Looking for what limits RPM

[andrew15]

I'm wondering what parts are the limiting factor for high RPMs and what they should be replaced with. Obviously for the Porsche flat 6, but in particular 2.0 - 2.7 engines.

ie 7400 RPM - Rod bolts need upgrading, 7600 RPM - need stiffer valve springs, 7800 - Upgraded rods, etc

There is a ton of info floating around - I'm just trying to put together a sheet that would list recommended upgrades for progressively higher RPMs

Thanks,
Andrew M

[jluetjen]

Speed is a function of money -- how fast do you want to go?

(What's your target?)

BTW - Valve springs will also depend on the cam which will define or be defined by how fast you want to spin your engine. The bottom end will also be heavily influenced by the crank stroke that you're using. It's easier to keep a bottom end with a 66 mm crank alive at high RPMs then a 70.4 mm crank.

[jgparker]

Along the same lines, why is the red-line on a carrera RS 7,300, but a 2.7 CIS is 6,400 even though they are basically the same long block? Same springs, valves, crank, rods, etc... Doesn't add up.

JP

[Wayne 962]

Somethings that need to be beefed up with the goal of reaching higher RPM:

- Valve springs ( to prevent valve float) and retainers
- Lighter valves
- Stronger rod bolts
- Stronger and lighter pistons
- Stronger rods

Mostly stuff that doesn't rotate, but moves in a sine wave with respect to the crankshart.

-Wayne

[Steve@Rennsport]

A few last, but VERY important considerations,..............

911 cranks and rods are oiled from the ends of the crankshaft and the oil gets centrifuged out at high RPM,.....7800-up RPM. The case, crank, and main bearings must be modified to live "in harmony" when operating at those engine speeds.

A few other things;

1) Rod loadings go up exponentially with RPM and total piston weight.

2) Valve train weight is a huge limitation on maximum RPM. That's why we use non-adjustable rocker arms, special valve springs, and titanium retainers and valves,...

3) The heads and intake must flow sufficient air to make power and the exhaust must be configured for the camshaft profile and RPM range.

Not as simple as it might appear,....Making an engine live at 10K RPM when a customer misses a shift is my main job,...

[snowman]

One of the FUNDAMENTAL reasons rpms cannot be increased above a certain limit is often overlooked or not even considered. There is a limit an absolute limit of how fast one thing can slide past another and still maintain an oil film to seperate them. The rod bearings, the crank bearings and the rings and cylinders all have to observe this limit. No ammount of super bolts, ultra light pistons, no weight valves can overcome this limit.

So with a given engine design you are limited. The limit will vary between 7.500 and 12,000 rpm for most high performance engines, depending on the exact stroke, size of the crank and rod journals, max piston velocity. A more likely limit for most common engines is say 8500 prm.

Ever wonder why there are 10,12, even 16 cylinder engines? Well if the displacement is limited (more cubic inches ALWAYS wins) The ONLY way to get more power out of a given limit of displacement is MORE rpms. To get more rpms and observe the limit of maximum piston velocity, before the oil gives out, requires shorter strokes. There is also limits on max piston weight, and or diameter, so the solution is to go to more cylinders with short strokes, and small pistons, ie more cylinders to get the same displacement. Just find the mininum number of cylinders required to observe all the known limits and wallah, a 12 or 16 cylinder engine is born. thats the main reason a Ferrari can be buzzed up to 12,000 rpm and a chevy cannot.