m42racer

Maybe I mis spoke here. what I have been told, is in a Turbo engine/head, the velocity of the air is affected by many other factors not found in a NA head. You can wish for the fastest velocity and see it on a Flow bench, but when all other parts are bolted up, everything changes. Porting or any re shaping of the Intake or exhaust ports for low lift will help here, but as soon as the backpressure rises, the shape will have little effect over volume. Yes, we can have smaller ports in a Turbo head, which will help the velocity, but you pay for it at higher boost. It is a trade off as stated. My point was poorly made, but the point I was trying to get across was that when the backpressure is up, and the Intake mmanifold pressure is up, it is all down to Comp air speed. any additional speed gained from the port shape will be very small if any at all. In a NA engine, typically the Cam centers are closer together to help with exhaust scavanging, exhaust pulses matched at the same time as the Inlet reversion wave, stuff. In a turbo engine the lobe centers are way further apart, negating and exhaust help. We cannot run the sort of overlap in a Turbo engine we do in a NA engine, or all the HP goes down the exhaust. It is really this scavanging that is added by port velocities. I think we are talking about the same thing here, but in theory we all want what we usually cannot have, or we think are getting it, when in fact we are never. I think this is a case of the same. I have seen on the Flow bench my head is tested on, the tubes fitted to measure the air speed, and they use some sort of dye to show just where the air is going. It is unfortunate that a turbo and running Exhaust system cannot be fitted to the Flow bench. I was told that on their Dyno they fit some sort of encoding wheel to the crankshaft which measures the rotational speed and the amount it jerks when the cylinders are charged. This way they can tell which cylinders are better charged, exhausted etc, and the best VE. I know that my 930 engine was fitted with a pressure tranducer in a special spark plug which showed the cylinder pressures, so I am sure that between the 2 measuring devices, we should be able to get the most out of these old heads. I hope so.

m42racer

oops, too many fingers. Sorry for the double rant.

Under Pressure Performance

Turbo Tommy wrote...

>>It was rightfully stated that there needs to be an air exchange from the intake to the exhaust. Does this imply that there needs to be some valve overlap at around TDC to achieve this? If not, can this exchange still be somewhat achieved?<<

This is a question that cannot be answered with a definitive yes or no - That does not mean it is a gray area either. Many think valve overlap is voo doo in a turbocharged engine, but it is not. Depending on the application you will have varying degrees of overlap. Some applications require very little, if any, while others require more. Without getting into the subject too deep, valve overlap becomes a requirement when trying to achieve certain objectives. A turbocharged "race" (fully prepared) engine running high RPM, perhaps larger valves, some head work, a cam, and higher boost will have significantly different needs than a slower revving street version of the same engine.

>>Does it even matter if the turbo hotside is restrictive when there is absolutely no overlap?<<

Depends "how" the hotside is restrictive - Restrictive because the turbine wheel is layed over for quicker spool, but becomes a restriction at higher RPM? Or a restriction because the hot side A/R ratio is wrong? Or because the exhaust system is too restrictive? If the housing and wheel are the culprits, then I would imagine the goal is low-end/mid-range performance. If that were the case, then the importance of overlap diminishes - Does it vanish, not necessarily, but again, it depends on the application.

>>Is there some small amount of valve overlap in stock form? Is this part of your (Scott) goal with your heads; to increase valve overlap?<<

In stock form, there is a small amount of of overlap. In regard to overlap being my goal, again, it depends on the application. I wish I had a better answer for you, but virtually everything I do is one-off, and I base how much overlap is required on the application at hand. It takes lots of math, empirical data, and real world experience/knowledge to get things right, but in the end, the results are well worth the effort.

Thanks for the questions, and thanks for reading.

Under Pressure Performance

J Chen,

The information you were given is "relatively" accurate, however, a head fully developed for a turbo application and a head fully developed for a N/A engine should have; slightly different shaped ports, different valve sizes, different valve ratios, different tulip angles, perhaps different seat angles, different seat materials, valve materials, valve to guide tolerances, valve spring pressures, exhaust heat provisions, and so forth.

So, to answer your question - In regard to port velocities, the general port function remains the same for turbocharged or N/A heads, but that does not mean that a fully prepared N/A head will perform well in a turbocharged application - Looking at flow and velocity numbers alone, in theory the prepared N/A "should" work well in a turbocharged application, but as I have mentioned above, there are too many differences between a good N/A head, and a good turbo head to us one in the other's place effectively.

Hope this answers your question.

Thanks for reading.

m42racer

Scott,

In all due respect, and I am not trying to be an *** here, but I have just read and re read your last 12 inches of post and I cannot understand what you are saying. I am dumb, I know that, but I cannot for the life of me get anything crediable or substantive out or your posts. I read it as if you talk all around the issue, but never to the point. You go one way then the other. Am I just not getting it. I understand there are many different conditions etc which change the design or spec's. But we are talking on this post about using a 951 2V Head or a 968 4V Head. Can we not discuss real specifics about each of these applications, their differences and how they will affect the outcome.

Under Pressure Performance

m42racer,

I doubt anyone thinks you are coming across as an ***, I certainly don't.

While I am doing my best to answer questions on an individual basis, I think my individual responses are being misinterpreted as direct posts on the subject of this thread, which they are not.

Within my posts collectively, I feel as though I have touched upon the major differences in the 16V and 8v heads. Having just read all my posts to this thread, I believe that they are informative enough to be used as a basic guideline for determining what one should expect from each head.

Since posting to this thread, I have had several VERY long telephone conversations with a few folks that have taken the time to give me a call to discuss the details of their projects very specifically. Which, quite frankly, is the point of all that I have posted here.

Here, there are too many variables, too many non-specifics to offer detailed, specific information. So, what I have offered here is the tip of the iceberg, a few thoughts to get folks thinking on the right track. It is not a menu, or a detailed road map, but a guide in the right direction.

Through the course of the thread, I was asked a couple of direct questions which I took the time to answer directly, and the best way I could, without any misinformation. I never mean to be vague with my answers, but simply put, it is impossible to offer "specific information" based on imaginary and hypothetical applications. There is a certain amount of data that needs to be plugged into the equation, without it, all we are doing is speculating based on hypothetical situations, which shouldn't be done - Which is my point exactly.

In regard to my last couple of posts, well, they were directed to individual questions and were slight tangents off the focus of this thread, but somewhat related nonetheless. While those posts may not have answered any questions you may have, I am hopeful I have helped those that asked them directly.

More specific details require a more specific application. Which is likely beyond the scope of what can be discussed here - If you would like to discuss specific details regarding your project, please feel free to give me a call - Ask anyone that knows me, I will spend as much time as it takes to discuss any concerns, questions, and thoughts you may have.

Sorry if any of my posts come across as confusing, or unclear, but I do my best to answer within the context of specific questions, and I do my best to stay within the scope of the thread, but from time to time things can go off on tangents a bit.

That said, nothing I have said is contradicting in any way - I have re-read all my posts and reaffirm my stand on the subject at hand. While I can see how some of what I have said can be a bit difficult to sort, as I present the pros and cons of both heads. Still, I believe my underlying points are relatively clear and straightforward.

Well, it is 4:30 am, and I need to get some sleep, so I will respond with some additional thoughts later tonight when I get home from the shop. Till then, if anyone has any questions about their project, please feel free to use the number below - I will be more than happy to be as specific as humanly possible.

As always, thanks for reading!

Russ Murphy

I am one of the beneficiaries of a lengthy phone conversation with Scott and I can say without a doubt that he is one of the most "credible" individuals I've ever spoken to in this enthusiast automotive realm. As is often the case, the more information revealed, the more difficult the choices become. If someone gives me a cut and dried, definitive answer to a complicated question, then I can be assured of a salesperson-like interest in the outcome being attached to or affecting the answer.

Jeff Lamb

To answer an earlier question in this thread, the compression ratio I have calculated for just bolting the 944S head on to a stock 944 turbo bottom end, is 9.32:1. However, I offer the following warnings:

* The stock 944 turbo piston dish is not optimal for the efficient combustion chamber of the 944S head. The 944S head works best with pistons that have a round dish and not the offset dish that the stock 944 turbo pistons have. In other words, if you bolt-on the 944S head without installing the properly designed pistons, you have messed up the squish area.

* The top of the stock 944 turbo piston has a flat portion on the exhaust side. Without modification, I think the exhaust valves of the 944S head might impact this portion of the pistons. You could install shorter rods to reduce the compression ratio and get you close to fixing the piston to valve clearance problem but you still will not have cured the squish area problem.

Therefore, in order to properly swap over to a 16 valve head, I think you should do the following:

* New pistons (with properly designed dish)
* New connecting rods
* Custom intake manifold
* Custom exhaust
* Eliminate the simplex chain and drive both cams with the timing belt

Don't get me wrong, though -> The 944S head does directly bolt-on to the stock 2.5 liter block, so there are no problems with that part of the equation. It's everything else that you need to address to make the head work that becomes the not so obvious problem.

Jeff

J Chen

Jeff,
Thanks for the info. Funny, the turbo
pistons that I've seen aren't like that.
The dish looks something like an oval
race track with parallel sides. When
I saw it , I said to myself good, I can
machine the parallel sides & dish it
like the S pistons that way I can
reduce compression as well.

Jeff Lamb

You need to take a closer look at a stock 944 Turbo piston. You will notice that the dish is offset towards the intake side of the engine and there is a relatively large flat area on the exhaust side. I have attached a picture of three pistons lined up next to each other. The one on the left is a 968 piston, the one in the center is a 944S piston and the one on the right is a 944 Turbo piston. I am not sure if there is enough material underneath the flat top portions of the 944 Turbo piston to machine very much away.

Jeff

David Nichols

Id like to ask a question in this thread since it came up.

Why not use the whole s motor except with shorter rods? (this is what I was planning) Is the crank weaker? block? Im just wondering why this wasent mentioned.

To me, this seems like the cheapest and best solution..

Duke

Well both the crank and the block are the same in both engines so it should be no problem!
But I don't the pistons of the S-engine are forged?

Other, but small, things are of course engine mount and oil return drain from turbo etc.

Jeff Lamb

You could do this, however, you would probably want to shorten the rods by as much as 2mm to bring the compression ratio back down closer to the 8:1 range. This is not optimal because the squish area has been compromised. However, this is better than running the 951 piston.

As Duke mentioned, the crank and block are the same as used in the 944 Turbo, however, I am not sure if the pistons are forged.

In total, you would eliminate having to buy new pistons but you would still end up doing the following:

* Buy used 944S complete engine
* New connecting rods
* Custom intake manifold
* Custom exhaust

You would probably save a little money by doing this (as opposed to my originally suggested approach), however, the savings would not be very much and you would achieve somewhat less than the optimal results.

Jeff

J Chen

Going shorter rods is not
a good option. It's to do
with rod angle ratio.
Theres not need to bring
compression don't to 8:1
9:1 will do unless you plan
to run very high boost.
The advantage with 16V
head is the design is way
much better than the 8V
heads.

Konstantin

hm!!!
five of the best Europen tuned 996 TT needs 16sec for the 80-240 km/h (about 48-149mph)
The New Carrera GT the Mc Laren F1 or even the GT1 with much less weight needs about 10 sec and this is using the 2nd 3rd and 4rth gear.
Are you sure the 951 can do that in 7 sec starting in 4rth gear?
maybe the stop watch was wrong . Better do teh test using some serius dataloging.
I can not imagine how a Gt1 can be over 30% slower wihle beeing so much lighter or teh Mc Laren with 627 HP and 1100 kg and an engine with excellense response and no TRurbo lag using all gears is so much slower??

I would check again. Maybe the car had a shorter ring and piniomn and another trany and shows much higher RPM that do not show the right speed at the right RPM

Konstantin