The Big Picture
Why do P29 pistons from D16A or ZC engines raise the compression ratio of a D16A6 when the bore and stroke are the same and the stock compression ratios are so similar?
Rik Gagala
Orlando, FL
Compression ratio is affected by every spec in your rotating assembly; you have to look at each and every spec of the motor to accurately calculate your compression ratio.
First, the deck height tells you how much rotating assembly you can fit in your bottom end. The ZC and the A6 share a deck height of 212mm. This deck height is made up of 43.5mm of crank (87mm stroke divided by 2), 137mm of connecting rods, and the rest is piston compression height and piston to deck clearance. The PM7/P29 piston's compression height is 29mm to the 29.5mm of the A6. So far, the A6 is in the lead.
The piston-to-deck height on the ZC is at .040 inches to the A6's .020 inches. The A6 is still in the lead.
It is until you get to the piston crown and combustion chamber volume that you'll find the reason why the PM7/P29's make so much more compression. The ZC's combustion chamber volume is 43.5cc to the A6's 38cc. This, combined with a bigger piston-to-deck gap, leaves a whole lot of room that needs to be filled up for the ZC to show similar compression to the A6. Honda took care of this by endowing the ZC with its legendary 7.2cc domed pistons. The A6, however, got stuck with a set of -3.4cc dished pistons. The difference in the piston crowns (10.6cc) is enough to make a 43.5cc chamber and a 38cc chamber put out approximately the same compression.
Now, with this knowledge fresh in your mind, what happens when we put a set of 7.2cc-domed pistons in a 38cc chamber? The compression goes up, way up actually, to about 11.3:1. Check out the budget D-series build we did in the May '06 issue of HT (p. 60) for more info on a build like this.
Wide Load
I just bought a wideband O2 sensor and I'm not exactly sure how to hook it up. Do I leave the stock narrowband in its bung and add another for the wideband? Do I get rid of the narrowband altogether? Wouldn't that throw a check engine code?
Tom Peters
Cleveland, OH
There are a few ways you can go about this depending on which wideband controller you are using along with your engine management tools.
If you're running a stock ECU, you have two options. The first is to use both O2 sensors simultaneously. The wideband would be used only for readout purposes and the narrowband would be used for ECU input. The other option would be a narrowband output on your wideband controller. Most wideband controllers have two analog outputs; one is a 0-5 volt output for wideband and one is a 0-1v output for narrowband. The Innovate Motorsports wideband controller we use actually has programmable outputs to do with what you please. In the case of a stock ECU, you'll want to run the narrowband 0-1v output into the ECU and the 0-5v output into your wideband gauge. This would negate the need for two O2 sensors.
The thing is, since you are installing a wideband, you're probably not using a stock ECU, and if you are, it's surely modified. If you couldn't tune with your ECU, what need would you have for a wideband?
If you're running a chipped stock ECU, the software used to chip the ECU will almost always be able to disable the O2 sensor making the narrowband input unnecessary. Aftermarket engine management systems like the AEM EMS will usually be able to accept wideband inputs anyway, so if you're running one of those you've got nothing to worry about.
I personally run an Innovate wideband acting as my wideband and my narrowband with my Hondata S300. So if you're asking what I would do, there's your answer.