At least we can breath it and that's something!

M42>> Would wet flow testing really reflect the effect with air.

 

Perfectly ok...I haven't ever attended.

Because of the misleading belief that a turbo "slams" the air in,it stays there,and then the intake valve close and you have a quadrillion hp. It doesn't work that way.

That's true... Yes,it does...but then again you just need the air necessary to fill the cylinder to 120%+ that one cylinder filling. If it flows too much,there is less restriction,it fills the cylinder really fast...and stops. Then the pressure in the cylinder is bigger than in the intake,because the air doesn't have the speed to "keep ramming it in",and your super-high-flowbench-CFM port then flow very well backwards...leaving a "static" 100%VE,because the pressure that WAS inside the cylinder filling it over 100% has blown back out past the open intake valve again.

It will,if the cylinder can trap the air there. If it flows more than the cylinder can "trap",then you lose potential power. It's not about big flow...it's about max filling every time. And that filling has to occur just before the intake valve closes,if it happens too fast,your gain in flow will literally just blow back out the same way it came in.

If you have a too-small,too-little-flowing port,that is correct. In this case,the opposite is the problem.

 

The bottom line is that the head (vavle flow/timing included)is always a comprimise, otherwise the powerband would start just off idle and would continue to oblivion.
If the velocities become too high resulting from too small openings, the static pressure drop WILL NOT be recovered by the ram air effect into the cylinder.

Skunk,
your making it sound like bigger openings don't offer more power.
They certainly do, but only at the higher RPMs and only if the ports were too small to begin with. You're probably right; that the 951 ports are big enough.
Now, if we increase displacement, bigger turbo, more aggresive valve timing; then all of a sudden the ports could be too small.

If a vendor wants to sell a P&P job on a stock 951, then that would probably be a waste of time and money.

On the issue of a spit shine polish; I understand it increases flow potential but decreases turbulence (which is not a good thing). Another comprimise.

Skunk,
I don't know what you meant by 120+
If you're referring to VE potential; I've never heard of anything more than 105%.

 

Understand we are talking about a turbo engine and not a N/A one. P/P on a N/A engine can produce gains but generally moves the HP and torque curve to the right of the scale. on a turbo engine, it allows for greater flow at lower pressure since the turbo is pumping air thru restrictions all the way to the cylinders. Also, in a N/A engine, the intake tract and port/valve design is critical to promote a good charge mixture. But with the turbo, the compressor impeller produces a significant degree of tumbling of the outgoing air that I doubt the the casting roghness of the of the head are necessary to promote good mix between fuel and air

Jpr

 

If you must port and polish then polish the outside of the head and buy a nice port wine and drink it.

The ‘good old days’ of porting and polishing died a while ago. The “bigger hole – more air – more power” theory does not apply. The interactions of the shape/texture/design of the intake ports and the production of power is seriously complex and, as any decent head man will tell you, not scientifically well defined.
The most important product of modern cylinder head design is the turbulence created in the combustion chamber – a pretty tough thing to measure. The propagation of the frame front can be greatly effected by the turbulence, by as much as a factor of two. The speed of propagation has every thing to do with controlling detonation and achieving favorable conditions for additional ignition timing. So if anybody thinks they can grab a die grinder and alter their ports to add turbulence to the combustion chamber good luck.

Back in the days when I helped out on the Trans Am circuit I got to see the real high dollar heads and you would swear that they wouldn’t work well. They were the result of a little science and a lot of trail and error. If you have 30 or 40 heads to experiment on (the results can only be shown on the dyno so you will need to assemble and test each engine) then you might be able to show some real improvement.

That being said – it doesn’t hurt to clean up any casting abnormalities and maybe clean up the valve guide casting area. A good valve job will get good results.

Chris White


PS - the turbo may create some turbulence at the compressor but after it has traveled through the intercooler and quite a bit of piping and the throttle body the turbulence is pretty much "straightened out."

 

Not only big enough...if you plan on running a 3.2 + -litre engine and boost 40 psi,it's about ok as it is,then you just need to make it flow at that cross-sectional area...
See above.

Unless that vendor shrank the port to match,it would be.


My "VE-record" on an engine built by myself is 126.2%...but by massive flow/wellmatched cam/intake alone that cannot be done,then I guess 105% is the best you can get.. To get more you need optimum port area/speed.

 

Oh yeah – one other issue on head texture – remember that at any kind of real load the fuel injector will be squirting fuel at a closed valve. 80% duty cycle refers to the injector firing for 80% of the complete 4 cycle. Batch fired systems fire twice per each 4 cycle (720 degree) occurrence. One shot will be during the intake cycle but the other one will be 180 degrees out. The texture of the walls can effect the ‘wetting’ factor. Very smooth surfaces tend to increase the amount of fuel that will come out of suspension.

Just as a point of interest – the Tec3 system even has an adjustment factor for the amount of intake port ‘wetting’. If you have a set up that really encourages intake wetting then you will see an initial lean out when opening the throttle. This is because the fuel is ‘sucked’ of the walls during a closed throttle decelerating condition and when you first crack the throttle a certain amount of fuel will drop out of suspension to rewet the walls.

Polish the outside….

Chris White

 

Hmm,
So if the port is too big... you need to make it smaller.


So... how do you make it smaller? Can you install a coating and then machine the the coating to the proper shape? Sleeve it?

I have been following these threads by Skunk Works and I think I realized why it is so hard to get power from the NA motor. The dam port is too big. Why did they make it so big anyway?

 

How about making a venturi & slip it between the valve guide
& the entrance of the port M758 ? Just like a carb.

 

What is this venturi system and "venturi" I keep reading about..what does it do and why does deleting it help?

-Richard

 

Hi Rogue,
That venturi & the venturi that I'm talking about
is two different things. The venturi delete that
you read about refers to the multitude of spagetti
hose under the intake manifold.

 

My guess is the ports are oversized because they also had to work in the 928... isn't the N/A 944 head identical to the ones on 16-valve 928's? Doesn't explain the turbo head though, since they went the extra mile with the exhaust ports anyway - couldn't have been that difficult to redo the intake side as well...

 

Sami I know it goes with the 928, but hell the 928 had TWO heads not one. It started life in 4.7L form which means each head supplied and even small displacment than the 2.5L 944 motors.

I have to think there is some reason the port is so big. Fuel economy, emissions, longevity, cheaper?

 

Does the 2.7L NA head flow enough to support a 3.0L T application or would “hogging it out” be beneficial? (Especially since it won’t be a problem opening up the exhaust). Additionally I would love to see some real data on how much gain or loss in spool would be found not using the ceramic liners in the exhaust port. While I can see the intent of the liners, especially with the distance between the turbo and the head, I’m not sure of any other P-cars (stock at least) that used them. What head did Porsche use on the 968 T? I’ve yet to do any research on this part of my project but want to get started before the fall. I’ve also pondered just keeping a 968 head and getting some bowl work and just cleaning up the ports a bit.

 

"The most important product of modern cylinder head design is the turbulence created in the combustion chamber – a pretty tough thing to measure. The propagation of the frame front can be greatly effected by the turbulence, by as much as a factor of two. "

Not true. well documented and tested. Testing the turbulence is now done with Swirl Meters, Port mapping etc. This technology is well used by many development houses now. What used to be THOUGHT is now known not to be so. In fact on performance engines it has been known for years that turbulence in the port is NOT a good thing. Yes it helps in the atomization of the fuel, but turbulence's take up huge amounts of volume in the port, thus lowering the mass flow. They also hurt the velocity. The new flow technology now can show this.

Whatever the design is, many other factors play roles here. Valve timing, Cam lobe design, Intake manifold, aspiration type etc all will help or hurt you, if the whole package is not in concert with one another.

"The ‘good old days’ of porting and polishing died a while ago."

Very true. I see very new approaches to what used to be thought as the best.
The question was asked if wet testing reflects what air does. Yes it does. Liquids and air do similar things under the same conditions.