New GT40 trans combinations?

Greetings,

Getting ready for a build using a 351W with a RBT ZF.

Interested in gear combinations anyone is willing to share.

Thanks

Tru
 

Rick Muck- Mark IV

GT40s Sponsor
Supporter
For the street we have found the 3.77 final to be nice. The 4.22 makes for a very short first gear. The 3.91 might be a good compromise but have never driven a GT with it.
 
Depends on what you plan on using your 40 for. Track use, highway cruising? As Rick said 3.77 diff is probably most common with 2.58, 1.61, 1.14, .846, and .642 gears. I like the tall 5th gear for highway cruising, but find it too tall for track days. If your primary usage is track days and racing I would go for a 4.22 diff and/or different gears.
 

Tim Kay

Lifetime Supporter
I'm planning on changing my ZF gears. I haven't a clue what Olthoff\Peterson were thinking when they built this ZF, 2nd & 3rd are too tall, 5th and maybe 4th are too low. So I can tell you what not to use:

4.00 final, 2.58, 1.47, 1.04, 0.826, 0.704 (1st thru 5th respectively)

Tim
 

Howard Jones

Supporter
I have two custom gear sets in my R21 and my G50. Both are more or less the same. I hope that's tells you something.

Without digging out the build sheets they are approximately 2.4, 1.7, 1.4, 1.0, and .8 with a final of 3.4

Three things. 1st gear is never used for anything but crawling along in traffic, loading on a trailer, pulling in the driveway, and transiting the paddock. In hind sight 2.4 is a little high for those purposes, especially with the touchy clutch in my GT40. 2.8 might have been better. Do not underestimate the need for slow speed traffic 1st gear stop and go. Select 1st gear for this, not a mythical hairpin corner on some track that doesn't exist in your region.

Second thing. About 2800RPMs @ 80MPH is a good SBF cruise engine speed in 5th and you can still use it if you have a nice long straight on track where you are running north of 140+. 2800 is right at the beginning of the power band so it will pull like a freight train if you just roll it on in 5th and if you run a little slower MPH on the highway, like 70ish, it's still nice and quiet.

Third thing. I have never needed 1st or 2nd on the tracks around me. I spend almost all of the lap in 3rd and 4th in my GT40 and if I ran slicks it would be even faster, 3rd 4th and 5th. 2nd is a good 35-45 MPH gear around town and a shift to 3rd will make you nice and quite for the cops/neighbors at about +40mph.

Lastly a strong 400hp V8 with a .8 5th and a 3.44 final will run north of 170MPH. If you run slicks on the track the car will run near the front of any group of track day cars. More than likely faster than 90% of them. If you want to run at the front. You will need downforce, slicks, and around 500hp. SLC..........?

The other grbx (G50) is going in my SLC. If that's not fast enough then I will need to spend some money on a drivers school.
 

Tim Kay

Lifetime Supporter
^Very well said and thought out Howard. I too have a G50 with very similar ratios and could not be happier for all the same reasons Howard mentions.

Heres my G50: 3.44 final, 3.15, 1.79, 1.27, 0.97, 0.76 and I might add it's been one of the most durable trans' I've owned. Has not been opened up since 2005, guessing 400 hrs (90% street). Thank you California Motorsports \ Bobby Hart.
 
At what RPM are you guys shifting at?
I used a calculator, and some of these gear ratios look good.
What are you looking for a close-ratio or a wide-ratio transmission?
 
I'm planning on changing my ZF gears. I haven't a clue what Olthoff\Peterson were thinking when they built this ZF, 2nd & 3rd are too tall, 5th and maybe 4th are too low. So I can tell you what not to use:

4.00 final, 2.58, 1.47, 1.04, 0.826, 0.704 (1st thru 5th respectively)

Tim

I have the same gears but with a 4.22 which probably makes a lot of difference. The rev drops between gears is the important thing. In lower gears you wnat to drop intot he Tq peak, and higher gears more closely spaced towards the Hp peaks. I shift around 7k rpm, but some corners, short straight etc by definition you shift lower, so there mtoro characteristics are important.

For a street car its the opposite, you want shorter lower gears for snap and taller higher gears for cruising.

IMo for track as some say the short 1st works for getting rollling and loading on to the trailer, for the rest lower gears are taller and higher gears shorter.

After that it depends on your engine setup, where tq starts to build as well as tire height. I run hoosiers which are 25.8 ins.. My motor comes on song around 3.5-4k rpm. Sometimes I find 2nd and 3rd to be a bit tall coming out of some bends, but then i also currently have too much throttle on oversteer.

One obvious answers are to carry more speed through bends which for sure I need to do, that will keep the motor more on song by definition. After that if its still not satisfactory I will change the cam and maybe go to webbers because the currrent 580 hp at 7k rpm is more than enough, and maybe sacrificng 50 hp up top to get more TQ lower (2500-3k on) and more linear precise throttle makes for a faster car overall.

But first I need to learn how to carry the correct amount of speed through bends, which is a suspension development/setup/skill issue.

At Monticello last time we ran around 2.43. Think stock 997.2 GT3. Changing springs better shock setup(more power on stability and sharper turn in) fresh tires there is a reasonable 5-7 secs more than that. Ultimatly to break 2.30 and get into viper ACR territory, well more skill for sure, slicks, brakes, and def further suspension sorting.

Probably the gearing I have is good for track. Most areas are 3rd, a few exits second and on the long straights I see the top of 4th and some 5th. Less shifts saves time.While 5th might be short for highway cruising I am still not exceeding 160 on the straights,(current revs and tires will theoreticaly max to 180) so taller there would be slower. More important than higher top speed right now would be stability under heavy braking at the end of long straights, not over braking and carrying more speed through bends. After that, as I said maybe webbers and a flatter cam for more low down oomph and precision.

As I write this I can see in my minds eye how to loose 15 secs with the current motor and gearing setup.

So to answer the question, it all depends on tire size, motor spec and most importantly is this for street or track..
 
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