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Old 11-11-2010, 12:14 AM   #1
AlexofNazareth
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Default Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

Ive been meaning to ask this for a while. This spring i was bored and was taking Automotive Tech classes at this one college and asked my professors about how to get rid of the speed limit on my car. They had no idea, so i came up with the great idea to disconnect the input and output speed sensors from my transmission.

I effectively removed my top speed limit...hell, i didnt have a speedometer that worked w/o those sensors plugged in. I got my car up to 115-120, but the weird thing was that the rev limit was changed from 5800rpm to 5k.

Is there any particular reason for that? Just wondering.

And for anyone with automatic trannies, dont do this. Your car wont work, haha

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Old 11-11-2010, 01:14 AM   #2
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

Quote:
Originally Posted by AlexofNazareth View Post
Ive been meaning to ask this for a while. This spring i was bored and was taking Automotive Tech classes at this one college and asked my professors about how to get rid of the speed limit on my car. They had no idea, so i came up with the great idea to disconnect the input and output speed sensors from my transmission.

I effectively removed my top speed limit...hell, i didnt have a speedometer that worked w/o those sensors plugged in. I got my car up to 115-120, but the weird thing was that the rev limit was changed from 5800rpm to 5k.

Is there any particular reason for that? Just wondering.

And for anyone with automatic trannies, dont do this. Your car wont work, haha
Sorry, I call BS.

There is just simply no way a SOHC Saturn will go 115-120 (MPH); at least not in stock form or without the assistance of gravity on a steep grade. I guess that way I see it, if a DOHC stuggles to get there it just seems virtually impossible (to me) for a SOHC to achieve the same with 24 less HP.

Also, IIRC the PCM is programmed to manage the engine as if the car was not moving when it loses the VSS signal and therefore limits the engine to 4,000 RPM under those circumstances.

...
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Last edited by ruley73; 11-11-2010 at 01:27 AM..

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Old 11-11-2010, 01:25 AM   #3
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

im just guesstamating the speed. Cause i can hit 100 in 4th gear at something above 4k, and i was over 4k in 5th doing this lil stunt.

Try it for yourself, i only did it once. And it probably too me a minute and a half to even get up there. I know when the cars not moving the revs are limited to 4grand, but its different w/o the speed sensors plugged in

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Old 11-11-2010, 01:58 AM   #4
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

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Originally Posted by AlexofNazareth View Post
im just guesstamating the speed. Cause i can hit 100 in 4th gear at something above 4k, and i was over 4k in 5th doing this lil stunt.

Try it for yourself, i only did it once. And it probably too me a minute and a half to even get up there. I know when the cars not moving the revs are limited to 4grand, but its different w/o the speed sensors plugged in
"guesstamating" is the key word.

The math says that 100 MPH in fourth gear should put you at about 4890 RPM; assuming you have the correct size tires on your car.

~5625 RPM = 115 MPH and ~5870 = 120 MPH in fourth gear

...
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Last edited by ruley73; 11-11-2010 at 02:03 AM..

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Old 11-11-2010, 04:09 AM   #5
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

Quote:
Originally Posted by AlexofNazareth View Post
im just guesstamating the speed. Cause i can hit 100 in 4th gear at something above 4k, and i was over 4k in 5th doing this lil stunt.

Try it for yourself, i only did it once. And it probably too me a minute and a half to even get up there. I know when the cars not moving the revs are limited to 4grand, but its different w/o the speed sensors plugged in
Common sense, sir.

The "car computer" detects speed by "speed sensors".

"Speed sensors" not plugged in= 0 MPH.

Car not moving= 0MPH.

There is no difference between the two. If you have an automatic transmission, it won't even shift without a proper VSS input.

"And for anyone with automatic trannies, dont do this. Your car wont work, haha "

Already knew that. Thanks for the intel.

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Old 11-11-2010, 07:38 AM   #6
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

Struggles to hit 120 with a DOHC? Hmmm...mines broke then...

...
The proper way to fix a S-Series automatic is to replace it with a 5spd O:)

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Old 11-11-2010, 09:44 AM   #7
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

yeah, i dont see SL2s having any problem getting up there. 5th gear at those speeds, i was sitting around 4300 and it wasnt going anywhere. Stock SL has got nothing in 5th gear going 110+, obviously. I did have my CAI on there tho. When i have time ill disconnect the sensors and record my results for the skeptics

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Old 11-11-2010, 09:55 AM   #8
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

If you have a portable GPS unit, it may have a speed display for an unbiased "witness". Some GPS units using satellite signals (and not the vss input) will automatically calculate speed and display it. My Microsoft Street map with laptop GPS software gave me speed info with its gps receiver.

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Old 11-11-2010, 10:12 AM   #9
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

I use my LG Voyager as my GPS, haha. Nah, ill just record the Revs in 5th gear. Itll be pretty easy to figure it out. Except I dont have my old SL anymore, i have an 2002 SL1 now with alloy rims.

Ruley, how many revs would my SL1 be at going 120 in 5th gear?

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Old 11-11-2010, 04:56 PM   #10
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

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Originally Posted by AlexofNazareth View Post
Ruley, how many revs would my SL1 be at going 120 in 5th gear?
~4380 RPM

I was using this calculator to figure this out along with the gear ratios found here. Compared to the speed vs RPM observations with my car, the calculator seems to be very accurate.

FWIW I've previously owned 2 other DOHCs which is where my thinking about twin cams and the speed limiter comes from:

1995 SC2 auto, dead stock, well maintained with about 60K miles on it: It got to 110 MPH OK in 3rd, but didn't do much after shifting to 4th. The fastest I ever got it to go was 115 MPH with a slight downhill grade.

1997 SC7 manual, dead stock, well-maintained with about 55K miles on it: I never did get it to hit the speed limiter. It was drag limited to ~118 MPH on the same stretch of road used to max out the 1995 SC2. It got to 110 MPH OK, but struggled to accelerate beyond that in 5th.

2000 SC2 manual (current car, see mods in signature): will hit the speed limiter. It gets to about 110 MPH pretty quick, but struggles to accelerate beyond that after shifting into 5th much like the 1997 SC2 did, but with a slightly more pulling power. When I had the smaller 205/50/15 tires on it, I could definitely hit the speed limiter quicker. Smaller tires numerically raise the final gear ratio a little bit which I felt noticeably helped with acceleration. I didn't have a way to verify this, but I felt that the increase in RPM with the smaller tires gave the engine more leverage/pulling power against drag. It didn't struggle beyond 110 MPH, like it does now. It pulled steady all the way to the limiter. Obviously, the indicated speed was 124 MPH, but the actual speed was about 118 MPH after calculating the difference with the smaller tires.

...
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Old 11-11-2010, 05:07 PM   #11
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

is it difficult to do an SL2 motor swap. Everyone is having a tiff over me modding my SL. Could i just get the motor, bolt it into my SL gear box, reflash the PCM and call it a day?

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Old 11-11-2010, 06:05 PM   #12
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

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Originally Posted by AlexofNazareth View Post
is it difficult to do an SL2 motor swap. Everyone is having a tiff over me modding my SL. Could i just get the motor, bolt it into my SL gear box, reflash the PCM and call it a day?
You could, but your cruise won't work if your car currently has it and you keep the SOHC transmission. A DOHC (MP3) manual transmission is strongly suggested, but not absolutely necessary.

...
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Old 11-11-2010, 06:35 PM   #13
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

lmao, never had CC.

So which generation Dohc should i get?

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Old 11-11-2010, 07:30 PM   #14
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

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Originally Posted by AlexofNazareth View Post
Everyone is having a tiff over me modding my SL. Could i just get the motor, bolt it into my SL gear box, reflash the PCM and call it a day?
Haha, dude it's not a tiff, just friendly advice from guys that have been here for a while and have seen people go down the same path.

Performance mods to the S series SOHC engine just aren't worth it, especially if you want to do things like go fast and stuff.

DOHC engines are, depending on where you are, plentiful, cheap, and are in some ways more reliable than the SOHC motors (eg, random head cracking).
They also come stock with more power and respond a lot better to performance mods. You get more bang for your mod buck, in other words.

As for your second question, yes. You could actually also do all of that in less than a day.

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Old 11-12-2010, 02:06 PM   #15
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

I understand. My buddy is gonna find me a rebuilt sl2 motor for about a grand and then ill pay him back when i get the money. What year engine should i get?

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Old 11-15-2010, 05:30 PM   #16
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

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I understand. My buddy is gonna find me a rebuilt sl2 motor for about a grand and then ill pay him back when i get the money. What year engine should i get?
2002. it's probably easier to keep it the same year as your car. it's probably even EASIER to sell your car and buy a 2002 sl2 for $1k more than you sold your car for. easiest "swap" ever.

and, please, do us a favor and DON'T test your top speed for our entertainment. you don't know how to properly drive a car at that pace and the sl is not made for those speeds anyway...


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Old 11-15-2010, 06:39 PM   #17
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

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Originally Posted by fdryer View Post
If you have a portable GPS unit, it may have a speed display for an unbiased "witness". Some GPS units using satellite signals (and not the vss input) will automatically calculate speed and display it. My Microsoft Street map with laptop GPS software gave me speed info with its gps receiver.
ALL Global Positioning Systems (GPS) rely on the government's satellite constellation. Velocity magnitude and direction are inherent in some of the messages provided by GPS receivers (see NEMA 0183 for GPS messages). Other messages will provide, for example, satellite constellation status and position data. My Gpsy software for Mac permits display of the NEMA messages.

Steve

Last edited by azrocketman; 11-15-2010 at 06:44 PM..

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Old 11-15-2010, 06:45 PM   #18
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

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2002. it's probably easier to keep it the same year as your car. it's probably even EASIER to sell your car and buy a 2002 sl2 for $1k more than you sold your car for. easiest "swap" ever.
More like $300-400, I'll bet. At this time in history, and the cars reaching the age they are, I doubt that the difference in price between a typical DOHC Saturn and its SOHC equivalent is more than a few Benjamins. Most people selling them these days don't even know the difference, and with fuel prices where they are (and where they're going), the fuel economy the OP's SOHC might actually be a plus. But I couldn't talk him into the idea earlier, in a semi-redundant thread to this one.

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Old 11-15-2010, 07:00 PM   #19
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

yeah, i dont have the cash now to buy another car or engine. Maybe ill swap engines if a totalled SL2 with a decent motor falls into my lap, but im just gonna make due with what i got. Ill post pics of my car after i put everything on it this thanksgiving break.

And ill see if my buddy gives me a free dyno run. Stock, should the sl1 be putting around 70hp to the wheels?

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Old 11-16-2010, 05:54 PM   #20
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Default Re: Removing the Speed Limiter: Disconnecting the Input and Output Speed Sensors

A couple points

GPS is the way to go for the speed. Calculating from engine RPM depends on the ratios being correct including the biggest variable, tire circumference.

The DOHC motor - A 2002 motor will have a different head than the earlier ones so you will be stuck with the plastic manifold and different bolt pattern for a header. If you want to do performance mods it would be better to have an earlier one.

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