Honda ecu question for fuel efficiency.

[Jesse]

Would it be safe to run a PM8(CRX HF) ECU in a Civic Si(D16a6)? I know it would be running lean, but it's not as if I would be beating on it since I am trying to conserve fuel. Thanks!

[Ryland]

They both have o2 sensors so it shouldn't run lean once it starts running in closed loop.

[Jesse]

So that would render this mod for fuel efficiency useless then? I was hoping to get a couple extra miles per gallon swapping out ecu's. Wish I could locate an hf engine/transmission.

[Ryland]

what year of CRX is it?
Even tho the HF wheels were steel, they were a light weight steel wheel, civic VX, HX, Honda Insight, or early 4 bolt Civic hybrid wheels will all bolt on, not sure on the weight of the SI wheels but the above mentioned wheels are all around 11-12 pounds each.
The stock HF front bumper had a lower lip, some years of the HF had aluminum rear brake drums, is the SI drum or disk rear brakes? Honda Insight aluminum drums bolt on and can be found used.
I realize people like to "make their engine breath better" but an exhaust sized for a 4 liter engine does not help, the HF had a small exhaust, not because it was cheaper but because it helped with mileage and low end torque, HF transmissions shouldn't be to hard to find, I use car-part.com all the time to find used parts, alot of them are close by, others are shipped via low cost shipping companies that are used to dealing with bulky parts being sent to body shops and mechanics.
But as it is, good low rolling resistance tires, synthetic motor oil in your transmision and a tune up should help you be on your way to 40mpg.

[Jesse]

I have located a few HF transmissions using car-part.com, the closest being in IL, I live in MN. Not sure if shipping would be cheap, thats why I am trying to find it locally.

I have a 1991 Civic Si, not a crx. I am currently seeing 38-41mpg in MN. Up from the 28-29 I would see before I started ecomodding. I deleted my pass side mirror, have a front air dam, undertray, roof extension and rear wheel well covers. I have a semi synthetic in trans(GM synchromesh) did that about 5k miles ago. Car doesn't need a tune up, I have a two year old dist/cap/rotor, ngk iridiums with 5k on them, new bosh 02 sensor, fuel filter, pressure regulator, too. I made a warm air intake which helped with warm up time, but thats it.

I would put synthetic oil in my engine if it werent for the fact the oil pan seal is bad, has a slow leak. I'm worried that it will begin to leak elsewhere if I go with 0w30 mobil one, so I just use cheap 5w30. Engine has 250k on it. I have thought about buying a cheap set of steelies and throwing on some 175s, the car currently has 185s on it on aluminum wheels. The gearing is what is killing me, though. I usually cruise at 65-75mph, so I see 3,000-3,800rpm on my commute to work. That HF transmission would help big time, or if I had more discipline and stayed at 55, which is ten below the speed limit I would probably gain a couple ticks as well.

[Ryland]

I haven't use synchromesh before, but I do know that most gear oils are not recommended for Honda transmissions because the sulpher reacts with the alloy in some of the bushings, that is why they call for motor oil or in more recent years Honda's own brand of MTF, I personally have 0W30 in my transmision and it's been working great.

That's a cable operated clutch, right? other option for a transmision would be the 4 speed out of the base model, I don't have the gear ratios in front of me but I seem to remember it being a bit better then the DX and quite a bit better then the SI.
You might almost find it worth buying a whole crx HF, I keep seeing them around with tons of rust, way to rough for any high school kids to want to mess with, find one that is no longer safe to drive then you would have a whole car worth of parts to play around with, that is assuming that you have the space.