Dear Brickmeisters:

I've found a donor car. But the dealer who has it raised an interesting question.

He says that ECUs for stick 240s are different from those with auto trans, because the ignition advance curves are different. Does anyone know about this? I've never seen this mentioned on the board, but maybe there's something to it.

Also: auto 240s (at least my 1992) have a 3.73/1 rear axle ratio. Does anyone know what the ratio is for stick 240s? I recently had to replace my rear axle and would hate to do it again if the difference isn't enough to mess up the operation. Also the replacement is a real low mileage unit and the axle on this donor car may not be as solid.

Thanks for any help you can provide.

Doug Harvey

My son's 81 has the 3.73 rear axle and M46/OD. RPM at 70mph is 3000, which feels reasonably relaxed. This is with the std 195/60-15 tires on Virgo rims. I've measured the 185/70-14 snow tires on it now - very close to the same rolling circumference as the others.

Your 92 automatic would have an OD ratio of .69 vs. the manual box's .80, so your overall top gear ratio will be a bit "lower" (by 16%). A little more engine noise, a little better acceleration or hill climbing ability, a bit lower fuel economy.

The electronic speedos take their signal from a "tone ring" on the differential carrier, so the only thing that would change speedo accuracy is tire size, not axle ratio. Older mechanical speedos took their cable input from the trans (or OD) output shaft rotational speed, which does vary with rear axle ratio.
--
Bob (81-244GL B21F, 83-244DL B23F, 94-944 B230FD plus grocery-getter Dodge minivan, MGB, and numerous old motorcycles)

The ECUs are the same.

While the ECUs may well be different, I cannot believe it will really MATTER any.

The ECU gets the vehicel speed off the tone ring in the rear end, so as long as you keep your 3.73s, you will be fine for speedo calibration etc.

When I went to change my '82 240 Diesel from A/T to M/T a couple "expierenced" mechanics told me that "you'll over-rev it and blow it up". Sure I said, how's that? The M/T is direct in 4th, and so is the A/T. "Yea, but the M/T is coupled with a clutch and not a T/C, that's the problem." Needless to say, I did NOT have any troubles, and neither will you.
--
JohnG 1989 245 MT @216,500

Doug,

I've done a couple of swaps on older cars (83s) and never had a problem with the ECU or distributor advance--but more modern cars may differ. Easiest way to check for the 92 would be to see if the parts folks list 2 different ECU part numbers, one for each application.

Regarding the ratios, I think the "manual" 240 NA cars came with a 3.31:1 rear. That said, the old turbo 240s used a 3.73 rear when equipped with a manual trans--so if you keep the 3.73, it will be similar to those old manual turbos (you trade off some fuel economy (15+%) for better acceleration and maybe a little more noise at highway speeds--BTDT).

The other nice thing is that by keeping the current rear axle, your speedometer should continue to read correctly . . . .

Hope this helps--and have a happy new year!

Steve A-

I do not think there is any truth to the ECU issue and 3.73 will be a nice ratio with the manual tranny. Peppy, but still cruising well.