question on 99 GSR
08-19-2003, 09:30 PM
#5
H-T Order of Merit
Re: question on 99 GSR (JL 74)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by JL 74 »</TD></TR><TR><TD CLASS="quote">Yeah, around 4400 rpm but still, nothing much happens. It's overrated IMO.</TD></TR></TABLE>
Then you don't understand what VTEC does. Many people think it is supposed to increase acceleration when it kicks in. That's wrong. What it does is to keep acceleration from dropping off as revs increase. This gives you better acceleration because of gearing.
Let's go back to the beginning. The first misunderstood fact is that acceleration is a function of torque at the wheels, which is engine torque times gearing, less any drivetrain losses.
The second misunderstood fact is that variable valve timing does not increase torque (or, as a result, acceleration) at higher revs. Its objective is to prevent torque from dropping as revs increase. When you look at the torque curve for a VTEC car like the NSX:
you can see that it's quite flat from roughly 3500 RPM to about 7500 RPM, and there is no huge increase above the VTEC crossover point of 5800 RPM.
The advantage of a wider, flat torque curve like this is in gearing. Cars without VTEC have torque dropping off as revs rise, and are forced to upshift by around 6000 to 6500 RPM for optimal acceleration; when they upshift, they lose acceleration because of the loss in gearing. This is why VTEC cars are so much faster than their torque numbers would imply - because they gain an acceleration advantage from their ability to stay in a lower gear before upshifting.
If you would like a more lengthy explanation, including how horsepower numbers reflect this, read this excellent article about the difference between torque and horsepower, and how gearing comes into play.
Then you don't understand what VTEC does. Many people think it is supposed to increase acceleration when it kicks in. That's wrong. What it does is to keep acceleration from dropping off as revs increase. This gives you better acceleration because of gearing.
Let's go back to the beginning. The first misunderstood fact is that acceleration is a function of torque at the wheels, which is engine torque times gearing, less any drivetrain losses.
The second misunderstood fact is that variable valve timing does not increase torque (or, as a result, acceleration) at higher revs. Its objective is to prevent torque from dropping as revs increase. When you look at the torque curve for a VTEC car like the NSX:
you can see that it's quite flat from roughly 3500 RPM to about 7500 RPM, and there is no huge increase above the VTEC crossover point of 5800 RPM.
The advantage of a wider, flat torque curve like this is in gearing. Cars without VTEC have torque dropping off as revs rise, and are forced to upshift by around 6000 to 6500 RPM for optimal acceleration; when they upshift, they lose acceleration because of the loss in gearing. This is why VTEC cars are so much faster than their torque numbers would imply - because they gain an acceleration advantage from their ability to stay in a lower gear before upshifting.
If you would like a more lengthy explanation, including how horsepower numbers reflect this, read this excellent article about the difference between torque and horsepower, and how gearing comes into play.
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