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993RS crankshaft in a 930/66 crankcase...?

Hi,
does anybody did that upgrade...?...is-it possible...2 mm longer.....?

The 993 RS crank is 76.4 mm stroke and the 930/66 crank is 74.4 mm... 2.6% shorter...

Thanks for helping....


I got a good opportunity on a 993RS crank stock, mint condition...
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Old 12-08-2011, 09:34 AM
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Pray tell why would you want to lenghten the crank?
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Lincoln Phillip
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Old 12-08-2011, 10:01 AM
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Linc,
Serge was referring to 2mm longer stroke that would give 3.4 vs 3.3 liter assuming stock bore.

Serge,
I have a 993 crank in my 930 engine but it was done by previous owner. I'm not sure about the dia of connecting rod since my engine has Carrillo rods which I don't have specs (sizing). You will have to use a different crank pulley (993 or aftermarket) since it's nose is much shorter than a 930 crank. When I bought my car it had a modified crank pulley which is a combination of a 930 and 993 pulleys (inner portion of 993 welded to outer portion of 930 pulley). I dished this modified pulley and bought Rothsports aluminum 993 crank pulley. I had to try different size (length) fan belts until one fits properly.
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Old 12-08-2011, 10:21 AM
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Thanks Mike for this very interesting answer...I knew about the crank pulley....but my doubts were about the risk for the valves because of the biggest stroke and also to have a too big volumetric ratio....

Here's the beast....


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Old 12-08-2011, 12:45 PM
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You're welcome.

I wish I knew the specs of the rods and pistons. I do not know whether the rods or pistons (location of pin relative to crown) were custom made for a longer stroke crank. If I did then I would have let you know!

Whatever you do...if you use that 993RS crank I would like to know the specs for future rebuilds.
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Old 12-08-2011, 01:20 PM
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Oh I see, well as far as the rod goes, I don't think you can use 930 rods past the 964 crank, for the 993 the rod journals are smaller.
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Last edited by SCHNELE; 12-08-2011 at 07:23 PM.. Reason: Info
Old 12-08-2011, 04:01 PM
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The 76.4 crank installed on the 3.0/3.2/3.3 case with 98mm cylinders (3.44 liters) is the biggest and best displacement for the early turbos. The longer stroke provides better torque for off boost performance and the 98mm cylinders can be Ni-resisted and are robust enough to withstand the higher cylinder pressures generated by high boost and big HP numbers. The 993 crank has a narrower rod bearing width and requires a custom rod because the stock rod is woefully inadequate. You will also need new pistons to go with that crank. This combination works well only if the revs are held to below 7500rpm. The crankshaft and rod bearing area is marginal for operation above that.
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Old 12-08-2011, 07:06 PM
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Thanks AWS.....very important infos....I checked on PAUTER specs and found the sizes...I just bought a set of rods for 930/66.....



Engine Series........................Pin dia .........B E bore........Length ...........B E width ...........Weight ................Part Number

911 3.2-3.6L (3.2-3.8L turbo) .905 ...........2.283 ........5.000............... .856 ...............575g ..................964-230-580-1270F

993, 996 turbo, 997 turbo ........905 ..........2.283 .........5.000 ................738 ...............545g ..................993-230-580-1270F
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Old 12-09-2011, 12:01 AM
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Yes, the 993 rods are narrower. Solution: new rods.
Then the crank "snout" -- where the pulley mounts -- is different. Solution: custom pulley or modify crankshaft (if you have someone qualified).
And now all your pistons are traveling up and down an extra millimeter each direction, so you need to watch your clearance to the cylinder heads and valves and to the crankcase webbing. Solutions: piston machining or new pistons, or head machining, or shim cylinders, or...

Adding stroke to the engine is not a simple bolt-in conversion.
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Old 12-09-2011, 02:08 PM
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Quote:
Originally Posted by aws View Post
The longer stroke provides better torque for off boost performance
Torque produced will be approximately the same for an oversquare, square or undersquare engine. Stroke will effect power (longer stroke = less engine speed=less power and vice versa) but not torque.
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Old 12-09-2011, 03:27 PM
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Power is a function of torque:
(Torque x rpm)/5252 = Horsepower
Can you elaborate how power is increased but not torque?
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Old 12-09-2011, 03:47 PM
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One of the reasons that the Porsche flat six engine took to turbocharging so well was the stoutness of the bottom end. Eight main bearings and relatively short, robust rods allowed for the tripling of the horsepower output without much penalty in reliability. The obsolete Hemispherical head design also benefited by the lower compression of a flat top piston. My opinion is the longer stroke (76.4) compliments the rev range of the larger displacement turbo engines and adds displacement without increasing the bore. The weak point in these engines is the cylinder to head connection and the larger bore (100mm) cylinders just do not have enough material to insure a bulletproof seal. The largest bore size the factory raced in the 935 days was 97mm---for a good reason.

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Old 12-09-2011, 07:17 PM
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Quote:
Originally Posted by TurboKraft View Post
Power is a function of torque:
(Torque x rpm)/5252 = Horsepower
Can you elaborate how power is increased but not torque?
Sure.
Look at the equation that you posted. The answer is there.
Torque and rpm are measurable physical quantities. Horsepower is a calculated quantity. If you look closely at that equation, the number 5252 is not any real physical quantity but a constant.
Say we have two engines that produce the same torque but can run a different maximum speeds. According to the arithmetic, torque stays constant but increasing the rpms will yield a bigger number for hp.
A real example would be a Porsche turbo six that produces 450 ft-lb of torque and has a max engine speed of 7000 rpm vs. a normally aspirated F1 engine which also produces 450 ft-lb of torque and has a max engine speed of 18,000 rpm. The Porsche engine produces calculated 599.77 hp while the F1 engine produces a calculated 1545.69 hp. In reality an F1 engine makes around 850 hp due to friction and other losses that aren't accounted for the simple power equation but the point is that if an engine spins faster, hp increases.

However, spinning an engine faster isn't a good real world solution so we are all doing things to our engines that increase torque. Torque is what you feel when you press on the accelerator. We are all conditioned to talk about hp but really what we are concerned about is torque.
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Last edited by 911nut; 12-11-2011 at 01:27 PM..
Old 12-11-2011, 04:21 AM
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Quote:
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The obsolete Hemispherical head design also benefited by the lower compression of a flat top piston.
The greatest weakness of the 911 engine combustion chamber is the offset placement of the spark plug. It's not as big a liability in the turbo engine as it is in a n/a high compression 911 engine due to the flat top piston and the superior atomization of the turbulant mixture that a turbo provides.

Quote:
Originally Posted by aws View Post
My opinion is the longer stroke (76.4) compliments the rev range of the larger displacement turbo engines and adds displacement without increasing the bore. The weak point in these engines is the cylinder to head connection and the larger bore (100mm) cylinders just do not have enough material to insure a bulletproof seal. The largest bore size the factory raced in the 935 days was 97mm---for a good reason.
When OEMs design engines the bore and stroke is primarily set by the area that the drivetrain has to fit into.
Increasing stroke puts additional stress on the crankshaft and increases parasitic friction. The 935 engine had to be based on the stock 911 engine per regulations, so the factory was stuck with the stock case and the 97 mm bore.
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Old 12-11-2011, 04:37 AM
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Quote:
Originally Posted by 911nut View Post
Sure.
Look at the equation that you posted. The answer is there.
RPM. I understand where you're coming from.

Quote:
Originally Posted by 911nut View Post
Torque is what you feel when you press on the accelerator. We are all conditioned to talk about hp but really what we are concerned about is torque.
AMEN! A notion much too often overlooked.
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Old 12-11-2011, 07:29 PM
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Maybe a useful before-and--after comparison:

Red line = 3.5L 930 (100mm bore) with EFI, usual bolt-ons (headers & muffler, intercooler, cams), mild head porting, small Garrett turbocharger. 0.9bar (13psi)
Blue = same engine with 76.4mm stroke crank added, pistons & heads modified to accommodate stroke. 1.0bar (actually 14psi).

The displacement and compression give it a healthy gain of 20-30ft.lb. up to ~3,300rpm. Rather different story above that when at full boost.

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Old 12-11-2011, 07:44 PM
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