I don't think this is the place to look at history, so let's not digress any further.

If we get back on topic, Shawn has done a great work developing the standard 8V head, which is even more remarkable considering his focus on a limited budget.
However, would we get more power and reliability with less resource by using a non-worked 16V head? The answer is yes.

Historically 16v race breed head was available 5+ years before big $$ GTRs.

There is no proof 16v head is more reliable. It is an assumption.

The 16V Le Mans factory head was very fragile, they ran only 375hp with reduced boost because of the issues during the various testing sessions. It makes sense that the factory was not interested to supply private teams with non-proven parts, especially when PAG itself had no official racing program. In this context it makes also sense that PAG supplied Holbert and Andial with blank castings to let them design the GTR head by themselves.

"There is no proof 16v head is more reliable. It is an assumption"

How long is a piece of string, etc.

Powerhaus/ David Raines build reliable 16v 3.0 turbo engines 20 years ago.....and not just one.

Porsche spent months on engine dyno torturing these engines to all extents possible and decided 16V head is not good enough for turbo application.

I don't know why, but they certainly do.

im sure they spent a ton of time working out possible permutations of parts and stuff but the 8v turbo was likely a result of budget considerations (reuse a bunch of parts from the NA engines, easy enough to tweak the head casting, chamber design figured out already) along with practicality for a fast street car (the feeling of thrust is what fast car buyers want, not really every last ounce of power at the top end!)

sohc torque tendencies + long runners + small turbine wheel = mega midrange which 80s 944 buyers would have wanted. think about even today there are people buying 944s who are afraid to rev them over 5000 because they think they are delicate.

If the factory was really doing all this back to back R&D to compare the various virtues of the two head configs one can only theorise as to what their findings were?

Also, there are a lot of changes available to us now that weren't back them. Different turbos, ECUs and possibly valves. Lighter pistons and rods too. Great that we have these options available for us to go either way.

This video is interesting from a turbo engine development perspective. Not everything that gets slapped together will work. It will fail more likely than not.

Now that this thread obviously have derailed I must add my 2 cents..

There is absolutely nothing in factory development for a street car that is aimed at maximum power. Turbo was primarily added to engines to give more power without maximizing the rest of the engine.

This "16v head was not proven to be reliable for a turbo" is a bunch of BS and there is nothing that supports this. The same goes for the ceramic insert in the 951 exhaust ports that also have turned out to be done mostly for emissions to get the catalytic converter up to temp more quickly...

Comparing the heads the 16v IS indeed much better to use for more reasons than power as the 8v head has a steam pocket that can and will create issues on a high hp engine. The 16v head does not have this problem so yes, it is a better head to use. Sodium valves are nice but usually heavy. My new 16v head gets the Ferrea SuperAlloy exhaust valves which can be said to be an improved inconel valve and removes the need for sodium filled vales.

Besides the fact that the head flows much better and produces the same power at lower boost levels the pent roof design with location of spark plug is much more efficient. And you can run a little higher static compression. So if you're interested in the full aspect of things you will get out more power compared to an 8v for the same energy level you put in (fuel).

Trying to analyze what engine parts are the "best" to use by looking at different racing programs some 30 years ago is simply not the right thing to do. It doesn't say anything about anything. The same reason the 968 Turbo RS have a special 8v head with tiny ports (and only produces 305 hp). It doesn't say ANYTHING about what is best. It is always a combination of marketing aspects along with rules.

If anyone wants to discuss this further I really suggest a new thread to be started.

Maybe Shawn should separate this part to new thread.

There are so many variables that without extensive testing it is all more or less guess-work. (in my opinion)

Part of full aspect of things is also the fact that 16V combustion chamber has more extremely hot surface than 8V head since the valve head combined area is larger and stock ex. valves are hotter than sodium valves. So is this affecting knock threshold and combustion temps or not?

There is an unbelievable amount of quality disinformation about these engines out there. Proportional to that, percentage of self destructing rebuilt/reworked engines is very high. Even from seemingly very reputable rebuild shops.

I was hoping the next poster in this topic would respect Shawns thread and start a new one. I started one here:
https://rennlist.com/forums/944-turb...rbo-myths.html

Dumb question for people running larger intake valves (47 / 48 / etc mm) in a 951 motor with stock pistons. Do you need to run a thicker HG to account for the larger valve face? I wouldn't think you would but want to double check.

Also - would 47mm or 48mm be a better choice for a street car? I'm shooting for 300-350 whp. I know 45mm valves will get me there, but I want to make that power at a lower boost level.

Edit: even dumber question. I just want to confirm that this is a compatible mod for a 2.5 (stock bore diameter). I was reading in another thread that this is not the case, due to the valve radius exceeding the 100mm bore.

The 48mm valve fits in a 100mm bore just fine, but you need to open up the port and valve seat to get it to fit.

Without appropriate backup work a larger valve can actually COST flow!

Thanks for the info, the price is right for the larger valves and just in time for my head to be rebuilt at a reputable local place. Would "opening up" the port as it reaches the valve get me there? I don't really have the experience to do a full blown port job on my good 951 head (or the wallet to pay someone).

This thread was my first attempt, much better info in the second updated thread:

https://rennlist.com/forums/944-turb...t-8v-head.html