looking for input for 2 issues with my 94 sc

gtpstyl

Registered User
First off...i recently bought a 94 sc 5spd which i've only got 55k on it. first question is simple i just can't find an anwser in the forums...my speedo reads way faster then i'm really moving...in a 65mph zone i can run 80 and nearly everybody passes me, but what bothers me is the fact that my odometer is turning faster then it should....for every 1 mile travelled...it registers approx 1.2 miles. being such a rare car with low miles i would like it to be accurate so it doesn't throw on all those undriven miles. So I guess the question is, is there a common problem that does this? Just wondering if anyone else has had this problem, if it was an easy fix or what Ford might charge. Also note, I've got the stock wheels with the proper 225/60R16 tires so tire size shouldn't be the cause.

Second Question...due to the LH cat blowing apart and all the peices plugging the 3rd cat where the pipes come together I had true 2-1/4 duels put on with resonators and an x-pipe. no mufflers yet as i didn't have the extra money for flowmasters at that time. Immediatly afterwards i noticed the car has lost 2-3lbs of boost compared to before the exhaust got plugged. is this just a normal result from a lack of backpressure? or did something else happen? i should also mention that when this occurred, while still being heavy on the gas, as the car first started sputtering i looked at the boost gauge to see the needle pointing straight down, well beyond the 15lb mark, which i immediatly let off the gas and the car died. i'm assuming that the increased pressure in the exhaust system backed up through the motor and into the supercharger causing the gauge to show so much boost. so, did something get wrecked by this happening, or is the 2-3lb loss of boost to be blamed upon opening up the exhaust system? thanks for any input!
-brendan
 
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2 answers

I. The spedo is off because the rear end gear ratio was changed but the spedo gear in the transmission that sends the info to the spedo was not corrected. Figure out what the ratio is now by jacking up the car and manually turning the wheels. Mark the tire somewhere where you can accurately see how far it turns and turn it one turn, slowly. Mark the crank and count how many turns it makes with one wheel rotation. If the crank turns under three revs, it's a 2.71 gear set, a hair over three revs, it's a 3.08, etc. I suspect that you have a 3.27 or maybe a 3.40. SC Performance sels the transmission correction gear.

2. The boost is decreased because your flow has been increased. (It's like trying to blow up a plastic bag with a hole. You can put in a lot more pressure when the hole is smaller. When the hole is larger, more air excapes even though the input pressure hasn't changed. But, if the hole is too big, you can't get any pressure at all.) Unless you notice a marked decrease in performance, you are in an improved state. If you keep a milage log, check and see if your MPG didn't just go up.
 
2 answers

1. Someone has changed the rear end gears but not the sending gear in the transmission. SC Performance sells the correction gears.

2. Your boost is down because your flow is up. It's like trying to blow up a plastic bag with a hole in it. When the hole is small, you can put pressure into the bag. When the hole is larger, the pressure in the bag is lower eventhough the input pressure hasn't changed. But, if the hole is too big, the bag doesn't hold pressure. Unless you've noticed a marked decrease in performance, you are in an improved state. If you keep a milage log, check and see if your MPG hasn't just gone up. I'm putting duals on my SC right now and expect to loose some boost but am adding a raised top to offset it. When I put a flowmaster cat-back designed for the SC on my 5.0 (it has a 3 inch pipe instead of the center cat,) my milage went up almost 2 MPG.
 
another quick question...for this test to figure out the rear end ratio by counting the crank revolutions, what gear do i need to be in? based upon this article, http://www.sccoa.com/forums/showthread.php?t=12473 i'm assuming i would want the shifter in 4th for a 1:1 ratio...is that correct? or even easier, could i just leave the tranny in neutral, mark the driveshaft and count the driveshaft revolutions? Thanks for the input on the loss of boost, that's a big relief!

Sharon Silver said:
I. The spedo is off because the rear end gear ratio was changed but the spedo gear in the transmission that sends the info to the spedo was not corrected. Figure out what the ratio is now by jacking up the car and manually turning the wheels. Mark the tire somewhere where you can accurately see how far it turns and turn it one turn, slowly. Mark the crank and count how many turns it makes with one wheel rotation. If the crank turns under three revs, it's a 2.71 gear set, a hair over three revs, it's a 3.08, etc. I suspect that you have a 3.27 or maybe a 3.40. SC Performance sels the transmission correction gear.
 
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gtpstyl said:
or even easier, could i just leave the tranny in neutral, mark the driveshaft and count the driveshaft revolutions?


You are right...This is the easiest way to do it!
 
rear speed

Sorry for the delay, and the double posting; I didn't think the first went through. It doesn't make any difference what gear the tranny is in, you are counting the revs of the shaft regarding only the wheels and the shaft, which doesn't change (the tranny changes the torque and speed applied to the shaft.) I found it easiest to tape a piece of yarn to the drive shaft and then turn the wheels. I think you need a 21 tooth tranny gear.
 
well i finally had some time to check the gear ratio...it turned about 3 3/4 turns so it's a 3.73 right? according to the faq section it sounds like a little too much gear for the M5R2. not that it really matters as i don't go to the track anyway. but in doing this i've got a bigger concern then my odometer racking up more mile then the car is really going...my rear diff is leaking like a siv! a month ago when i had my exhaust redone after buying the car i inspected the whole underside twice along with the guy at the exhaust shop......once when they looked at it to see if they would be able to run true duals and again a couple days later when they were all done. everything under there looked as it should and was actually better then i expected and the guy even commented on how clean the car was and how good a shape it was in. anyways, when i was under there it looks like the leak is coming for where the abs sensor is attached...but with my research on here i'm guessing it's the halfshaft seal. sound like the probable thing to you guys? thanks in advance!
 
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