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From: egorr@interaccess.com (Eric Gorr) 

TWO-STROKE TUNING WITH COMPUTERS

BY ERIC GORR

The design evolution of two-stroke engines was greatly accelerated by the

proliferation of computers. Tom Turner is one of the most experienced two-stroke 

tuners in America. He markets his own line of design software. Some of the programs 

are based on Dr. Blair's work (but "Improved Greatly") but most are TSR original designs. Turner 

receives feedback from other tuners who use his programs for applications ranging from mx,

road racing, watercraft, ATVs, and snowmobile racing. He uses this feedback to

continually fine tune the programming code for better engine performance.

I use Turner's Programs everyday in my tuning business. Here is a case study of how I used 

computer design software to improve the performance of a Suzuki RM250 (1992-95)

ANALYZING THE STOCK ENGINE

The design of the cylinder head, port timing, and pipe must be coordinated to

peak simultaneously in the RPM band. There is a proper order to the design

process. First I start by measuring the engine specifications and inputting

the dimensions into the TSR Computer Programs, to look for shortcomings in design.

The cylinder's exhaust port time area peaks at a relatively low 7,200 rpm. The

transfer port time area is just the opposite. The ports are very wide and have

high time area and a high rpm peak. However they don't start to flow properly

until about 5,000 rpm. The problem with the RM250 is that the exhaust port is

too small and the transfers are too big. That is why the stock engine hits hard

in the mid-range, because the exhaust and transfer ports come into sync right

when the exhaust valves open. However the transfers are too large to flow

adequately at low rpm to give the engine tractable power and the exhaust port

is too small to give the engine any over rev. This is an easy problem to fix.

REDESIGNING FROM THE INSIDE OUT

This is the proper order for redesigning the engine specs; exhaust port time-

area, blow down time-area, transfer port time-area, cylinder head, reed valve,

and pipe.

EXHAUST PORT

The chordal widths of the exhaust and transfer ports, and

the port opening timing were input into Turner's PORT2000 program to determine

the rpm band width of the stock ports. Time-area is a calculation

based on the width of the exhaust port, the point when the port opens, and the

total port open duration in relation to RPM. It is a very difficult

calculation to do on a hand-held calculator but the computer performs it fast,

accurate, and easy. The program enables the tuner to adjust the exhaust time-

area for the power band we are tuning for. On the RM250 the exhaust port's

timearea was too low and the exhaust opening timing and width had to be increased

to allow the engine to peak at 8,800 rpm.

BLOW-DOWN TIMING

With the exhaust port in the correct position, we

need to set the opening time of the transfer ports and time-area so the cylinder has

adequate blow-down time area. The blow-down phase occurs from when the

exhaust port opens to when the transfer ports open. Too little blow-down will

cause burnt mixture gasses to flow into the crankcase when the transfers open and

pollute the un burnt gasses. Turner's PORT2000 program has an

automatic "FINDIT" feature that is used to determine the optimum blow-time-area

of the blow-down phase. FINDIT tells you the distance from TDC where the transfer

ports should open. Now that you have the opening timing of the transfers, you

can use the PORT2000 program to find the optimum width dimensions of the

transfer ports. The RM250 transfers were way too wide and the original

transfers didn't peak until 10,000 rpm. I narrowed the widths of the rear

transfer ports with epoxy in order to reduce the transfer port time-area to

peak around 8,800 rpm.

CYLINDER HEAD

Once you have determined the optimum opening timing for the

exhaust port, you will have the effective stroke length. That is the distance

from TDC to when the exhaust port opens. That is the key to designing the

combustion chamber because it is linked to the compression ratio. Turner's

COMPRESS program enables you to design the combustion chamber with variables 

http://tsrsoftware.com/compress.htm such as; outside air density or altitude, peak rpm, 

fuel octane rating, squish band area, and compression ratio versus bore size. 

The output of this program tells you the trapped volume and the squish band width 

so you can cut the head to specifications on a lathe. The SQUISH program enables you to

calculate the Maximum Squish Velocity (MSV) of turbulence in the combustion

chamber. This is an important design tool for finely targeting the "hit" in

the power band of mx bikes. I have experimented with this program extensively

and determined a link between power band and fuel. For example, a high rpm

road race engine requires a hemi-shaped combustion chamber with a narrow

squish band and unleaded fuel of about 104 octane. The MSV of this head design

will be about 13 meters per second (m/s). That contrasts greatly with a

super cross engine which demands an explosive mid-range power band and an MSV

rating of 23 m/s. Two-stroke engines that burn methanol require even higher

squish velocity because methanol is a relatively slow burning fuel.

EXPANSION CHAMBERS:

Turner's NEWPIPE program is a combination of an

expert system with look-up tables for specified applications and a mathematical

calculation program. These features enable you to specify certain variables like; the

type of induction system, engines with exhaust power valve systems, exhaust gas

temperature, metal thickness, the width of the tuned rpm band, port timing, port

diameter, etc. The program draws the pipe on a graphic screen, prints the dimensions

of the pipe, and a companion program (LAYOUT) prints the dimensions of the

individual cones. This feature makes it easy for a custom pipe builder o fabricate

a pipe. A third program that is part of NEWPIPE package (SCRNCONE) will

quickly give you cone angles off of any pipe you can measure lengths and

diameters. A forth program (VELOCITY) does a study of the exhaust port

diameter to rpm, to see if port is sized right to do what you want. The NEWPIPE program 

is very accurate and corresponds closely to actual inertia-dyno tests. To date this

program has made several large company's LOT$ of money!

REED VALVES

Run REEDVALV.exe and test the frequency range of

reed valves and petals. The effective area of the reed valve ports, the petal

material and thickness determine what RPM range that the reed valve will be

best tuned for. The 1993 RM250 reed valve has a 30 degree angle which helps

low end power but the reed material (fiberglass) is heavy and flutters at about

6,000 rpm. Carbon fiber reeds are lighter and stiffer so they resist

fluttering. The program allows us to quickly check the affect of different reed

materials. Often times the motorcycle manufacturers design the reed valves for

the wrong range. For example, in 1993 I computer-designed an engine for top

American enduro rider Kevin Hines. The 1993 Husky 250cc WR has a giant reed

valve with carbon fiber petals. Kevin wanted the engine to peak at 7,000 RPM

with a "four-stroke" type power band. The stock reed valve has a frequency

range of between 8,000 to 13,000 RPM. Using the computer program, I determined

that I needed to fill epoxy in the outside corners of the reed valve to reduce

the total area and install #615 Boyesen Dual-Stage reeds from a 1982 RM500

Suzuki. This simple mod dramatically improved the low to mid-range power to

make the engine more suitable for enduro riding. Yamaha uses a common reed

valve for the 125 and 250 engines. The reed valve is actually designed to work

on the 125. When installed on the 250 cylinder, the reed stop plates block-off

the rear transfer port. Installing the Boyesen RAD valve on the YZ250 yields more

top end power because it is properly designed for the requirements of the engine.

Plus RAD valves don't use reed stops so the crankcase transfer ports aren't blocked.

1) This is a calculation print-out of the stock RM250 exhaust port's time-area.

The port is maxed out at about 7,000 rpm where the BMEP reaches 150. This port

must be raised and widened in order to push the rpm peak to 8,800 rpm.

2) This is a calculation of the transfer port time-area for the stock RM250. The

TSR programs enable a tuner to examine the effective rpm range of the

transfers. The stock ports are too wide causing the transfers to peak at 10,000

rpm. A 250 mx or enduro bike needs to pull from about 3,000 rpm and peak around

8,500 rpm. The rear transfer ports were narrowed using epoxy to shift the rpm

range of the transfers lower in the rev range.

3) This is an example of TSR's COMPRESS program, used for designing cylinder

heads. This program gives helpful design information that enables a tuner to

adjust a cylinder head for any application, even for air density/altitude. This

program saves time in trial and error machining and testing.

4) This is an example of TSR's NEWPIPE program. It enables tuners to design

expansion chambers that correspond closely to changes in other engine

components like a tuned cylinder. The program even calculates the dimensions of

individual cones of the pipe. ALL New ‘NEWPIPE software in 2002’.

5) This is a drawing of the modified cylinder porting specs for a RM250.

6) The computer programs require precise measurements of engine components. Here

an inside divider is used to measure the chordal width of the exhaust port.

7) This is an example of a measurement of the transfer ports.

8) A burette tube is used to measure the volume of the cylinder head for use in

designing the cylinder head.

9) Jesse Williams of MOTOWERX in America demonstrates how a pipe is fabricated and

fitted to the bike. Jesse offers a custom pipe building service using a

combination of TSR SOFTWARE Programs and an inertia dynamometer.

NOTE: TSR Software is continually being improved so check with TSR for latest Software.

Eric Gorr --- E-mail:...info@eric-gorr.com

This material is for personal use only. Republication and redissemination, including posting to newsgroups, is expressly prohibited without the prior written consent of TSR Software.

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