speed calibration for 5-speed swap? - Page 2

mbtoloczko · #16 05-25-2005, 07:37 AM

Before I get this for you, are you sure that you're not running a mismatched set of front and rear diffs? Judging from your other thread, mismatched front and rear diffs seems like the problem.

If you can jack up the rear of the car so you can get under it, then you can rotate the driveshaft by hand and watch how many times the rear wheels turn. You can get the rear diff ratio from this. You might want to mark the wheels and the shaft with some white paint to make it a little easier.

#17 05-25-2005, 07:44 AM

Quote:

Originally Posted by huck369

The Speed sensor screws into the tranny where the speed-o cable normally goes, and converts the rotation of the Speed-o gear into an electronic signal the ECU can read (as the SVX doesn't use the Speed-o cable)

Cool, thank you.

Quote:

Originally Posted by mbtoloczko

Changing diff ratios causes the speedo to read incorrectly because the speedo reads off the 3rd/4th driven gear cluster on 5MTs. Any change to the diff ratio then affects the relationship between the rpm of the 3rd/4th gear cluster and the rpm of the tires.

Hmm, so I guess what I'm confused on is how the car will move at a different speed than the transmission is allowing it to. Let's make up some numbers. If at 3000 rpms with 4.111 gears both axles should be going 60 mph, and at 3000 rpms with 4.444 gears both axles should be going 50 mph. If you were to mismatch the diffs, say with the front as 4.111, and the rear as 4.444, how would the speedo read 60mph from the transmission, but the car move 50 mph from the rear differential? Am I even in the right ballpark?

GreenMarine · #18 05-25-2005, 07:53 AM

Quote:

Originally Posted by Earthworm

When I did my swap I kept the WRX speed sensor with no issues. I've also run with the SVX sensor in there (currently)

I run the WRX one too and my speed is right on... Possible alittle slow because of my bigger wheels and tires (which are forsale BTW

)

mbtoloczko · #19 05-25-2005, 08:24 AM

Quote:

Originally Posted by THAWA

... Am I even in the right ballpark?

Remember that the driven shaft in the tranny first goes into the center diff where the rotation is then sent to the front and rear drivelines. The center diff will allow the mismatched diffs to work. From the perspective of the driven shaft in the tranny, a 4.111 front diff and a 4.444 rear diff would have an effective ratio of (4.111 + 4.444)/2 = 4.28.

huck369 · #20 05-25-2005, 10:08 AM

Quote:

Originally Posted by mbtoloczko

Remember that the driven shaft in the tranny first goes into the center diff where the rotation is then sent to the front and rear drivelines. The center diff will allow the mismatched diffs to work. From the perspective of the driven shaft in the tranny, a 4.111 front diff and a 4.444 rear diff would have an effective ratio of (4.111 + 4.444)/2 = 4.28.

But running it that way will ruin the center diff.....

mbtoloczko · #21 05-25-2005, 10:54 AM

Quote:

Originally Posted by huck369

But running it that way will ruin the center diff.....

Yep, didn't mean to imply that it was ok in the long term.

huck369 · #22 05-25-2005, 11:02 AM

Quote:

Originally Posted by mbtoloczko

Yep, didn't mean to imply that it was ok in the long term.

I knew you knew, but wanted to make sure the original poster realized if that is the case, he's wearing out his center diff, every time he drives it...

#23 05-25-2005, 02:52 PM

I am useing a 4:11 tranny and diff from my Forester. I did not actually watch my mechanic do the diff. But he claims he used my Forester 4:11 and put the LSD from my SVX in it. I did see my old SVX diff sitting in his shop gutted. But I didn't count the teeth to verify wether it was my old 3:55 gear. I trusted that he did it correctly. I guess it is possible that he accidently put my old 3:55 gear in or some other gear laying around. I doubt that he would dileberatly ruin my AWD. I've been useing him to work on my Subarus for a couple years and have found him to be trust worthy.

I think my driveline issue is a worn out carrier bearing. I would like to try a good OB driveline to see if that solves that problem. Hopefully it isn't my AWD going bad.

I think my speedo is off because my tranny and diff came from a Forester which has taller tires stock than a SVX. I think that would account for the extra 10% error. I really need the speed calibration box for my Forester more than I do for the SVX. I will try the calibration box on the Forester first since it is reading 14% too slow. If I like how it works I'll get another one for the SVX. I'm assuming the calibration of this box is adjustable like the yellow box. Which is what I'm looking for in the Forester. Since I'll be putting larger tires on later on and don't want to buy another box for everytime I modify gearing.

#24 05-25-2005, 04:22 PM

Ok I did some tire size calculations. By useing a 4:11 tranny/diff from a forester with 26.2" stock tire hieght. And putting it into an SVX with stock tire hieght of 24.8" it should add about 5% extra to the speedo/odometer. I'm not sure how to calculate the speed for a mismatched diff. Anybody know what would the mismatch diff have to be in order to add another 5% to the speedo. I also remember that when my forester was stock it added about 2% to speedo. So actually all I need to account for is the extra 3%.

mbtoloczko · #25 05-25-2005, 04:52 PM

Just take the average of the front and rear diff ratios, and then divide that by 4.11.

#26 06-05-2005, 05:27 PM

I changed my driveline out for a 95 Legacy driveline. It isn't bucking like it was before. I think the CV joint may have been bad since it wasn't sitting in there straight. Also while I was under the car I turned the driveshaft and it took a little over 4 revolutions to turn the the tire one revolution. so it appears I have the correct 4.11 rear diff. I'm still interested in getting a speed calibration box. Could you PM or post info about the product, thanks.