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Old 17-10-19, 07:13 AM   #1
Seeker
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Default Yamaha MT125 VVT

This new 125 from Yamaha has variable valve timing. I thought the idea of VVT was to produce more power/torque but this model produces less torque than the none-VVT, dropping from 9.1 ft lb to 8.5 ft lb. Maybe it helps with emissions?



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Old 17-10-19, 08:30 AM   #2
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Default Re: Yamaha MT125 VVT

On bigger engines VVT is usually used to improve tractability at low revs while still maintaining gas flow at high RPM.

In a regular car engine the valve timing used to be designed(*) for smooth running at low RPM, which resulted in insufficient airflow at high RPM and therefore reduced power. At the other extreme, a race engine has valve timing is designed for maximum power at high RPM, but they are lumpy and un-driveable at low rpm.

My old Toyota had VVT using two different cam profiles. At low rpm the engine used an inlet cam with small openings of the valve to provide smooth power for regular driving. Around 5700 rpm the ECU activated a solenoid valve which used oil pressure to switch to a high lift cam. This provided improved gas flow and more power at high RPM.

VVT has also been used on some models to reduce emissions across the rev range, but that's no fun.

Just my thoughts,

Keith

(*) These days it's all about economy and emissions. Boring!!

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Old 17-10-19, 03:55 PM   #3
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Default Re: Yamaha MT125 VVT

In most modern car engines the variable valve event controls are used to regulate "internal EGR" by changing the valve overlap period. This lowers raw NOx production. It can also be used to modify the underlying cycle characteristics to reduce pumping losses etc at part throttle, improving fuel efficiency.
Depending on how flexible the system is, it can be used to various ends, including optimising the volumetric efficiency across the speed range as described by keith above.
I suspect the principal reason for it on the 125 Yamaha will be for regulated emissions under the new regs. We will se a lot more of it used for this reason.
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Old 19-10-19, 08:15 PM   #4
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Default Re: Yamaha MT125 VVT

what do you think ? Hero XPulse 200 is better than Yamaha MT125 VVT or not ?
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Old 20-10-19, 05:59 PM   #5
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Default Re: Yamaha MT125 VVT

Quote:
Originally Posted by Seeker View Post
This new 125 from Yamaha has variable valve timing. I thought the idea of VVT was to produce more power/torque but this model produces less torque than the none-VVT, dropping from 9.1 ft lb to 8.5 ft lb. Maybe it helps with emissions?
With a manual transmission, peak torque is less important than spread of torque.
Variable valve timing can be used to boost torque throughout the rev range.
But in a restricted 125cc bike engine I fear its more of a marketing gimmick.
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