Porting Tech Article
Eric Gorr
An Overview of Basic Principles and Processes
By:
www.eric-gorr.com
The process of cylinder porting is a funny paradox. The people in the
market to buy it are looking for information and the people in the market
of selling it are hiding information on porting. So much myth and
misinformation is associated with this complex machining and metal
finishing process. Yet the tooling is easily available and the design of
the ports is actually quite straightforward with resources like computer
design programs. This article is an overview of how porting is performed
and how it can benefit your performance demands.
Two-Stroke Principles
Although a two-stroke engine has fewer moving parts than a four-stroke
engine, a two-stroke is a complex engine with different phases taking place
in the crankcase and in the cylinder bore at the same time. This is
necessary because a two-stroke engine completes a power cycle in only 360
degrees of crankshaft rotation, compared to a four-stroke engine, which
requires 720 degrees of crankshaft rotation to complete one power cycle.
Two-stroke engines aren't as efficient as four-stroke engines, meaning that
they don't retain as much air as they draw in through the intake. Some of
the air is lost out the exhaust pipe. If a two-stroke engine could retain
the same percentage of air, they would be twice as powerful as a
four-stroke engine because they produce twice as many power strokes in the
same number of crankshaft revolutions.
The following is an explanation of the basic operation of the two-stroke
engine.
1. Starting with the piston at top dead center (TDC 0 degrees) ignition has
occurred and the gasses in the combustion chamber are expanding and pushing
down the piston. This pressurizes the crankcase causing the reed valve to
close. At about 90 degrees after TDC the exhaust port opens ending the
power stroke. A pressure wave of hot expanding gasses flows down the
exhaust pipe. The blow-down phase has started and will end when the
transfer ports open. The pressure in the cylinder must blow-down to below
the pressure in the crankcase in order for the unburned mixture gasses to
flow out the transfer ports during the scavenging phase.
2.Now the transfer ports are uncovered at about 120 degrees after TDC. The
scavenging phase has begun. Meaning that the unburned mixture gasses are
flowing out of the transfers and merging together to form a loop. The
gasses travel up the backside of the cylinder and loops around in the
cylinder head to scavenge out the burnt mixture gasses from the previous
power stroke. It is critical that the burnt gasses are scavenged from the
combustion chamber, to make room for as much unburned gasses as possible.
That is the key to making more power in a two-stroke engine. The more
unburned gasses you can squeeze into the combustion chamber, the more the
engine will produce. Now the loop of unburned mixture gasses have traveled
into the exhaust pipe's header section. Most of the gasses aren't lost
because a compression pressure wave has reflected from the baffle cone of
the exhaust pipe, to pack the unburned gasses back into the cylinder before
the piston closes off the exhaust port.
3. Now the crankshaft has rotated past bottom dead center (BDC 180 degrees)
and the piston is on the upstroke. The compression wave reflected from the
exhaust pipe is packing the unburned gasses back in through the exhaust
port as the piston closes off the port the start the compression phase. In
the crankcase the pressure is below atmospheric producing a vacuum and a
fresh charge of unburned mixture gasses is flowing through the reed valve
into the crankcase.
4. The unburned mixture gasses are compresses and just before the piston
reaches TDC, the ignition system discharges a spark causing the gasses to
ignite and start the process all over again.
What is Porting?
Porting is a metal finishing process performed to the passageways of a
two-stroke cylinder and crankcases, that serves to match the surface
texture, shapes and sizes of port ducts, and the timing and angle aspects
of the port windows that interface with the cylinder bore.
The port windows determine the opening and closing timing of the intake,
exhaust, blowdown, and transfer phases that take place in the cylinder.
These phases must be coordinated to work with other engine components such
as the intake and exhaust system. The intake and exhaust systems are
designed to take advantage of the finite amplitude waves that travel back
and forth from the atmosphere. Porting coordinates the opening of the
intake, exhaust, and transfer ports to maximize the tuning affect of the
exhaust pipe and intake system. Generally speaking porting for more
mid-range acceleration is intended for use with stock intake and exhaust
systems. Porting for more high rpm power is intended for use with
aftermarket exhaust systems and especially clutching mods.
Terminology
These are some common words and terms associated with porting.
Ports - Passageways cast and machined into the cylinder.
Ducts- The tube shape that comprises the ports.
Windows- The part of the port that interfaces the cylinder bore.
Exhaust Port- The large port where the burnt gasses exit the cylinder.
Exhaust Bridge- The center divider used on triangular shaped exhaust ports.
Sub-Exhaust Ports- The minor exhaust ports positioned on each side of the
main exhaust port.
Front Transfers- Transfer ports link the crankcase to the cylinder bore.
The front set (2) of transfers is located closest to the exhaust port.
Rear Transfers- The rear set of transfers are located closest to the intake
port.
Auxilary Transfers- Some cylinders have a minor set of transfers located
between the front and rear sets.
Transfer Port Area Ratio- The area of the crankcase side of the transfers
divided by the area of the port window.
Boost Ports- The port or ports that are located opposite of the exhaust
port and in-line with the intake port. These ports are usually by-pass
ports for the intake or piston and sharply angled upwards to help direct
the gas flow during scavenging.
Port-Time-Area- A mathematical computation of the area of a port, divided
by the displacement of the cylinder, and multiplied by the time that the
port is open. The higher an engine revs the more time-area the port needs.
The higher the piston speed the less time available for the gas to flow
through the port.
Duration- The number of crankshaft angle rotational degrees that a port is
open.
Opening Timing- The crank angle degree when the piston uncovers the port.
Crank Angle- Measured in units of degrees of crankshaft rotation. On a
two-stroke engine there are a total of 360 degrees of crankshaft rotation
in one power cycle.
Port Side angle- The side angle of a port measured at the window, from the
centerline of the bore with the exhaust port being the starting point (0).
Port Roof angle- The angle of the top of the port at the window.
Port Height- The distance from the top of the cylinder to the opening point
of the port.
Top Dead Center (TDC)- The top of the piston's stroke.
Bottom Dead Center (BDC)- The bottom of the piston's stroke.
Chordal Width- The effective width of a port, measured from the straightest
point between sides.
BMEP- Brake Mean Effective Pressure.
Loop Scavenging- Scavenging is the process of purging the combustion
chamber of burnt gasses. Loop scavenging refers to the flow pattern
generated by the transfer port duct shapes and port entry angles and area.
The gasses are directed to merge together and travel up the intake side of
the bore into the head and loop around towards the exhaust port.
Blow-Down- This is the time-area of the exhaust port between the opening
time of the exhaust and the transfers. When the exhaust port opens the
pressure blows down. Ideally to below the rising pressure of the gasses in
the transfer ports. Blow-down is measured in degrees of crank rotation and
time-area.
Effective Stroke- The distance from TDC to the exhaust port height. The
longer the effective stroke the better the low end power.
Primary Compression Ratio- The compression ratio of the crankcase.
Secondary Compression Ratio- The compression ratio of the cylinder head.
Compression Waves- Pressure waves that reflect from the end of the intake
or exhaust system and return to the engine.
Expansion Waves- Pressure waves that travel from the engine and out to the
atmosphere.
Tools of the Trade
There are two main types of tools used in porting, measuring and grinding.
Here is an overview of how these tools are used.
Measuring
The basic measuring tools include a dial caliper, an inside divider, and an
assortment of angle gauges. The caliper is used to measure the port height,
the divider is used to measure the chordal width of the port, and the angle
gauges are used to measure the roof and side angles of the ports. Calipers
and dividers are available from places like Sears or industrial supply
stores. Angle gauges are fashioned from cardboard and specific to
individual cylinders.
Grinding
The most common grinding tools are electric powered. They consist of a
motor, speed control, flexible drive shaft, tool handle, and tool bits. The
power of these motors ranges from 1/5th to 1/4th HP with a maximum rpm of
15,000. Popular manufacturers include Foredom, Dremel, and Dumor. Each
company sells a full compliment of accessories for all sorts of hobbyist
activities. The most popular source for cylinder porting tools and
accessories is CC Specialty in Tennessee (1-800-762-6995).
The tool handles and bits are the secret to porting. There are two types of
tool handles; straight and right angle. The straight tool handles are used
for machining the port ducts. The right angle tool handles are used to gain
access to the port windows from the cylinder bore. Over the years I've
tested hundreds of different tool bits and arrived at some simple materials
and patterns for finishing the different surfaces of a cylinder. The
materials of a cylinder range from aluminum as the base casting material,
to a cast iron or steel liner, or nickel composite plated cylinder bores.
Here are the basic tool bits used for porting; tungsten carbide works best
for aluminum, steel, and cast iron, stones are best for grinding through
nickel composite. The tungsten carbide tool bits are available in hundreds
of different patterns and shapes. The diamond pattern is the best
performing and the shape of the bit should match the corresponding shape of
the port. Stones, or mounted points as they are termed in industrial supply
catalogs, are available in different shapes and grits. The grits are graded
by the color of the stones. Gray being the most course and red being the
most fine. The finer the grit the faster it wears but the smoother the
finish.
Making Ports Bigger
Generally speaking, if you're trying to raise the peak rpm of the powerband
with an aftermarket exhaust system of clutching on a snowmobile, the ports
will probably need to be machined in this manner; widen the transfer ports
for more time-area and raise the exhaust port for more duration. Most OEM
cylinders have exhaust ports that are cast to the maximum safe limit of
chordal width. Often times widening the exhaust port will cause accelerated
piston and ring wear. In some cases the port will be widened so far that it
breaks through into the water jacket. Transfer ports should be widened with
respect to the piston ring centering pins. The ports should have a safe
margin of 2mm for the centering pin.
Making Ports Smaller
Ports are purposely made smaller for several reasons. One or more of the
ports could have been designed too big, or a well meaning tuner may have
been overzealous, or a customer may of asked for more that he could handle.
There are performance gains to be had from smaller ports, for high altitude
compensation or for more punch for trail and snowcross riding. The exhaust
ports can be made smaller by two ways, either by simply using a thinner
base gasket (Cometic makes graded gaskets) or by turning-down the cylinder
base on a lathe. The other method is by welding the perimeter of the port,
although that entails replating the bore. Transfer and intake ports can be
made smaller with the use of epoxy. Brand name products like DURO Master
Mend or Weld-Stick are chemical resistant, easy to mold to fit, and can
withstand temperatures of 400F. Master Mend is a liquid product and
Weld-Stick is a semi-dry putty material. The epoxy can be applied to the
roof of the ports to retard the timing and reduce the duration. It can be
applied to the sides of the transfers to reduce the time-area, and it can
be applied to the ducts to boost the primary compression ratio (crankcase
volume).
Porting for Big Bores
WISECO offers big bore piston kits for most popular snowmobiles. The
average increase in displacement is 50cc per cylinder. This requires that
the cylinder be over-bored 4-8mm. Because the ports enter the cylinder bore
at angles, when the bore size is increased all the ports drop in height.
The steeper the port angles the greater the port height will drop. Lower
port heights mean retarded timing and reduced duration. The exhaust port
gets narrower and the transfers get wider. A larger displacement cylinder
will require more port-time-area. Normally the exhaust port needs to be
raised higher than stock to compensate for the compression ratio. If you're
adding a set of performance pipes at the same time as the big bore, you'll
need to compensate the port timing to get the best gains from the pipes.
It's a complicated thing. Sometimes tuners use thicker base gaskets to
compensate for big bores, but ideally the port-time-area needs to be
calculated before any serious porting changes are made. If you are strictly
trail riding at high altitude, you can just have the cylinders bored and
replated because the porting will inherently change to suit that type of
application.
Computer Design Software
The best-kept secret in high performance tuning is the use of computer
design software. These products became popular about six years ago when Tom
Turner adapted the SAE programming code and added in his own empirical data
from his career as a motorcycle drag racer and tuner. Tom's products are
named TSR software (www.tsrsoftware.com) and available in MS-DOS format for
PCs. The programs cover individual engine components. The most popular
program for porting is PORTTIME 2000 and it sells for $200. It features
target specs for all sorts of vehicles including snowmobiles. Basically a
tuner types in engine spec dimensions like the bore and stroke plus all the
individual port measurements. The program runs mathematical calculations in
order to provide a simulation of how the porting changes will affect the
engine's powerband. TSR's programs will get a tuner 90% to the engines
potential. The next level of programs includes 1D and 3D gas dynamics
simulators. Dynomation is a 1D simulator that sells for about $500
(www.audietech.com). It enables tuners to combine all the engine components
together to simulate dyno runs on the computer so they can save time
machining metal and swapping parts on actual dyno runs. Dr. Gordon Blair
programmed the 3D simulator named Virtual Two-Stroke, the most well know
two-stroke engine researcher. His program is marketed through Optimum
(www.optimum-power.com) through lease programs. Priced at $12,000 a year,
it's intended for use by engine manufacturers.
Adam H.
Thanks for the article.
Adam
Pete Plassmann
I still wouldn't do it myself, anymore. (Had bad experiences with porting
TS125s and RD350s about 15 years ago. Evil Bikes!)
Thanks, Eric!
Pete P.
Burt B. Lamborn
--
Burt Lamborn Phone: (435) 797-4403
Space Dynamics Laboratory UMC 9700 Fax: (435) 797-4562
1695 North Research Park Way email: burt.l...@sdl.usu.edu
North Logan, UT 84341-1947
Jay C
news:3857CFF7...@excite.com...
> Wow! Great secrets from the Gods!
> I still wouldn't do it myself, anymore. (Had bad experiences with porting
> TS125s and RD350s about 15 years ago. Evil Bikes!)
>
> Thanks, Eric!
<snip 15k of EG quote>
Hey Pete - when you want to post a "neeto" reply to a 15k post, PLEASE don't
quote the whole thing!!
Thanks -
Jay (still on a phone line)
Mike W.
Eric Gorr wrote:
>
> CYLINDER PORTING
> An Overview of Basic Principles and Processes
Thank you for a really excellent article.
In
> the crankcase the pressure is below atmospheric producing a vacuum and a
> fresh charge of unburned mixture gasses is flowing through the reed valve
> into the crankcase.
Given analogies I'm familiar with from high-speed electronics, I would
imagine that starting and stopping the fuel-air mass cannot be done as
well as one would like at higher RPM... particularly if you also want
something to be effective at low and mid range, though I'm only
speculating. One of the technologies that led to higher speed switching
was one where instead of constantly shoveling electrons into and out of
a device and the parasitics of it's local power-environment was a family
called ECL/PECL which constantly flowed the electrons into the region
and simply alternated what bucket it put them in once they got on-board
the device. Sort of like, fill bucket A on this 180-deg phase and then
fill bucket B on the next. Crude analogy but close enough. Would there
be some benefit to keeping the mass of the slug (that's redundant) of
air-fuel mix in motion and distributing it to two smaller pistons? Could
there also be some useful moving-mass considerations in the piston/crank
assembly too?
> Computer Design Software
> The best-kept secret in high performance tuning is the use of computer
> design software. <snip> It features
> target specs for all sorts of vehicles including snowmobiles. Basically a
> tuner types in engine spec dimensions like the bore and stroke plus all the
> individual port measurements. The program runs mathematical calculations in
> order to provide a simulation of how the porting changes will affect the
> engine's powerband. TSR's programs will get a tuner 90% to the engines
> potential. The next level of programs includes 1D and 3D gas dynamics
> simulators. Dynomation is a 1D simulator that sells for about $500
> (www.audietech.com). It enables tuners to combine all the engine components
> together to simulate dyno runs on the computer so they can save time
> machining metal and swapping parts on actual dyno runs. Dr. Gordon Blair
> programmed the 3D simulator named Virtual Two-Stroke, the most well know
> two-stroke engine researcher. His program is marketed through Optimum
> (www.optimum-power.com) through lease programs. Priced at $12,000 a year,
> it's intended for use by engine manufacturers.
He's giving it away. Anyway, one of the biggest unrecognized trends in
my industry (high-speed/high-precision digital electronics) has been the
escalation of engineer's (and their organizations) mindless reliance
upon simulation results in situations where they go so fast that they
outgrow the models capabilities to deal with all of the little
parametric issues that become significant at speed. The mathematics and
any behavioral data that underlie the models (for electronics, and
likely mechanics too) is specific to certain
conditions/assumptions/operating scenarios but that is unfortunately as
widely recognized as it needs to be. The result has been serious gaffes
in designs and system/device interoperabilites when things are finally
prototyped (or worse, built in sufficiently large numbers), and a class
of practicing designers who don't understand nearly enough about the
actual real-world high-speed behaviors because they use simulators and
are at most, only hands-on (actually measuring the parametrics) in
trivial ways. Are there ways of measuring what is happening inside of
the intake-combustions-exhaust path other than just the overall system
measurements on the dyno?
Again, thank you for a great article! Maybe you should get it reviewed,
add some graphics and submit it to a magazine.
Mike W.
96 XR400
BRC/AMA
Pete Plassmann
The magazines wouldn't print it. Too many advertisers wuldn't want their
"secrets" revealed.
Pete P>
"Mike W." wrote:
> Eric Gorr wrote:
> >
> > CYLINDER PORTING
> > An Overview of Basic Principles and Processes
>
Smiles
(hack them is more like it) until I started reading Eric's stuff in Dirt
Rider ... then in some UK mag (I did use other resources ... but Eric's
stuff didn't require my engineering degree to understand what he was
saying). My engine's started to rock after that ... I got out of it because
I didn't have a name ... was not marketing my self correctly and had to
compete against Eric who has a reputation, great prices and does excellent
work. I still do my own stuff for fun:-)
s
http://boardnbike.com
BMX and Sk8 Boards
Pete Plassmann <ppl...@excite.com> wrote in message
news:385928E2...@excite.com...
> Mike:
>
> The magazines wouldn't print it. Too many advertisers wuldn't want their
> "secrets" revealed.
>
> Pete P>
>
> "Mike W." wrote:
>
> > Eric Gorr wrote:
> > >
> > > CYLINDER PORTING
> > > An Overview of Basic Principles and Processes
> >
Mike W.
Pete Plassmann wrote:
>
> Mike:
>
> The magazines wouldn't print it. Too many advertisers wuldn't want their
> "secrets" revealed.
>
> Pete P>
>
Hey Pete. The standard way around that is to align with one of them as a
sponsor and place it in the article. Then you not only get published,
but paid well to do so.
Mike