Honda CRX Si JDM ZC Swap

Last updated: 5/26/06
Email comments or questions to Eric Sanborn


I completed this swap in the first few weeks of December 2004.  Our old engine had about 190k on it and was sucking coolant somewhere.  I thought it might be the water pump.  It was due a timing belt anyway so I decided to fix it.  When I needed a breaker bar to get the spark plugs out I decided it was actually time for a new motor.  Since we painted the car a year or so ago it was coming time to freshen up the drive train anyway.  I had researched ZC swaps a few years ago, but the stock engine was doing so well we could not justify the expense.  The engines are now harder to come by and more expensive, but as you will see below I still feel worth it.

For those not in the Japanese car community the ZC is a double overhead cam engine that came in the Japanese domestic market (JDM) CRX SiR.  It is similar by not the same as the engine that came in the late 80s Acura Integra.  The engine makes about 20 more horse power than the stock US market CRX Si engine did.  The story goes that due to emission legislation in Japan engines are pulled after only 40k to 50k miles.   Therefore these JDM engines are supposed to have only about that many miles.  Now since there is no way to prove that then it is anyone's guess how many miles they really have.

I got my engine from Woodfins in Richmond, VA.  They turned around and got it out of a yard in NC.  Cosmetically the engine is a wreck.  If I were to put the 190k engine next to it on a stand and ask you to pick which one has "40k" and which has 190k 10 out of 10 people would guess wrong.    It had a chipped flywheel pulley (discussed later), dented oil pan, 2 broken off vacuum lines, broken TP sensor, rusted up fuel pressure regulator and unusable throttle body.  As a result I reused a number of times off the old engine like the fuel rail and throttle body as well as a number of brackets.  My recommendation to someone buying an engine is to get the yard to email you a picture of the engine you are getting.  I would not have purchased this one.  However as you will see later the engine runs like a top so some time put into refinishing the valve cover should work wonders in cleaning up the engine cosmetically.

Engine it its ugly form.  Soon I hope to blast and paint the valve cover.

Compared to my Audi swap of a turbo engine this was a walk in the park.  The ZC pretty much bolts in where the Si engine was.  I chose to upgrade to the 90/91 clutch setup which is 212 mm diameter instead of 200 mm.  At the same time we got a 6.5 lb Fidanza aluminum flywheel.  I had figured the stock one would be at the end of its useful life.  It was rather blue and scored a bit so I think it was a good choice.  As I recall the stock flywheel was 22 lbs when I weighed it.  This whole upgrade is  tougher for those with 88 CRX Si cars as the number of splines on the transmission is different than the 89-91.

Below is a list of things that are different between the two engines when it comes to installation:

  1. No fast idle solenoid on the ZC.  I just tucked the connector away.  The engine idles like a dream even without it.  Even when the A/C is cycled on.
  2. Slightly different Vacuum ports on the back of the intake.  I just plugged one of the lines going to the charcoal canister purge system.  Maybe I will get a T and fix it right sometime.
  3. The distributor functions on the Si engine are done in two connectors.  With the ZC it is done with 3.  You will have to do a little bit of soldering to split out two wires for the other connector.
  4. Hood clearance can be a problem.  I put some washers between the drivers side engine mount and the engine to lower the engine some.  I have had no hood clearance issues so far.  I would not recommend running without the cam gear cover.  If you do the first thing to rub is your timing belt.
  5. Need to use an '89 Integra upper radiator hose.  All other hoses are the same to the radiator.  I did have to use a different hose to the idle stabilizer valve, but I just used cut to length material.
  6. The vent line between the intake pipe and the valve cover is a different diameter and does not fit as securely.  I just used one of the factory style full wrap hose clamps.
  7. The black plastic cover for the fuel injector harness needs to be flipped around.
  8. The exhaust will likely need some work.  My engine was missing the down pipe so I could not use the 4-2-1 factory manifold and had to reuse the Si 4-1 manifold.  This requires a little bit of grinding as the exhaust ports are slightly higher in the ZC.  I just got an 89 Integra exhaust gasket, marked it with a sharpie marker and ground it to match with my die grinder.

The ZC appears to have a power steering pump although my engine came with zero accessories. As a result the crank pulley is a bit different.  The Si pulley has a damper of some sort in it.  My ZC pulley was quite banged up, but the pulley was so much lighter (about 6 lbs) than the Si pulley I chose to touch up the chips and reinstall it.


The most notable difference with the new engine is how smooth the car drives.  I am still running just the stock Si computer and it is amazingly smooth.  The only problem currently is a strange idle fluctuation when you first come to a stop for maybe 10 seconds.  This was not there for the first week, but is now.  Mid range power feels improved somewhat while power above 5000 rpm is quite an improvement.  This is definitely the engine that should have come in the car from the factory.

So far fuel economy has been lower than I hoped, but the motor had a bad thermostat in it that was stuck open.  I have changed it now and hopefully there will be an improvement.  The old engine averaged about 36 MPG with our mixed driving.  The first full tank through with the new engine was 30.5 MPG.  My wife has a small road trip planned soon so that should be a good test of highway fuel economy.

On a side note I will never again build an engine without an aluminum flywheel.  Then engine revs so easily and I am sure it has contributed to how smooth the engine is.




I noticed the number of hits on this page has gotten fairly significant. I feel bad I have not put any updates on it in ages. We probably have about 10,000 miles on the ZC engine. I don't have the figures in front of me,but I believe it has settled about at about 34-35 MPG with around town driving. My idle fluctuation turned out to be a nicked wire in the distributor. Sometimes on hot days when the engine is just turned on it idles low, but other then that it is just like you would expect with a new car.

The car pulls like a freight train above 4500 RPM. Between the light flywheel and the light car it took me some time to get used to how fast it reved in first gear. I was often hitting the rev limiter. Above about 5500 the car puts down some black smoke. I think this is because the stock Si computer doesn't give enough ignition advance. The stock computer also has a lower rev limit. To fix both these problems I am planning to reprogram the ECU with the help of the knowledgable people at I am trying to get an extra ECU so I don't have to take the car off the road while I work on it. I plan to load the JDM ZC original program as a first step. Then I will likely add the USB functionality and along with my Wide Band confirm everything is perfect.