2.8 vs 2.6ss
 

Ok, I need some opinions here... I've been collecting for a 2.8 for the last several years. Parts list includes: nos mahle 92s mated to 92 ln engineering nickies, GT3 oil pump, early Al 68 case with 993tt squirters, crower ti rods, cross drilled 2.4 crank, 906 center oil cams w/early 4bearing trays, heads are TP with soon to be Cnc porting, ect.

The goal was to lighten the rotating mass as much as possible to shift the toxic harmonic up.

However, given the reasonable cost of undergrinding a 66 mm crank from 57 to the 53 mm for the 'J2.4' rods (around $600), I'm wondering what is the better option?

Just for detail, I realize there is 2.2 mm 'unaccounted for'. Plan would be to either face 2 mm from the case or face the tops of the cylinders then use a set of the Supertec (0.080") spacers between the camtray and head. Chain length now normal.

Benifits would be improved rod ratio from the 70.4 stroke, no rpm 'artifical' rpm limit, enlarged fillet radius.

Disadvanges would be reduced displacement and a bit more cost (in the noise at this point!).

Finally, given the huge mass difference in the crower rod vs stock (340g vs 720g) and the change in mass from undergrinding the rod journal what about using an uncounterweighted T crank instead? That is a lot of mass removed: about 8.3 cc or 60g per throw (assuming I did my math right). Anybody know the big end mass for a 2.4 rod? The ti rods are 245g...

Thoughts anyone (or what did I miss)?

Thanks a bunch...

You know my opinions, but for the record, I think taking a CW 66mm crank and offset underrgrinding it to a custom stroke of 68mm would be the perfect ticket. You then have lots of meat for knife edging the crank to weight match your Ti rods and pistons for 8K+ RPM potential (this operation is not typically advisable for motors with steel rods), you get a harmonic range shift from the 2.4mm shorter stroke, displacement becomes 2.7 and the rod length and compression difference is easier to recover from flycutting the heads and chain length is easier to recover with thinner cambox spacers. To top it of you get bragging rights for having a truly unique configuration: an RSR spec 2.7SS would be something to behold!

Hi Tadd,

I would personally do one of two things;

Continue to build the 2.8 as is with the parts you have. Nothing wrong with that build. The Ti rods may reduce the harmonics at the upper rpms. The problem with the 2.4-2.7 crank IS the width of the rod. The cross-section of the throws had to be thinner to accomodate.... as a result you get a flexy crank. Choose a cam that will be finished by 7500 rpm and you are set.


Sell the parts specific to the long stroke and focus on building the engine with the proper length rods, proper compression, etc.

Trying to fudge parts from one into another is an immediate compromise and you may be dissappointed in the result.

The problem with the 2.4-2.7 crank IS the width of the rod. The cross-section of the throws had to be thinner to accomodate.... as a result you get a flexy crank.

This intrigued me as I hadn't ever heard this thought. All I've come across is that the journals have less overlap with the mains, the pesky 'Vend' diagram. So with my trusty digital calipers I measured two cranks webs. A 2.4 counterweighted and a 2.0 non counterweighted from a 914-6. Average for the 2.0 was 7mm thickness and 6.0 for the 2.4. The forged 2.4 was much tighter with 5.90 the thinnest and 6.2 the thickest. The 2.0 was 8.05 vs 6.55.

The fix for the design issue was the 3.0 nine bolt crank. Three changes were made. The rod width was reduced 2mm to help this area out. Journal dia increased 1mm as well. The 9 bolt flange was created to solve any future problems with flywheels falling off. However, the real fix was the reduction in rod width.

The same frequency problem happened again with the 964. The engineers again reduced the width again to fix the problem when the 993 was released. This problem IMHO was due to increased piston weight and stroke increase.



Running a lighter reciprocating mass will definitely help the frequency issue with the 2.4-2.7 crank. Possibly eliminating it for your RPM application.


A nice mod to do on a 66mm crank is to grind it for the SC dimension and run a 130mm length rod from the GT-3 or an aftermarket with similar dimensions. Try to avoid going smaller than the 3.0 liter as the oil passages going to the throw can get close to the surface. Altering stroke as Kenik has stated is also a viable alternative for the 66mm stroke vs the 2.4 rod. Oil passages would still need to be checked to verify that the journal diameter and the stroke increase does not cause a problem. I personally would run a 3 liter type of rod and bearing instead of the 2.4 to get the most strength out of your crank.

Porsche currently runs the SC journal dimensions on the GT-3 so it looks like they seen to like it.

Burn-Bro:
Thanks, that was a nice piece of history. Lots of little things now make good sense.
T