Far out transmission possibility?
 

What brave soul amongst us shall be the first to boldly go where no man has gone before in a V8 Miata? http://tinyurl.com/kb53gnc

Page two, column C gear ratio options paired with a 3.73 rear gear would mimic the vintage, always-rowing experience of a 2.20 low Muncie rockcrusher with the addition of intergalactic warp speed capability.

7 speeds? 3 overdrive? I rarely use 6th now. but makes for an interesting proposition if/when FI is introduced to the equation....

So here's where it gets interesting. If you have the most common T-56 it would have the standard, wide-ratio gear set. Ratios 1st to 6th are 2.97, 2.10, 1.46, 1.00, .74, .50, which figures out to a 5.1 to 1 reduction between the highest and lowest gear. Saying the same thing another way, at a constant rpm on the input shaft, the output shaft turns 5.1 times faster in 6th than it does in 1st. So that is it's available range of gear reduction.

Looking at the available close ratio choice for the 7 speed transmission (column C in the brochure), the ratios are 2.29, 1.61, 1.21, 1.00, .82, .68, .45 which figures out to the exact same RANGE of reduction as the T-6, only there are 7 speeds packed within that reduction range instead of 6.

As I alluded previously, if you've ever driven a 2.20 low Muncie rockcrusher 4 speed in a late 60's Chevy muscle car (with straight cut gears that whine like a newborn baby), you know that "always on the boil" feeling of very small rpm drops between upshifts. Kinda' like a sport bike. It's fun, if you like to shift often and butter smooth.

The three overdrive ratios in the 7 speed are deceiving. If you ran a shorter geared differential (roughly 3.73), you could have the exact same road speed per rpm in 1st and 7th that you do with the T-56 in 1st and 6th with a 3.27 rear gear. Just closer ratios in-between.

Do we NEED 7 closely packed ratios in these cars? No, absolutely not. But if you are focused on "need" and "practicality," you're probably in the wrong forum.

This unit is pretty new and I believe only currently available in Chevy configuration. But I figure by the time I win the lottery or save my lunch money long enough to buy one, they'll have an aftermarket version for the big Blue Oval, too.

If I was going to add another gear, it would be at the bottom. Like a 3.25'ish 1st.

Indeed!

I do like very close ratios for their reduced rpm drop, but in my application I have a hard time even getting myself to want. Part of what I love about my current set up is sticking it in 4th (1:1) and leaving it there. I find it has plenty of oompf to dig me out of 70* turns with the tail end wagging ever so slightly and gets the rpms up enough to keep me from going *too* much over the acceptable speeds.

The one thing that does really appeal to me about this gearbox....it's just cool! "Hey brah..I got a 7 speed behind my LS3 in my Miata. What ya got?" Ya...one upmanship for the win! :loser:

To turn to the practical...isn't the 6060 dimensionally larger? I suspect there'd be fitment issues. Nothing a plasma cutter, some sheet metal, and a welder couldn't fix.... :)

That's another cool thing about this box. You can choose the total range of reduction. If you look at the table on page 2 of the linked brochure, column C lists the gearset ratios for the close ratio box I've been talking about. But if you wanted greater ratio range, you could choose either the ratios in column B (which are very similar to the close-ratio T-56 box plus a taller gear for 7th), or even column A (which is very similar to the wide-ratio T-56 box with again, an additional, taller 7th speed).

Really, it is only the total range of reduction and the number of gears within that range that determines your launch/cruise possibilities and intermediate driving characteristics, not just the specific, numerical ratios employed. For example, if you chose the box in column B, you could simply install lower (shorter) differential gears that give you the same overall gearing in first that a 3.25'ish first gear would give you with your current differential ratio. Then, the 7 speeds mean you still have closer ratios between all the gears compared to a T-5 or the standard, wide-ratio T-56, along with a taller overdrive for cruising. That's the "have your cake and eat it too" possibilities of adding a gear and changing the driving characteristics.

To expand on that thought from another angle, we are conditioned to think of any ratio numerically lower than 1:00 to be an overdrive, only applicable to cruising or when dropping numerically into the 40's, maybe the Bonneville salt flats, LOL. In reality, the only thing that matters is how fast the wheels turn versus engine speed in each gear, right? Wheel speed is a product of engine speed divided by transmission ratio times differential reduction. It doesn't matter whether the transmission is gearing engine speed up or down (underdrive or overdrive) to arrive at a given transmission output shaft speed, it really only matters what axle rpm is generated by output shaft speed times differential reduction ratio.

So as long as the box has sufficient torque capacity, which this one does, we don't "have" to think of the three top gears in a 7 speed as all "cruising overdrives" unless that's what we want them to be. We simply choose the overall range of reduction we think we need from column A, B, or C, and then select a differential gear ratio to match our requirements. In other words, determine what overall reduction you desire in first gear and top gear, then look behind door A, B, or C to get the overall reduction range that fits your needs.

All of which is, as previously suggested, logical justification for being able to nonchalantly say, "My little old Miata? Oh, it just has a 425 HP V8 with a 7 speed manual transmission." :)

Actually, it does matter.

If you do the math, you can come up with a similar FDR for a given rpm regardless of how many OD gears you have. And for gentle cruising that works out fine.

But overdrive gears divide torque, and underdrive gears multiply torque. And that matters. When you're pushing the car hard (at the track) you keep the rpm's up by holding the shift. It's not because the engine makes more power up there - it probably doesn't. But the lower gears will multiply torque better, and pull you out of the corner.

A chassis dyno measures torque, and calculates HP. The test is done in whatever gear is 1:1, because that most accurately reflects engine torque. Try doing the same test in 2nd or 3rd, and see how different the torque curve is.

With that same thought in mind, imagine launching the car with a 3.25 1st vs. a 2.90 1st.

Short answer: I suggested, “It doesn't matter whether the transmission is gearing engine speed up or down (underdrive or overdrive) to arrive at a given transmission output shaft speed, it really only matters what axle rpm is generated by output shaft speed times differential reduction ratio.” You suggested it does matter. You are correct if we don’t have the ability to adjust the differential ratio; I am correct if we do.

Long answer: Your point is well-taken that the generally-accepted 1:1 transmission ratio will produce a different dyno graph than an alternate transmission ratio. However, when we are trying to optimize our driveline to produce real-world performance, I will continue to suggest that what really matters is the range of overall reduction between first and top gear that matters, not necessarily their specific numerical ratios. This is true because we also have the ability to manipulate the differential ratio to compensate for those specific, numerical transmission ratios and achieve the overall driving characteristics and performance we desire.

Dyno graphs give us a good way to measure engine torque. (I am ignoring the parasitic driveline losses incurred from measuring at the wheels because that won’t change significantly with different transmission gear ratios). However, dyno graphs don't accelerate the car. Rather, (assuming traction), acceleration is accomplished solely by the force applied at the tire contact patch. Once we have selected the tire height we will use, we have locked in the lever arm that converts rotational motion to linear motion; at this point the sole determinant of acceleration will be, not torque at the crankshaft and not torque at the transmission output shaft (via underdrive or overdrive), but rather torque at the driving axle shaft. Obviously, that is the product (multiplier) of crankshaft torque x transmission ratio x axle ratio.

You correctly alluded to this principle when you suggested the engine may not necessarily be making maximum torque up near redline, but staying in a lower transmission gear may result in the car pulling harder out of a turn due to the additional reduction (torque multiplication). That’s absolutely correct, because when you desire acceleration, all that matters is axle torque. If the increased percentage of torque multiplication of staying in a lower gear exceeds the percentage of engine torque fall-off from revving above peak torque, the axle torque will be higher and the car will accelerate harder. What you said.

However, when considering possible gearbox alternatives, it’s really easy to be fooled about the performance possibilities of a transmission if we view absolute ratios in a vacuum. Those ratios are only one third of the axle torque equation, and we can still change axle torque (and acceleration) by varying crankshaft torque or axle ratio. We can pretty much eliminate crankshaft torque from this discussion because we typically will choose “as much as we can get.” That leaves differential ratio to balance out the gear ratios in the transmission we select and ultimately determine axle torque.

So, an example: The standard 5.0L Mustang T-5 has a 3.35 low and a .68 overdrive. That gives it a reduction range of 4.92 to 1.

Now, look at the closest ratio version of that 7 speed box, 2.29, 1.61, 1.21. 1.00, .82, .68, .45. What jumps right out at you is three, tall overdrive ratios. So this box must be for high speed Autobahn blasting? Yet this box actually has an overall reduction range of 5.08, only about 3% wider than the standard, 5 speed Mustang T-5. If you change to a shorter differential gear with the 7 speed to give the same speed per engine rpm in first gear as the T-5, you will be going only a couple miles per hour faster per rpm in that super-tall .45 overdrive than in top gear in the T-5. Again, that’s why the range of reduction is what really matters, because you can adjust for the absolute numerical transmission ratios by tuning the differential reduction .

In this comparison between the T-5 and the close ratio 7 speed, with rear gears selected to normalize vehicle speed per rpm in first gear, as I suggested originally, the fact that the 7 speed box uses three overdrive ratios really doesn’t matter. You have the same vehicle speed in first gear and top gear, but the 7 speed narrows the splits between ratios and allows you to ride right on the fat part of the engine’s torque curve. This can be fun when enjoying street driving, effortlessly riding that torque wave without straining the engine, and it can be effective in optimizing a maximum effort drive out of a corner by allowing the perfect match between the engine’s torque curve and ground speed.

The REAL trick is selecting the optimum overall range of reduction so that the transmission gearing best utilizes the shape of the particular engine’s torque curve. There is no one set of transmission ratios that is “best.” It depends on the torque curve of the engine.

Many of us spend great amounts of time thinking about each and every engine part and how it will contribute to power and thus performance. Yet the engine is only one-third of the total equation in delivering torque to the axle, which is what determines the vehicle’s actual performance. So I’ve always thought “bench-racing” the rest of the driveline was as important and effective as the effort invested in the engine.

Boy, isn't that the truth! :)

And you're right, most builders tend to focus on the engine, and put a lot less attention on the rest of the car.

I built my engine to make a very broad and flat torque curve. Consequently, I don't really need a close ratio transmission. I find the gear ratios on the TKO to be just about perfect. Except for the 4-5 shift. It's about a 1,000 rpm drop between the two. I think an in between OD gear like a T56 has would be perfect for some of the back roads I drive on.

I have an older TKO, too. 1st gear is about 3.29 or so, IIRC. I really like that low first gear. really nice in traffic, parking lots, loading on a trailer, etc.

For me, the perfect trans would have 2 OD gears like a T56, a low 1st, and a fairly even ------ in between. Couple that with a 3.08 or a 3.27 rear gear, you would have almost the perfect all around transmission.

Now, a track only trans would be a different story. :)

Bob, I looked at the dyno graph posted in your intro thread. The torque curve of your engine would pull a three-on-the-tree like a freight train!

Somebody here must have flunked math class. Oh, wait, that was me.:eek4:

The reduction range between the standard ratio T-56 and the close ratio, 2.29 low 7 speed are NOT the same. That T-56 has 5.94 overall reduction; the 7 speed is 5.08. So the close-ratio version of the 7 speed actually covers a 14.5% NARROWER reduction range than the standard T-56!

I was originally thinking about various transmission possibilities during a recent airline flight. The cocktail napkin math I later recalled when writing the post above was actually a comparison of the standard ratio, 3.35 low with .68 overdrive T-5 and that close-ratio 7 speed. This is the pair of Tremec cousins that actually have very similar overall reduction, with the T-5 at 4.93 and the 7 speed at 5.08. Even more interesting, perhaps.

Although it is the close-ratio model that captures my imagination, there are two other ratio choices in the 7 speed, the 2.66 low medium ratio and the 2.97 low wide ratio. With overall reduction of 6.19 and 6.33 respectively, they are only 4% or 6.5% wider than the standard T-56 while offering the extra gear for narrower splits.

Drivetrain note to self: Engage brain before putting mouth in gear.

Exactly! And that's why I would want a lower first gear, and leave the two OD's the same. A transmission like this is really designed for a street car - what race car can use 3 OD's? So a low 1st - almost a granny gear - is really handy for a lot of stuff. Parking lots, trailers, starting on a steep hill, donuts, burnouts, etc.

Close ratio with a lot of gears just means a lot of shifting. That's fine for drag racing, but not much else.

But, that's just my opinion. I could be wrong. :)

Lots of shifts are bad for drag racing. Those guys like 2 speeds. :eek4:

But in a road race scenario it means less rpm drop allowing you to stay in the HP with less rpm drop. Also allows for choosing that perfect gear to get good drive out of turn 4...

Street is the last place it's "needed". One more gear won't get me to work or the ice cream shop any faster.

all in my opinion..of course. :p

And for bench racing...7 is always better than 6. :naughty:

I for one would love a 7th gear. I'm not exactly sure what I would do with it though. In my car 2nd is just a hair too tall for autocross and on the track the jump from 3 to 4 could be better and 4 to 5 especially is too big. So I guess for autox I'd tighten up 1,2 and 3 with it and for the track I'd tighten up 3,4 and 5. On 2nd thought I'll take 2 and and leave 1 and 6 (8) alone and close the gaps of everything in-between. Oh, and while you are at it lets get reverse off of 1st where it belongs rather than the high /wrong end of the pattern! http://i58.tinypic.com/2zswk9c.jpg

For drag racing the current trans is fine especially if you use a 3.27 diff. Start in 2nd, shift to 3rd and done. 11 sec at 125 mph and if anyone is http://i61.tinypic.com/200sqhk.jpg enough to whine about your girls car just tell them to meet you to a real track...

or on the street for a roll race...start in second at 4k rpm and tell anything w/o $$$ sunk into it good bye. When they say, "its still a Miata", say, "Thank you. Please tell the purists at Miata.net that for me..." :biggthumpup:

yes...7....sounding better every day...:beer:

McLeod Racing Reveals New Muscle Car 5-Speed and More

SEMA 2014: McLeod Racing Reveals New Muscle Car 5-Speed and More - Chevy Hardcore

It looks physically about the same size as a T-5. Its gear ratios are the same as the T-5Z, so I'm wondering if they are using T-5 innards, perhaps with some aftermarket upgrades in the bearings, shafts, heat treating, etc. to get to their 500 ft/lb. claim? It will be interesting to learn more about this unit as the info becomes available.