I appologize for not having paid better attention at the most recent Callas tech session, but this is a bit of a revelation to me.

Did anyone else as ignorant as me assume we had DOHC/24V?

Wasn't SOHC a bit retro even in 1994? Was that a design limitation due to space, maybe on the heads themselves?

Has anyone in the aftermarket (9M?) attempted to retrofit a DOHC system with any success?

 

Crap, pushrods were retro decades ago but yet the Corvette still uses them and that car is the best bang for the buck. In otherwords, it has no overhead cam period.

Why change what works?

And no, I always knew that the 993 was a SOHC 2 valver just like my Duc.

 

Hey Chuck,

New to the Rennlist 993 forum. Have been a member of the flat-6 forum for about four years, as long as I had been looking for my 993. Finally got it in March of this year (2008): 1997 993 C2.

Anyway, I have been following the Porsche 911 since the late '60s, early '70s time frame (yeah, I'm an "Old Fart"). The reason that the Porsche air/oil cooled engines are SOHC/12V I believe has to do with cooling the heads. I think I read this in either Road & Track or Excellence. Only when Porsche changed to water cooled (996) did the valvetrain move up to DOHC/24V.

Also, if you want to know everything about your 993, I suggest you get the book "Porsche 993 The Essential Companion" (all 640 pages!).
You'll find out lots of other interesting bits about your car, like the engine has a dry sump oil system, and the six cylinders are actually horizontally opposed (sorry, I couldn't help myself!).

I have seen 993's advertised for sale on AutoTrader from dealers and they specified that the engine was DOHC/24V. I actually e-mailed one (the big one in Houston) and let them know that the air/oil cooled 911s are SOHC/12V.

Gary

 

Chuck,

Porsche used DOHC racing engines (air-cooled) in various forms with 4, 6, 8, and 12 cylinder versions. These were all 2-valve engines as there is no room for 4 valves in an air-cooled head due to cooling problems. All production air-cooled motors were 2-valve engines for that reason.

SOHC technology goes back a long way and simply represents a cost vs efficiency compromise for engines that will not generally live at high RPM's. Porsche used 4-cam racing engines since they spent their lives from 6K to 8K+ to help contain valve float issues.

It would be frighteningly expensive to manufacture a 4-cam kit for a 911 engine,............ I don't think anyone would be willing to pay for something with such dubious benefits at this juncture.

 

Steve,
Any reason that Porsche has never used positive close as well as positive open valve technology like Ducati uses today with their Desmodronic valve system and Mercedes used in the golden days to combat valve float? Or is this merely a gimmick in the automotive world? Sure, it would probably be a challenge adjusting valves on a 2 valver let alone a 4 valver cylinder x 6 vs 2.

 

Thanks, Gary and Steve. Actually, my post was born out of a little frustration after spending time with Adrian's otherwise wonderful book this morning. I looked through two engine sections as well as the index for "valves" and "camshafts." Hoping for a cutaway or some diagram of at least the heads. Maybe something in the turbo and or RS, but nothing on normal 3.6 NA valve configuration. Maybe the topic was too elementary!

 

Check out the engine rebuild articles and the photos of the internals of a 993 engine on www.pcarworkshop.com ; go to the photo sections. There are some good photos of the heads as well (especially in the engine rebuilding section).

I know, I took them! :-)

Cheers,

Mike

 

Gary---

Where in Colorado are you?

Brian

 

Porsche didn't use Desmodromic valve control since they didn't spin their engines as hard. The Auto-Union's used coil springs and never went over 6000RPM; 4-cam Carrera's used similar twin-valve springs as that did the trick for those small valves @ 7300 RPM. They were able to avoid extra the compexity of such systems since a dual-coil valve spring design did the job.

Those M-B W125 6.6 litre straight eights were spun beyond 7K so they required Desmodromic valve gear since coil spring metallurgy was not very good at that time.

Desmo valve gear is no longer needed since we now have really good valve springs available. Together with lighter valves, these do the job much more efficiently than using hairsprings and cam lobes to close as well as open the valves,...

The tedious and stringent maintenance aspect of Desmodromic valve actuation has to be experienced to be appreciated,.....

 

Hey Brian,

Way up in Longnont. So you're at the foot of Pike's Peak? Manitou Springs? My Wife and I have lived in Colorado since 2000, and everytime a family member comes for a visit, we take them up to Pike's Peak on the cog train. I think we've been up there four times now. Nice area for sure, Garden of the Gods is cool also.

Hey it looks like you are (or were) a British car masochist like my brother. He had a nice Austin Healey (100-6?) that was broadsidded by a young girl that just got her license and didn't know what a flashing red light meant! I think he still has his MGA FHC that he will someday (yeah sure) restore. I think he has or had one other one, can't remember what it was (MGB?).

I just got my 1997 993 C2 in March. It has 18" turbo twist wheels, H&R lowering springs and sparco seats. The lowering makes it pretty tricky on driveways etc. trying not to scrape the front end and chrome exhaust tips.
The airbox cover is also drilled out (like with the Motor Sound option), and also hogged out a bunch. It sure makes some nice induction sounds when getting on it, especially from around 5k on.

It's been a frustrating summer trying to keep it clean though! Every time I take it out it seems to rain just enough to cover it with dirt spots.

It's alot of fun though, I've wanted a 911 since the late 60's, earyly 70's.

Talk to you later,
Gary

 

Gary, i remember seeing your car on AutoTrader a few months back when a friend was looking for a 993. Nice car, please post pics in a different thread.

Chuck, in answer to your question, I've always known that "our" engines were behind the times in more than one way. But what a sweet way to do so!

 

Tim,

Thanks, I agree, it's pretty nice (it's as I bought it)! Yeah, it's the one you saw in AutoTrader back in March at the latest. I picked it up in California March 12th.

So your friend that was looking for a 993, did he pass on this one because of the price? It was kinda high ($39,999.) but had pretty low miles (34,070 miles) and some upgrades.

I've had a busy spring/summer since getting it (including three weeks in the USVI!) and haven't had a chance to take some really good pictures. The best ones of it that I have are from the previous owner (including my avatar).

Gary