Head Porting Study

Transcription

Head Porting Study

Phone Toll Free: (877)FOR-928M
Website: www.928motorsports.com
Cylinder Head Improvements for the Porsche® 928
CNC Porting – Polishing – Milling - 3 angle Valve Job – Larger Valves
The natural progression of engine development always brings a builder into the heads at some point.
Sometimes sooner, sometimes later, but ultimately no development of headers and exhaust products,
camshafts and superchargers would realize their full potential until the air flow at the heads are taken
into account.
Now to begin – we need to say that Porsche has blessed us with some pretty damn fine heads on the
32v 928. The combustion chamber is nearly ideal in shape, uniform, with a centered ignition source
and a short flame front. The 4 valves per cylinder breathe well in the stock condition, and have already
shown that they will support 500 HP without any modification at all.
Two-valve engines, on the other hand, must use intake valves more than twice the size of ours to get
the same flow, and find it difficult to prevent a large pent-roof combustion chamber and long flame
front as a result of the space envelope needed for those valves. They can compensate with longer camshaft durations and more valve overlap, but then they have added still more characteristics – not all of
them good – to the engine.
No, our friends at Porsche engineering did us proud with this head design. But can it be better? Can
they be improved? Certainly. One benefit in after-market that we have over production is we can
swing the importance further towards ultimate output with less regard to cost of manufacturing than the
OEM producers can. We can afford to spend more time on a set of heads than anyone in a production/manufacturing environment would think prudent.
And we have.
Take our recent development of a system to improve the air flow (measured in Cubic Feet per Minute
or “CFM) of the 32v 928 heads.
Valves and Seats: Starting with the valves
themselves – we looked for the largest valves the
combustion chambers could carry without increasing the size of the combustion chamber itself. An upgrade from the 36mm stock 928 intake valve to the “40 mm” intake valve (which is
more accurately 39.5 mm) was performed.
Funny how the 39.5mm intake valve actually has
a 928 part number and is used in the stock Porsche 968… raising eyebrows that the next generation of the 928 was planned – indeed larger/better intake valves had been made, when the
928 was discontinued.
Copyright 2009 928 Motorsports, LLC All Rights Reserved
The 968 valve is an excellent fit in the 928 head, requiring only removal of the small valve seat and milling to
install the larger valve seat needed.
Still, that is not to say that it cannot be improved upon
We have developed both a stainless steel valve and a titanium valve for the 928. They are lighter than stock (even
though larger), and they have smaller stem diameters
than the 968 valve.
The lighter weight reduces stress on the very long 928
timing belt, and permits the use of lighter valve
springs and higher RPM’s.
The smaller stem diameter increases air flow to the
cylinder, and as a result our valves flow more than the
968 model. If you are interested in reading more
about these valves, please visit:
http://www.928motorsports.com/parts/32v_valves.php
Have we forgotten to address or enlarge the exhaust
valves? No. They were too large already! Tests have shown that for all-out race engines, maximum
horsepower is obtained when the exhaust valve is about 75% of the size of the intake valve. When we
had a 33mm exhaust valve and a 36mm intake, the exhaust valve was almost 92% the size of the intake.
The improvement to the 39.5mm intake valves improves the ratio in the direction we want – and now
the exhaust valve is 83% of the size of the intake.
That’s the largest we can make it without increasing
the combustions chamber size in the head (which
would be a mistake) and lands us in a sweet spot for
boosted 928 engines too. The one exception to the
75% valve ratio rule is boosted motors – where a
ratio closer to 85% is ideal.
Porting: Multiple port profiles and shapes were made of a
single set of heads, creating 8 different intake and exhaust
porting profiles to measure. These were done by an experienced performance head shop, based on all their accumulated
knowledge from years of hot-rodding engines.
The heads then went to be flow-benched; to separate what we
“thought” would be the best flowing profile in each intake and
exhaust port, from those we could prove were the best. From
these flow bench tests, the very best intake profile and exhaust
profiles were selected to be modeled.
Usually, porting a set of heads is up to the skill of the tool operator as each intake and exhaust port is
hand-milled to open them up. In many cases, this is the only way it can be done. But no matter how
expert the operator, making the intake and exhaust ports match from cylinder-to-cylinder is just about
impossible. And with that irregularity, you are building in variables from cylinder to cylinder that will
negatively affect the engines smoothness, power, and longevity under high loads.
To correct this and improve on the handported performance head, 928 Motorsports LLC has developed and uses a
CNC milling program on a 5-axis mill
for optimizing the Porsche 928 heads.
This digital milling process guarantees
not only maximum flow, but repeatability from cylinder-to-cylinder.
Now, with 5-axis CNC machining, we
can confirm that each cylinder and its
ports are an exact match across the
head – giving you greater engine output
as well as smoothness and increased
safety from detonation in high-output
engines.
Here is a photo of our head-milling fixture mounted in place – it gives us digital accuracy on every set of heads by
locating the combustion chambers precisely off the locating pins.
Neither the
program nor
the fixture
were cheap to
have made!
Flow bench analysis before-and-after our CNC porting and Hi-Flow valve installation shows that you can
expect the following gains:
Comparison: The 928 Motorsports LLC modified Porsche 928 heads compared to Small Block Chevy
Brodix GB2000/2001 racing heads (NASCAR):
NOTE: maximum valve lift on the 928 with the most aggressive cams possible is only .442 on the intake,
.398 on the exhaust. For more information on our Camshafts, visit
http://www.928motorsports.com/parts/camshafts.php
Surface Flow Characteristics Designed in:
An ancillary benefit to this method is that we
are also able to design in the precise surface
finish that we want on the wall of the runner.
Time was, whenever you heard the word
“porting” it was closely followed with
“…and polishing”; so much so, that you
would think they were synonymous and inseparable. It has been only in this last decade
that we have gained the knowledge that a
smooth-bore finish is not always optimal.
Now we know that we benefit from a certain amount of roughness on the runner walls. The nature of this
rough finish determines how thick the boundary layer will be, and hence, the ‘hydraulic diameter’ of the
runner (the useable diameter in the runner that has flow is the “hydraulic diameter”).
The rough finish brings two benefits with it. First, the boundary layer it creates near the runner walls helps
prevent fuel pooling before the combustion chamber. Even fuel injectors aimed at the back of the valve
(like our Porsche 928) will flume to the sides of the runners and also hit the splitter between the valves. If
the finish were too smooth, the boundary layer would be too small, and the fuel would re-condense from
aerosol into liquid again in those spots. Obviously, this hurts air/fuel mixtures, performance, and makes tuning difficult.
Secondly, the boundary layer that the relative
roughness creates functions aerodynamically for us
where the air flow must bend around curves within
the runner. If the head passageways were smooth,
the air flow would separate from the surface easily
wherever a bend occurs, disrupting the hydraulic
diameter and making it smaller. With the relative
roughness provided, however, the air flow can resist separation and follow those bends much better
without detaching, and flow is improved as a result.
It is worth noting that this can be carried too far. A
more aggressive relative roughness makes the
boundary layer thicker, taking away from the
hydraulic diameter of the runner.
Finished combustion chamber with oversized
vales fitted.
So, while some is good, more is not necessarily better.
Our CNC port milling process allows us to specify the surface finish to achieve just the roughness we desire
to control this benefit. Not too much, not too little.
In contrast to the runners, a smooth finish in the combustion chamber is still the best bet, where the smoothness prevents carbon deposits from forming and assists in a nice even flame propagation.
Here is the finished result – before-and-after photos of the intake and exhaust ports. Note how tight radiuses to the valves have been removed and the valve splitter has been reduced and sharpened. These are easy
to pick out, while the improvements in profile and arc are far more subtle.
More information on our CNC Head Porting service including pricing is available here:
http://www.928motorsports.com/services/32_valve_heads.php
Can you guess what’s next? The 32v High-Flow intake manifold that can make full use of
these heads!

Head Porting Study

Transcription

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