- Wed Nov 21, 2007 5:33 pm
#30762
I believe the (OBD2) D15B to be the best D-series motor ever created (for a street car*).
The first stage is 12 valve mode. One of the intake valves is almost all the way closed. This increases air velocity into the combustion chamber at low RPM and also swirls the incoming air in order to burn all the fuel. This avoids the compromises that are made at low RPM on a normal motor, resulting in more power, better economy, and better emissions. It feels like you are already in the power band right from the start. The ECU also switches to 12 valve mode during cruising. You don't need power when cruising, so decreasing the amount of incoming air allows you to decrease the amount of fuel being burned.
The second stage is 16 valve mode. This is just like a normal non-VTEC motor except that the cam is optimized to work between 2500 and ~ 5000 RPM (as opposed to 0 ~ 5000 on a VTEC motor or 0 - 6800 on a non-VTEC motor. As you know, one cam can't do the job of low RPM and high RPM w/out serious compromises, hence the invention of VTEC.) When switching from 12 valve to 16 valve mode, the feeling is stronger than normal VTEC, but there isn't much of a difference in exhaust sound.
The third stage is your conventional SOHC VTEC mode occurring around 4500 - 5000 RPM. This switches the intake valves to high lift, longer duration, and more overlap resulting in the same peak numbers as the more familiar D16Y8.
The result of the 3 stage VTEC isn't seen at peak hp. But when you compare the dyno graph to a D16Y8, the 3 stage motor stomps it from the beginning all the way to the VTEC crossover, and then matches it from there to redline.
One thing I want to point out is that although 12 valve mode is very similar in operation to the D15Z1 (VX) and D16Y5 (HX) motors, it's not the same. The 3 stage VTEC motor doesn't have roller rockers and it doesn't seem to have been designed for lean-burn mode, at least not to the same extent as the federal (non-CA) VX models. I think in this implementation, Honda was trying to get better performance at low RPM with VTEC-E the same way they were going for better performance at high RPM when they implemented VTEC.
The 3 stage VTEC motor uses a 1-wire O2 sensor. This sensor can detect a stoichiometric 14.7 : 1 AFR or whether the AFR is leaner or richer than stoich, that's it. It doesn't know how much richer or leaner it is from stoich. In order to run at the ragged edge of lean on the federal VX's, Honda used a UEGO O2 sensor. This sensor can detect an accurate AFR anywhere between 10 : 1 and 18 : 1. This is critical when the motor runs fine at 17.1 : 1 but will not burn fuel at 17.2 : 1. I think Honda may have abandoned this extreme lean-burn mode because of what it does for the owner's perception of reliability. You see, any small problem ie a vacuum leak would lead to major problems because it pushes you just over the limit during lean-burn mode. This is how I got my car so cheap. It was bucking like crazy.
*For a pure race car that never sees street duty, a non VTEC motor ie the D16A6 with a high RPM race cam would be my D-series of choice.
The first stage is 12 valve mode. One of the intake valves is almost all the way closed. This increases air velocity into the combustion chamber at low RPM and also swirls the incoming air in order to burn all the fuel. This avoids the compromises that are made at low RPM on a normal motor, resulting in more power, better economy, and better emissions. It feels like you are already in the power band right from the start. The ECU also switches to 12 valve mode during cruising. You don't need power when cruising, so decreasing the amount of incoming air allows you to decrease the amount of fuel being burned.
The second stage is 16 valve mode. This is just like a normal non-VTEC motor except that the cam is optimized to work between 2500 and ~ 5000 RPM (as opposed to 0 ~ 5000 on a VTEC motor or 0 - 6800 on a non-VTEC motor. As you know, one cam can't do the job of low RPM and high RPM w/out serious compromises, hence the invention of VTEC.) When switching from 12 valve to 16 valve mode, the feeling is stronger than normal VTEC, but there isn't much of a difference in exhaust sound.
The third stage is your conventional SOHC VTEC mode occurring around 4500 - 5000 RPM. This switches the intake valves to high lift, longer duration, and more overlap resulting in the same peak numbers as the more familiar D16Y8.
The result of the 3 stage VTEC isn't seen at peak hp. But when you compare the dyno graph to a D16Y8, the 3 stage motor stomps it from the beginning all the way to the VTEC crossover, and then matches it from there to redline.
One thing I want to point out is that although 12 valve mode is very similar in operation to the D15Z1 (VX) and D16Y5 (HX) motors, it's not the same. The 3 stage VTEC motor doesn't have roller rockers and it doesn't seem to have been designed for lean-burn mode, at least not to the same extent as the federal (non-CA) VX models. I think in this implementation, Honda was trying to get better performance at low RPM with VTEC-E the same way they were going for better performance at high RPM when they implemented VTEC.
The 3 stage VTEC motor uses a 1-wire O2 sensor. This sensor can detect a stoichiometric 14.7 : 1 AFR or whether the AFR is leaner or richer than stoich, that's it. It doesn't know how much richer or leaner it is from stoich. In order to run at the ragged edge of lean on the federal VX's, Honda used a UEGO O2 sensor. This sensor can detect an accurate AFR anywhere between 10 : 1 and 18 : 1. This is critical when the motor runs fine at 17.1 : 1 but will not burn fuel at 17.2 : 1. I think Honda may have abandoned this extreme lean-burn mode because of what it does for the owner's perception of reliability. You see, any small problem ie a vacuum leak would lead to major problems because it pushes you just over the limit during lean-burn mode. This is how I got my car so cheap. It was bucking like crazy.
*For a pure race car that never sees street duty, a non VTEC motor ie the D16A6 with a high RPM race cam would be my D-series of choice.
Last edited by suspendedHatch on Wed Nov 21, 2007 6:13 pm, edited 1 time in total.
