The L15A VTEC seeks to retain as much of the fuel economy and low emissions of the i-DSI as possible while delivering a higher level of power for a higher performance Fit / Jazz...

The City VTEC presentation calls the L15A VTEC 'the latest version of Honda's unique VTEC engine technology', emphasizing that it combines VTEC with new intake ports and technologies that delivers lower internal friction and that the objective is to deliver 'a high level of balance between dynamic and powerful performance and fuel economy at mid-to-high engine speeds'. So as can be seen, the L15A VTEC is now focussing on the mid and high rpm range as compared to the low and mid rpm range of the i-DSI versions. One good way to see the two variants of this L-series, the i-DSI and the VTEC is to see it as two different extremes of implementation: the i-DSI are extremes on squeezing maximum fuel economy while the VTEC extremes on squeezing maximum power output. Both of course are implemented on the L-series blueprint which is fundamentally a fuel efficient and low emissions design and which sets the overall physical bounds on what both i-DSI and VTEC can deliver.

The VTEC mechanism on the L15A VTEC is that of a 1-valve/2-valve system for the intake side only. The exhaust side always functions as a 2-valve system. So the L15A VTEC is a 12valve/16valve system. Here again, misunderstandings and prejudice abounds. Firstly because the original B16A engine that pioneered VTEC was a 3 rocker/cam-lobe design, while the first 12valve/16valve VTEC implementation was on the VTEC-E D15B engine, many have since labelled the 12v/16v VTEC mechanism as the 'economy VTEC' version while reserving the 3-rocker arm mechanism as the 'true power' version. Again if taken too strictly, this is wrong. We must first understand the working principle of VTEC.

In a naturally aspirated (NA) engine, the air-fuel mixture that enters the engine will always be at around normal atmopsheric pressure. In a 4-cylinder 1.5l engine like the L15A, this actually places an upper limit on the amount of air that can enter each cylinder, i.e. approx 375cc (actually slightly more) per cylinder. It's just physically impossible to move much more than 375cc into each cylinder. Of course all of this 375cc goes in through the intake valve(s) when they open during each intake cycle. Now the problem is again like the case of the air-fuel mixture combustion above, the rate of air-flow into the cylinder is more or less constant but the intake valves can only open for a certain portion of the 4-stroke cycle, basically when the piston is on its downstroke during the intake cycle. Because the piston speed actually increases as engine rpm increase, this means the absolute amount of time the intake valves can open actually gets smaller and smaller as engine rpm increases. So there is less and less time for the 375cc of air to get into the cylinder as rpm increases. Since the rate of air-flow is more or less fixed, to ensure we get as much as possible of that 375cc of air into the cylinder, the only mean is to ensure the air-flow passage is large enough; i.e. we get a larger valve and/or we can open the valve more and/or longer, i.e. adjust the valve timing. But the physical properly of air-flow is such that when we optimize the valve size and timing for maximum air-flow at high rpms, it actually hinders that optimal filling of the cylinders at lower rpms. So for lower rpms, we want smaller valves or we lower the valve timing - open the intake valves less or shorter while for higher rpms, we want the reverse. In a standard non-VTEC engine, even those engines with normal variable valve timing mechanisms which only adjusts the relative position of the intake and exhaust valve opening cycle, we have to accomodate by compromising both valve size and valve timing in order to get acceptable performance in the low, mid and high rpms. Honda's VTEC system however is a mechanism that enables us to actually control the intake air passage so as to optimize air-flow at all rpms.

If we look at this principle in relation to whether a VTEC mechanism is full 16valve 3-rocker arms or a 12v/16v 2 rocker system, then we can see that there is no physical law that dictates that one must be for power or the other must be for fuel economy. The 16v 3-rocker system adjusts valve timing whereas the 12v/16v system actually adjusts valve sizing. But both are merely different means to achieve the same end. I.e. the 12v/16v system actually works on the principle of increasing the effective valve size at higher rpms but valve timing remains the same whereas the 16v 3-rockers system works with a constant valve size but adjusts the valve opening amount and duration. Both works with the objective of allowing more air-flow into the cylinder. So in relation to the 12v/16v system, yes, the original D15B VTEC-E engine runs 12valve to enable lean-burn (or what some likes to call 'stratified charge') mode in order to run at very lean air-fuel ratios. But it is equally possible to implement a 12v/16v wild-cam VTEC system, i.e. either 1 or both intake valves are working off a very wild, racey cam timing.

Indeed, a 12v/16v system is capable of almost doubling the size of the intake valve once VTEC 'opens' the secondary valve. Of course there is a physical limit to the absolute size of both valves (dictated by the bore of the cylinder) so the 12v/16v system is restricted by this limit in terms of increasing the effective valve size. But on the other hand, the 16v 3-rocker system has a lower limit in effective valve size eventhough it can adjust valve timing because both valves are always operating so it cannot deliver a very small valve size without impacting air-flow at high rpms. So ultimately a 12v/16v VTEC system will not be able to deliver the same power as a 16v 3-rocker VTEC system but it can be quite respectable too. Conversely, the 16v 3-rocker system will not be able to get anywhere near the fuel economy as the 12v/16v system but again it can be quite miserly in fuel consumption too. Honda did offer us the best of both worlds of course and that is in their incredible 3-stage VTEC D15B engine, an engine which has a VTEC system to offers 3-modes, a 12v, 16v and then 16v with higher valve lift and longer valve opening. It was indeed a technological showcase in its time. In the current generation of L and K series engines, that engine is effectively replaced by this L15A VTEC we are looking at now as well as the 12v/16v K-series engines.

So the L15A VTEC uses a 12valve/16valve VTEC system. A nice by-product of a 12v/16v system is the offset placement of the intake valve during 12v mode and this gives us a nice property of inducing swirl in the intake air-flow besides enabling two effective valve sizes. At low-rpms, only 1 intake valve opens while the other is basically 'dead' or not operating. By using this with optimally designed intake ports and piston heads, a swirl is induced in the incoming air-fuel mixture and this helps to enhance the efficiency of the combustion just like in the i-DSI versions. It is not possible to achieve the same level of efficiency as with the i-DSI system of course, but it is good enough to enable the L15A VTEC to achieve almost the same levels of fuel economy as the older D-series engines, the VTEC-E and the 3-stage VTEC D15Bs.

L15A-VTEC Power and Torque

At higher-rpms both intake valves opens and the engine now operates in 16valve mode. During my test-loan of the Jazz 1.5VTEC from Honda Malaysia, I thought the switch to be at approx 3,500rpm based on the subtle intake note change. When this happens, it effectively doubles the size of the intake valve, enabling almost double the amount of air-flow into the engine. The nett result is much better mid-range and high-end power on the L15A-VTEC, allowing it to deliver 110ps or almost 25% more power than the equivalent L15A i-DSI unit. In operation, it does not feel like running out of breath at all when entering high-rpms. Subjectively, there is a nice power surge after around 4000rpm and the power curve published by Honda (on the left) actually shows this surge. Indeed the profile of the power curve is not that much different from the B16A DOHC VTEC for e.g. which is supposed to be a 'true' 3-rocker arms VTEC engine. To enable this high level of power output, the exhaust system for the engine is actually upgraded in comparison to the L15A i-DSI system. So the rate of flow for the VTEC engine is now 55litres per second as compared to 50 litres per second for the i-DSI and this is achieved by increasing the diameter of the exhuast piping from 38.1mm to 42.7mm while the rear muffler volume is increased from 8 litres to 10 litres at the same time.

As an interesting aside to this discussion, I did ask the Honda R&D engineers why the L15A VTEC did not implement both VTEC and i-DSI at the same time. Surely that would give us the best of both worlds, good maximum power with fantastic fuel economy, much like the incredible 3-stage VTEC D15B of old. I actually already knew the answer beforehand but it's still best to get the confirmation from the experts. The truth is the valve size on the L15A VTEC needs to be large in order to generate enough air-flow for the rather healthy output (110ps is as good as equivalent 1.5l DOHC performance oriented engines, most of which won't be able to deliver anywhere near the same level of fuel economy). So with 4 valves in each cylinder, there really is no space left to put anything more than 1 spark plug in, certainly no space to implement the diagonally opposite placement that is a key feature of the i-DSI system.