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 !
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.
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
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.
Randy Wells
Automotive Writer/Photographer/Filmmaker
www.randywells.com/blog
www.hotrodfilms.com
Early S Registry #187
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.
Kenik
- 1969 911S
- 1965/66 911
- S Reg #760
- RGruppe #389
Generally speaking thats good advice,.....Originally Posted by kenikh
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,...
Steve Weiner
Rennsport Systems
Portland Oregon
503.244.0990
E-mail: porsche@rennsportsystems.com
http://www.rennsportsystems.com
I am not an engine guru and do not understand all the implications of what I read, but I recall reading the the cast iron rockers and the forged rockers had different ratios. I am uncertain if it was the rsr racing rocker or the early forged production rockers that were being discussed. What exactly is the ratio? was it the racing or production rocker and how does it affect the engine preformance?
Tom
67S soft rear window
60 356 Cab
70 914-6 3.2 short stroke twin plug
05 Audi S4 Cabrio (commuter)
05 Audi Alroad (family driver)
Aprilia SR 50R (Sanibel scoot)
So I've found.
I have one of them....now to find eleven more.
it's built on 3.0 turbo case, with a 66mm crank
light rods and pistons.
Interesting set of problems and there must be a solution but cost could be prohibitive.
It would be possible to carry out a detailed torsional analysis of the crank, ords, pistons and flywheel to look at inrtia torques without too much expense but modelling gas torques would be a bit more difficult and would need some expensive software.
It would also be useful to run a motor on a dyno equipped with a torsional analyser and look at rhe vibration signature.
It may then be possible to design a suitable damper to help reduce the damaging vibrations. (I would guess that most of these are higher order).
Ti rods and light weight pistons would also probably help as they effectively increase crank stiffness by reducing forces.
If custom crankshafts were used, and I guess we maybe reaching the point where this is becomong viable, there are several improvements that could be considered.
Design tools have improved greatly in the last 30 years and I am sure that counterwight design could be optimised to help reduce vibration levels.
Material selection could also be enhanced and 'EN40B' Aero Release materials with full nitirding rather than Tenifer style heat treatment would give improved fatigue life and could be comined with generour fillet radii.
The clamping forces holding flywheels could be improved by increasing the number of bolts (Cosworth BDA engines now commonly use 12 bolt flywheels as opposed to early generation 6 bolt designs)
It really needs a detailed review of the design and the problem areas as a starting point.
I don't think cast steel flywheels have any significant damping, other than, perhaps due to differences in mass/inertia. Grey Cast Irons have higher damping capability than steel due to the presence of graphite flakes and this means that large castings that used to be used for machine tool bases tend transmit less vibration than fabricated components that are now relatively common but I don't think this type of material would affect torsional vibrations in an engine. I would be very interested in any data.
'Solid' 935 Rockers are available but are extremely expensive. They have been designed to work with 'Motorsport' Rocker Shafts that can still be ordered from Weissach.
We can also manufature suitable lash caps for these rockers.
That's the puppy. I have one, someday maybe I can find 11 more.
I found a 962 66mm stroke crank and mixed it with a 77 3.0 turbo case good forged rods and pistons. I'm convinced it's as solid as I can get it, but worry about the valve train
I've talked with a few 2.5 challenge guys that say they twist their motors to 8500/8700 with the early rockers and just change them more frequently?
Till I get it sorted out, I'll keep it dialed back a bit, but I sure would like to find some solution for the rocker worry
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