Which one has better FE? CRX Si or DX?
 

Here's the deal. I owned a 91 CRX DX automatic(I know it sucks) and have a chance to pick up a 91 CRX Si parts car. I was thinking about just swapping the Si engine and transmission over to my car, definitely for the 5-speed. Just want to know if I should keep the DX motor with the transmission, or just go for the whole shebang.

Might just hunt for an HF transmission for the longer gears.

The Si gears are really tight on the CRX (I have a bone stock 88 si) and at 60 mph you are already at 3000+ rpm's. A DX manual has longer gears (different final drive ratio) and would be better in FE in my opinion, and a HF even more.

I disagree about the HF transmission. I actually tried using an HF transmission with a DX motor that originally had an SI transmission. And flat-out MASSACRED my fuel economy. Given the fact that the SI motor is actually more aggressively cammed than the DX motor, I would expect things to be even WORSE with that motor. If you want the HF transmission to help you with mileage, it MUST be used with a motor that is designed to take advantage of that tall gearing. So your best bet would be either a complete HF drivetrain swap OR (better) a D15Z1 swap with the HF transmission. Otherwise, if you want to use the SI engine, just use it with the SI transmission. You might be able to pick up SOME economy with DX manual transmission on the SI engine (or DEFINITELY with your stock DX motor). And, given that you could probably get one for free, why not experiement?

That will kill your in town driving but the hwy mpg should have gone up. What guys on the CRX forum do is swap the HF 5th into the Si trans. They keep the Si final drive ratio so 5th overall is still not as tall as in the HF, but it is a big help on the highway. It maintains good gearing for around town but on the highway you have a sort of "super" overdrive. Most of them report getting over 40mpg highway with that hybrid transmission.

Yeah, it's a matter of where the engine makes it's torque. The D15B2 (DX motor) makes peak torque at 4500 RPM, the D15B6 (HF motor) makes peak torque at 2800 RPM. The HF transmission's gearing keeps engine RPM in the 'B6's range of usable torque. If you bolt the 'B2 to the HF transmission, the engine will be outside its usable range and so will struggle to produce enough torque to maintain highway speeds. It's that struggling that kills the FE.
The D15Z1 is spec'd as having peak torque at 4800 RPM, but it's a VTEC-E motor, so it changes cam profiles in the middle of its RPM range. It uses an HF-like low-end torque profile at low RPM, but switches to a more DX-like profile above that. The result being a more or less flat torque curve, but it hits a little peak at 4800, so that's where the peak torque spec lies.

Highway MPG didn't go up. It went down. As for swapping an HF fifth gear in an SI transmission, this only makes fifth as tall as a stock DX transmission - a far cry from fifth in the HF.

Then something else was wrong
But still much better than the Si.

Try these two threads

https://crxcommunity.com/viewtopic.php?t=16427
https://crxcommunity.com/viewtopic.php?t=15913

Well, it would be a good combination then.
The DX 5th gear/final drive combo is very similar to the DX auto's 4th gear/final drive combo in terms of RPM-to-speed. I've gotten milage in the low 40s with both the D16A6 (Si engine) and D15B2 (DX, as I said earlier) engines, and nearly 46 mpg with a D16Z6 though the DX auto transmission.


Top gear ratio * final drive:

Si: 0.771 * 4.250 = 3.277
DX manual: 0.771 * 3.888 = 2.998
DX auto: 0.780 * 3.933 = 3.068
HF federal: 0.694 * 2.954 = 2.050
HF Cali: 0.694 * 3.250 = 2.256

Si/HF hybrid: 0.694 * 4.250 = 2.949

So the Si/HF combo is geared just a hair slower than a DX.

I'd look for an hf and sell your dx :)

They're still out there.... I am picking up a rust free HF tomorrow morning :D