Charles@dundonmotorsports

The change to the finger follower/rocker arm is most likely for several reasons

-To reduce valvetrain mass directly above the valve (like a typical bucket style lifter)
-the rocker arm with the HYD lash adjuster in the cylinder head normally allows for better oil and contol to the lash for precise clearance and not allowing the lash adjuster to collapse at higher rpms
-the rocker arm geometry allows for faster valve opening and closing velocities even with an overal weight penalty from the valve train having more overall moving mass. The bucket style lifter directly acting on the cam lobe has physical limitations for duration and valve velocities.

Honda has always opted to run a rocker arm style valve train over conventional lifter bucket design because they feel the faster opening and closing ramp/valve rates is more than worth the added valve train mass.

Lifter bucket designs may be cheaper to produce as there are less parts involved which generally means less manufacturing costs.

Porsche choosing to go with the rocker arm finger follower design shows that they were probably near the limitations of the old style valve train and to increase performance (mass flow intake) they needed higher valve velocities to allow more time for the valves to be open at higher lift.


I see this change as a positive note because this means more performance potential.

Petevb

Getting to 9k in the new motor vs 8.5k in the old GT3 motors seems primarily due to the changes in the top end, not the bottom end.

Porsche runs a relatively short stroke, which means piston speeds, the main limitation on the bottom end, are relatively low. Higher the mean piston speed directly correlates to a higher G force on the pistons and rods. Because the limitations are materials related (ie everyone is using Ti), mean piston speeds are relatively consistent across all types of high performance engines:

Formula 1 V8: ~26 m/s
Audi RS5: 26.3 m/s (current production car record)
2012 Z06 Corvette: 23.7 m/s
BMW M3 GTS: 23 m/s

In comparison it's clear Porsche is on the lower end of these numbers, and isn't pushing the limits of their bottom end as far as Audi or Corvette:

GT3 3.8: @ 8400 rpm: 21.4 m/s
GT3 RS 4.0 @ 8500 rpm: 22.8 m/s
2014 GT3 @9000 rpm: 23.3 m/s

It seems clear that even on the old GT3 motors Porsche could have designed to increase peak rpm fairly easily (9000 rpm would have been just 22.9 m/s on the 3.8 GT3). The issue was the top end with hydraulic lifters couldn't handle it.

Switching to this top end seems to give the new motor significant potential the Metzger couldn't have.

Carrera GT

That is a classic video. Such talent. One of those competitive types who was in it to win it, but always the good sport. Born with raw ability and he practiced to perfection. Peter Perfect.

A tragic day for the whole country when died. We lost a true hero and a great bloke. Back in the days of that video (VN "dunny door") he was living in Melbourne and shared a fence with my brother's house. He'd won every race there was and yet he'd be home on the weekend, mow his own lawns and lived simply and quietly.

BGB Motorsports

I never said anything about the bottom end being the change. I said that the 9A1 motors don't have titanium parts, rods and pistons and therefore THEY aren't as safe over 8000RPM as the GT3 engines. The new GT3 engine which in 997.2 trim always had those parts and in 991 trim now has a 9K RPM, got new valvetrain love because when they went to the 4.0L, all GT3 Cup teams reduced revs from 9K to 8200 due to the stroke change of the 4.0L and Porsche's being nervous about higher RPM levels. I think you thought I meant that the new car got a bottom end change to spin to 9K. The reality is that those motors were already spinning close to those levels.i think its more looking forward to when they decide to try and spin the 4.0L higher, if they do make a 991 stroker.

GrantG

Doesn't the RSR rev to 9,400 with 4.0? But maybe the teardown intervals are very short...

Cupcar

I found the paragraph in the 991 GT3 press kit that is shown in the red box below interesting. In looking at diagrams of the A91 I noted that the engine is split case like Mezger but with removable sump that in the 991 Carrera is "an integrated dry sump" the design of the oil pump and sump is such that it would be easy to convert the engine to a true dry sump which it appears from below they have done in the GT3.

The only other question I have is whether the GT3 has Nikasil cylinders, which with the closed A91 deck design would be relatively easy to install like Ferrari, etc. do.

Simplified split case with water jackets cast in + true dry sump w/removable sump + Ti rods + Mahle forged pistons + Mahle Nikasil cylinders along with the finger followers would be one nice engine.

Boxster Coupe GTS

Hey Grant -- do you know if the new 991 GT3 engine can be re-built like the Mezger engines?

Petevb

The 3.8s and DP motors were higher rpm than that. But 30 hour rebuild schedules...

GrantG

I think all the water cooled motors "can" be rebuilt to some extent, but it was cheaper for Porsche just to replace the non-Mezger motors, rather than make warranty repairs. There are shops that will rebuild the M96 motors, afaik.

Cupcar

If one does their homework and has built an aircooled 911 engine then a Mezger is not that big of a deal. A lot of special tools required though.

If A91 has press in Nikasil liners like Mezger it should be even easier than Mezger to rebuild.