Hot 2 liter engines

[Jeff Higgins]

Yup, I can definitely see that - we would be adding the weight back to the rocker arm tip that we were able to lose with the elimination of the adjustment screw and lock nut. Maybe even more weight, with the axle, roller, and change in profile on the rocker arm to accommodate.

I do suspect that the available valve spring materials of the time were a crucial limiting factor. We have seen tremendous advances in that arena in the intervening time. 8,000 rpm back in the 906's day was really something for even a small bore motor such as that, where today we see big American pushrod V-8's turning over 9,500 rpm for 500 miles in the unrestricted NASCAR short track stuff. The NHRA imposed a 10,500 rpm limit on their 500 cubic inch Pro Stock motors - the well funded teams were hitting 12,000 rpm. With a 500 inch pushrod V-8... with 2.25"+ diameter intake valves... so, yeah, the material sciences have met these demands.

I have to think that the demand just isn't there for our motors. Everyone racing them is restricted to in-period equipment, so there is no real reason (or ability) to take advantage of these modern day developments. We do, of course, see all of this applied to modern racing Porsches, where it rightly belongs. Just not in our wonderful old air-cooled world.

[stout]

Yup, I can definitely see that - we would be adding the weight back to the rocker arm tip that we were able to lose with the elimination of the adjustment screw and lock nut. Maybe even more weight, with the axle, roller, and change in profile on the rocker arm to accommodate.

I do suspect that the available valve spring materials of the time were a crucial limiting factor. We have seen tremendous advances in that arena in the intervening time. 8,000 rpm back in the 906's day was really something for even a small bore motor such as that, where today we see big American pushrod V-8's turning over 9,500 rpm for 500 miles in the unrestricted NASCAR short track stuff. The NHRA imposed a 10,500 rpm limit on their 500 cubic inch Pro Stock motors - the well funded teams were hitting 12,000 rpm. With a 500 inch pushrod V-8... with 2.25"+ diameter intake valves... so, yeah, the material sciences have met these demands.

I have to think that the demand just isn't there for our motors. Everyone racing them is restricted to in-period equipment, so there is no real reason (or ability) to take advantage of these modern day developments. We do, of course, see all of this applied to modern racing Porsches, where it rightly belongs. Just not in our wonderful old air-cooled world.

Would love to hear more from Gordon or others on the above. Been running a 2.2 with Solex cams, Webers, headers, and a bit of head work that's restricted to a "mere" 7200 rpm for a few years now. While I like it, I wonder if I'd like a 2.0 or 2.2 built to stretch to 8000 rpm on short gears even more. I've always viewed 8k air-cooled engines as too exotic for the street, but the post above makes me curious.

Modern street cars have spoiled me with 8000-9000 rpm redlines, but their pace has me less and less interested in mph and more and more interested in the experience these days—and the small-bore sixes sing in a way very few engines do. One that could spin to eight grand—on the street—would be quite a gem, and extend the reach of those short gears in the process.

[gled49]

Porsche’s split 3 grove valve keepers allow the valve to rotate, when the rocker strikes the stem off center it rotates the valve allowing it to hunt, helps keep the valve and seat clean. Modern valves use pinch keepers that don’t rotate. In race engines doesn’t seem to be a problem. I’m getting ready to put a 8k rpm motor on the street, I guess we’ll see. G

[Jeff Higgins]

Good point - roller tips can not force the valve to rotate and re-seat in a different spot every time, leading to "hot spots". I dealt with the results of this many years ago in, of all things, an early Toyota Land Cruiser engine. Basically a metric Chevy "stove bolt" six. Valve job every 30,000 miles or so as that one "hot spot" burnt every valve seat. The "cure" was to eventually swap out to their improved "2F" engine, which had incorporated the "rotating valve" idea. Rocker tips contacted the valve stem just far enough off center to rotate the valve. Made it really fun to set valve lash, as that was done with the motor ticking over at idle. It really chewed up the feeler gauges as it kept trying to spit them out...

I can see where in a race motor no one would really care whether valves rotated or not. There are plenty of other issues that will come to bare forcing a refresh long before valve seats. Funny, though, these roller rocker tips are quite commonly used in street applications on big pushrod V-8 hot rod motors. I suppose they really don't get driven enough to matter anyway.

[TargaFlorio]

Would love to hear more from Gordon or others on the above. Been running a 2.2 with Solex cams, Webers, headers, and a bit of head work that's restricted to a "mere" 7200 rpm for a few years now. While I like it, I wonder if I'd like a 2.0 or 2.2 built to stretch to 8000 rpm on short gears even more. I've always viewed 8k air-cooled engines as too exotic for the street, but the post above makes me curious.

Modern street cars have spoiled me with 8000-9000 rpm redlines, but their pace has me less and less interested in mph and more and more interested in the experience these days—and the small-bore sixes sing in a way very few engines do. One that could spin to eight grand—on the street—would be quite a gem, and extend the reach of those short gears in the process.

Keep in mind that Porsche quoted 30 race hours (5000 kms / 3K miles) on the 906 engine between rebuilds. They also advised that a rebuild may be necessary before 30 hours, especially if the car was used in long-distance races. By rebuild they meant "disassembly of engine" in order to check on internal parts.

You'd get more hours out of it with street use. Modern oils / materials may help in that department as well.
At the end of the day higher revs have a habit of shortening the lifespan of everything, something my race engine builder happily reminds me of anytime I see him...

Ciao,
n.

[Jeff Higgins]

Would love to hear more from Gordon or others on the above. Been running a 2.2 with Solex cams, Webers, headers, and a bit of head work that's restricted to a "mere" 7200 rpm for a few years now. While I like it, I wonder if I'd like a 2.0 or 2.2 built to stretch to 8000 rpm on short gears even more. I've always viewed 8k air-cooled engines as too exotic for the street, but the post above makes me curious.

Modern street cars have spoiled me with 8000-9000 rpm redlines, but their pace has me less and less interested in mph and more and more interested in the experience these days—and the small-bore sixes sing in a way very few engines do. One that could spin to eight grand—on the street—would be quite a gem, and extend the reach of those short gears in the process.

The technology to achieve exactly what you are after has certainly been around for a good long time. We see it applied in a broad variety of applications outside of our air cooled motors. I just don't think the market is there to warrant specific development for our application. Like I said earlier, anyone racing these is doing so under vintage rules, and could not take advantage of these technologies anyway. I believe the street going hot-rod market is just too small to attract the investment necessary.

Thinking out loud, I think we would require a sturdier rocker arm to be compatible with the heavier valve springs and higher rpm's. The current design is meant to break in the event of an over rev or valve to piston contact, saving the valve and piston in so doing. That limits the rpm's and seat pressures they will withstand. Beyond that, I'm not aware of any other mechanical constraints. Piston speeds, for example, are still pretty moderate by today's standards with either a 66 or 70.4 mm stroke, so that should not be a limiting factor. As a point of reference those NASCAR V-8's run a 3.25" stroke, or about 83 mm, and rev well up into the upper 9,000 rpm range. Maybe there is some other mechanical limitation in our motors - oiling, bearing sizes, something like that? Or maybe cylinder head design, or more specifically, port size, shape, and placement limit breathing to an extent that such rpm's are impossible. I know there are some super trick (possibly billet?) heads out there that address a lot of the shortcomings of the original designs, but are they enough of an improvement to raise rpm's substantially?