What brings RPM limits...

[RS-GT]

I'm reading and reading about the best motor configuration for my 72 narrow body ; what is needed to bring the rpm limits to 7500rpm or so ?? I know a good builder (!), but why RS 2.7 is lower then this and it's even worse on the SC 3.0?

I'm thinking about a 3.0 (built on SC base) Keep the Nikasil cylinders but with higher compression JE piston, twin plug and MFI ?!? I like the quick reving of the 2.2S but I need more low end power to drive around...

Thanks for your help !

[boba]

There are many here who can give you more technical detail but I will try to get you started.

There are a number of things you will need to turn high RPM.

The most important item is a lot of air.

This requires an induction setup and heads that can flow the volume of air to keep the motor going. When a motor starts to fall flat it is due to a lack of air.

You will also need a valve train that can handle the high revs, this means light weight, springs, retainers, keepers.

Agressive cams which are dialed in for high rpm (lots of air at the top).

You will need to run a higher compression ratio than is found with most stock motors. 9.5:1 would be the lowest I would think would make sense. Over 10.3:1 and I would recommend twin plug.

The bottom end of the motor needs to be able to also handle the high revs. This means lighten and balance the recipricating bits.

You will need a exhaust system that is balanced with the induction. You need to get the air out as well as get it in.

Getting it to turn 8 or 8.5 is one thing getting it to live is the second.

As you can see this is a system, no one mod will get you a high RPM motor. If you want it to live and make good power there is a lot to do. Many have specific thing they like to do and feel make their configuration better but you need to address all the elements.

I hope others will chime in with their knowledge.

[pu911rsr]

I don't think it is any big deal to get a 70.4 stroke motor to rev to 7500 RPM and survive. If you want it to make a lot of power that's where correctly configured motor makes all the difference. Any good Porsche race shop can build you a slightly detuned race motor, not cheap but hell it's only money.
Phil

[a911s]

Great suggestions from Bob.

An early S motor or 2.7RS motor will rev to 7500 no problem. Beyond that is where it starts to get tricky. To rev to 8500 and last you will want to do all the stuff Bob suggests, plus cross drill the crank and put in a really big oil pump.

[kenikh]

Other things to consider:

Mechanical limitations are a real bugger, especially on a 70.4mm crank. First, I have had more than a few expert builders tell me that the harmonics of a 70.4mm crank over 7400 RPM are troublesome, leading to flywheel shearing at the bolts for the early 6 bolt 70.4mm cranks in extended high RPM usage. This problem doen't exist on the later 9 bolt crank. So basically, you really don't want a 7500+ RPM 2.7. A 3 liter will be OK, but 7500 rpm really is the limit for the long crank.

If you want more, you'll need to build a motor on an early turbo case w/ a 66mm crank. You can get up to 3.1 liters on a 66mm crank with one of those, although you are most realistically limited to 2.8 liters by the air you can into max sized ports at 8000+ RPM. Just as fair warning, all things being equal, this may be the most expensive motor you can build.

[Steve@Rennsport]

Other things to consider:

Mechanical limitations are a real bugger, especially on a 70.4mm crank. First, I have had more than a few expert builders tell me that the harmonics of a 70.4mm crank over 7400 RPM are troublesome, leading to flywheel shearing at the bolts for the early 6 bolt 70.4mm cranks in extended high RPM usage. This problem doen't exist on the later 9 bolt crank. So basically, you really don't want a 7500+ RPM 2.7. A 3 liter will be OK, but 7500 rpm really is the limit for the long crank.

Generally speaking thats good advice,.....

For the record,.....the 70.4mm crank doesn't shed its flywheel until 8000 RPM. Keep it at 7900 and you'll never have a problem with that unless there are other issues. Further, when one uses a pull-type clutch, the flywheels stay attached north of 8K.

I would tell someone that running a mag-cased engine with the 70.4mm crank above 7500 greatly increases the chance for cracks in the bulkhead behind the #3 cylinder and thats a bigger issue for a race engine than the above,...

[304065]

Dr. Weiner, that is an interesting fact. I had to look up which was a pull-type clutch, the 901 cup style flywheel or the 911/915 flat style. (the 911/915 is the pull-type according to the web site of one expert who resides in the 503 area code). So presumably the flat flywheel has something to do with it.

[a911s]

Generally speaking thats good advice,.....

For the record,.....the 70.4mm crank doesn't shed its flywheel until 8000 RPM. Keep it at 7900 and you'll never have a problem with that unless there are other issues. Further, when one uses a pull-type clutch, the flywheels stay attached north of 8K.

As always, my personal experience (although far more limited) matches Steve's. We used all the mods Bob and Steve have suggested on my 2.8L race motor (built on heavily modified 70.4 crank and early aluminum case). Everything internally is as light and balanced as possible. The heads are a piece of art. Puck clutch is mated to 6 bolt '73 RSR flywheel. Depending on the cam, it will keep making max HP to 7500. I let it rev to 8500 rpm occasionally (national events), but usually set the rev limit at 7800 (programmable via Motec). Have had no problems so far.

Like Kenik says, this level of motor is very time consuming and expensive to build.

[Grady Clay]

Yes the flat flywheel has something to do with it. The issue is when you use the forged steel RSR flywheel and turn the engine 8300 and above. We went to using the cast steel (stock) flywheel, lightened close to RSR weight and never had another problem.

The problem was one of resonance. The different ‘natural frequency’ and dampening characteristics of the cast steel was enough to eliminate the problem. The type of clutch also influences the resonance.


Porsche solved the valve train limit with titanium retainers and forged, non-adjustable rocker arms (lash caps on the ends of the valves). With stock springs and installed height, there isn’t any ‘valve float’ at 9000 rpm.

The biggest limiting factor is the stroke and the mass of the piston and rod. The forged, slipper skirt racing pistons from Mahle are great. Porsche’s forged titanium rods reduce the mass further. The rods are exceedingly expensive and have a limited service life. (Porsche replaces them every 40 hours in a Cup car.)

Best,
Grady

[911pcars]

As I recall, the factory increased the radius of the fillet of the main/rod bearing journals of the RSR crank (I forget which one; maybe both). That should tell you something about what the factory thought about the longevity of the stock crank at high revs., aftermarket mods notwithstanding.

Sherwood