INTMD8

I'm sure this has already been thought of but I can't find any threads about it so here it goes.

I've read that there was no 4 valve air cooled engine as it could not be reliably built because of heat issues.

I am curious, what were the heat related failures?

Or said in another way, how is it a naturally aspirated 993 is limited to low 300's hp (cup/rsr) because of heat but 993 turbo can make much more with the same air cooling?

Not to mention the power levels achieved both naturally aspirated and turbocharged by aftermarket companies.

The thought I had was, why not a 3 valve? Over time, with 2 valve engines, more and more power has been found by decreasing exhaust valve size to accommodate more intake valve.

So, would seem maybe 3 valve would be the best compromise. Substantially increase intake flow, exhaust still flows as much as it needs to (while minimizing heat transfer area vs 2 ex valves) and the geometry allows for one centrally located plug?

Keep same cam design and use a forked intake rocker. Flatten valve(s) angle would increase rocker ratio/valve lift while increasing compression.

Probably many reasons other than massive cost that it's a bad idea, just thinking out loud

pp000830

It may or may not be true but my understanding as to why air-cooled engines became obsolete had to do with combustion chamber temperatures where air-cooled engines run a lot hotter and as a result, produce combustion byproducts that are incompatible with evolving and evermore stringent emission regulatory mandates.

Andy

Watson

Correct. Was the problem with NOx emissions being too high? (higher combustion temperatures will increase NOx emissions)

MarinS4

I've found it's not the HP number that creates happiness with a power plant but the personality. I've driven big powered 993's that were like two stroke dirt bikes. Fun in a straight line but not so much in the twisties where the 993 should shine. The TPC SC on the other hand does not make impressive numbers but still lives on in my mind as one of the happiest 993's I've ever driven.

Creating a "happy" 993 does not require going to such extremes as a 3v design. Protomotive tune with simple rebuild, zero decking cylinders (or whatever is ideal spec) with healthy cams like DR21's or DC24's will not break the bank and give you a HUGE smile!

JasonAndreas

There is currently a 6 cylinder-4L, 4 valve, and 4 cam naturally aspirated air-cooled engine running around Germany in a 964 undergoing long-term testing. 500HP+ and ~350lb.ft of torque...

paranoidandroid

I think the two main factors are cost, and like Andy indicated, emissions. Porsche wasn't exactly in great financial shape at the end of the air cooled run and probably had very little cash outside of their investment into developing the 996 and Boxster.

Heat can be effectively managed with proper engineering. There are plenty of air-cooled, 4 valve boxer engines out there. But more valves will allow for higher revs, and now you need to consider noise on top of heat too.

71-3.0-911

Wish Jeff Gamroth and Steve Weiner were still around here.... If you've got the pockets, give Rothsport a call....

-Scott

ToSi

Any technical details available for this one? Curious how they're addressing cooling in the area between exhaust ports and around the valve seats. Ignoring cost, I suppose modern additive manufacturing technologies could make things possible today that weren't conceivable when these engines were originally developed.

ToSi

It was probably hydrocarbon, NOx wasn't as tightly regulated back then as it is today.

HalfGerman

I thought Porsche did 4 valve water cooled heads with the 935 and 959.

Watson

+1

Andy Hodapp

I have worked on numerous engine rebuilds and upgrades. You can extract over 400 HP out of a 3.6, the problem is that power can only be very very high in the rev range. On naturally aspirated engines, we figured out how to make the maximum peak power quite a while ago, all modern engines do is make that power more usable.

Being able to adjust cam profiles and cam timing allows for more optimal breathing at a variety of RPM. Same thing with ignition timing and the ability to change runner length and such. None of this stuff actually helps to improve the peak power, you could lock all of that in their optimal position and get the same power. The problem is the low and mid-range suffer significantly.

These are old engines that live up to “There is no such thing as a free lunch.” When building high horsepower 964 and 993 motors and keeping the same displacement, there is really no way to not affect the low and mid-range. For a racecar this is fine, they are always going full tilt and are geared to always stay near peak power. Not ideal for road cars and the only way to change that is with modern technology.

Four-valve vs two-valve is an improvement as it increases the valves' overall surface area, allowing for more air and the valves are smaller and lighter, giving you better valvetrain control at high RPMs. By switch to a twin-cam setup and a way to control cam timing relative to each other, we can change the overlap and get better breathing at different RPMs.

So even with going to a four-valve design, unless things like cam timing and lift are also adjustable, you are going to end up with a rather peaky engine.

Searching for power with these cars is not easy and for road use, I really wouldn’t recommend it. There are several tuners on here that would be a good start, as well as a lightweight flywheel. If you are looking to go a little further, I would recommend having the heads rebuilt, have them polished up, RS valves and cam will get you a majority of the RS power without having to open pandora's box.

Once you start going it gets hard to stop, there are so many components that need upgrading when you search for high horsepower. You’ll want to move the power curve up, raising the redline, and to do that you really need to replace the rods. While you're at it the GT3 crankshaft offers better oiling and has wider rod bearings that also have a smaller diameter which reduces the surface speed of the bearing as the distance it travels is shorter. GT3 oil pump will also be needed. I would not recommend slip-in pistons so you’ll want to increase the spigot size to 109mm. Get rid of the hydraulic lifters to increase valve control and allow for more aggressive cam. Lighten up the valvetrain with some titanium bits possible, though this turns things into more of a wear item with the engine needing to come apart periodically, same if you go for titanium rods. You’ll need something like a MoTeC to run it and better hope you can find a competent tuner that can also make the vehicle driveable at low speed, high altitude, high and low temperatures.

Overall a lot to do to make power. You are better off buying a 996 Turbo and tuning it or just throwing a supercharger on a 993.

Watson

Yes, there were not so much money to develop the 993. A new cylinder head with 2 intake valves would probably have meant also redesign of the combustion chamber which has big consequences for proofing emissions and full load performance. I think Porsche would have designed a 4 valves cylinder head (or 3 valves) without any problem if cost was not an issue.

I agree 3 valves per cylinder would have made a lot of sense.

MarinS4

Andy you’re one of the few people who gets it. Most don’t realize what they want is a broad healthy torque curve not a XXXhp number on one spot way up in the RPM band.

Unfortunately forced induction is the best way to extract a healthy torque curve out of these engine. If you combine solid NA flow principles with proper cams and low boost you’ll get a personality that is somewhere between a NA and turbo.

Greg Bell

It also had a lot to do with noise. An internal combustion engine with a water jacket is far less noisy. As an aside, I understand US models have different gearing ratios on lower manual gears to reduce noise.