kawasakijockey
Pro
Posts: 1876
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posted January 16, 2010 11:24 AM
quote:
quote:
i heard you get better numbers if you dyno in 4th vs 6th..
On my dyno 4th 5th or 6th allways makes the same HP ..
That is pretty much true. The reason most shops dyno in 4th is to keep the drum rpm down.
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Get on the shortbus boys 'cause its time to get schooled.
2007 ZX-14
1.38 60ft
9.03 @ 149mph
8.95 @153 small shot n2o
8.68 @160mph 5lbs boost
  
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Shane661
Needs a life
Posts: 11494
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posted January 16, 2010 11:44 AM
I'm sure that someone just made this up:
Gear ratios:
When conducting "roll-on" power tests using a rear wheel dyno, the transmission gear used and final drive ratio can affect the power readings. For example, a power reading recorded in Fifth gear is generally higher than one performed in Fourth gear. Additionally, a lower final drive ratio such as 3.15:1 will often show a higher power reading than a higher 3.37:1 ratio. However, power differences may diminish as engine displacement increases. Generally, the lower the transmission gear or final drive ratio, the greater the engine loading and higher the power reading will be. Since the lower gear ratio slows the engine's acceleration rate, less power is required to accelerate the rotating and reciprocating parts. Furthermore, the air/fuel mixture has more time to stabilize within the intake tract similar to a step or steady-state test, thus resulting in a higher power reading.
Although no formal standard exists, fourth gear is normally the de facto standard for conducting roll-on dyno pulls. Some tuners elect to use Fifth gear because they prefer to load the engine more believing that hard to detect problems may be identified. Astute engine builders have been known to deliberately perform rear wheel dyno tests in Fifth gear to maximize power readings. However, that doesn't necessarily mean the power readings are wrong. Instead, it only illustrates that rear wheel dyno charts cannot be accurately compared when using different gear ratios. A Dynojet output report lists engine rpm per one mph. The exact rpm for each mph is dependent on the transmission gear, overall drive ratio and rear tire diameter. When comparing rear wheel dyno charts, make sure all roll-on dyno tests are made using the same transmission gear.
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BobC
Pro
Posts: 1736
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posted January 17, 2010 12:33 AM
 Edited By: BobC on 17 Jan 2010 11:37
With some ECU's, which change the OE map according to gear, it's useful to see what happens in 4th, 5th and 6th. My bike was being prepped for LSR and we took time and trouble with it. It was a one-off, the first 14 on our dyno and there was no ball park map to start from. If you measure success ultimately at the track you could say it worked just fine.
Perhaps 4th gear has become the accepted gear for a dyno run because it used to be the top gear on a lot of bikes some years ago, when they had skinny rear wheels and slippage was a problem in the lower gears? I know we still get it on bikes which make a lot of power, we had a turbo 'Busa on the roller recently which was a 500bhp bike and it was slipping at around 350bhp, no matter how hard we strapped it down.
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Candy Thunder Blue 2006 ZZR1400
Stock wheelbase
Max: 205.4 mph in 1.25 miles
2012 ZZR1400 in Golden Blazed Green
Brock CT Full System. etc
Max: 203.1 in 1 mile (so far)
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kingtramp
Expert Class
Posts: 121
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posted January 17, 2010 02:11 PM
Edited By: kingtramp on 17 Jan 2010 22:13
When testing on dyno most operators will select the best ( 1:1) ratio for final drive, when the chassis type interia dyno was first introduce By DJ most bikes were 4 or 5 speed gearboxes. For a 6 speed gearbox
5th gear is consider a good & efficient chose. First DJ dyno was the Model 100 in 1989.
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