How Does Vtec Really Work
Thread Starter
Junior Member
Joined: Oct 2002
Posts: 344
Likes: 1
From: St. Louis, MO
How Does Vtec Really Work
1) Does vtec only work on the intake cam? I've heard that the exhaust cam doesn't have another set of lobes for vtec.
2) It's obvious that vtec is engineered to give a boost in power at high rpm but does it work better for low rpm torque too? Meaning is the low rpm lobes just like a standard camshaft engineered to make power from idle to redline or was it designed specifically to make power from idle to 4400 or whenever it switches to the secondary lobes? I guess the question is was the vtec cams made to where it's a standard (b18b) cam with some power boosting extra lobes or were both sets of lobes designed to make power specifically for when they're engaged in the rpm range.
2) It's obvious that vtec is engineered to give a boost in power at high rpm but does it work better for low rpm torque too? Meaning is the low rpm lobes just like a standard camshaft engineered to make power from idle to redline or was it designed specifically to make power from idle to 4400 or whenever it switches to the secondary lobes? I guess the question is was the vtec cams made to where it's a standard (b18b) cam with some power boosting extra lobes or were both sets of lobes designed to make power specifically for when they're engaged in the rpm range.
H-T Order of Merit
Joined: Jan 2000
Posts: 23,478
Likes: 2
From: Chicago
Re: How Does Vtec Really Work (Brett)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by Brett »</TD></TR><TR><TD CLASS="quote">It's obvious that vtec is engineered to give a boost in power at high rpm but does it work better for low rpm torque too?</TD></TR></TABLE>
People really misunderstand VTEC and the difference between power and torque.
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. Not horsepower per se (but see more below).
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. If it didn't have VTEC, you would see the torque drop rapidly as revs rose above that point.
The advantage of a wider, flat torque curve like this is in gearing. Cars without VTEC would be forced to upshift by around 6000 to 6500 RPM for optimal acceleration, and 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.
Another informative website, that explains how Honda's VTEC system works, is here, with links to similar topics on that site.
People really misunderstand VTEC and the difference between power and torque.
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. Not horsepower per se (but see more below).
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. If it didn't have VTEC, you would see the torque drop rapidly as revs rose above that point.
The advantage of a wider, flat torque curve like this is in gearing. Cars without VTEC would be forced to upshift by around 6000 to 6500 RPM for optimal acceleration, and 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.
Another informative website, that explains how Honda's VTEC system works, is here, with links to similar topics on that site.
... lol.. good work nsxtasy.... PEACE
H-T Order of Merit
Joined: Jan 2000
Posts: 23,478
Likes: 2
From: Chicago
Re: How Does Vtec Really Work (..::91TEG-G2::..)
Hey, no prob, and no diss intended.
Many people think that there is a lot of extra acceleration when you hit the VTEC point, because the engine gets a lot louder. But when you see acceleration plotted on a g-meter or similar device, you can see that the rate of acceleration stays pretty constant. Still, it's a good thing and it is a performance advantage for the VTEC cars, because of the gearing.
My VTEC cars:
91 NSX
94 GS-R
01 ITR
Many people think that there is a lot of extra acceleration when you hit the VTEC point, because the engine gets a lot louder. But when you see acceleration plotted on a g-meter or similar device, you can see that the rate of acceleration stays pretty constant. Still, it's a good thing and it is a performance advantage for the VTEC cars, because of the gearing.
My VTEC cars:
91 NSX
94 GS-R
01 ITR
H-T Order of Merit
Joined: Jan 2000
Posts: 23,478
Likes: 2
From: Chicago
Re: How Does Vtec Really Work (Brett)
If your question is, does knowledge that VTEC will permit torque to be maintained at higher RPMs have an ancillary benefit on low RPM operation by allowing the Honda engine designers to concentrate on that and design the engine so that its torque curve has additional torque at low revs, knowing that the extra cam will take care of higher revs - it's possible, but the only people who can really answer that question for sure are the Honda engineers themselves...
Honda-Tech Member
Joined: Mar 2002
Posts: 5,449
Likes: 0
From: Yeah IM from Jersey, big wup, wanna fight about, it, USA
Re: How Does Vtec Really Work (nsxtasy)
WOW good info nsxtasy. Also I hate you for the mere fact that you have an NSX, ITR and GSR. 3 sweet rides.
Trending Topics
Honda-Tech Member
Joined: Mar 2001
Posts: 199
Likes: 0
From: Upper East Side, NY, USA
Here's your answer:
http://auto.howstuffworks.com/question229.htm
oh wait didnt read above post. haha
Modified by 2kGS at 5:14 AM 7/14/2003
http://auto.howstuffworks.com/question229.htm
oh wait didnt read above post. haha
Modified by 2kGS at 5:14 AM 7/14/2003
Joined: Jul 2003
Posts: 2,799
Likes: 0
Re: How Does Vtec Really Work (deepgreengsr)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by deepgreengsr »</TD></TR><TR><TD CLASS="quote">Ya, but my phone has vtech and I was wondering how that worked?</TD></TR></TABLE>
yea ok man.. dont be a tool...
yea ok man.. dont be a tool...
Thread Starter
Junior Member
Joined: Oct 2002
Posts: 344
Likes: 1
From: St. Louis, MO
Re: How Does Vtec Really Work (nsxtasy)
<TABLE WIDTH="90%" CELLSPACING=0 CELLPADDING=0 ALIGN=CENTER><TR><TD>Quote, originally posted by nsxtasy »</TD></TR><TR><TD CLASS="quote">
If your question is, does knowledge that VTEC will permit torque to be maintained at higher RPMs have an ancillary benefit on low RPM operation by allowing the Honda engine designers to concentrate on that and design the engine so that its torque curve has additional torque at low revs, knowing that the extra cam will take care of higher revs - it's possible, but the only people who can really answer that question for sure are the Honda engineers themselves...
</TD></TR></TABLE>
That definetly answered the second question. The chart answered the first. I appreciate the good info.
If your question is, does knowledge that VTEC will permit torque to be maintained at higher RPMs have an ancillary benefit on low RPM operation by allowing the Honda engine designers to concentrate on that and design the engine so that its torque curve has additional torque at low revs, knowing that the extra cam will take care of higher revs - it's possible, but the only people who can really answer that question for sure are the Honda engineers themselves...
</TD></TR></TABLE>
That definetly answered the second question. The chart answered the first. I appreciate the good info.
Thread
Thread Starter
Forum
Replies
Last Post
