3.90 LSD/Poconos

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Weaver7

New member
I going to be running at Poconos May 18&19 and was wondering if a 3.90 LSD would be ok to run? I ran a test day there last week with the 4.88 welded rear (in the rain) on the North Course which is quite short. The car ran great but at the end of the straight I was pushing near 7K RMP's. Any opinions??
Thanks in advance for any help you can give me.
 
It depends on your setup/power band, but most RX-7 guys think it's just getting good at 7K.....and many shift around 7700-8200......

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Jake Gulick
ITA 57 RX-7
 
I was gonna say... sounds like you've just about got the short track figured out. Now you just gotta get faster in the last turn before the straight so you can get up above 7K!

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Scott
It's not what you build... it's how you build it

rx7sigpic.jpg

PFM Racing
 
Originally posted by lateapex911:
It depends on your setup/power band, but most RX-7 guys think it's just getting good at 7K.....and many shift around 7700-8200......
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I quess what I'm trying to say is I'm coming up on 7K in 5th gear before the turn in. When we run the whole course I think I would run out of motor long before the end of the straight. I have a chance to get a complete axle assembly (3.90 with LSD) if I switched it over to that rear end would I still be alright to run with the pack. I'm new to this so please forgive me for sounding stupid. When driving the car it seemed to come alive around 5500 and start to fall on it's face around 7200. I know that most RX7 guys are saying 8+ but even the previous owner said he felt that after 8000 it was just making noise. Also everyone tells me that a welded diff. is a pain to drive and that a LSD will improve my lap times. I just don't want to turn the car into a dog. Thanks again for your help.
 
In 5th!!! Wow!!! 4.88's are steep!!! With our 3.9's, I never even got close to 7K at the end of the front straight at Gateway and thats coming off of turns 3 and 4 and down the straight of the circle track!
 
Two formulas to use to figure out your MPH at said RPM:

1. MPH = [(RPM x TIRE DIAMETER)/(OVERALL GEAR RATIO x 336)]

2. RPM = [(336 x OVERALL GEAR RATIO x MPH)/(TIRE DIAMETER)]

No offense to anyone, but TIRE DIAMETER is the height of the tire. 1st Gen trans in 5th gear @ 7000 RPM with a 4.88 rear. .825 5th gear, and tire dia. of 21.74 (185/60-13) means you are going 112.5 MPH. (128.5 @ 8000 RPM)




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Corey L. Clough
Ex-1st Gen, Pre-2nd Gen
[email protected]
 
I think I might be confused......

You say that at the end of your straight you are approaching 7K. Correct me if I'm wrong, but i always felt that you wanted to gear the car so that you are just hitting the redline (Whatever that is!!) in your top gear at the end of your longest straight.

So, if you think your engine is pretty much done at 7200, then that would be your "redline/shift point". And if you are just getting to that point now, then changing to a 3.9 will put you way lower rev-wise, if my math is correct. Which would essentially make your 5 speed into a 4 speed!

The LSD is a great way to go by all reports....the locked diffs do have their issues, as do your average LSDs to a lesser degree....too bad the ultimate is way expensive!

A couple thoughts on power curves and peaks. My engine, which has an unknown past, put out it's max power at just over 7100 when i dynoed it, then dropped just a little before my self imposed 8k redline. I have been known to stretch it to 8200 now and then to save a shift.....

I'd be interested to read what some of the other guys have found power curve-wise, and where they shift and what they consider their redline to be.


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Jake Gulick
ITA 57 RX-7
 
Originally posted by lateapex911:
I think I might be confused......

You say that at the end of your straight you are approaching 7K. Correct me if I'm wrong, but i always felt that you wanted to gear the car so that you are just hitting the redline (Whatever that is!!) in your top gear at the end of your longest straight.


Jack I understand what you mean but this was on the short test course. When we run the long course the striaghts are much much longer. I know some guys running 2nd gen rx7's that run their stock I believe 4.10's and just don't use 5th I thinking that maybe I could do the same. I must say also I really don't care for the welded diff. and this is another reason why I'm considering switching to the 3.90. I priced a 4.4 and the 4.88 with LSD and they are WAY over my budget right know. I still have to get used to this car so we'll see what happens. By the way what ratio are you running?? As for the power curve my car turns into a rocket somewhere around 5500 . I quess it depends on the motor but it seems like everybody else is diffrent.
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