Mini-me and Engine Theory
More information about mini-me and SOHC performance.

We also recommend you check out xtensive's mini-me article.

daven


I've seen a lot of questions recently regarding the conversion of a D16a6 to become a sohc WITH vtec. This setups is commonly referred to as a "Mini-me." So I'm going to try and cover all the various angles to making one of these little D-series screamers right. NOTE: This isn't a how-to article, it's more of a broad ideas type pointer...for the down and dirty hands on, see Xtensive's earlier article.

In theory, it's very simple. Take a D16a6 bottom end and "just" bolt on a D16z6 or D16y8 head. Okay, if you have no idea why somebody would bother with this, raise your hand...
For starters, more horsepower, more torque and it's cheaper and easier than a complete engine swap. Especially if the only thing wrong with your current engine is in the head. Why replace a perfectly good block, unnecessarily.
Secondly, vtec is cool. It has a kind of a Jekyl/Hyde thing going on. Below 5000 rpm it's incredibly efficient yet peppy but once it hits the higher rpms, it opens up two extra valves per cylinder and makes an extra oomph of power.

According to knowledgeable engine builders, the a6 block was/is the best D-series civic bottom end. However, the heads from the newer z6 and y8 are without a doubt a better design...so why not have your cake and eat it too? You can realistically expect at least 120hp at the flywheel without a loss of driveability or fuel economy. In fact, my MPG went up. There's even more power there with some aftermarket parts and tuning. However, if tuning an air to fuel ratio or fiddling with your engine's timing is a daunting task, you might be better off just getting a nice port and polished D16a6 head because you will have to do at least rudimentary tuning. The only drawback I can see is that the higher compression makes it forced induction unfriendly. And I don't think I've met an import enthusiast yet who didn't, on some level, want to go turbo or supercharged somewhere down the line. Not to say it can't be done, it'll just be another hurdle. So consider that before you go ahead with the surgery.

Choices:

Priorities:
Chief concerns in making one of these setups work well is...(not in any particular order)

    Figuring compression ratio and keeping detonation in check

    Tuning the fuel and timing

    Deciding on a way of activating the vtec solenoid

So compression...an a6 block and pistons with a z6 head nets a low to mid 10:1 compression ratio (CR). The y8 head mated to the same block/pistons comes out to be high 10, low 11:1 CR. The stock CR on a d16a6 is 9.1:1 so as you can see, it's a significant increase, as far as CR goes. Higher CR makes more power so more is better, right? Well, to a point. You go to high and you run into a nasty little engine-breaking problem called detonation. Basically, the high pressure in a cylinder ignites the gas/air mixture before the engine management system sends a spark. This out of place ignition really places stress on a system that relies heavily on precise timing to maintain longevity.
If you want that simplified-detonation is bad, avoid it all costs.
A higher octane rated gas is more resistant to detonation so this may be required to keep your engine from pinging. There are ways to lower the CR, which we'll cover under the Components section.

Fuel and Timing...these are important because they both have the potential to result in detonation if botched. If you run lean(not enough fuel) you get detonation and if your timing is advanced to far, again, detonation. Too much fuel and you foul plugs, o2 sensors and the catalytic converter. And timing retarded to far means weak power. With the Mini-me, I would suggest a setup for maximum adjustability so you can tinker with tuning these variables. And tuning is a necessity. This setup was never made by Honda and it takes some trial and error to get it running 100%.

And vtec? There's a little solenoid on the backside of the head that sends the signal to the valvetrain to open the extra valves. On a stock z6/y8, this is controlled by the computer. The ecu monitors temperature, oil pressure, and finally rpm. If the rpms go above X amount, and the oil pressure and temp are right, you get vtec. You can solve your problem this way, but it will require converting to the newer wiring harness, ecu, and sensors. After you've done all this, it will work great but it has significantly increased cost and time involved. So most people opt for one of the alternatives.
On the cheap end, you can fabricate a rpm sensitive switch to the vtec solenoid so anytime you go above a specified RPM it trips the solenoid. There are aftermarket computers that can do this for you also. They vary significantly in features and cost so we'll cover them more in depth in Components.

Components:
There are several components to consider when building a Mini-me. These are the categories I consider important and several option in each one. This is not to say it's all the solutions(or even best), just the ones I have researched and considered.

    Pistons.
    Stock a6 pistons. They're already in there and the most cost effective option. Only problem I have with the a6 pistons is the CR is getting kind of high on the y8 head. If you go with the a6 pistons, you'll want to explore options for compensating for CR with fuel and timing adjustability.
    Aftermarket pistons. You can pick up a set of pistons designed for turbos that typically drop the CR a whole point. This would put it at a very safe street/ daily driver level. Expensive though.
    Other Honda pistons. The z6 pistons lower CR by about half a point over the a6 pistons. Inversely, ZC and 1st gen Integra pistons raise it by a half point. I picked up a set of new z6 pistons for a little over $100.

    Rods.
    Stock a6 rods. If you're not tearing your block apart, stock is fine in the interest of saving money.
    ZC and 1st gen Integra rods. They're a little beefier and relatively inexpensive.
    Aftermarket rods. Plenty to choose from but again, expensive.

    Headgaskets.
    Since you're removing the old one, time to upgrade. Do not use the a6 gasket, there are plenty of better choices. The a6 was a metal composite type gasket that did not hold up as well as full metal ones. The thickness of the headgasket can also marginally effect CR(thicker gasket equals lower CR).
    The z6 gasket is a full metal replacement. Cheap and it works well. In fact, it's what my Honda dealer uses (instead of the a6 gaskets) on a6 setups.
    Avoid the y8 gasket because it's supposed to be thinner than the z6, unless you're trying for a higher compression.
    Aftermarket copper gaskets. Initially attractive as they are extremely durable, available in different thicknesses and cheap. After some research, I found several people who said the block or the head would need to be machined to have these fit properly...uh, too much work. No thanks.
    HKS makes several different thickness metal headgaskets for the D series motors.

    Head studs vs Head bolts.
    Technically, you're not supposed to reuse the old bolts because they stretch, but people do. So if you're going to do it right, pick up a set of head studs from ARP. They're just a little more $ than a new set of standard bolts from Honda. At first I didn't understand the difference, aside from being made of a better quality metal. Then a mechanic friend used this somewhat simple-stupid analogy. Imagine standing in hallway. Your job is to physically hold a door shut. A guy walks up and tries to go out the door by pushing. You're pulling with both hands. If he pushes hard enough the door will open. In this case, you are a bolt. Now if you were a stud(ha) you could reach one arm across the hall and use another door's handle as an anchor. Make sense? I hope so because I couldn't think of another way to explain it.

    Adjustable Cam Gear.
    Really as a precaution more than performance enhancement. This will give you a little more room for adjustability when it come to dialing in your timing. Get the gear for whatever head you're using. Stick with a known brand...AEM, Skunk2, DC sports, STR...etc.

    A Air-to-Fuel meter or gauge.
    This will help you determine where you are at as far as fuel mixture. Having a mixture that's off will either lead to all sorts of secondary problems, rob you of power, or result in detonation. Get one that has a good range. The kind that's commonly used by the old V-8 crowd, the one with three LEDs for rich, stoichmetric, and lean won't be any help. I picked an Autometer gauge that has something like a 26 light range from rich to lean and was warned after the fact it wouldn't be very accurate but to date, I have no complaints with it's performance.

    Vtec controls/ computers.

      A'PEXi V-TEC AFC.
      A great unit that piggybacks on the standard ecu that has a vtec option and fuel adjustment capability thru the whole rpm range. Not to expensive either...

      Fields SFC-VTEC.
      Basically the same thing as the A'PEXi unit. It allows for fuel and vtec control.

      Zdyne.
      You send them a stock CRX ecu and they reprogram it. There are several options here.
      Cheapest is their One Wire Conversion. They basically just put a vtec feature on. This is a good option because it doesn't rely on any piggyback units to activate the vtec. The bad is that it's not an adjustable unit so you will need something like an adjustable fuel pressure regulator to lean or richen the fuel mixture.
      The most expensive is the Gold Secu. It's fully adjustable. You can change fuel AND timing, move vtec activation, control NOS or boost etc. Very nice but prohibitively expensive if you're on a tight budget. A good investment if you plan on keeping the car or changing your setup in the future. It pretty much does it all.

      RPM switch.
      Very cheap, very easy...but you get what you pay for. You are going to have to get a fuel pressure regulator so you have some control over mixture. The adjustable cam gear is also a good idea because with this, the only timing adjustability you'll have is how far you can turn the distributor, which is (more often than not) not enough.

My results:
I ended getting 106hp and 101 ft/lbs. to the wheels on a dyno. The cool thing though, it was making 99 ft lbs at 2200 rpms. It's really great for moving thru traffic, roll on power is impressive. It's actually making more torque than a stock B16 and I'm still getting 30-32mpg in town. I'm toying with the idea of some headwork and maybe a performance cam. The car is a sleeper, down to the stock exhaust. I'm sure there's still some serious Hp to be made here...we'll see.



Another informative gem by daven. We can't thank you enough. Or can we?. Ponder.