Short stroke options

[saintdave]

Hi all, i need some advice on what piston and cylinder sets are suitable and available for building a short stroke motor on a 2.4S case retaining the small spigots. Also which rods to use with a 66mm crank, S or normal, 2.0 or 2.2.

Thanks Dave.

[304065]

What sanctioning body? That is the absolute threshold question as winning race engines are built to the limit of the rule book.

2,4S would be a 5R case, you may want to consider a 7R as they are stronger. Figure about $1000 of machine work to any magnesium case to make it suitable for a short stroke. Personally I would much rather use a later 7R than cut the spigots on a magnesium 2,4S (diminishes its value and not as strong)

What P&C? Well. . .

Mahle Nikasil (RS type, low compression)
LN Engineering "Nickies"
JB Racing Aluminum Fin with Iron Sleeve
QSC Chinese-Made
Porsche Biral 84mm with 1mm Overbore for 85mm.

Pistons- either stock Mahle or JE, compression either 9,5 single plug or 10,3 dual plug.

Rods- I would use 2,2 rods, they are stronger than 2,0, and the nitrided S rods are just expensive collectors items. Of course these rods are heavier. ARP rod bolts are a must.

If you want custom rods like Pauter, you may also want to consider having the crank reground to take Chevy bearings- this entails regrinding the crank journals to 2" size for reduced resistance (down from about 2.25" stock) AND you get to use NASCAR bearings which are a hell of a lot better than the OEM bearings for the factory crank. This is BIG $$$.

Anyway, assuming this is a street engine here is what I would do-

Late 7R case with oil bypass mod, align bore, case-savers, cut for big oil pump
964 Oil Pump
89mm Mahle Nikasil Cylinders (will make 2,463CC with crank below)
9,5 JE Pistons
2,2 normal Rods with ARP bolts
66mm crank
2,7 Heads
PMO 40 Carbs

If you have an extra three thousand or so in the budget then I would twin plug and go 10,3 to 1. If your budget is higher, port the heads.

Camshaft selection is a VERY involved topic-- if you use the lower compression Mahle RS pistons and cylinders, you want 911T cams, these close the intake port earlier for a higher dynamic compression ratio that makes up for the loss of compression. If you have higher static compression, then I would consider a modern grind like a Supercup lobe on 102 centers-- this is what I am using in my restored '66-- Solex duration with slightly higher lift and a modern lobe profile for quicker opening, 102 lobe center like the late 911E cam. Others have their favorite cams, this is a long topic.

Good luck!

[MarkS]

Check the # on the case, a 72-73 2.4S will be a 7R with small spigots.

[BBausser]

I think John C gave good advice. I just went through a similar thought process with my 67N. Only thing I would add is that while your in there and spending the money you should go twin plugs. A crank fire system and head work should come in a tad below $3K. I went so far as to source and buy a new Mahle 90MM P&C's and some other parts, that I will be selling when I get organzed, before I decided to go with a 3.6 conversion. Good luck with your project.

Bill

[jhora]

Bill..let me know when your ready to sell the Euro 90mm P&C...Thanks

[kenikh]

FYI, the biggest pistons you can get in the small spigot bore case is an 87.5mm piston, using Nickies only as the cylinders.

[304065]

FYI, the biggest pistons you can get in the small spigot bore case is an 87.5mm piston, using Nickies only as the cylinders.

^without cutting the case

[saintdave]

Thanks for all the advice, the case is a 3R very early 2.4S.

[Flieger]

Why are NASCAR bearings better than Porsche factory stuff? Materials? Design?

The under-grinding must be done carefully to keep the fillet radius between the bearing journals and the crank throw, otherwise stress risers and metal fatigue failures can occur.

[Neil '67 911S]

"Rods- I would use 2,2 rods, they are stronger than 2,0, and the nitrided S rods are just expensive collectors items. Of course these rods are heavier. ARP rod bolts are a must."

John, why would you not want to use nitrided rods in a performance engine?

thanks!