Tune vs tire size
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From: POULSBO WA.
Tune vs tire size Question: my car is tuned by Pcmforless and at the time i had 18 and 19" wheels with 3.23 gear running 13.9 @.102-3. Then i did 3.73 gears and i went to 13.62 @ 105.49. I have not had it tuned for the 3.73 gears yet and i was wondering if with the new gears am i running lean or rich? Why did my mph go up that much but my time is poor? When i had 17" rims i was in the 13:4x @102, can 1" in tire size really effect me that much? I was thinking of getting a handheld programer for going to the track to put 16 or 17" rims on but i have a 93 with a different prom and i am not sure if it will work. Pcmforless offers a dual tune any body have it? Thanks sorry for so many questions.
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From: POULSBO WA.
Its off by 7 mph but my time slips are correct. So after making a tire size change i don't have to send in for a retune? So they are just ajusting my speedo? Which they cannot do on a 93 with a 700r4.
I would imagine its more to do with shift points and speedo calibration. I think theres a certain function they do and then you have to enter what it equates to in the tables to get a certain shift rpm. I dont think theres a "shift @ XXXXrpm" table lol, Hence why they "target" a 6200rpm shift. I would imagine going from a certain gear to another gear, with an automatic with a stall converter, you could run into some issues if it was a jump like 3.23 to 4.10.
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From: centerville, ohio
I would imagine its more to do with shift points and speedo calibration. I think theres a certain function they do and then you have to enter what it equates to in the tables to get a certain shift rpm. I dont think theres a "shift @ XXXXrpm" table lol, Hence why they "target" a 6200rpm shift. I would imagine going from a certain gear to another gear, with an automatic with a stall converter, you could run into some issues if it was a jump like 3.23 to 4.10.
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Thread Starter
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From: POULSBO WA.
From what i understand i have a 700r4 and it is cable driven, not electronic like the 4l60e from 94 on. I leave the line at 1400 rpm 60ft is 2.00 and i have always shifted at 6000 on the stock cam. I just want to make sure i am not running lean with the gear change. Any body have any ideas on the second part of the question why my times are so poor with such good speed? I guess with a ss3600 stahl i should be able to get in the mid 12's with speed @105.5. Yes? Let me know which way i should go thanks.
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First off, we need to know the difference in tire size, not the difference in rim size. Although, a bigger wheel diameter generally means less sidewall height. With smaller sidewall height in the tire, you will get less forward traction (generally speaking) because the sidewalls can't flex as much. The harder you leave the line, the more this will become evident...that's why the fast guys don't run 19" wheels. I personally have 19" wheels, but I didn't build my car for drag racing. I put my stock 17" SS wheels on if I do make the occasional trip to the track.
Second, gear ratio in the rear-end has nothing to do with the "tune" or "air-to-fuel" ratio. It is simply the ration at which the axles spin in relation to the driveshaft. I have r2ear ends, one with 4:10's and the stock 3:42's. The only thing affected between the two is the ratio the speedometer is programmed for. If I change rear-ends, I have to reprogram the speedo. The tires I chose for my 19" wheels are very close in height to my stock 17" wheels (less than .3")...however my speedometer would be slightly off when I switch.
Third, there are a lot of variables that can be causing the minimal gains. First, human error...can you make the same pass everytime. Track temp, Air temp, tire temp, tire air pressure, humidity...
I also have pcmforless tune. Much better than a programmer, not quite as good as a dyno tune.
Second, gear ratio in the rear-end has nothing to do with the "tune" or "air-to-fuel" ratio. It is simply the ration at which the axles spin in relation to the driveshaft. I have r2ear ends, one with 4:10's and the stock 3:42's. The only thing affected between the two is the ratio the speedometer is programmed for. If I change rear-ends, I have to reprogram the speedo. The tires I chose for my 19" wheels are very close in height to my stock 17" wheels (less than .3")...however my speedometer would be slightly off when I switch.
Third, there are a lot of variables that can be causing the minimal gains. First, human error...can you make the same pass everytime. Track temp, Air temp, tire temp, tire air pressure, humidity...
I also have pcmforless tune. Much better than a programmer, not quite as good as a dyno tune.
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Joined: Jun 2007
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From: centerville, ohio
From what i understand i have a 700r4 and it is cable driven, not electronic like the 4l60e from 94 on. I leave the line at 1400 rpm 60ft is 2.00 and i have always shifted at 6000 on the stock cam. I just want to make sure i am not running lean with the gear change. Any body have any ideas on the second part of the question why my times are so poor with such good speed? I guess with a ss3600 stahl i should be able to get in the mid 12's with speed @105.5. Yes? Let me know which way i should go thanks.
Thread Starter
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Joined: Oct 2006
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From: POULSBO WA.
I just looked up the specs on tire size diamerter ect...the 275/45/17 the tires i use to run and the 285/35/19 i run now are practically mirror images of each other. The first tire is 26.74 and 4.87 sidewall the second is 26.85 and 3.92. I have no traction problems at all, in fact it is really hard to break them loose. I do data log all my runs with a Mac Taskmaster and my shift points are really pretty close, there is room for error i am not perfect. I do not know how to calculate DA or what equipment to use, any sugestions on equip? My 60fts were pretty consistant at 1.94 to 1.95 then tire change went to 2.0 then gear change and no change still 2.0 although the pull through second gear made up alot of time. Thanks for all the input guys.
