Help Me Pick Gears For My Supercharged 2v

I've heard many people complaint about not having a low enough gear with the centri blowers. I would recommend getting a stickier and taller tire to fix the problem.

3.55

if your going to change gears, go with 3.73

I dropped from 4.10 to 3.73 and overall I am happy... The car is alot more 'tame' in terms of driveability. With the 4.10's it was real snappy, just a lil bit of gas would make the car lurch, and 1st gear was pretty much useless...

dont get me wrong, i miss my 4.10's... the car was a blast to drive on the street, but I was runnning out of gear in the quarter. As a matter of fact, I would have been happier with 3.90's for my current setup.

What are you currently trapping in the 1/4 and how many RPM are you willing to turn?

I agree with sneaky: We need more info.

I did some back-of-the-napkin calcs. I assumed a 25.9" tire diameter, a trap speed of 115MPH and a redline of 6000RPM. Going through in 4th gear (1:1) would require a theoretical gear ratio of 4.02:1.

3.73s would have you at roughly 5600RPM. 3.90s would be at about 5800.

Don't know if these are close to your requirements but it should give you an idea...

Car should make 700rwhp. I'd go with a 28inch tall tire and a 3.55 gear.

JymboSlice said:

Im shooting for 600 rwhp.. if it makes 700 id be more than pleased. Never drag raced it before, first time i will when i get it all together. Tuner and a bunch of other people said it would be about a 6500-6700 rpm redline as well.

I want to use as close to a 28" tire as possible.

Hope this other info helps, leaning more towards a 3.55 gear ratio. I would hope my trap speed is more than 115mph too.

Click to expand...

You're going to trap in the low 130's minimum with that setup. Even in full street weight. 18-22psi is a lot of power through a built motor with heads/cams. Just for perspective, my 402 CI motor will make about 900-925rwhp at 15psi with the same blower you have(maxing out the blower, got sponsored by paxton and it is sitting in my living room). That 281CI with those heads and cam at 4-6psi more should easily make way over 600rwhp. That is 130's for sure. I would guess the car ends up trapping 135mph-140mph.

Okay, so now you're on 28" tall tires and, assuming because you still haven't actually specified a redline number, a 7000RPM redline on your "built motor": With a 135MPH trap speed you could have 4.30s in the rear with the engine just touching redline in 4th gear.

You're going to need a tougher gearbox than a '3650...

trinity_gt said:

Okay, so now you're on 28" tall tires and, assuming because you still haven't actually specified a redline number, a 7000RPM redline on your "built motor": With a 135MPH trap speed you could have 4.30s in the rear with the engine just touching redline in 4th gear.

You're going to need a tougher gearbox than a '3650...

Click to expand...

Good post. I think you would know a lot more about the 3650 than I do. I run a stock 6060, and at 800-1000hp(crank), it is now giving me problems after 10k and 30 passes and 100 mexico runs.

I would listen to what the guys have to say about the tranny. I have not calculated your mph at the top of 4th with your gearing(3.55-4.30) at a certain MPH, but if Trinity or yourself could, that would be extremely helpful.

Also remember, if you ever do any street pulls, as fast as your car is, game is over if you run out of gear. I run a 9.6@148-150mph with my gear in 4th, but my car does 172mph in the same gear, so I am not geared for the 1/4 in anyway. I could probably run a 9.4 with a better gear, but once you get to a certain power level, you want a gear that will run a big MPH if you ever choose to(which I do weekly on the street). Your car is going to be extremely fast, so gear choice isn't going to be as much of a deciding factor IMO

Winters98GT said:

I would listen to what the guys have to say about the tranny. I have not calculated your mph at the top of 4th with your gearing(3.55-4.30) at a certain MPH, but if Trinity or yourself could, that would be extremely helpful.

Click to expand...

In general, the relationship between speed and RPM for a 4th gear of 1:1 can be summarized as follows:

EngineRPM = (AxleRatio * MPH * 336.135) / TireDiameter

where

- AxleRatio is, well, the ratio of the rear axle (e.g. 4.30)
- MPH is the vehicle speed
- 336.135 is a constant
- TireDiameter is the height of the tire in inches

You can also re-arrange to solve for other terms:

MPH = (EngineRPM * TireDiameter) / (AxleRatio * 336.135)

AxleRatio = (EngineRPM * TireDiameter) / (MPH * 336.135)

Just plug in your numbers as required to get the desired output.

Again, this is for a 1:1 (4th) gear. If you choose another trans ratio you'll need to add a term to the equations.

I've actually got a racing calculator built in Excel that includes all the gearing stuff. After entering your constants (rear gear, transmission gear, and tire height), you can calculate either the speed at an RPM or the RPM at a speed.

You can also enter your torque data from the dyno chart, gearing info, and shift points, and it'll graph the torque on the wheels (which is exactly correlated to how 'fast' your car is). Plus, you can do this with two different vehicles, making it easy to compare two vehicles.

For example, here's the graph of the torque on the wheels of a new Boss 302 vs. a Terminator (Boss in blue).



And you can see why a new Boss runs better than the Terminators do....

If anyone wants that calculator, PM me your email and I'll send it to you (unless someone knows how to post it up here).


And back on topic. Here's a little chart I made with the gears/rpms/speeds.

7000 rpm, 25.66" tire
Rear gear--------4th Gear Speed------RPM to go 180 mph in 5th
3.27--------------------163-----------------------4780
3.55--------------------151-----------------------5190
3.73--------------------143-----------------------5452
3.90--------------------137-----------------------5700

7000 rpm, 27" tire
Rear Gear-------4th Gear Speed------RPM to go 180 mph in 5th
3.27--------------------172-----------------------4543
3.55--------------------158-----------------------4932
3.73--------------------151-----------------------5182
3.90--------------------144-----------------------5418
4.10--------------------137 -----------------------5696

7000 rpm, 28" tire
Rear Gear------4th Gear Speed------RPM to go 180 mph in 5th
3.27--------------------178-----------------------4380
3.55--------------------164-----------------------4756
3.73--------------------156-----------------------4997
3.90--------------------150-----------------------5225
4.10--------------------142-----------------------5493
4.30--------------------136-----------------------5761


This is assuming the stock 3650 gear ratios. As you can see, it entirely depends on what tire you plan on running on the street vs. at the track, whether you want to shift to 5th or not, how fast you want to go on the street (I assumed 180, although I hope you aren't planning on doing that fast....), etc, etc. BUT, this should give you the info you need.

This also illustrates a good reason why a 6-speed tranny with a gear between 1:1 and OD would be nice. If that were the case, the top of 4th would be good for the track, and the top of 5th would be perfect for some Mexico runs.