Topic: 4Banger Jeep Speed (Ha! :)

[chardrc]

ya the tbi system doesn't have a speed input to the ecu.. 

[Jeffy]

ya the tbi system doesn't have a speed input to the ecu.. 

No one cares about you guy's.   

[stan98tj]

Not correcting your speedometer will have an effect on performance as the PCM uses the vehicles speed to adjust the timing and I believe the fuel curve.  Not to mention its a cheap fix.

at what size tire do you need to adjust the speedo? are 31s barely big enough to make a difference?

[sharpxmen]

at what size tire do you need to adjust the speedo? are 31s barely big enough to make a difference?

i think 31 is 10% off - that means when you're seeing 60mph you're travelling at 66 - same for mileage, your odometer changes by 1000 miles you traveled 1100. mpg as well btw, so your mileage is actually better.

[Jesse-James]

at what size tire do you need to adjust the speedo? are 31s barely big enough to make a difference?

Yes, 31's are enough to throw it off 5-6 mph at highway speed.

[chrisfranklin]

I don't know but I feel like I've been having problems at highway speeds when I went up from the original tires to 29ATs, then 31MTs.  Pretty small leap in size I know, but even with the 29ATs, it seemed like the engine would start "dialing" back at higher speed (especially noticeable when it was under load on inclines ).  

[TrailsLessTaken]

is it 65 what your speedo shows or based on gps or adjusted speedo reading?

If I remember I think that is adjusted, I never bothered fixing mine, cause I mainly just drive mine to the gas station or run in to town to get some food, so I never really get going more then 45 or so

[chrisfranklin]

Well it looks like I am down for a 38 tooth according to the chart -- 4.11, 31".  Of course, I am going to measure the tire-diameter aired up to what I use on the street just to make sure the tires are truly 31s. 

[Jesse-James]

To be the most accurate, measure from the ground to the center of the hub and multiply by two.

[chrisfranklin]

The 31MT Firestone Destinations I've got measured 31" at 38psi.  So I went ahead and ordered a 38 tooth speedo gear and gasket according to the 4banger chart

Here's the unexpected deal though: I sit there and looked at sites like Quadratec and their chart shows 4:10/31" should use a 36 tooth!    Then I read around and apparently some believe that Quadratec is errantly alternating between charts and that 38 tooth is the way to go for those aforementioned parameters.

[Jeffy]

Wheel width along with air pressure will also come into play as a wider wheel will make the tire sit flatter while a narrow tire (at the same pressure) will be a bit overinflated.  Not to mention height differences between brands and AT and MT', Bias Ply and Radial Ply...

[Torch_Ind]

Timing isn't controlled by the distributor.

yes it is. why do you think people with performance v8's advance the distributor for more high end output and they call it advancing the timing!!!

they also had centrifugal weights in the older distributors to advance timing at higher rpm!!

[Jeffy]

yes it is. why do you think people with performance v8's advance the distributor for more high end output and they call it advancing the timing!!!

they also had centrifugal weights in the older distributors to advance timing at higher rpm!!

You cannot adjust the timing on the Jeep.  You even said it yourself.  The way you can change the timing is by adjusting the CPS.  Then the PCM backs or advances the timing.

[sharpxmen]

yes it is. why do you think people with performance v8's advance the distributor for more high end output and they call it advancing the timing!!!

they also had centrifugal weights in the older distributors to advance timing at higher rpm!!

sorry man but you're wrong , the timing on the Jeep is controlled by the PCM on a predefined map (a table, not the map sensor) - it uses inputs from sensors to calculate the advance timing based on rpm, map sensor reading, tps and i think the IAT. the table is stored on 2 dimensions rpm and vacuum (map reading) and i think the tps and IAT come in play to adjust the predefined advance. not sure if the coolant temperature is used or not for the Jeep but i believe i saw somewhere that is used on some vehicles. You are right about some v8 distributors that you can rotate them to change the initial advance and also change the weights and/or springs to adjust the curve - all late ECUs use an advance map and calculations to determine the correct degrees of ignition advance though.

you can do it like Jeffy said by changing the cps position which will basically tell the PCM that the TDC is at a different location than it actually is - the downside with that is that you might get out of sync with the cam sensor but i'm not sure what the difference would be to affect the PCM so it could be safe. I don't know on top of my head what the diameter of the flywheel is on the 4banger but guessing here if let's say it is 20'' then you will need to move the CPS about 3/16 of an inch (0.1875'') to get 1deg change in advance - for that it should be fine but if you want to retard or advance the ignition like 4 degrees that can be a problem since you'll run out of room on the bellhousing opening (you'll need to move the cps 3/4'').

[Jeffy]

you can do it like Jeffy said by changing the cps position which will basically tell the PCM that the TDC is at a different location than it actually is - the downside with that is that you might get out of sync with the cam sensor but i'm not sure what the difference would be to affect the PCM so it could be safe. I don't know on top of my head what the diameter of the flywheel is on the 4banger but guessing here if let's say it is 20'' then you will need to move the CPS about 3/16 of an inch (0.1875'') to get 1deg change in advance - for that it should be fine but if you want to retard or advance the ignition like 4 degrees that can be a problem since you'll run out of room on the bellhousing opening (you'll need to move the cps 3/4'').

I've heard it's good for a few degrees at most.  There isn't much room to do more then that anyway.  otherwise, the best option is to get a programmable PCM and control it all by computer.