I have a relatively stock '91 Evo Softail. Jetting, true duals, free-flowing slip-ons. (I have a 'bagger' exhaust).

I am considering adding a 6-speed transmission. I see a lot of guys say their similar bikes don't make the 'beans' to pull a six speed. Want to make sure I understand so I don't do something too dumb, as I'm puzzled...

My bike makes its maximum torque in the mid-2K's I believe. From there, it's all downhill (literally on the graph).

The 6-speed I'm looking at has almost identical ratios to the stock 5-speed, in gears 1 through 5. Doesn't that mean I wouldn't notice a torque disadvantage, compared to the 5 speed, in these gears? (Seems pretty logical).

The area where there's a difference (in gear ratio) is 6th gear. The 5-Speed is 1.0:1.0 in 5th. The 6 speed is .86:1.0.

My question is, if the .86 ratio lowers the engine RPM into a range that actually results in more engine torque (versus 5th gear in the 5-speed) wouldn't I be better off with the 6-speed in terms of torque, than I would the 5-speed?

My fear is I have something messed up in my 'equation' (e.g., that the Evo engine develops max torque down low, and it falls off steadily above that), or I'm missing something really obvious.

At this point, it seems to me I won't notice any difference in 'grunt' between the two transmissions in gears 1 through 5, and would actually have a little more grunt in 6th gear so long as I shift to sixth at a speed that will place the engine RPM at or above the max-torque RPM; as long as I stay above the max-torque RPM, the 6-speed at a given MPH will always beat the 5-speed in terms of torque available.

My intention is to only use 6th out on the freeway, for cruising. I have no desire to go 100 (or even 90) in 6th. Around town I'll stay in 1st to 5th.

BTW, I have a 'California' model bike, which the HD Owner's Manual says produces max torque, 80 ft/lbs. (I think that's pretty optimistic...or a typo, as the max HP numbers are down about 15 HP on the Cali bike versus 49-state), at 2500 RPM. I think the 49-state bikes develop max torque around 4K. So on my Cali bike (which has really wimpy HP numbers, even compared to the 49-staters), if I lower the RPM towards 2.5K I'm increasing torque, whereas on a 49-state bike, I'm decreasing torque with the same downward RPM shift. I guess the thing that's got me concerned is, is if there's any mods to the cam on my carb'd bike (which would likely raise the max-torque RPM), and is the Owner's Manual accurate about the torque/RMP? But presuming these are accurate, does anyone see a flaw in my logic, i.e. reducing RPM toward 2.5K at cruising speed will increase my engine/rear-wheel torque?

BTW, I'm looking at the Rivera Primo LSD complete transmission. The Bakers seem pretty great. But that 3K+ price-tag for a full tranny is pretty proud. I want a full tranny (versus a gear set) so I can revert to the 5-speed if the experiment doesn't work out. I understand the Rivera gear-sets are made offshore, assembled here in the USA by Primo, and warranted for 2-years and unlimited mileage.

Thanks...

Alan

 

People that say their motor does not have enough HP for a 6 speed...do not understand how a transmission works...you have to shift it...lol. A six speed transmission on a low HP stock motor will work just fine...as long as you have it in the correct gear for the speed you are going. You will only use it as your stated intention...on the freeway. Been running one on a basically stock motor for six years ..no problems...love it..best mod a ever made to my bike.

 

If the 6th gear is a higher ratio than you currently have, yes the engine RPM will be lower in 6th than they currently are with in 5th on your current transmission. As long as you only go in 6th when the speed keeps the RPMs out of the "lugging" zone, then worst case, you may have to down shift if you want to get into the sweet spot.

 

Gear reduction multiplies torque available to the rear wheels, so a .86 ratio yields less torque to do the work than a direct drive.

Starting in 1994, Evo touring bikes shipped with a 3.15:1 final drive ratio. Prior to that, they were 3.37:1. In 1995, Soft Tails went to 2.92:1. Applying a .86:1 6th gear ratio to your 1991 would give it a final drive of 2.90:1, or about what a Soft Tail 95 and newer would be in 6th, with the option to downshift a gear to your 3.37:1 when you need to.

 

The above posters are correct IMO. Yes, it's possible by going with a higher final drive ratio you would drop your motor into a higher torque range. But remember, with a higher final drive/overdrive you have negative torque multiplication. So more than likely (you have to run the actual numbers) whatever you would gain in getting the motor to a higher torque output RPM, you would loose in torque multiplication. And remember you need torque to get off the line/pull a grade; but you need horsepower to keep going at speed. So you may be in a lower HP RPM with the higher ratio.

Again what tom84FXST said, use it on the flats when cruising and downshift when need power.

 

Thanks to each of you (literally) for your responses and help. It sounds like this ought to work fine (6-speed) for my application...so I'm going to give it a go.

Ah... .86 x 3.37 ~ 2.90

Alan

 

Quite right and perfectly true.

As pointed out your 6th gear will reduce torque at the rear wheel, not increase it. The .86 ratio is literally an overdrive gear, just like of old, in which the output from the trans pulley is rotating slower than the input, meaning the clutch.

Max torque usually occurs in the mid range, a little above the revs most riders seem to ride in.

You will actually have less grunt in 6th.

6th is there for highway use, so you are right about that, as for going fast your bike will certainly do around 100mph, probably in 6th, 5th and 4th!

 

I went through the same thing a couple years ago. It is kind of a long story, but The short version is I changed the cam and installed parts to support the cam change as well as a fresh set of rings. I milled the heads not only to increase the compression but to equalize the combustion chamber volume since factory tolerances and variance was very sloppy. The result was a much more powerful engine that really delivered at the rpm's I ride. It also ran so smooth due to the head work that I no longer felt the need to add a sixth overdrive gear.

I have a parts list laying around along with prices if you are interested.

The stock N grind cam is lame.

 

Not all 6 speed trannys are Overdrive..In fact many are not...1:1 is 1:1 adding ratios to get there amplifies torque.

 

The trans is a done deal...it will arrive Friday. Prot, thanks for the offer. If I find there's insufficient power after the switch to the 6-speed I'll look you up and take you up on the parts list offer if it still holds. But frankly, my 'hot rod' days are behind me. I like to 'putt' along these days. So I expect I'll be ok.

Racepres, I looked at the Baker DD6, which is 1:1 final, but uses a modified engine sprocket to reduce the final ratio to .86 to 1 for overdrive. It's just the damn cost for one. I don't doubt they're worth every penny, and worth more if you have trouble with a cheaper one you've purchased. But Rivera is a well respected name. I'm banking on the fact they have done their due diligence, and ensured the transmission they put their name on will stand up. We'll see... I average 10k to 12k miles a year. I'll know more in a year, or two.

Thanks again, each of you guys, for your help.

Alan