark - thanks for the info. I wanted an idea of what combo's do what before I dive into figuring out what I'm going to do. I've got access to a couple of S4 motors, set of 951 pistons, few different blocks and short blocks etc..... So if I can bolt my US heads onto an 85-86 block and get around 9:1 - that will be perfect. A friend of mine has a set of 85-86 pistons he doesn't need, another friend has a couple of S4 blocks he doesn't need. Since my crank and connection rods are the same - I have my motor!

John.. - for this car I'm going to make do with what I have in front of me. No need to reinvent the wheel since whatever I do this round will change by the end of next year.



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