ptuomov

I may be starting too many new threads given my short tenure here, but I would very much like to reopen a focused version of the ported and polished by committee thread. That is, minus the various arguments about work quality of vendors etc. Just porting and polishing.

Some topics to raise specifically. All this is about the 4-valve heads.


1. Valve sizes. Do we really need bigger valves for the heads?

From A. Graham Bell "Four stroke performacne tuning" 3rd ed.
engine cc; cylinder cc; max inlet valve diam. mm; max exhaust valve diam. mm
4800 600 38.1 32.5
5600 700 40.6 34.3
6400 800 41.9 35.6

The answer that I read from here is that the 5.0 probably doesn't need new valves whereas 5.4 and custom strokers need a new inlet valve.

What designs could we copy?

One of the top high-performance forced-induction engines of today is Subaru wrx sti 2.5L EJ25. By a coincidence (well, actually, this is why I chose this Subaru engine as an example), it has the same bore and stroke as the 928 s4 32v 5.0.

This engine has "big valve" Cosworth heads with all sorts of modifications. See pics in the links:
http://www.harmanmotive.com/catalog/images/KK3670-9.jpg
https://rennlist.com/forums/928-foru...uby%206low.jpg
http://www.vividracing.com/catalog/c...1.html?image=0
http://www.speedalliance.com/catalog...%20%208low.jpg
http://i149.photobucket.com/albums/s...worthHead1.jpg
http://i149.photobucket.com/albums/s...worthHead2.jpg

The engine with the Cosworth heads and new software but with stock turbo and pistons gives about 400hp from 2.5L. With different turbo, pistons, and other mods the heads support 800hp max power in somewhat usable racing engines (not pure dyno queens).

Guess what valve sizes the Cosworth "big valve" head uses. That's right, 37 mm intake and 33 mm exhaust. Sounds familiar? Those are the stock s4 valve sizes.

All this has lead me to conclude that the stock valves are just right for a turbocharged or supercharged 928 5.0 32v engine.

The current theory of why Porsche didn't use the 928-coded 968 larger intake valves in the GTS is that GTS would have made more power. I don't belive this is true. I think they did it because the 37mm is large enough for the GTS 5.4 engine, given the stock intake manifold.


2. What other designs can we copy from the Subaru wrx sti 2.5L "big valve" Cosworth head?

The sti head is good to start with. Here's what Cosworth CNC program does to the heads:

- They start with the OEM head and CNC port it.
- Hand blended finish work, although I don't know what value this adds since the heads are sold separate from the intake manifold. Guess just quality control et.c
- +1mm SS intake valves (1800 degrees max.) This takes the intake valve to 37mm diameter.
- +1mm Inconel exhaust valves (2400 degrees max.) This takes the exhaust valve to 33mm diameter.
- 4-angle inlet valve job
- 3-angle & radiused exhaust valve job
- Back-cut inlet valves
- Notive no back-cut exhaust valves
- Valves are lapped
- Knife edge port divider. This is controversial. It might be because the injector is so close that a bullnosed divider might catch a puddle of fuel.
- Stiffer valve springs
- Titanium retainers
- Hardened steel spring platforms

Results are
- Intake 10-20% increased flow at max lift
- Exhaust 20-30% increased flow at max lift
- These heads seem to flow about 300 CFM, although I couldn't find independently verified numbers.


3. Any safe modifications, low hanging fruit with the 4-valve head?

Quoting A. Graham Bell:
"Main flow improvement [in a 4-valve pent-roof head] comes from work in the inlet port valve bowl area. Valve seat inset overhanging the valve throat, the sharp lip where the machining meats the rough cast and port 'dingle berries' all upset flow."

I think that in the previously closed thread there was some debate whether to blend in the lip. The book, which otherwise is conservative, says to blend it.


4. What about shrowding. How concerned should we be about the combustion chamber and the other valve blocking the flow thru the valve?

Small valves, small problems; big valves, big problems?

My theory is the following. The valve area is approximately equal to the the throat cross sectional area. For a 37 mm valve with a throat diameter of say 33 mm that is 8.55 cm^2. Subtract the valve stem diameter from that and you'll get 8.17 cm^2. The valve has a circumference of 11.62 cm.

The curtain area exceeds the port cross-sectional area at lifts greater 7.03 mm, after which point by my little theory more lift doesn't help as much as as before. But that's a digression.

Now, if we want to create a 8.55 cm^2 area around the 37 mm valve, we need to solve x from the equation (((3.7+x)/2)^2)*pi - ((3.7/2)^2)*pi = 8.55, which is about 1.25 cm or 12.5 mm.

By my interpretation is that for completely uninterupted flow, we'd ideally want the combustion chamber and the cylinder wall to be more than 12.5 mm away from the valve edge. More clearance wouldn't help. Less clearance doesn't necessarily hurt, but if the clearance is less than 12.5mm, then one should investigate if in fact smaller valves would flow better.

Has anyone measured the minimum valve clearance from the cylinder wall at say 7 mm lift for the stock 37 mm intake valve? It would be interesting to know how whether this is a constraint.


That's all I got. I made up much of it on the spot, so don't believe it.

Please debate, extend, and discredit!

Caveat emptor, as always. And let's keep enough on topic that this porting thread stays open.

BC

Without a flow bench, and 100s of hours, all we can do is look at those ports on the subaru and say "that looks great. Maybe our ports should be that wide"

Understand that the S4 design is currently being held to at least 850chp on pump gas. Its holding together just fine, but Todd is controlling detonation to the point of fastidiousness.

ptuomov

I recall you porting your heads without a flow bench, right? I may be confusing things. My memory is that you blended in the lip in the bowl area where the machining ends and casting starts. How did it work, any idea?

That modification, by the way, is the one thing that A. Graham Bell's book recommends universally as an improvement to all pent-roof 4-valve heads. That's the kind of low hanging fruit is what this kind of board could serve.

Also, there's the valid question whether the popular stroker and gts mod of installing the larger intake valves helps. Sterling says that it's good for a double digit number of hp without any tuning. Might as well then go with the larger valves if one needs to change GTS valves anyway. Again, low hanging fruit.

BC

I took the chances because I was in a position to make some slight changes. I did the 80/20 stuff and hopefully it will be good. But I have no idea right now.

I have his books. I have read them several times over.
If you are running a stroker, you by all means must increase port size and valve size. But most of all you must ditch the intake.

GregBBRD

I've had 928 heads, with bigger valves, that flow less air than a stock head....

ptuomov

Very interesting.

Did they flow less at all lifts? Was the flow volume only slightly lower and the velocity much higher?

And then the bottom line: Did they make less power?

Or were they just simply bad with no redeeming qualities? And if they were just bad, why?

My naive belief was that unless there were other modifications as well, valve shrouding is the only way in which larger valves can hurt. Was it because of shrouding?

Of course, if other modifications were made to the head, those could always make the head flow worse (less volume at lower velocity). A rookie like me would think that the impact of the larger valve would be only minor unless the throat is modified as well. And once the throat is modified, whether the whole procedure is an improvement or not depends on how it's modified.

ptuomov

Do you know exactly what was changed as well? Was the stock valve seat machined to the larger valve or was a new seat installed? Was the throat expanded? What else?

Cheburator

The stock seats cannot take the bigger 39mm 968 valve. I installed 968 valves in my GTS heads, ceramic valve seats and new bronze valve guides. The throat was expanded and we also paid attention to the lip where the valve seats meets the intake walls - we smoothed it to perfection. Lastly, we made sure that all major casting imperfections were taken out.

My head guy works on high performance ///Ms, AMGs, Lancia Deltas, VAGs and GMs and he said that the 4-valve 928 head is one of the best designs and production quality items he has ever seen...

RyanPerrella

I was weary about reading another thread by you i must admit......

That being said, there is already some great info in here so thanks for posting it.

I too am curious about Sterlings post in regards to large intake valves in an otherwise stock GTS. I would love more specifics on this. Also maybe you can comment on the cost to have the valve cutouts enlarged to make room for the larger valves, something i recall you saying was necessary on the GTS as you had done mock-up of them.

PS: I also have this book and have no way to discredit it. But i have seen this book referenced here before as "The Bible" (Not that i believe in that either) and that all things in this book are all truths. Like i said, i have no way to discredit this book, it maybe 100% correct, but I think there is something inherently wrong with using JUST THIS BOOK as your means to making modifications. Take that for what its worth, i just think you need to open yourself up to other sources too then just Mr. Bell's ideas. Of course they could all be 100% correct, just an observation.....

kary4th

I haven't started any in months... here, take my bandwidth.

ptuomov

Another observation. One big difference between the 928 32v 5.0 and the Subaru wrx sti 2.5L cosworth is camshafts.

Subaru wrx sti 2.5L cosworth (with ACAS off):
- Intake duration 278 deg. / lift 10.7 mm
- Exhaust duration 274 deg. / lift 10.0 mm

928 s4:
- Intake duration 200 deg. / lift 9 mm
- Exhaust duration 190 deg. / lift 8mm

I think it's safe to say that the 928 camshafts are conservative. Even the high-performance aftermarket camshafts.

The point being that the right way to port the head is probably different for <200 degree cams than for >270 degree cams.

BC

If you want me to open the thread, I will.

ptuomov

I know the 4-valve heads heavily prioritized the intake flow, which makes sense. The exhaust got the short end of the stick and has to make a sharper turn. That said, it seems like a reasonable compromise.

What was their complaint about the exhaust port? The valve is pretty large for NA engine, perhaps it was the velocity of the flow (which I agree with you on, it's important)? What did they end up doing?

They could quote both CFM and the exit velocity. Come to think of that, if we know the cross-sectional area and we know the CFM, don't we know the velocity as well then? I think this ties nicely to one of the porting objectives: Get as much flow as possible without increasing the cross-sectional area.

What are your cam specifications for this head? I am wondering if the exhaust port flow problem has something to do with you running better cams.

FBIII

Are there many more 944/968 tuners that have proceeded down this road already?

ptuomov

That's an excellent point. On general porting, the 944s2 heads and 968 heads have a little more data thant the almost equivalent 928 head.

However, the hp oriented half-motor pcar crowd is mostly working witht he 2-valve 951 turbos. There are some 968 3.0L turbo conversions with 4-valve heads, but I could find any photos or data on those.

From what I've understood so far, and I don't really understand what I am writing about, is that the 928 head needs only some cleanup a la BrendanC. A short-runner single plenum intake and real camshafts are what will make the power.

Someone posted this to the forum and I copied the link right away.
http://www.norcal928.org/928cup/928CupSpecs.pdf

There's some dynamite info here about Porsche's mods for the 928 cup car. I think this doc is money. I am quoting:

"Pistons. The compression is upped by 1 mark."

"Valves. Standard valves, not especially made."

"Camshafts. Camshafts are specially made. The engine reacts very sensibly to comshafts."

"Intake system. The intake system has been specially designed for our racing engine. It resembles the Indy engine somewhat. We have shortened the intake distance considerably. This is one of the most important steps to make HP on the 928 engine."

"Cylinder head. The intake ports have been carefully ported and polished."