Exhaust Time Area

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Exhaust Time Area Programme

This programmme calculates the specific time-area and specific blowdown time-area for the exhaust port of an engine. From this information the programme calculates the potential power capability of the engine. If you require an increase in power from your engine, you can quickly ascertain what modification will give you the required increase.


Exhaust Port Tune Levels
TuneComments
AMaximum Power with narrow powerband
BHigh Power with modest powerband
CMedium Power with wide powerband

The "A" level tune is typically found in engines requiring absolute maximum power. Such applications include motorcycle gp racing (125cc and 250cc classes), gearbox kart racing, snowmobile drag racing and small capacity scooter racing. All these applications have either a multispeed gearbox or CVT transmission due to the relatively narrow powerband.


The "B" level tune is typically found in engines requiring a modestly high power output. Such applications include motocross racing (65cc, 85cc, 125cc and 250cc classes) and production snowmobiles. As with the "A" tune, these applications tend to have either a multispeed gearbox or CVT transmission.


The "C" level tune is typically found in engines requiring a medium power ouput but with a wider spread of power than the "A" and "B" tunes. Such applications include classic and vintage air cooled motocross bikes of various sizes.




Example 1 - Tuning a 50cc Engine

Suppose you have a 50cc motorcycle and you would like to go racing. After fitting an aftermarket exhaust plus bigger carburettor you attend a practice session and find your engine is well down on performance. You estimate if you had an extra 25% power then you may be competitive.

First measure up the dimensions of your exhaust port carefully. In our example there is one exhaust port with a width of 26mm and corner radii of 5mm ( top and bottom ). The exhaust duration is 180 deg and the transfer duration is 126 degrees.

Torqsoft - Exhaust Time Area Programme

The potential power output based on the specific exhaust time area is 12.54 hp at the crankshaft - labelled 1 in above image. The "C" beside the specific time area value indicates that this engine has a C level tune.

Examination of the potential power output based on the blowdown exhaust time area is 11.02 hp. Ideally these numbers should be identical so we can conclude that the blowdown time area is suppressing the power output.

One method of increasing the blowdown time area is to raise the exhaust port - this will open the port earlier. First try an exhaust port opening of 86 deg atdc. The results are shown below.

Torqsoft - Exhaust Time Area Programme

The potential power output based on the specific exhaust time area has now increased to 15.14 hp at the crankshaft - labelled 1 in above image - and the state of tune has been raised to a B level. The potential power output based on the blowdown exhaust time area has increased to 14.12 hp, representing an increase of 28% compared to the standard exhaust port. Hence this modification looks very promising but first the transfer time area needs to be investigated.



Example 2 - Yamaha TZ250B Road Racer

The TZ250B is a V-Twin Road Racing Motorcycle produced by Yamaha for the 1991 racing season. The cylinders have a bore of 56.0 mm and a stroke of 50.6 mm. There are 3 exhaust ports - One large central port flanked by 2 smaller side ports ( boosters ). Peak power occurs at 12,000 revs/min.

The calculated specific time-area and blowdown are shown below. It is evident that the exhaust port has an A level tune. The potential power based on the calculated blowdown time-area is 45.1 bhp at the crankshaft, 90.2 bhp for the full engine.

Torqsoft - Exhaust Time Area Programme - TZ250B



Tuning a YZ80-YZ85 Motocross Engine

Here we are tuning a YZ80 motocross engine for use in a motorcycle road racer. By using the Torqsoft exhaust time-area programme, it was possible to raise the power output of the engine by 10% just by a simple modification to the exhaust port.

The YZ80 has a bore of 47.00 mm and a stroke of 47.80 mm. The standard engine has an exhaust duration of 185 deg and a transfer duration of 126.5 deg. In standard trim the engine gives 18 hp. However, after fitted our road race pipe the engine gives just over 21 hp, as seen in the dyno curve below.

Torqsoft - Exhaust Time Area Programme - YZ80 Dyno

This pipe is designed to peak in the region of 12,750 revs/min. It can be seen from the dyno curve that the second peak of 21.0 hp occurs at 12,600 revs/min and that the first peak of 21.4 hp occurs at 11,250 revs/min. This double peak is often a sign that the engine requires more blowdown time area. This turns out to be indeed the case when we run the exhaust time-area programme, with the red cross beside the calculated specific blowdown time area indicating that the value is on the low side of optimum.

The potential power at 12,750 is calculated as 24.97 hp at the crankshaft. Multiplying this number by 0.9 to account for gearbox and chain losses gives a potential power of 22.47 hp at rear wheel.

Torqsoft - Exhaust Time Area Programme - YZ80

The exhaust time-area programme was then used to "raise" the exhaust port until our desired power increase was realized. Shown below is the calculated time areas for an exhaust duration of 189.8 deg. The potential power is now 28.1 hp at crankshaft (25.29 at wheel). This represents an increase of 12.5 % compared to the standard exhaust port. From the dyno curve it can be seen that the modified exhaust port results in a peak power of 23.6 hp at 12,950 revs/min.

Torqsoft - Exhaust Time Area Programme - YZ80

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Measurement of Exhaust Port

The image below shows the measurement of exhaust port chordal widths using dividers for an TZ250 engine.

Torqsoft - Measurement of TZ250 Exhaust Port

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