Deciding on rear gear

[Feyd]

I need help making the decision on which gear to choose. I made the spread sheet using tremec's calculator. While not a true race car, I am using my car on the track for HPDEs often; 6 days booked this year and plan to do a similar amount annually. The 2.73 is the current gear and I was sold on the 3.73 but now I'm not so sure.

Roebling Road Raceway is my home track (and the only one I've been on so far) and I stay in 3rd for all but the straight. I could stay in 3rd the whole time because the governor tops out at 115ish before redline but I'm trying to not abuse the engine more than necessary. I'm hitting the governed top speed about half way down the straight. At the same time I do the gear change I will also be installing an aluminum driveshaft and a tune to get a bit more power, adjust the speedo and eliminate the governor.
So based on the spreadsheet;
3.31 gear seems like a decent choice because I will have better torque in 3rd through the whole track but may need to shift to 5th on the straight esp. when I go to VIR.
3.55 seems wrong because although 3rd will be lots stronger I may be near redline before even hitting the straight, either slowing me down or risking unsettling the car shifting during a high speed turn, and will definitely need to go to 5th.
3.73 gear would get me slightly better into the power band in 4th than the current 2.73 in 3rd so I could stay there for the whole track and shift to 5th for the straight.

Am I thinking about all this correctly? I would love to have this done before I go to VIR in May but it might have to wait.

 

[Phil II]

Why would you need to go to fifth with the 3.31 ratio? Did intend to write 4th?

Whose track day program are you using?

At first brush I'd consider the 3.31. What was the guy on corner carvers using ?

The best thing may be to drive another track. Have you considered Carolina Motorsports Park? Its in the middle of SC. I enjoyed it a lot.

You should be able to install a one piece driveshaft yourself. There ought to be a decent thread on how to do it. Svtperformance.com would be a good place to look.

 

[Feyd]

If I need to go over 125mph on a straight it will require a shift to 5th with the 3.31

I'm going through Hooked On Driving

I will have to dig back through corner carvers

Eventually I want to try as many tracks as I can. I get a printout of my work schedule for the entire year so I looked at the events that lined up with my time off and this year it was RRR, VIR and AMP. At VIR the Car And Driver reporters said this car rode the governor for 12 seconds per lap. I probably won't be that good but will still spend a lot of time not being able to go as fast as I should.

I will definitely do the driveshaft myself and use a Bama X4/SF4 to tune but I'm not going anywhere near the rear end lol. I'm thinking of checking out a local shop that makes custom driveshafts but may just go with Driveshaft Shop.

 

[Phil II]

As your 5th gear is 1:1, aka direct drive, I would not expect a problem using it on a road course.

 

[Feyd]

What I was getting at is instead of only needing to shift once btw 3rd and 4th and back for the entire track I would need a 2nd shift on the straight and then a two gear downshift into turn 1. Am I mistaken in thinking you would just skip 4th on the way down and just go straight from 5th to 3rd?

I've pretty much ruled out 3.73 as this would just put me running virtually the same rpm in 4th and 5th instead of 3rd and 4th with no real advantage. So its just btw 3.31 and 3.55. I'm kicking myself for not having a data logger with me at the track lol. I will be better prepared at VIR and after reviewing the data I will be able to make a more educated decision.

 

[Phil II]

More shifting is more work, but the right amount is the most rewarding. Yes, you can skip gears on downshifting. Mechanically the same thing is happening when driving on the street. Whether or not you need/want to skip gears going down will become a matter of preference and talent.

Here are two videos that, in addition to awesome driving, demonstrate the value of dancing on the pedals. BTW I'm nowhere near this talent. I just know its something to strive for.

 

[Lonnie-S]

Your task is similar to the one I'm doing myself with respect to my project car, which should be fairly similar to yours. Mine's basically a street car with planned occasional track sessions more for testing and tuning than competition purposes. I don't plan to race.

I think I could add a little something (maybe), but use some information too. I did my rough (call them ballpark) calculations the old fashioned way using a calculator and pencil and paper. I used the formulas on page 193 of the first edition of Paul Van Valkenburgh's book, "Race Car Engineering Mechanics." That's in Chapter 9: Gearing and Differentials. Because I don't currently have a torque curve for my engine, I based it on speeds, not staying in a particular power band, which I believe is the preferred way to do it.

Also, if you're most interested in improving your lap times, the best way to cut time off your lap is to have better exit speeds from the corners. Of course, that implies you're better set up going into the corners too. However, I think it is fair to say that performance in corners on that track, going in and coming out, should be your targets.

I'm interested in the TREMEC calculator you mentioned. I've been running "what ifs" by hand using the gear ratios for the V6 version of the T5 (what I have now) and also for the Ford Racing close ratio gear set for the T5. I plan to go to the latter in Phase II of my project along with an engine in the 250-300 HP range. By the way, I came to the conclusion that a 3.31 rear axle looks best overall for me and that is my current plan unless I get better information before I purchase.

If you know where I can find a torque curve for a stock '97 Mustang 3.8 engine, I'd appreciate knowing about it.

Cheers,

Lonnie

 

[Feyd]

http://www.tremec.com/calculadora.php or just google tremec gear calculator. I was also able to find a dyno test for my car on youtube to study. I was surprised at the amount of torque available at 3000rpm and the relative flatness of the curve, the HP however is a very steep ramp and I figure I need to stay btw 4000-6500.

This will probably make you laugh but I've been trying the diff gear ratios on VIR in Forza 6 to try to get a feel for them. unfortunately the closest I could come to my car is a 2015 GT which obviously has lots more power. I'm still going to reserve judgement until after I've been on the track but I'm starting to lean toward the 3.55 because I think I will enjoy it more during daily driving which is where most time will be spent.

I read through the entire corner carvers thread again, tons of good info. Made me really think about how I'm going to walk the line btw track toy and DD.
Priorities: 1, rear gear, tune, driveshaft. 2, dedicated set of wheels and tires for track. 3, full exhaust, CAI, possibly TB and new tune. That's all I really want to do with this car with little things like a new set of peddle covers to make heal toe easier and strut tower brace thrown in whenever.
The dream for the future is to get a C6 Z06 when they become affordable and make that a dedicated track monster. For now I'm learning how to take the corners as Lonnie said and just loving every minute of it.

I tried uploading video of some of my laps but I guess I need to put it on youtube and share the link... I'm kinda techtarded. coming soon-ish