final gear ratio selection

[Fezzek]

MK4 Roadster, 3.27 rear end with Torsen T2 and IRS, 17" wheels.

Engine will be a 351/stroked to either 408 or 427, EFI with stacks. Aiming for about 450rwhp.

Gearbox will be a TKO600, but I can't decide between a 0.64 or a 0.82 final gear ratio.

By my calculations, with the 0.82 I'll be at 2459rpm@70mph (motorway cruising), and with the 0.64 I'll have 1919rpm at the same speed. This is very attractive to me.

But, the other side of it is that I live in the UK, where the race tracks have longer straights. The car will mainly be a street car, with 4-5 track days a year, with 1 long road trip a year to Nuburgring or Le Mans.

I'd feel disappointed if I ran out of gears (assuming I had big enough cahones to hold my foot down that long).

So my head says go for the 0.82 (change to 5th around 105mph) and risk running out of gears, and my heart says go for the 0.64 for lower rpm cruising and less risk of running out of gears.

The idea of going 130mph in an open top car scares me as of today, but I'm sure that as I get more comfortable with it I'll get more and more comfortable with the idea of going faster.

Thoughts?

Martin

[MikeKelly]

Originally Posted by Fezzek

The idea of going 130mph in an open top car scares me as of today, but I'm sure that as I get more comfortable with it I'll get more and more comfortable with the idea of going faster.

Thoughts?

Watch this video, pay attention to the shifts to 5th. Especially the one at about 2:05.

I think that's about what you're going to get with a .64 5th.

My personal experience is that I enjoy 1st through 3rd so much that I very rarely make a hard shift to 4th, much less 5th... (just something else to think about).

HTH,

Mike

http://www.youtube.com/watch?v=qW-bc8FFZ1A

http://www.youtube.com/watch?v=qW-bc8FFZ1A

[weendoggy]

This will get interesting. It has been hashed many times, and comes down to personal preferance.

I run a 3.27 IRS rear with a T5 and .67 5th behind a 331 stroker. First is good to 45, second to 75, third to 120 plus, you get the picture.

For cruising, it is great. 5th gear @70mph is about 2000pm and great mileage. 4th does about 2400.

On the track I use 2nd through 4th most always and suits me fine. I really don't think you'll run out of gears with either choice.

[MikeKelly]

Originally Posted by Fezzek

(change to 5th around 105mph)

Originally Posted by weendoggy

third to 120 plus, you get the picture.

Somebodys' 3.27 calculations are off...

Or maybe we swithed to KPH?

[Frank Messina]

Glenn is right in that this will largely come down to personal preference. Often the .82 5th is chosen for better top gear pull, however with a well tuned 408 or 427 you should have plenty of torque available so that you can have your .64 for good cruising mileage and stiill be able to pull away strong in 5th. Just my 2¢.
Frank

[Fezzek]

How fast do these cars get to on a regular track?

I know at 130mph the aero dynamics start to kick in making it very hard to get above 130mph without some aero mods.

I wouldn't want to go for the 0.64 to make sure I have enough gears and never realistically get above say 140mph even on a long straight.

If I shift at 105mph into 5th and 6500 redline at 140mph with the 0.82 and it could do 130mph in 4th, I'd probably be better off with the 0.64 5th as I'd never use the 5th gear in normal usage.

There is no perfect gear ratio for track and street, which is why it is debated all the time on here. I guess I'll just have to pull the plug one way or another soon.

Martin

[MikeKelly]

Originally Posted by Fezzek

I guess I'll just have to pull the plug one way or another soon.

The aero is another often discussed topic, I'm not going there, lol.

You might consider the T56, that gives you both gears (but the bar where the handbrake mounts has to move, TKO600 is going to be real close to it anyway...)

Mike

[cobracruz]

Have the .82, would prefer .64.

When I'm tracking at Willow Springs, I get to the top of 4th gear on the front straight, so I never use 5th. Even if I ever run a track with longer straights, I don't think I would attempt much faster than 145mph in the roadster. Also, by the time you shift into 5th, you'd be right in the middle of your torque band and have plenty of pull.

However, I cruise on the freeway much more frequently. In LA it's common to drive 75-85, so I would've preferred a lower RPM. Probably the best of both worlds is the T56, have that sixth gear. I've heard it's a bit more expensive and more complicated to install.

[Frank Messina]

Depending on what size rear tire you have you should be able to get real close if not over 140 in 4th gear, but you'll be buzzing the motor to about 6000 RPMs if it can take it.

Frank

[Fezzek]

Originally Posted by Frank Messina

Depending on what size rear tire you have you should be able to get real close if not over 140 in 4th gear, but you'll be buzzing the motor to about 6000 RPMs if it can take it.

Frank

Rear tyres are 315/35/17.

I think I'll be setting the rev limit at around the 6000 mark anyway, even if the engine can take more.

Martin

[Fezzek]

Originally Posted by cobracruz

Have the .82, would prefer .64.

When I'm tracking at Willow Springs, I get to the top of 4th gear on the front straight, so I never use 5th. Even if I ever run a track with longer straights, I don't think I would attempt much faster than 145mph in the roadster. Also, by the time you shift into 5th, you'd be right in the middle of your torque band and have plenty of pull.

However, I cruise on the freeway much more frequently. In LA it's common to drive 75-85, so I would've preferred a lower RPM. Probably the best of both worlds is the T56, have that sixth gear. I've heard it's a bit more expensive and more complicated to install.

I did look at the T56, but the issue I have with it is that even though I have IRS and I can fit it in, the drive shaft will be short which I've bene told can lead to binding issues being quite common.

The TKO600 just seems to be a better fit in the FFR Roadster. I went standard width as well to avoid some of the problems that the pin drives guys have with getting wheels/brakes to fit even though I would have preferred the pin drive look. I don't want to solve one problem and introduce another problem by going down the T56 route.

But yes - the T56 was my preferred option to begin with, even with the extra weight. My car won't be a light weight regardless because of other options I have gone for so I can get away with a bit more hp.

Martin

[Fezzek]

That's a great video to highlight your point. Thanks for posting it. At that shift at 2:05, the engine seems to "stall" as the fifth gear doesn't quite have enough oomph to it and the other cars pull away a bit.

I'm not so concerned about the fastest lap, more just about having fun.

I'm tending towards the 0.64 at the moment although all the advice seems to say 0.82 is better. I just don't think I'll be at 130+mph often enough to needs the 0.82 pulling power, but I'll spend a lot more time at 70mph on the motorway.

Martin

Originally Posted by MikeKelly

Watch this video, pay attention to the shifts to 5th. Especially the one at about 2:05.

I think that's about what you're going to get with a .64 5th.

My personal experience is that I enjoy 1st through 3rd so much that I very rarely make a hard shift to 4th, much less 5th... (just something else to think about).

HTH,

Mike

Shelby Daytona Coupe at LeMans #2 - YouTube

[turbodon]

I have a Coupe. 410 , 3.27 , tko500 with .68, 315 x 35 rear on 17" rims.

6200 rpm in 4th is 130.

70 mph in 5th is about 2000

[rscocca]

If the car is going to be driven mostly on street, .64 is fine. Its used primarily for highway use. Local driving i never come out of 4th.
Yes in fifth the RPM drops off but with enough grunt it will pull away nicely.

On Highway I get about 22MPG. at 70 MPH I'm at roughly 2000RPM.
TKO600, 3.27 IRS

[Fezzek]

Originally Posted by rscocca

If the car is going to be driven mostly on street, .64 is fine. Its used primarily for highway use. Local driving i never come out of 4th.
Yes in fifth the RPM drops off but with enough grunt it will pull away nicely.

On Highway I get about 22MPG. at 70 MPH I'm at roughly 2000RPM.
TKO600, 3.27 IRS

Is 2500rpm too loud for journeys of 2-3 hours? I've never driven an open top car before, never mind a v8 open top car at 2500rpm for a few hours.

Martin

[CHOTIS BILL]

According to my gear program, which maybe wrong, using 25.7 inch dia tires, 3.27 R and P, and 6000 red line:

4th gear tops out at 140, .82 fifth gear tops out at 171, and .64 fifth tops out at 219.

Bill Lomenick

[BigLeo69]

my .82 has a lot of foot left in fifth
you may think of what the carb is doing for you at top end.
i am running 3.73 gears 70mph is around 2650 doesnt bother me i can't hardly hear anyway

how is that cobra doing did you change out the the drivers to Passenger side yet?

[cobracruz]

I use ear plugs for almost all driving. It's almost habit now, plenty of posts in regard to hearing loss. At highway speeds, the wind noise, as well as other car noises are almost as high as the engine. Plugs make for a much better driving experience.

[rscocca]

Originally Posted by Fezzek

Is 2500rpm too loud for journeys of 2-3 hours? I've never driven an open top car before, never mind a v8 open top car at 2500rpm for a few hours.

Martin

With the custom side pipes I have it is very nice to drive. At 70 or so mostly wind noise unless I'm in left lane and the wall is there.
I have high back seats that helps a little with the winds noise

I have driven to Lake George 4 plus hours, ok, I've driven to FFR 3+ hours ok. You kind of block it out after the first half hour.

[Sanford]

"I did look at the T56, but the issue I have with it is that even though I have IRS and I can fit it in, the drive shaft will be short which I've bene told can lead to binding issues being quite common."

I believe that with the IRS you will have no binding issue, only with a live axle will it.

[weendoggy]

Originally Posted by MikeKelly

Somebodys' 3.27 calculations are off...

Or maybe we swithed to KPH?

My math calculations are not off, I can carry my little putt-putt to 120mph + easily in third gear. Do I do it a lot, heck no, but it will do it with ease. Like I said, it'll carry to the speeds mentioned, but is NOT done for every shift. On track, I mainly use 2,3 and 4. There's no need for 5th as you lose a lot of torque.

I get mine to 140mph at Willow Springs main straight and that's fast enough for me. The car does tend to get a "light" feel. Laguna Seca and Sonoma only get to 120mph (4th).

I told you this would get interesting.

[MikeKelly]

Originally Posted by weendoggy

My math calculations are not off, I can carry my little putt-putt to 120mph + easily in third gear.

Did it occur to you that I chose to use the term "calculations", rather than "observed" for a reason (that had nothing to do with you)?

[KC Wildcat]

Sanford is correct, potential binding with a T56 is why you can only use it with IRS. The IRS pumpkin is bolted in the cage and does not move, only the half shafts (axles) & hubs move. The whole rear end moves on a solid axle.

[Fezzek]

Unfortunately the car is still sitting in the US at the moment as we are making a few other mods. LHD to RHD is just one of the mods we are doing. With any luck, we will ship it in October. I'll be posting a lot of photos once it gets here.

Originally Posted by BigLeo69

my .82 has a lot of foot left in fifth
you may think of what the carb is doing for you at top end.
i am running 3.73 gears 70mph is around 2650 doesnt bother me i can't hardly hear anyway

how is that cobra doing did you change out the the drivers to Passenger side yet?

[Fezzek]

We've got the high back seats to help with wind noise as well. I've got GasN side pipes, and I'm waiting for a set of quiet pipes from Breeze as well.

I have been pondering this for a couple of years now. Despite all the good advice, I don't think I am any closer to a decision.

Martin

Originally Posted by rscocca

With the custom side pipes I have it is very nice to drive. At 70 or so mostly wind noise unless I'm in left lane and the wall is there.
I have high back seats that helps a little with the winds noise

I have driven to Lake George 4 plus hours, ok, I've driven to FFR 3+ hours ok. You kind of block it out after the first half hour.

[Fezzek]

Just to close this out.

The internet is full of conflicting information on the various gear boxes.

I spoke to FFR about it - I trust FFR.
1) The T56 will fit, the T56 magnum will not as it is too big
2) The T56 only works if you have IRS. Without IRS, you will have binding issues

I spoke to Mike forte - I trust Mike Forte.
1) Tremec changed the naming and hate calling it the T56 Magnum, and the 6060 is the uprated version.
2) The T56 Magnum uses the same case as the TKO600
3) The T56 Magnum will fit no problem

I spoke to Tremec. They said:
1) No c*bra kit car will be able to fit any of the T56. It is too big
2) The T56 and the TKO600 use the same case, except that the T56 is about 2 inches longer

To make matters worse, I read dozens of other web sites, threads in other forums etc.

I know people have managed to get the T56 to work in their IRS cars. My concern is that so many people talk about the T56, the T56 Magnum, and the 6060 as if they are the exact same thing. They may well be, but I am getting so much conflicting information I'm more confused than ever.

I think the short answer is that it's either the TKO600 with the 0.82 final gear - it is one of the most tried and tested combinations out there.

I may still consider a T56, but it's unlikely. I like the fact that it has the extra gear, that it shifts a bit smoother etc. But without getting specific model numbers I'm not going to risk it.

Martin