333pg333

My .02c. You don't need to knife the crank. You can improve the oiling situation without cutting too much away from the crank. A good oil (not too thin...not M1 imho) and improved oil cooling is more important.

Do plenty of research and look for poster michaelmount123 who is a very experienced engine builder / racer who has specialised in 16v n/a motors. Perhaps figure out your budget and ask his opinion on how that effects your rpm range....seriously. They're pretty much directly related to each other.

I have a 2.5 turbo motor that we can rev to 8000rpm+. Sure, it's designed for it but it still runs a stock crank that hasn't been knifed. It has been lightened marginally but within what was considered safe limits. The downside to a knifed crank in my experience is increasing the possibility of ugly harmonics through excessive crank whip.

Good luck!

924srr27l

Oh, I'll be trying out a Lindsey stage 3 Knifed Crank in about 15 Weeks time in my 2.7 Litre Motor (3 Litre block / 2.5 Head)

I've had the crank / Fly and pulley balanced......when you say there can be
"Ugly Harmonics and Whip" what and why is this?

Roger.

924srr27l

Are 968 Rods longer than 2.5 ones? or is it just the S2 / 968 Piston' have a shorter skirt?

On the right a Stock 951 100mm Piston and a Wossner 105mm for my 3.0L block being used with a 951 2.5 Crank and a capacity 2707cc





Roger

V2Rocket

Rods for all 944 variants are 150mm long.
The 3.0 pistons had the piston pin raised to accommodate the 88mm stroke.

SloMo228

What about a longer rod without increasing stroke? From my (very rudimentary) understanding of the topic, increasing rod length will reduce the amplitude of the second order vibrations that the balance shafts are designed to cancel... Perhaps making removal of the balance shafts less of an issue.

Edit for clarity: second-order vibrations are affected by the rod/stroke ratio in inline 4-cylinder engines (maybe others, but different engine configurations have different inherent balancing issues). The higher the ratio, the lesser the amplitude of the vibrations. So, increasing the rod length without changing stroke should make the second-order vibrations (which is what the balance shafts are there to counteract) less severe.

V2Rocket

It'd have to be custom, there aren't any rods that are ~154.5mm long with a 24mm piston pin and 52mm rod bearing...I've been looking for a long time

Carillo will make anything you want for $1500/set of 4.

333pg333

I'm sure there are plenty of people out there running knifed cranks that haven't had issues. I believe my crank was actually sent to LR for lightening. Was meant to be medium but came back a full knife. We ran that for a while in a 3.1 turbo motor. Eventually that motor had a catastrophic failure which we put down to harmonics shaking it apart. Cranks shift off their axis (crank whip) and cause worse things to happen. We put this down in part to the knifed crank. No actual proof but enough smart / experienced people took a look at it and that was the general consensus.

V2Rocket

If you do want a knifed crank, there is a guy in San Diego craigslist selling one for $350.
It's been on there for a while so he's probably desperate to move it.

Arominus

I'd look to shed weight in the flywheel, rods and pistons before getting into the crank.

Get your hands on the Isihara Johnson crank scraper for your windage concerns and keep running the oil pan that came with your S. If its like my S was, the inside is fairly polished/finished (unlike the 968 pan which is as cast), this along with the factory plastic insert, scraper tabs cast in the pan and the IJ scraper should do a very effective job of keeping the oil off your crank.

Humboldtgrin

Question, if using a crankshaft (stock or lightened) with crankshaft scraper, should piston squirters be installed due to less oil splash up to the piston walls? I heard that's how Chevy 8v engines would have camshaft bearing failure if a crankshaft scraper was used, not sure it's true. It wouldn't have any affect on the camshaft bearings on the 944 engines but maybe a problem with less fresh oil on the Pistons walls. Was adding piston squirters Porsche's idea to help fresh oil to get to the Piston walls with the added crank scraper built into the 968 oil pan and baffle?

SloMo228

Just spitballing here, and most of these ideas on this thread would blow my budget anyway... but it's an interesting discussion at least.

I know the hybrid stroker idea is based on offset grinding the crank to increase stroke on the 2.5L crank (78.9mm stock, seems most go to a stroke in the mid-80s). Presumably, then, it's possible to grind the offset in the other direction, to decrease stroke? If you went with a 104mm bore and decreased stroke to 75mm or so, you're still displacing about the same as the stock 2.5L. Generally, a shorter stroke engine is able to rev higher than a longer stroke one, at least to my understanding.

On the other hand, I don't know if this would have any benefit at all in terms of improving the engine's ability to rev, since you still have oiling issues at high speeds.

V2Rocket

Unless you had a race class to compete in with a turbo multiplier factor, I wouldn't ever build a smaller-displacement 944 engine, especially not a 16v.

944 is so lacking in low-end power, and it's worse on the 16v, it needs all the displacement it can get.

SloMo228

It would be the same displacement, though. The shorter stroke would be offset by the larger bore.

Arominus

If your going for a pure HP via RPM motor, sure, but shorter stroke generally means less torque.

If your doing anything with offset grinding a crank, go bigger as the rpm limit is really 7kish without dry sumping and an external oil pump. Then you have to get into valvetrain.

Displacement is your friend here.