hf tranny swap into an si

[sharpe]

i was wondering if i swap the tranny in my 91 crx si with an hf tranny what kind of mpg difference can i expect. i know it will be slower as far as acceleration but i dont care. its winter time and i just installed a hot air intake so my si is getting around 39-40 mpgs concistantly but i want mor because i now have a 60 mile commute.

[StorminMatt]

DON'T DO IT!!!!

Last year, I swapped an HF transmission into a Civic DX tht had an SI transmission, expecting some increase in mileage. It DECREASED instead. Remember that most Honda motors are optimized for higher RPM cruising. And lower revs don't seem to mean higher efficiency. Unless, of course, the motor is actually designed to operate efficiently at low RPM (ie D15B6, D15Z1). So unless you want to lose both power AND economy, keep what you have. Unless, of course, you decide to swap.

[GasSavers_bobski]

Like matt was explaining, the Si motor is tuned to make good power and torque in the 3500-6500 RPM range. The HF redlines at 4500 RPM. If you swap in an HF transmission, you'll end up cruising below the Si's ideal range, resulting in inefficient operation. You could cruise in a higher gear, but then why bother with an HF transmission?
If you want lower RPMs on the highway, take a look at a DX trans. It's the same 1-5 gearing as the Si, but with a longer final drive, so all the gears end up a bit longer.