Vtec?

[Street Racer]

Ok, not sure if this is the right forum but... Ok so VTEC doesn't make horsepower or torque right? But, if the valves open and stay open for a longer time allowing more fuel and air to enter the piston, why would the engine not make more horsepower and/or torque? And what is the difference between say a b18 vtec and b18 non-vtec engine (which one would win a drag race)?

 

[ZohsixGT5]

www.honda-tech.com

Probably a better place to ask.

 

[Bopop4]

VTEC allows the engine to have high rpm and low rpm performance.
The non-vtec might have to trade high rpm power for low rpm drive ability.

But if it's only tuned for high rpm, then it will have less weight and more speed.

 

[Street Racer]

VTEC allows the engine to have high rpm and low rpm performance.
The non-vtec might have to trade high rpm power for low rpm drive ability.

But if it's only tuned for high rpm, then it will have less weight and more speed.

But, what is the point of having high rpm and low rpm performance, if it does nothing to performance? Does it have better fuel efficiency? And would vtec or non-vtec (same car) win in a drag.

 

[Watt2159]

I'll just let Eric explain it, before this thread is filled with.......THAT joke

 

[Street Racer]

I already watched this video. It says that the valves open for a longer duration. So, wouldn't more fuel and air be able to enter the engine creating more horses and torque?

 

[1241Penguin]

Valves open longer = greater intake = bigger bang.

 

[Street Racer]

Greater Intake Boom! wouldn't that have more air entering the engine therefore more power?

 

[Bopop4]

But, what is the point of having high rpm and low rpm performance, if it does nothing to performance? Does it have better fuel efficiency? And would vtec or non-vtec (same car) win in a drag.

The point of having it is that you have the best of both cam worlds.

At low rpm a smaller cam is more desirable because it will be better on gas mileage and general engine performance.

At high rpm a large cam is more desirable because there is less time to get the air/fuel into the engine, so you need a bigger cam to make up for it.

What VTEC does is give you both cams in the same engine, it allows you to have a high performance engine, that retains its driveability.



Imagine having to live with a sports car engine, hard to do at low rpm city driving, on the flip side, a normal engine won't have the power of the sports tune.







And the race winning depends on if you took out the large lift, or small lift portion of the cam. If you took out the small lift part then it should be lighter and rev faster, but the difference would be small.

 

[Street Racer]

So a engine with a small lift portion of the cam vs a vtec engine. Would the vtec have more hp. Because at high rpm's, more air would be entering the cylinders?

 

[Bopop4]

Yes.
If you are thinking about taking it out for street use, don't, there really isn't a point when you gain barely anything for a large loss on the bottom end.

And uh...
<_<
>_>
4-AGE rules!

 

[alexlam24]

V-TEC basically switches cams, some witch craft happens and you go faster

 

[Street Racer]

Yes.

Ok... so why do people say that vtec does not help produce hp. I know that it depends on the car but about how much more hp would vtec vs non-vtec produce(same car)?

 

[Bopop4]

If you are running the same cam profile as the large lift portion of the VTEC, then you will make the same amount of power.

But if you have a 7000rpm 1.6L Non-VTEC, and a 7000rpm 1.6L VTEC, then the manufacture of the non=VTEC motor has to make the engine driver friendly at low rpm, and thus sacrifices high rpm power.

The VTEC motor won't have to make a sacrifice on the power side for the driver, it can be super easy to use, and then switch to race mode in an instant.

 

[Street Racer]

So basically the powerband would be different? The vtec would have the power coming later on in the rpm range?

 

[Bopop4]

Edit Yes and no, the power would start to drop off for the non-VTEC because of the small cam.

The VTEC would switch to the big cam and not lose anything


If you want to look at it like a dyno chart, the non-VTEC would be ok in the low range because that's what it's built for, it would still make some low end power sacrifices for the high end though.

The VTEC can have the best low end cam, and then have the best high end cam, it doesn't have to make any sacrifices.

So the VTEC would be better in all areas, especially the top end.

 

[Azuremen]

So basically the powerband would be different? The vtec would have the power coming later on in the rpm range?

Yes, thus the term "V-tec kicking in" which is when the cam profile changes.

A dyno chart showing a VTEC change over point

 

[1241Penguin]

Forgive me for this dumb question, but is that sudden drop in power/torque in the chart caused by the slight lag when switching cams?

 

[Street Racer]

So, it's just the power band that is different. It would be nice to see a vtec vs non-vtec powerband. And when the cams get switched, vtec helps keep the power in a straight line for a longer time on the powerband?(so the hp doesn't drop quickly)

 

[Eunos_Cosmo]

Yeah it's almost a vertical vertical asymptote during the crossover. I never felt that in my B16a civic. Must be imperceptible in practice.

My personal opinion on vtec: It rocks. None of the low rpm response issues that big-cam engines have, yet all of the top end. Two of the best engines I've ever experienced were both vtec units. F20C and B16A. For a response whore like me, it's hard to do better. I still prefer rotaries because they are lighter, more compact and can produce more power while being far smoother (even the S2000's engine feels rough up top) at high RPM, but the Vtec units are extremely good.

 

[Street Racer]

Yeah it's almost a vertical vertical asymptote during the crossover. I never felt that in my B16a civic. Must be imperceptible in practice.

My personal opinion on vtec: It rocks. None of the low rpm response issues that big-cam engines have, yet all of the top end. Two of the best engines I've ever experienced were both vtec units. F20C and B16A. For a response whore like me, it's hard to do better. I still prefer rotaries because they are lighter, more compact and can produce more power while being far smoother (even the S2000's engine feels rough up top) at high RPM, but the Vtec units are extremely good.

I still don't get what makes vtec so good. I'm not saying its bad just what makes it good. Does it help make a better powerband for the car?

 

[1241Penguin]

I still don't get what makes vtec so good. I'm not saying its bad just what makes it good. Does it help make a better powerband for the car?

It means when you drive it sensibly, you will get good fuel economy. At the same time, you get power when driving it enthusiastically. (I admit finding this info on a forum, so don't take my word for it.)

Watch these videos:

 

[Street Racer]

So, what vtec basically does is help you maintain fuel economy? But what about what about this

If you are running the same cam profile as the large lift portion of the VTEC, then you will make the same amount of power.

But if you have a 7000rpm 1.6L Non-VTEC, and a 7000rpm 1.6L VTEC, then the manufacture of the non=VTEC motor has to make the engine driver friendly at low rpm, and thus sacrifices high rpm power.

The VTEC motor won't have to make a sacrifice on the power side for the driver, it can be super easy to use, and then switch to race mode in an instant.

 

[Azuremen]

I still don't get what makes vtec so good. I'm not saying its bad just what makes it good. Does it help make a better powerband for the car?

Like others have pointed out, it basically gives you the benefits of having a high lift, high performance cam profile while preserving torque and drivability at lower rpms.

As for your other bit, I am starting to think you aren't really reading what people are posting. In order for a typical engine to have low RPM torque and smoothness, high end performance will be lost because of the cam profile and timings. Thus why entry level cheap cars typical make the most of the power and torque down low, because it is more accessible in the real world.

 

[Street Racer]

Like others have pointed out, it basically gives you the benefits of having a high lift, high performance cam profile while preserving torque and drivability at lower rpms.

But, does it help improve hp since the second cam would allow more air into the cylinder. Wouldn't the hp of a non-vtec engine be less than the hp of a vtec engine (saying that the engines are similar models)?
EDIT:

In order for a typical engine to have low RPM torque and smoothness, high end performance will be lost because of the cam profile and timings. Thus why entry level cheap cars typical make the most of the power and torque down low, because it is more accessible in the real world.

This helps. But, still does it help produce hp. And so it does change the powerband.
EDIT:I finished watching the two videos and the guy states that vtec does produce more power. And why can't you just run on the larger camshaft the entire time? It would be less fuel effiecient, but wouldn't it help produce even more torque (more air would enter)?

 

[Eric.]

What has you so fixed on thinking VTEC doesn't improve performance?

Let's say you have a typical non variable valve timing four cylinder. Say that it is 2.0L in displacement. Rev limiter cuts out at 7000rpm. Peak power of 160hp comes in at about 6500rpm.

Horsepower can be increased by increasing airflow, and one way to increase airflow is to increase RPM. With this engine, we start losing horsepower after 6500rpm, so we need to make changes to increase the high RPM efficiency. So we put in some cams designed to take us up to about 8500rpm. Now we're able to make about 190hp with the extra rpm. Except now the engine is set up to operate well from 3000rpm to 8500rpm, the aggressive cams give a rough idle and poor power and torque off idle (taking off, cruising at low rpm).

VTEC gives you the best of both worlds. It operates on the low rpm cams until passing a point where the high rpm cam has better performance. Its a win-win-win for economy, drivability, and performance.

 

[Street Racer]

Well, that helps clear it up. I've heard people on different sites saying that vtec does not improve power it just helps fuel efficiency and stuff like that. But, why would the aggressive cams give a rough idle and poor power and torque off idle?

 

[Eunos_Cosmo]

Overlap.

Blatantly stolen:

As the engine cycles, there is a period when both the intake and exhaust valves are open at the same time. This valve timing is known as "overlap." Think of this as the exhaust and intake cycles overlapping each other.

The valves are timed so the intake valve opens slightly before the piston reaches top dead center (TDC) on the exhaust stroke. Likewise, the exhaust valve is timed to close just after the piston starts down on the intake stroke.

The objective of overlap is for the exhaust gas which is already running down the exhaust pipe, to create an effect like a siphon and pull a fresh mixture into the combustion chamber. Otherwise, a small amount of burned gasses would remain in the combustion chamber and dilute the incoming mixture on the intake stroke. This valve timing is a product of the cam's duration and separation specs. For more information on these cam specs see the Cam Specs & Effects page.

The science involved with overlap is quite complex. Pressures, runner lengths, temperature, and many other aspects influence how well the overlap effect works. The advantage to using software like Engine Analyzer Pro is that the software does all of the calculations for you. Simply enter a different duration and separation in the computer and the software will show you whether or not to expect more power from your engine.

With high lift, long duration cams you end up having exhaust dilute the air-fuel mix at low RPM, causing poor performance and often stumbling. Which is heard as 'lope' on a piston engine or 'brap' on a rotary.

Lope:


Brap:


My knowledge isn't as extensive with piston engines, but with a rotary that idle sound is caused by the air/fuel mix being fouled by exhaust and not firing at all. For a complete revolution on both rotors. So you hear 6 (2 rotors x 3 combustions per revolution) bangs (heard as essentially one bang) followed by 6 non-fires. brap-brap-brap

 

[Perfect Balance]

Why is everyone making this thread so complicated? This whole thing could have been answered in one post. Eric did a good job summing it up, but I'll repeat it again in an even shorter manner.

Vtec allows you to have the benefits of a larger cam (more power at higher rpm) without ruining your fuel economy and driveability at lower rpm. That is all.


EDIT: This is what happens when your car has cams that make it run as if it's in Vtec all the time. You tell me why this wouldn't be ideal for driving around town and getting 30mpg.

 

[1241Penguin]

EDIT:I finished watching the two videos and the guy states that vtec does produce more power. And why can't you just run on the larger camshaft the entire time? It would be less fuel effiecient, but wouldn't it help produce even more torque (more air would enter)?

I believe it's because the gear-ratios of the cars should generally suited so that it will be in the VTEC zone whenever you change up from redline. So there would be no point in keeping the car running with the larger lobes.