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Discussion Starter #1
What rear gear i should run in my 95gt
331ci
cam 230/237 duration 3000 to 6500rpm
Tko 600 RR .82 5th
has 3.27 gear in it now
Was turbod now going na, changing cam and rear gear.
was thinking of going to 3:73 good idea or not?
 

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Would depend on which course(s) you'll be running most frequently, methinks. You might find that going to 3.73s ends up putting some of your shift points in awkward spots on Track A, but not at Track B. Maybe at Track A, 3.55s or 3.90s would be better. If you run Track A most of the time, and only hit Track B once a year, then go with what works better for Track A.

So maybe hold off on changing the gears until you see how the 3.27s work, and then do the math to see how other ratios would work out?
 

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I agree with Patrick, don't do anything until you try what you have. I just went thru the same sort of process because I was needing a new differential and thought I would change the gear while I was in there. I go to one track way more than others so I tuned to the most frequent track. I ended up going from a 355 to a 373 which is not a huge jump but it made a significant difference. Had I gone to the next gear I would have found myself having to upshift and then immediately having to downshift at one place and other problems elsewhere.
 

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Discussion Starter #4
Thanks for your input on this i mainly run at Road Atlanta and Vir. Ive been kind torn on this because the cam people all say must run 3:73. But then i think about that will equal more gear changes every lap.
I ran it at road atlanta back in 08 n/a with 315whp and 3.27 rear gear never got into 5th.
Im hoping engine will now make 380 to 400whp so may be different now
Road atlanta 3.27 gear 315rwhp
https://youtu.be/lJvw1dMBc3I
 

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Plot it out- where it really matters is corner exit. You don't want to be exiting Oak Tree at say 6,000 rpm and need to shift before you get off the rumble strips. Look at your slow corner speeds (VIR T1 & 4, Oak Tree) and run the numbers with different rear gear and what trans gear you would be in. You'll also want to look at your fast speeds, front and back straight, and see where you're at there. A lower gear does automatically equate to being faster, case in point my car: I run a conservative redline and with a 3.73 rear gear I need to shift into 5th gear, which is a miserable 0.63 bog city. Running a 2.93 lets me run 4th right to redline at the braking zone, and puts me nicely in the power band in second gear out of the slow corners. Now, a .82 5th like you have is very useable, and not getting into it is probably leaving some rear gear on the table.

Wallace Racing - Calculate Speed for given RPM, Rear Gear Ratio and Trans Gear Ratio
 

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. . . because the cam people all say must run 3:73.
Unless you really hammered home the point that this was for road course duty, you're going to get axle gearing recommendations based on street and maybe drag race uses.


On edit, that Wallace calculator is like all the rest I've ever seen - it calculates speeds that are about 3% higher than actual (or what a GPS-based system like Aim's Solo device measures). The reason is tire tread compression in the circumferential direction as it passes through the contact patch, which the simplified math does not recognize. While this doesn't matter much for street purposes such as speedometer correction, it can make a difference on the track.

A 3% rpm difference (a typical value rather than a fixed correction) could make the difference between needing to shift into 5th because the tach and your ears are telling you that you have to . . . as opposed to not upshifting because the calculator says that you shouldn't be having to. This could even happen for an upshift to 4th depending on the track. A 4% rev increase forced me to add one upshift and one downshift at one of my home tracks and made it a more reasonable thing to do at the other . . .


Norm
 

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Discussion Starter #7
Good point Norm, i used that calculator
3.73 165mph @6700rpm
3.55 175mph @6700rpm
3.27 188mph @6700rpm

im guessing my cam will redline at 6700

dont think car my 3200lbs 380-400whp would not pull past 165 at vir or road atlanta
 

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Good point Norm, i used that calculator
3.73 165mph @6700rpm
3.55 175mph @6700rpm
3.27 188mph @6700rpm

im guessing my cam will redline at 6700

dont think car my 3200lbs 380-400whp would pull past 165 at vir or road atlanta
I can hit about 145 going into T1 at 2800# and probably a touch less power (TW 170 head'ed 347 turning 5,800) and the aerodynamics of a brick, literally.

So lets say you hit 150 at VIR, with a 3.73 in 5th gear you're turning 6,300 rpm. With a 3.27 you'd just be hitting the limiter in 4th. If you had a dyno graph you could do the math to see which would be better: torque * rear gear * trans gear.

Also- stock or aftermarket block? I'd be wary of turning a stock block 6,700 for extended periods of time.
 

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Discussion Starter #10
Thanks for your info that the kind of data i was looking for.

its a boss 302 block.
When it was turboed it pulled to 7100 with a 25.4 .82 5th 3.27. but that was with 770whp

not sure how much power it will make
331ci 9.6cr 11r 205 56cc eld victor jr intake 90mm tb 1 3/4 lt 3"x 3"maxflows 230/237 .570 cam
 

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Putting my 25" tire in at 24.5" makes the calculator "work" for me.
Yes, you can "tweak" your tire dimensions (or maybe the tire size on some apps) to make it agree with the tire mfrs' revs/mile data or any reliable data that you might have determined yourself (that 2% "correction" is certainly within range). But that's what the app should be doing for you. At the very least it should be giving you enough information on the page so that you know that a correction factor is involved, together with an idea of its magnitude.

In their current state of development, you have to know more than the app's own designer did. You have to at least suspect that the calculator is missing something even if you don't know what that might be. I'd be surprised if more than a couple other members here ever stumbled across anything on this level ↓↓↓ , which I've had in my automotive library for something like 45 years now. SAE papers and similar level stuff.




Just to introduce another wrinkle, at high speeds where a lot of power needs to be put to the ground just to be at that speed there's another little correction involving tire slip (kind of like slip angles for cornering except it's "straight slip" somewhere between none at all and the point where any more at all results in a sudden snap into a smoke show. Maybe another couple percent or so.


Norm
 

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What rear gear i should run in my 95gt
331ci
cam 230/237 duration 3000 to 6500rpm
Tko 600 RR .82 5th
has 3.27 gear in it now
Was turbod now going na, changing cam and rear gear.
was thinking of going to 3:73 good idea or not?
All good stuff above, I would just add that tire size matters just as much as rear gear. Personally, I gear so I top out in 4th at most tracks, 4th is ~2% more power efficient than 5th. I run a TKO600, 3.27 gear, 275/40/17 tires. Below is a 1:36 lap I put down at RA. At that time, engine was a 331 with 335rwhp, 6300rpm redline. I just touch redline in 3rd before T2, and again before T5, exit T7 at low end of peak torque plateau at 3800rpm, redline in 4th before T10a, sitting on the torque plateau through 10a/b. Little time lost shifting. At VIR, I will grab the .82 5th on the back straight.

For your rpm range, 3.55 might be ok, but I don't think you'd be happy with 3.73.

 

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Discussion Starter #13
I ended up going with 3.73 after I changed the cam it pulls hard 4800 to 6800rpm.
The 3.27 were too long, it was difficult choosing between the 3.55 & 3.73.
Thanks for everyones input
 
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