SPF ZF Gearing

[DBLDREW]

Too, many new car customers like to cruise long distances at extremely low rpm, so a tall highway cruising gear is pretty much standard equipment on most cars now. In most cases, there is a HUGE jump between the first four (or five) gears and the single, tall overdrive top gear. Too, they tend to put extremely low gearing on the lower gears, so that the car will be able to get out of its own way when it is launched. My first experience driving such a car was with a 5.0 Mustang; first gear was so deep that you had to shift before you got halfway across the intersection, yet fifth gear was so tall that you could get decent gas mileage, and cruise on the freeway at 2000 rpm or so. It was far from optimum for performance driving, but it worked okay on the street.

The stock 5.0 mustang ran a t-5 trans that had a 3.35:1 first gear and a 3.08 ring and pinion. So what that means is that with a 26” tire at 6000 rpm you could do 45mph in first gear. With SpyderMikes set up he would be able to do 50 mph in first gear at 6000rpm. So in a 3200lb car with 225hp and 300lb/ft that first gear is way to low, but in a 2300lb car with 400-500hp and 400-500lb/ft, fist gear is not low enough?

Big V8s don’t like “high” rpms, but 3000 rpm isn’t “high”. In fact, most aggressive camshafts don’t even ‘turn on’ until something above that number.

It’s not that they don’t like high rpm and I’m not saying that 3000 rpm is exactly high. My point was that for longevity (and gas mileage) it’s better to spin it slower when you are just cursing at highway speed. Having lower gears gives you more engine options as well. Spinning a 302 at 3000-3500rpm on the highway is a bit high but not really that big of a deal. But compare that to running a stroker motor like a 408. Running a 408 at 3000-3500 would be the same as running a 302 at 4000-4500 as far as the piston wear goes.

So let me qualify my earlier statements and say that IF you are primarily looking for supreme dominance on an American racetrack, or IF you bought the car principally so you could enjoy driving it on twisty mountain roads, and IF your engine is capable of spinning to, say, 6000 rpm, and IF you are equipping it with standard-height (26 inch) tires, then you would almost assuredly be better off and happier with a Pantera-spec gearing setup, i.e. 4:22 rear end, and a closer spacing of all five gears with a 2.23 1st and a 0.705 5th

If you are looking for supreme dominance on a racetrack as in a dedicated racer then wouldn’t the better coarse of action would be to put in a straight cut dog box rather then a syncro box? Another point is that 99% of the time these cars are street cars not dedicated racers, you cant race these professionally, they don’t fit in any scca class.

I guess the last point is that we can assume what would work better based on your experience racing other types of cars, or we can look at how well the originalGT40s did in real racing. Well I seem to remember them having a good string of luck back in the 60s, and I do believe they where all geared to do over 200mph and some with only 4 gears.

I’m sorry if SpyderMike took offense at what I wrote, or if anybody else thought I was being rude. I meant no offense or insult to any individual; I’m just offering a bit of a reality check about gears. When I consider a GT40, I tend to think of it strictly as a driving, performance car, i.e. an open track or backroads car.

SpyderMike posts his gearing and you post how you and Lloyd where laughing at them for those gear ratios, I don’t know why you would be surprised that people would interpret your post as rude.

Listen I have no problem with you posting your thoughts and ideas, it’s the way you went about it that got me a little agitated. You apologized so water under the bridge.

As far as considering a GT40 as a driving performance car, I agree. I put it in the same category as a Ferrari, Lamborghini, Porsche, Ford GT, Vipers, etc And I think SpiderMikes gear ratios he chose puts his car in the same category as the above cars.

 

[Russ Noble]

when you are just cursing at highway speed.

Glad I'm not the only one who curses at highway speeds......


Cheers,

 

[Pat Buckley]

It may have been rude, but I thought your attack was just as rude.

 

[DBLDREW]

Your right my post to Mike Drew was rude, but that was my intention. He apologized so I will as well.

Mike Drew sorry if I was a little rude in my posts towards you…

 

[Ross Nicol]

Here is my perspective on this matter and my experience with an original M1 ZF-5DS-25/2 and later RBT which I have been using for over a year now. I have driven the car on the street a little but mostly race it wheel to wheel against Porsche,Ferrari,Lambo,Maserati,Corvette,Viper,MGBV8,TR8 and more.We are lucky in Australia to have a level of racing that accepts replica cars.
Now before I get into detail I read a previous post that all ZF boxes in GT40 race cars had the same final ratio, but if you read John Allen's book GT40 super profile he has tabled 2- ZF final drives 4.22 (sprint) and 3.2 (enduro).Now this makes perfect sense to me as the 3.2 would definitely be used at Le Mans for 200mph and the 4.22 at much shorter tracks where top speeds above 160 mph could not be acheived due to short straights.
I first bought the M1 ZF and was allowed to split it to check condition before handing over the money. I should have checked further but it was only later I found 5th gear was not as shown on the spec plate, nor was the final drive which I observed to be a 3.2 (plate 4:1). 1st gear was the normal 2.23 and fifth was supposed to be .917 but turned out to be .74. So I had a very tall 1st gear and a massive gap between 4th and 5th. I changed the 5th back to .917 but was still left with the tall 1st gear and standing race starts were still a challenge.The very good thing about this box was the close ratios 2nd to 5th which seemed motorbike like when racing.
Now onto the new RBT with standard Pantera ratios- 1st 2.23----5th.704
final drive 4.22. Time spent with gear calcs proved changes to o/all car speed minimal compared to old box. I think I lost some track times with the wider spaced ratios but I can pass quite a few cars into the first corner and those who know racing will understand how important starts are. I don't change down through all gears when racing,I have such confidence in the shift and gate tension that I'll change from 4th into 2nd as I enter a slow corner by-passing 3rd.It is not a dainty/delicate shifting box by any means but I maintain it fits the GT40 brute image.You will be able to determine how effective and quick my shifts are when I get my in car race video on line after the next meeting.Nothing wrong with synchros for racing in my opinion.
To sum up I am convinced that a gearbox with 5 speeds will face compromises when starts and revs for top speed road driving(fuel consumption) are considered and a lot of these compromises fall away when a 6th gear is added, so the comparison with Ford GT, Viper etc are not realistic as they have 6 gears.I believe 2000 revs is good for cruising in a V8 where my car with 4.22 final drive and with .704 5th it's too high at 3000 but then my car is better on the track.
I can understand that 3.77 is a good final ratio with a .704 5th for road touring and the need to take the 2.23 1st gear higher numerically but personally I would live with it having driven many starts (on the road) with a 2.23 first and 3.2 final I reckon just by changing the diff ratio to 3.77 or 3.9 would be pretty good for road driving.
Ross

 

[Randy]

Well this had made me think about my CAV that is on the boat, my zf has been on orded for mounths. Mike as a fellow Pantera owner you gave me great concern. The gearing in the 3200LB Pantera is great, but after a lot of thought I think the 4.22 in a 2300LB GT40 coupled with 400+ HP will mean shifting alot. The track time I have in my Pantera tells me to try the 3.77 so I can spend a little more time in each gear.My Pantera only had 325HP and when shifting at 6000RPM the ratio's are close!
Randy

 

[DBLDREW]

To sum up I am convinced that a gearbox with 5 speeds will face compromises when starts and revs for top speed road driving(fuel consumption) are considered and a lot of these compromises fall away when a 6th gear is added, so the comparison with Ford GT, Viper etc are not realistic as they have 6 gears.

Well just to point out that if we ignore 6th gear in all of my examples and look at 5th the Corvette has a (depending on year) .71 5th w/3.42rp, .74 5th w/3.42rp, and .75 5th w 3.45rp. The FordGT has a .76 5th w/3.36rp. and the Viper has a .74 5th w/ 3.07rp (rp= ring and pinion)

So all of the above cars have 200mph gearing (or very near) in 5th. Just some food for thought.

 

[johann hamm]

Gents

With 560 ftlb. which comfortably spins to 7k matters work this way
in a 2400lb car:
60mph comes in 4 secs cos I dont have to shift yet, then 4secs later comes 100 mph for shift to 3rd, which lasts another 4 secs by which time the party flies beyond 140mph and shift to 4th, nail it and hang on till 6k where there is no time to even look at speedometer for bragrights. Thusfar nothing has kept up to see their faces. This included twin turbo Porsches and some blown bling sting Beemers.
As for track use I only need 2, 3 and sometimes 4th. Makes concentrating on what is coming easier. Cruising at 100mph with 2800rpm gets distance done, which is the purpose of something calling itself a GT. Exclusive freedom. Ever stood under desert skies next to your 40?? I live in a country where I can and yes, covered 6000+ miles within first year of owning it. Much of it beyond 100mph.
Only drawback you have to know your stuff in 1st with urban traffic. But then I avoid it and plan my escapes at times when density is low. Not particularly interested in being seen, with actual emphasis on LIVING the nature of the beast.

Didnt those MK 2s and J-cars run 4 speed trannys??

Njoy your drivetime Spydermike, Life is short
JH
CAV106

 

[Ross Nicol]

Looks like I'll have to eat humble pie DBLDrew.However if the 5th gear in these cars with low "3" diffs is around .7 then the gear ratios are as wide as the ZF/Pantera ratio box, with my assumption that 1st gear would be between 2 and 3.This would confirm in my mind that in the Viper's case 6th gear is such a massive overdrive that it is only there for the car to pass the emissions and drive by noise tests.That is what I've heard.
Wow 560ftlb JH wide gear ratios no problem with that torque.How has your drive train handled it?
Ross

 

[Kirby Schrader]

At the risk of stirring up another hornet's nest....

I have been driving my Mk II now for a week or so, on and off. I've got some niggly things I'm working through.

The ZF in my car is an ex-Pantera model that Pantera Performance modified to fit my GT40. I am extremely happy with how the gear ratios have worked out.

First - 2.23
Second - 1.47
Third - 1.04
Fourth - .846
Fifth - .642 (special gear I asked to be put in)

Final drive - 4.22

I'm using the Avon 295/50 x 15 tires and they are nominally 26.5" diameter.
Redline is 6000rpm (I have roller hydraulic lifters) and the above works out to a top speed of 175mph.
It cruises along beautifully on the highway at 2400rpm (70mph).

I am very pleased with the result.

FWIW,

Kirby

 

[Ross Nicol]

The only difference between your box and mine is the 5th gear ratio.Mine is .704 and yours is .642. This allows you to tour in 5th with a nice low 2400 rpm whereas mines at 3000 rpm. Absolutely the right change to make for a road car and better than changing the diff ratio IMHO.
Ross