Manual trans swaps: Cheapest vs Easiest vs Best Performing vs Best Driving Experience
Cheapest vs Easiest vs Best Performing vs Best Driving Experience - which swap fits these categories in your opinion?
One thing worth considering when swapping in a M/T is your minimum driving speed. I don't know if this is important to you, but it was to me, for parading purposes, and inching along in traffic.
With a M/T you don't have a fluid coupling, so the engine, in gear, is married to the tires. If your engine won't idle, and pull, below 750, then it will require a pretty big rear gear to get down to walking speed.
For example, say you had 3.23s to keep the Rs down at freeway speeds. Then, your minimum speed might be ;
(rpm x tire diam. x pi)/(1056 x R1 x R2)=mph
Where the Rs are your low and rear gears.
pi is 3.1416
So, with a standard A833, and 27s;
750x85/1056 x 2.66 x 3.23=7.03mph.
If your cam is running near the bigger end of what's recommended for street, then loaded at this mph, it is already starting to get jumpy. By 650rpm/6.1mph, you will be well into the bucking.
There are only a few things that I know of, that can alleviate this bucking;
1) a lower first gear
2) less idle-timing/ less cylinder pressure
3) a lower rear gear
4) slipping the clutch
As to #1
If you don't already have a trans, then your first option is to get one with a lower first gear, like the Commando box with it's 3.09 low gear. This gets your starter gear down to; 3.09x3.23=9.98/6.05 mph @750 rpm
As to #2
less cylinder pressure, can be achieved by; A) a lower compression ratio, B) a later closing intake,or C) retarded idle-timing.
A) is counter productive to every reason for making power, bad choice
B) comes with a longer overlap period, increasing the engine's operating rpm, and consequently reducing it's minimum operating rpm, bad choice
C) is the only one to consider. By retarding the idle-timing, peak cylinder pressure is reduced, AND more importantly, it comes later in the cycle when the piston is already well on it's way down the cylinder. So the bucking sorta gets lost in the ever growing space created by the falling piston. The penalty is a huge reduction in idle-power.
But now, if you reduce your idle speed by electronically manipulating the idle-timing,
(see note #1) leaving the transfer slot exposure alone, then perhaps you can get your in-gear idle speed down to 550rpm, and now your minimum roadspeed is; 550x85/1056x3.09x3.23=4.44mph, still higher than walking speed.
As to #3
To get to walking speed,about 3.5 mph, your last option is the rear gear, and requires a new rear gear of 3.23x (4.44/3.5)= 4.10s for
550x85/1056x3.09x4.10=3.49mph
This indicates a starter gear of 3.09x4.10=12.67, and the engine doesn't care how you get it. It could be as above;
3.09low x4.10rear, or it could be
3.24low x3.91rear,or
3.40low x3.73rear,or
3.57low x3.55rear,or even
2.66low x4.76rear
So now, for a streeter, the low gear becomes really important, as Mopars only have these three available;3.09,2.66,2.47. I think the T-56 is available in 3.49 and or near 4.xx, but not sure at all.
OR
if the next smaller cam idles down to 500, then
500 x 85/1056x3.09x3.55s=3.67mph;
OR
if you buy a fast-rate of lift cam with short clearance ramps, then the bucking starts at a slightly slower rpm in the first place;
(see note2)
As to #4
this is not really a long term solution. But in a pinch......... you know; following pom-pom waving Barbie, daymn the clutch.
note #1;
As to the electronic timing device, my combo gets down to 500/550 @5* idle-timing, with a 230*@.050 cam.
I'm not really a fan of idling that low, but I did several mods inside my engine to encourage it's survival. It seems to be working,since the changes were done in 2004 or 5. To get my idle-timing down to 5* from 14*; I use an in-cab,dial-back, timing retard device, that I originally bought for tuning the power-timing. It has a range of 15 degrees, that I can set anyway I want. I set mine to;
9* retard/ 6* advance. The 6* advance comes in handy when the engine is cold cuz 14+6=20, and the engine warms up nicely on that without a choke.
Note #2
So, IMO, this is another great reason to chose one of those fast rate-of-lift cams. It's been said that this type of cam has a shorter lifespan, but IDK. My Hughes HE 3038AL was new in 2004, and the ramps on it are advertised at 276 less 230=46*; not particularly fast. But I installed the 1129 Dual-springs on it and shimmed them up enough to where the engine spins 7200 pretty regularly. So it's no slouch. That cam has over 60,000 miles on it, and I wish the slugger would die already, cuz I wanna try a solid.......... they have faster ramps,lol.
Note #3
As you can see, covering all the bases with a 4-speed takes some pretty interesting ideas. Ideas that a 5-speed instantly does away with. With a 5 (or more) you can run pretty much any gear you want to.
As a side note;
my combo (see my sig) will idle down to 500/550 in gear,with an ancient 750DP. That is in 3.09x3.55 low gear. But it has zero extra power. The parking lot has to be pretty much flat, and level, and hard. The slightest incline, and she starts dropping Rs. I compensate by adding timing up the hill and then subtracting it when the Rs begin to climb. But I have to be alert to watch for the bucking, and if it starts, then I have to put some slip in the clutch. My cylinder pressure is a tic under 180,and the Scr is just a tic under 11/1;so Ima thinking that has something to do with it too.
You cannot do this with speed-screw adjustments, which screw up the transfer fuel delivery, and then the engine just stalls. You have to do it with timing...... just like your outboard boat motor does. On the smaller 2-cycle engines, the first 1/4 throttle ,or more, is just advancing the timing.
