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daniel · Joined: 18 Jun 2009 Posts: 686 Location: Sydney, Australia

Hi All
I am in the process of rebuilding my race engine and have selected a cam that will result in peak power at ~ 7500rpm. I have already taken the following precautions:
- Pauter rods
- forged pistons
- lighter weight valves & appropriate springs.

I am concerned about the flywheel and lack of harmonic balancer on the front of the crank - has anyone had experience getting these engines to spin safely and reliably at this speed?

Cheers
_________________
Over the top of skyline, total brake failure.... hit the wall at over 200 kp/h at the dipper, so anyone who has to brake for the esses is a pussy.
1977.5 Race Car, CAMS Group S Spec
1989 944 Cabriolet

Cedric · Joined: 27 Aug 2004 Posts: 2805 Location: Sweden

924-76 · Joined: 05 Jun 2006 Posts: 1489 Location: Ontario, Canada

924RACR · Posted: Fri Mar 04, 2022 11:34 pm Post subject:

I'd definitely be concerned about the lifters, if you haven't replaced the stock buckets. I've had them split right through the adjuster holes at 8000rpm.

I wouldn't be concerned about the lack of a harmonic balancer - just get the rotating assembly/bottom end balanced by a good shop.
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

daniel · Joined: 18 Jun 2009 Posts: 686 Location: Sydney, Australia

Hi Vaughn
Yes bottom end has been balanced - will get it checked again before reassembly. The cam buckets I have are new from a group buy. They are the ones that take the adjusting screw. I was going to ask the shop doing my head to machine down some of the boss that the screw sits in and use shims however if you have a better suggestion??
_________________
Over the top of skyline, total brake failure.... hit the wall at over 200 kp/h at the dipper, so anyone who has to brake for the esses is a pussy.
1977.5 Race Car, CAMS Group S Spec
1989 944 Cabriolet

924RACR · Posted: Tue Mar 08, 2022 10:18 pm Post subject:

Seemed to me the weak point is the holes for the screws in the outer barrel; removing the screws won't fix that.

So I'd recommend finding a shim-based solution without holes in the sides...
_________________
Vaughan Scott
Webmeister
'79 924 #77 SCCA H Prod racecar
'82 931 Plat. Silver
#25 Hidari Firefly P2 sports prototype

MikeJinCO · Posted: Tue Mar 08, 2022 11:19 pm Post subject:

The cam followers come from ACM in Sweden cost would be less than doing any machining, the last ones I got were about $120 per set. They weigh about 53gm vs 85gm+ so a significant weight reduction. This alone will decrease the load on the cam by 15-20%.

Hopefully you have a crank scraper or at least a baffle in the pan. The guy at crankscraper.com has a very sophisticated setupbut I had delivery problems with him several years ago. I made a flat baffle and welded it into the pan, a copy of early Mittlemotor. Someone in UK or Europe did a similar one that was curved to follow the crank closer. I can mail you a pattern if you like.
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Mike

'67 MG Midget Dp
'71 Ocelot Dsr Kawasaki 1000(under rebuild)

Mike9311 · Joined: 14 Dec 2004 Posts: 1798 Location: Chicago-ish

Balance the flywheel dynamically on a pre-balanced custom shaft. Then bolt on clutch pressure plate and balance that with flywheel on same shaft. By having your motor balanced separately from the items you might change more frequently like the clutch, you are better off. This is also something Porsche worried about themselves. Its why there is a tuned damper in the torque tube. It wasn't so much new they were worried about but component swapping later like a new clutch not balance matched to engine.

Too bad you are so far away. I just did this and could do it for you

Edit: I also second everyone above with lighter valvetrain. The inertial response is improved tremendously every step you take to lighten. Lighter parts can change direction easier (better acceleration / + & - directions) which helps raise float ceiling i.e higher rpm capability. If the spring is kept the same, it will control things easier. It acts like stronger spring but with positive effects like less load on cam as MikeJinCo pointed out above. Basically win win other than setup. I also like that its less of a load on the contact line between the tappet and cam nose so the oil/additive package has a better chance of lubricating at that exact moment of peak force on that contact line.
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1980 931 since 1989
1981 Ideola 931 Club Sport
1982 931 Entwicklungsfahrzeug
1979 924 NA ohne 650 mit 471
1982 931 Red Resurrection - 951 IC
1982 931 parts car / resurrection?
1980 924 NA (R&D lightweight)
1982 931 wana-be GTR race car