Why I Dislike LS/VTEC

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http://www.hstuners.com/forums/showthread.php?t=321

LS/VTEC- Don't Believe the Hype

If it was all that great, Honda would have done it in the first place. Submitted for your approval, my thesis on why LS/VTEC is a bad idea.

What is LS/VTEC?
Why would Honda do that?
What is R/S?
Why a low R/S is bad for reliability
What is power, exactly, and how do Hondas make it?
B Series, by the numbers
How VTEC works, and why it lives at high RPMs
Why it doesn't all fit together


What is LS/VTEC?
A quick tutorial for anyone who doesn't already know.

LS/VTEC is using a B18A or B18B block (referred to as an LS block, even though it was found in the RS, LS, and GS) and mating it with any of the DOHC VTEC heads- the B16A, B17A, or B18C. The principle is to use the larger displacement of the LS block (READ: higher torque) and mate it with the high end power of VTEC. I'm also sure you've heard of CR-VTEC, which is a very similar idea. It uses the B20Z block of the CR-V (NOT the B20A of the Prelude Si, for reasons that will become obvious later) to achieve the same effect, only on a grander scale. What you end up with is an engine commonly referred to as a "Frankenstein" setup, and it's all the rage these days.

Why would Honda do that?

So why in the world would Honda put us in such a situation- having to build these incredible motors all by ourselves? Why would they knowingly decrease displacement and torque in a car being manufactured to be faster than its lower-trimmed breathen?

Look at it, too, from a manufacturing standpoint- Honda is already making the higher displacement B18A and B blocks (blocks are identical, only difference was in the head), so why go to the extra time and expense of developing and manufacturing a separate block, especially if it will decrease output?

The answer is easy: R/S.

What is R/S?

R/S is the abbreviation for rod to stroke ratio. It is the ratio of the length of the connecting rod to the length of the piston stroke, or the distance the piston travels from the top to the bottom of its stroke. As the ratio gets lower, the amount of stress on engine internals increases exponentially, killing long-term reliability. The higher the number is, the slower the piston is traveling, killing power output.

The ideal R/S is 1.75:1 (Three cheers for the B16A, at a near-perfect 1.74:1!).

Why a low R/S is bad for reliability

A low R/S means the rod will be closer to a horizontal angle on its upstroke. This means that more of its force will be pushing the piston horizontally, rather than vertically. What does this mean for your engine? Two things.

1. There will be more stress on the sides and in the center of the rod, rather than on its ends, leaving the rod more vulnerable to breaking. Picture a straw. This is no special straw, just an ordinary drinking straw. Is it going to be easier to bend this straw by applying pressure onto its ends, or at its center? Now think of your poor connecting rods.

2. There will be more stress on your cylinder walls. Once again, the rod is pushing the piston at a more horizontal angle- right into your cylinder walls, rather than up and through them. The risk here is double: A. Putting that piston right through the cylinder wall. B. The cylinder wall will actually flex under the pressure, causing the shape to turn from a circle to an oval or oblong shape. This causes the loss of the seal created by the piston rings. What happens? A small amount of oil could slip past into the combustion chamber. Bad things happen from here: The oil gets combusted, leaving nasty carbon deposits in your combustion chamber and exhaust ports- not a good thing for flow or valve sealing.

It's also important to note that as the RPMs increase, so does the amount of stress on your engine's internals.

What is power, exactly, and how do Hondas make it?

Warning: Once you see this, you will never look at horsepower and torque readings the same again, especially after you think about it.

P= (TR)/5252

P= power, in horsepower
T= torque, measured in lb/ft
R= Engine speed, in RPMs

Therefore:

Horsepower= (torque x RPMs) / 5252

Try it- pull out a dyno and see what you get.

So from this, we can conclude that if we increase torque or engine speed, we will get more power, right?

Remember that, it's important...

Now how do Hondas make power? Our tiny little 1.6-1.8L engines aren't exactly oozing spare displacement and creating gobs of torque, are they? Hondas make power through revving, and revving high. So why does everyone place so much emphasis on creating torque? It's because all these bolt-ons you see advertised won't raise your redline, but they will increase torque. There's nothing wrong with squeezing every last ounce of torque out of your engine- you should. But trying to get torque from more displacement in a Honda is like trying to fill a swimming pool using a squirt gun. You'll never get enough for it to be useful.

B Series, by the numbers

Let's take a closer look at the B series engine blocks.

In the B18 blocks, Honda increases displacement by using a larger crank and increasing stroke (the B20Z also has a slightly larger bore, which is bad for reasons I won't go into here). This, of course, lowers the R/S, since the rod length remains (almost) the same.

B16A:
Rod length: 134 mm
Stroke: 77 mm
R/S: 1.74:1
Displacement: 1587.12 cc

B17A:
rod length: 131.87 mm
Stroke: 81.4 mm
R/S: 1.62:1
Displacement: 1677.81 cc

B18A-B; B20B-Z:
Rod length: 137mm
Stroke: 89mm
R/S: 1.54:1
Displacement: 1834.47 cc

B18C:
Rod length: 137.9 mm
Stroke: 87.2 mm
R/S: 1.58:1
Displacement: 1797.36 cc

B20A (Older Prelude Si)
Rod length: 141.7-142.75 mm
Stroke: 95 mm
R/S: 1.49-1.50:1
Displacement: 1958.14-2056.03 cc

Now you see two things: Why the B20A is widely regarded as a not-so-great engine, and why Honda decreased the displacement from the B18A-B to the B18C. Honda decreased the displacement in the B18C by decreasing the stroke, improving the R/S. This allows the B18C to rev higher, and (Hey!) increase output.

Making sense? I bet you can see where this is going. But wait, there's plenty more...

How VTEC works, and why it lives at high RPMs

A quick crash course for anyone unfamiliar with VTEC:

VTEC stands for Variable Valve Timing and Electronic Lift Control. The premise is that at low RPMs and at idle, a less aggressive cam grind is necessary to prevent "loping." Ever hear a pro drag car staging up at the gates? Sounds like it's about to stall. It's because he's running aggressive camshafts, and since the cam is spinning more slowly at idle, the intake valve is still open after combustion has completed. That's what causes loping. At higher RPMs, a more aggressive grind is desirable. The idea is that you want to cram as much air and fuel mixture (A/F) into that combustion chamber as possible, so that when it's ignited you get as grandiose an explosion as possible. So what is good at low RPMs is bad for high RPMs. So what do you do?

If you're Honda, you invent VTEC. What VTEC does is simply to employ different cam grinds at different RPMs. A less aggressive grind at low RPMs for a smooth idle and low to mid range power, and a more aggressive grind up high to produce that high end pop. At a strategically placed "VTEC crossover point," the camshaft switches grind from the less aggressive to the more aggressive.

What determines this point? Hours and hours dyno testing and tuning. If it is set too low, the more aggressive grind will kick in early, bogging down the engine (think "loping" at 3500 RPM). Too high, and the engine is missing out on valuable time it could be spending with the VTEC engaged. So all those fools who spent $400 on a VTEC timer running stock camshafts just so they could get their VTEC to kick in earlier- they're idiots. They just cost themselves a ton of midrange power. The stock crossover point is optimized for stock camshafts.

So when is a VTEC timer necessary? Easy- when you're no longer running stock camshafts.

If you want big power all motor, you go with one of the big players in the cam game- Toda Spec B and C, or Jun Stage 2 and 3, and you accept no substitutes. All (or at least 95%) of the 225+ all motor whp B18s are running these camshafts.

How does this relate to VTEC crossover point? Well, the VTEC grinds on these cams are so aggressive, that the VTEC point needs to be moved up- way up- usually to 6500-7000 RPM. These cams will also make power to 9500+ RPM (READ: Built motor). Run these in conjunction with high compression pistons (at least 10.5:1), and you'll have yourself an all motor wonder. And this, friends, is where torque in Hondas comes from.

Why it doesn't all fit together

So here's what we've learned:

The LS/VTEC suffers from a bad R/S, due to the fact that it utilizes an LS block with a R/S of 1.54:1.
A bad R/S is bad for the engine, especially at high RPMs
Hondas make power through revving, and high power through revving higher, high compression, and aggressive camshafts

Because of its R/S ratio, it is not recommended that you rev an LS/VTEC past 6750 RPM on stock internals- the redline of a stock B18A-B. With a fairly built bottom end, it is still not recommended that you rev an LS/VTEC past 7800 RPM.

As we just discussed, in order to get any considerable power out of an engine, aggressive camshafts are a must. In order to get any benefit from aggressive camshafts, the ability to rev the engine high is a necessity. How valuable is VTEC if you can only use it for the top 1000 RPM of your powerband?

LS/VTEC is a fad, and I predict that it will be all but a pleasant memory in a few short years. As soon as kids start snapping rods and putting pistons through cylinder walls, they'll realize how important good engine geometry is. Add that to the fact that they're running stock cams (because it's all their engine can safely handle) and getting burned by kids running Todas or Juns, and they'll wish they had just stuck with their trusty B18C.

Never have I said that the LS/VTEC is an awful motor, and that no one in their right mind should build one. LS/VTEC has been hailed as the biggest thing to hit Honda tuning in years. Import Tuner, Super Street, Honda Tuning, and more have praised the ingenious design, marveled at its torque, and even showed you how to construct this beast in the comfort of your own home. Never do they mention its drawbacks or its subsequent limitations. The intention here is not to discourage someone from building an LS/VTEC. Rather, it is intended to give someone considering one of these hybrids some things to keep in mind before diving into the costly process of building an engine.

Like I said, if it was all that great, Honda would have done it in the first place.

Sources/ Influences

Larry at Endyn
"The best engine will be the best series of compromises."

Katman at FF Squad
"So please... Don't steal any of our cars again. Thanks."

Tuan at SHO
"The more you learn about the factors of making power, the more amazed you become at how good the stock engine really is."