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swapped gears...now I'm slower?


spork

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Well, I've got an 83 280zxt with bwt5 tranny and had 3.54 gears in the back. I swapped in a set of 3.90 welded gears that I had lying around and my MPH dropped. I had been running 106-107mph, and I've been to the track 3 ties now on the 3.9 and the best I've seen is 102mph. To me this doesn't makes sense. My et's by the way are off as well (though I'm pulling 1.8 60fts now). I guess I'm going to swap the old gears back in but I was wondering if someone could explain what happened. I heard someone mention once about when swapping gears on a turbo motor you have to be careful because the turbo won't have anything to work against and you'll make less power. Anyways, any input is appreciated.

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Ha, perfect back to back real testing data to confirm this. Thank you!

 

A turbo uses load in order to spool. With the 3.90 you have less load. As you say, you had a better 60' time due to mechanical advantage, but the rest of the track you were losing out. Chances are you weren't seeing close to full boost in 1st and 2nd gear with the 3.90, right? Going through them too fast?

 

Load up that turbo, put in the 3.54's and work on getting your 60' time down with those gears installed... BTW, what was your best 60' time with the 3.54 installed?

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the best 60fts I had with the 3.90's was a 1.9...but most were 2.0's and that was street tires, vs slicks I'm using with the 3.9's. I never had a chance to try the slicks with the 3.54's. And actually, it seemed like third gear is where I lost out. First and second were absolutely amazing (I think that was mostly due to the slicks) but third gear it wouldn't run real hard, picked up in fourth all the way to red line where I cross the finish line. As far as full boost in the lower gears...couldn't tell you, I went through them quick...but I had a hard enough time keeping the car going straight let alone enough time to look at my boost gauge until about the top of 3rd gear. I would try taller tires, but with the amount of squat I get...and having nine inch rims in the back (for the slicks only) I can't go real tall or they'll rub on my launch.

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my rpm is right about 6000...maybe just a hair over. It's not worth switching to 5th gear or I loose both time and mph if I do. I've typically been shifting at just over 6000 rpm also, I've found that with my set up, it yields both the best mph and et.

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Guest freakypainter

i dont think turbo has anything to do with mph loss in your case. If you want the extra mph get a taller tire. Ask any one who races on a regular basis, ever notice how tall the rear tires are on a 300 mph funny car? They run a very low gear to get the most acceleration and a tall tire to get the mph. 1 revolution of a 30 inch tall tire travels farther than 1 revolution of a 25 inch tall tire. I hope I explained that right.

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i dont think turbo has anything to do with mph loss in your case. If you want the extra mph get a taller tire. Ask any one who races on a regular basis, ever notice how tall the rear tires are on a 300 mph funny car? They run a very low gear to get the most acceleration and a tall tire to get the mph. 1 revolution of a 30 inch tall tire travels farther than 1 revolution of a 25 inch tall tire. I hope I explained that right.

 

The way I understand it is trap speed is an excellent indication of horsepower. I don't believe an N/A car will register a higher trap speed by simply going with a taller tire (or a lower gear ratio), with no other changes. Tthe way I look at it is there is no difference between going to a taller tire or lowering your rear end ratio. You end up with the exact same distance traveled per turn of the drive shaft, so it's the same "ratio". In my mind, that only leaves the increased load with the lower ratio increasing the torque of the motor (the turbo's role) as being responsible for the higher trap speed.

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well, I swapped the 3.54's back into the car last night and took it out for a run, The car feels like it's back to normal. I toasted a newer transam and a brand new roush (sp?) stage 2 mustang.

 

James

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