Selecting gear ratios

[RebekahsZ]

I'm sure this is a complicated question, but I'm looking to start simple:  I'm at 174mph and want to start working toward 200mph in the standing mile at Ohio. I have found that f-body.org/gears/ is very accurate to my car when it comes to modeling tire size, rearend ratio, trans ratios and rpm. I understand that I will need a lot more power to make the jump, and that I will need to make the safety mods required in the rule book. What I'm looking to work out at the moment is rear gear and transmission ratios. The help I need from you smart and experienced guys is: what rpm do I need to gear for?  In other words, do I want to go they the timing lights at the rpm peak on my dyno curve, or some other rpm?  Currently my car goes thru the beams at 174mph at 6200rpm. My peak torque is at 4150rpm and my peak hp is at 5450. With current gears, Im at my peak hp rpm at the 1/2-mile point. So, for the second 1/2-mile, I'm racing with progressively less hp as I approach top speed and the timing lights. Please offer advice.

[SPARKY]

Think in terms of Tractive Effort for the speeds you are trying to run.

Torque x Transmission Ratio x Rear Ratio x Tire Correction


BASE LINE---Use your existing rear tire RPM ( revs per mile) as a value of 1 and compensate from there

needless to say dyno sheets help

RPM is your biggest friend

[kiwi belly tank]

It's a waste of time spinning your engine past the point it noses over, 5500 in your case. You either need to up the usable rpm or run enough gear to get the job done. It takes less power to turn a tall tire than a tall gear so go to the tallest tire you can fit under there then go back to the calculator & see what axle ratio you need to get the job done in direct drive not overdrive if you have one. If you don't have enough HP to pull it you'll have to find some more.
  Sid. 

[RebekahsZ]

Thanks to you both for your replies. Kiwi belly tank-you and I are jiving. Exactly what Im talking about: over the next couple of years Im hoping to work down the chassis and setup issues, then in the last year (I'm hoping), the final issue will be a power race to build a powerplant to get the job done. I've clearanced the rear fenders to allow a 28" tire-no way I can get taller than that on this car. Next is available rearend ratios: I ran to 174 with a 3.54 gear, now I have acquired a 3.36 rear gear (the longest legged complete diff available from the production line without going into the performance parts catalog and trying to setup a diff from scratch).  If I follow the gearing chart that I have proven (at least to 174mph), with this gear I should be able to go 201mph, but I have to make sufficient power to 7000rpm in 5th gear (a 0.84 overdrive).  I guess my follow on question is: do I then build an engine to make power peak at 7000rpm or some higher or lower value?  I will run the numbers on that 3.18 R&P before I ask any more questions.

[SPARKY]

I know you wanted a simple answer---but the formula I posted will let you work through that ---you are going to have to have enough TE to accelerate to  200+ to

We are on all the same page

do you have some torque figures and lets work the formula so you can get a feel for the issues

[Stainless1]

The simple answer is yes you need a lot more HP and yes it needs to happen an a higher RPM.  There are several calculators around that will tell you how much HP you will need. 
 

[Captthundarr]

Just for kicks.... We run a 91 Camaro in the Ohio Mile. 424.24 hp 402.39 torque. @ 6250rpm, 350auto, factory 10 bolt with factory 3.08 w/mini spool, 28" tire. Amy shifts at 6500, Best speed in June 180.577. What have you done in the areo department?

[RebekahsZ]

Ran the numbers for 3.18, which is an R&P ratio that I can get from Nissan Motorsports.  It looks like this drops my 200mph point down to 6400 engine rpm.

SPARKY-I want to crunch the numbers on that formula, but I need some clarification first (BTW-thanks for helping me).  The max engine torque my car made on the dyno is 435 at 4150rpm. The engine torque available at higher rpms is much less (at 6200rpm it has dropped to about 300 if I read the dyno chart correctly).  So do I crunch numbers with max torque or the torque at my goal timing light velocity and associated rpm?  Secondly, do I calculate the tire correction by figuring corcumpherence and dividing that into a mile?  And I will have to figure in my .84 overdrive.

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Sparky replied

Associated rpm 

I put the formula on my XL   dream chart  per 100 RPM steps


RPM    Torque x Transmission Ratio x Rear Ratio x Tire Correction = TE

6200 l   300    l   1.00   l     3.54     l   1.00  =   1062  # ft. TE

now lets assume you shifted at 6200 you will fall back to 5200 rpm  guessing your torque will be some where around 375

5200l  375      l    .84    l    3.54     l   1.00 =     1115  # ft.  TE

6200 l  300     l   .84     l    3.54     l   1.00 =      894   # ft.  TE

This is the red neck way of being able to compare Apples and Oranges

Dave Dahlgren has a trick software programs as well as others but I print mine on a spread sheet so that I can figure and change gears with tires  I have 24 25 26 28 and which will cover the 10% steps in GM rear axels


now lets us assume when you shifted it at

[RebekahsZ]

Captthundarr-thanks for offering your specs-that helps a lot. I think we would have similar power if I just used a cam to raise my peak. Perhaps I will see you at the Sep 2016 meet?  That's my next anticipated participation. I expect to still be a 170-ish car depending on wind speed and direction. I had stability trouble when lifting after the timers this past June, and trying to correct this will be the big project for the winter. My only aero so far has been an airdam (which also functions as a 1/2-belly pan), but I hope to return with an adjustable splitter and a rear spoiler this year and spend the entire 3 days, not trying to go faster, but trying to get the car to go to top speed (whatever that is) and back to staging without me having to go to the port-a-potty to rinse out my fire suit.  If I get to it, I also hope to install a chute, and to start learning about how that affects things. Also, if I get to it, I may try the 3.36 rearend. I guess I'm hoping for a head wind in order to stay out of trouble for exceeding my safety classification. I hope to have that sorted to the 200mph specs for 2017 season. I guess I'm in this for the foreseeable future.

[panic]

Part of the gearing conundrum is whether you will make more than enough power to do 200, in which case running above peak RPM is helpful. If not, winding it past where it noses over doesn't work (as kiwi said above)

[Captthundarr]

I believe we met in June,  first while admiring you car, second trying to bum a front tire/wheel as we had one blow at 160 good luck with the areo.

[SPARKY]

Depends on how fast it falls off

 thats why you use the chart--- it will tell you where the cross over point is

[Interested Observer]

Worrying about gearing is the least of your problems and a waste of time at this point.  That will fall out once you have produced a power curve with sufficient power to go 200 mph.
Interpolating from the data you have cited, it would appear you utilized about 325 hp to go 174mph.
Until you generate 325hp*(200/174)^3 = 495 HP,  and the rpm at which that might occur, you don’t know where to start on the gearing.

[SPARKY]

OR if you know what gears and tires will get you there --Then you will know what you are going to have to have for an engine  other wise you are just trying to optimize what you can do with your eng.

[RebekahsZ]

Yeah, that was me trying to loan you a tire.  I'm gonna be chatting you up about chute issues in the spring.  I had so much fun at the May and June meets just trying to sort out handling, and going 13mph faster as reward for the trouble.  Really excited about the new 3-day format to allow more testing (and more hanging out getting to know folks).  I don't expect to make another record attempt for a couple of years.  Looks like its gonna require that 3.18 R&P.  I'll be spending my time looking for head/intake/cam packages that will provide power gains with the peak power moved up significantly.