Why 2 Camshafts in a 911 engine?

[Porschephile]

I know to many this is a dumb question but I'm curious as to why the 911 engine was designed to utilize two camshafts. I know the 912/356 engine uses a single camshaft so what's the advantage of two?

Thanks,

-Matt

[dickster]

matt,

one for each bank of cylinders, therefore less moving parts (and no pushrods).

[YTNUKLR]

DICKSTER BEAT ME

[dickster]

scott,

i wouldn't say its not possible (many v8's use one - ok they're not flat), just not beneficial.

[MichiganMat]

pushrods = crap

[Hoffman912]

the proto type for the 911 engine (the type 745) had 2 cams and pushrods (one cam over the crank for intake valves and one below for exhaust valves). when dyno'd it only made 120 hp at 6500 rpm, which was short of their goal of 130. becauseof that, and that it was not reliable to have a pushrod motor go to 7k rpm in racing situations, it was decided this motor would be canned when the designer Klaus von Ruker left in 1962. His sucessor Hans Tomala helped bring about the revison, the type 821 with single over head camshafts.

note.. the most interesting thing about the prototype 911 engine, the type 745, was that it had 2 smaller belt driven fans, rather than one, like the 911 and the 356/912 (the type 616).

[Joeaksa]

Matt,

Some of the really high performance engines have one cam per valve set, ie one exhaust cam and one intake cam. This would relate to the four cams per engine on the aircooled Porsche engine.

Ever hear of a "four cam engine" on a Porsche? This is usually the old 356 four cylinder version with two per cylinder bank...

JoeA

[Wil Ferch]

What we're all skirting around and not saying "directly" to the poster...is that a single cam, for example mounted in the engine center ( like pushrod V-8's), would then require a way to transmit the actuation force to the valves via long, heavy pushrods. This additional inertia ( an object at rest tends to stay at rest....an object in motion tends to stay in motion) would limit the rpm potential of the engine.


So...we ended up ( in the final 911 design) with one camshaft on top of the head...real close to the valves...so this inertia can be minimized. Taking this further, we could have used two camshafts over each head ( for a total of 4 camshafts), so that you have a dedicated shaft for the intakes..and another dedicated shaft for the exhausts.....and you'd do away with the rocker arms and the associated inertia of *that*.

It was decided that the extra complication, weight, size, and cost to go this extra step was not worth it, so we got a single overhead cam ( SOHC) instead of dual overhead cam ( DOHC) design. Th Porsche folks did indeed build a prototype of a dual cam engine, and this 911 variant...the 916 2 liter engine...became the basis for the later racing 908 that had 2 more cylinders ( eight)....but that's a story for another time

---Wil

[Porschephile]

Very informative everyone! Thanks to everyone who replied!

-Matt

[klaucke]

Wil- I think the 911 engine stayed SOHC as opposed to DOHC (of which there were prototype and racing engines) is because there is no real benefit of DOHC over SOHC unless you are using 4 valves per cylinder. With aircooling, there was not enough head material to adquately absorb and radiate the heat with such a small head surface in the combustion chamber with 4 valves. Hence the aircooled 911's =2 valve per cylinder, liquid cooled 911's (and boxsters) use DOHC and 4 valves per cyliner.

[Wil Ferch]

The 4 valve vs 2 valve argument is correct as far as head temperature limitations go....but it is totally independent of choosing between SOHC vs DOHC.

A DOHC acts on the valves directly, without the need for rocker arms....which introduce more valve train inertia. So, in theory, a DOHC arrangement can spin faster. It's as simple as that. Whether they act on one valve per side or two per side is irrelevant. However, there are cases where two valves for one function ( say, the intakes) may be overall lighter for the flow they produce than one intake valve. In that case, a 4 valve DOHC would have even less overall valvetrain inertia than a 2 valve DOHC engine.

Test question.....how come Porsche said we can't have air-cooled 4 valve heads but there are tons of Japanese motorcycles like this...hmm.?

Wil

[klaucke]

I think it is because the Japanese bikes are much smaller in displacement, that their cylinders can be cooled much more effectively and allow for 4 VPC. Also, I think the bikes are generally under less strain than a car because of the light weight.

[Wil Ferch]

Klauke:

All true....and is likely the proper answer.

Still, I wonder if anyone has done a true thermodynamic study of this. The bikes put out lots more hp/size....and have absolutely no aux fan, which we still do. Most have fairings too which impede low speed cooling.

I also recall some aftermarket outfit ( CMW?) offering 4 valve air cooled 911 heads.

---Wil

[KobaltBlau]

Originally posted by Wil Ferch
Test question.....how come Porsche said we can't have air-cooled 4 valve heads but there are tons of Japanese motorcycles like this...hmm.?

Wil

I mentioned this in another recent thread in the engine rebuilding forum:

4 valve 911 heads??

the 4valve heads were Advantec, I believe. There was a mention of htem in that thread as well.

[BURN-BROS]

I believe the initial reason was valve placement/port design. Splaying the valves in such a way creates a more linear path to the combustion chamber. The only way to achive this was overhead cams. the chrysler hemi and the 356 needed parts to move in odd ways.

[Randy Webb]

Word for the day: desmodromic

[KobaltBlau]

Originally posted by Randy Webb
Word for the day: desmodromic

what about it? Seems to work well on the ducatis. Not sure there's room in the 911 cam tower.

[wastintime]

I'd think a better question was why didn't they use 4 cams? In '63 Porsche technically went to a "hemi" engine. lol, it's great to make fun of guys in huge trucks with that one, but yes 911s are Hemis... they use hemispherical combustion chambers. The problem is that hemispherical combustion chambers limit you to using only one cam per bank of cylinders because you simply don't have enough room for more valves. The 911 cam setup is elegantly simple when compared to american V8s. Still, most modern cars have found ways to employ 4-5 valves per cylinder and not use the hemispherical chambers, and most people agree this gives you much more bang, it's just the evolution of motors through technology... anyway, that's just my $.02

[tdatk]

I had a Fiat X-19 once with a single OHC directly acting on the valves. High rpm motor though, and adjusting valve to cam clearance was achieved with "puck" type incremental discs inserted onto a cup style keeper over the valve spring. Pretty efficient, but crappy engineering on Fiats part as these cars required weekly adjustments to keep running ,not to mention the electrical issues and rust. Although it drove like a mid-engined go cart.

HTH

Tim

[KobaltBlau]

Originally posted by wastintime
I'd think a better question was why didn't they use 4 cams?

My summary: a 4 cam flat engine is significantly wider whan an engine like the air cooled 911. with the room between those rails and mechanical lifters... you get the picture. Maybe it's an urban legend, but I've heard the easiest way to change the plugs on a 4 cam 356 is to drop the engine. Grady Clay would know.

If you don't go to 4 valves, there isn't much benefit to 4 cams; a DOHC 2 valve like an older ALFA has a similar combustion chamber to the 911. Reduced lifting mass on the valve assembly is one, but with porsche's racing rockers this apparently is not a problem with pretty high max revs.

I still think that a 4 cam 2 valve flat engine will be harder to air cool than a 2 cam 2 valve flat engine, but certainly if you go to 4 valves the combustion chamber cooling is difficult; a problem nobody has solved with an air cooled engine in an automobile. As I mentioned in another thread, at a minimum you would have to make a much narrower angle to the pent roof than modern 4 valve/cyl engines like the 996 have. This would require a big angled dome on the top of the piston which doesn't make a great combustion chamber.