333pg333

So I was on another (non Porsche) forum and there was a detailed thread about the virtues and differences between 2 and 4 valve heads. Some pretty wise and experienced guys were on there discussing a wide range of topics in this regard.
I don't really think there was a conclusion in regards to which head produced the greater torque but for me it was an interesting discussion.

So I managed to slip in a few pics of a modified 944 head for their perusal. These are some of the comments that it received. Bear in mind that I couldn't furbish them with great detail but gave some specs off the head on my 3L motor. Being a 104.5 bore with 49mm inlet and 40mm ex.

Some quotes:

"I dont know porkas but i reckon the intake is small, a max effort 4.1" bore on a wedge headed v8 would probably be around 2.1" but the two may not be remotely comparable."

"These don't look like a very good head. Shrouded, parallel valves, little squish, protruding plug boss and so on."

"Yea the valves are all wrong the chamber edge is very sharp and there is not much squish."

refresh951

I am looking forward to the discussion in this thread.

333pg333

By comparison one guy showed a pic from a Ford V8 racehead. Here are some of his descriptions of this one:

"These are a high spec 2 valve Ford Cleveland/Windsor CHI chamber-pretty much state of the art- proven to make 90 lb/ft plus per litre with horsepower dependant on the engine's size and how high you want to rev it.
Designed and made in Melbourne and as good or better than the aftermarket US heads."

"The inlet valve is also canted towards the bore centre line which improves the flow and swirl. Even with modest head sizes as a percentage of bore area the port flow is good- and relatively unimpeded, with most of these engines having 18 mm or more of lift."

"That's nice how the valves use pretty much all of the chamber. They are almost touching. Where as on the 944 head above that wont be possible as the inlet valve is already being shrouded by the bore, while there is a huge gap to the exhaust valve."

rlm328

What is squish? I'm trying to envision a valve that isn't parallel.

333pg333

I said to be fair that the i4 Porsche head wasn't designed to go racing and it probably goes back to somewhere in the early 1970's for it's design as half of the 928 motor.

From what I can tell we can't go up much more in size due to the location of the valve stems. In other words they can't get closer due to the exterior wall of the chamber. So it would look impossible to get an inlet valve that is half the size of the bore.

The main aim of their thread was to derive which of the two heads gave greater torque. As we have come to know in here, it's always been said that the 2 valvers give better torque but the 4's breath better in the upper rpms and are more suited to some race cars.

333pg333

So do I Shawn! Most of this stuff is above my pay grade but I find it very interesting. Hopefully some of our more knowledgeable types might contribute.

Here's a different head. Not sure what it's off but I suspect Japanese.

chrenan

The 951 also needed to fit between the 944 and 911 in terms of performance. The engineers wouldn't have had free reign to make things as good as they potentially could be, the car needed to fit into a marketing window. Porsche has always worked this way.

refresh951

Computational fluid dynamics really picked up steam in the mid 80's, probably a bit late for our cars. CFD allowed serious progress in design. Anyone know if CFD was used in the turbo head design?

Dave W.

Squish is also known as 'quench zone'. It's a flat area around the edges of the head where the piston almost touches the head when the piston is at TDC. It reduces knock since knock starts at the edges of the combustion chamber, and the squish band tends to cool the gasses in that area. It also pushes more of the air/fuel charge inward while adding a little turbulence.

blade7

Bearing this in mind are there any 40 year old designs that are significantly better , and what's the opinion on the 944 16v heads ?. I wonder at what revs the benefits of the newer 8v designs come into play, would putting a head designed for 8k+ see much gain on a 944 turbo bottom end ?.

Weston Dillard

Bob,

On a Cleveland Ford (and I'm sure others, but that is the example he showed and one I used to mess with), the valve stems are not parallel. The valves are "canted" inward for better flow. The valve stems, and rocker assemblies are offset:

http://www.trickflow.com/pressreleas...PortCleveland/

West

Landjet

So with all of the above being said about the (not so optimal design of our head), this must have been the reason Porsche went the forced induction route on our cars as you can can over come a lot of bad design with forced induction.

refresh951

Micheal Mount discussed 2 areas wrt porting that make a significant difference to intake flow. It would be great to have some detailed discussion on this.

Also, in running a non-interference design Sid and I are running a bit down in the hole (approx 0.130") which obviously effects squish. I want to understand better what the tradeoffs are here wrt combustion.

Dave W.

Thanks for posting the various pics. It's interesting to see our head next to others.
What's the stock valve size?
Patrick, did you open up the combustion chamber to match the larger bore size?

Dave W.

Do you have a link to his comments?
Reading this thread makes me want to remove the head on my car and do some work!