5MT vs 6MT spreadsheet

[NSFW]

When you wrote "the number of cylinders is entirely germane" and you wrote "all things being equal" it was not at all obvious that you were talking about engines with different total displacement.

 

Assuming 0.5L cylinders, I would expect a 6-cylinder 3L engine to make around 50% more power than a 4-cylinder 2L engine (all other things (like intake pressure, rpm, etc) being equal).

 

However I'd also expect a four-cylinder 3L engine to make around 50% more power than a four-cylinder 2L engine. I'm not sure what your point was when you said "the number of cylinders is entirely germane."

[Vimy101]

^ We need a definition of terms. A similar displacement V6 would be the old Mazda 2.5. Similar power to a 2.5 boxer.

[kopy_kat]

power aside...

 

i definately wouldn't mind a 6th gear for long range cruising..

[John M]

John, if you can install the 3.272 differentials from the GT AT, you'll get 90mph in 5th gear at 3k, so 75mph would be a little under 2500rpm. You'll lose some acceleration in 1st gear, but you could gain about 20% in fuel economy.

 

Now that's an interesting proposition, especially if you can actually do it (bolt-on swap) and not screw up things like the speedometer. The 5spd Stealth could top 70 in 2nd gear and 120 in third. That was quite useful

 

While we don't have the absurd 1st gear the STi owners have to live with, I'm still pretty sure the change in axle ratios wouldn't hurt first gear performance that much. The turbo will spool that much quicker (more load), yielding the same results.

[Prod]

http://spreadsheets.google.com/pub?key=pnPjNqBluy31HYtJ2uAdlRw

 

Updated with GT gear ratios.

 

John, if you can install the 3.272 differentials from the GT AT, you'll get 90mph in 5th gear at 3k, so 75mph would be a little under 2500rpm. You'll lose some acceleration in 1st gear, but you could gain about 20% in fuel economy.

That can't be right. I have a 5EAT. It's 3k at 80mph and 3.5k at 90mph. 2.5k is about 70mph.

[NSFW]

John M: I make no promises, but that's what my math tells me (and 110 @ 6500 in 3rd). Do your own math before you buy though, just to be sure. The idea is not entirely mine - years ago I heard from a guy with a BMW track car who put an AT diff in his MT car, though if I remember right that got him better acceleration and lower top speed. (Whole other set of ratios in that case.)

 

If the speedo gets its signal from the wheel then the diff swap would not affect it. That's an "if," not a promise, I don't have any idea where the sensor actually is.

 

Prod: The automatic and manual transmissions themselves have different ratios in 5th gear, in addition to having different ratios in the differentials. I'm not saying that a stock AT should do 90 @ 3k, I'm saying that an MT with AT diffs probably would.

[NutBucket]

Now that's an interesting proposition, especially if you can actually do it (bolt-on swap) and not screw up things like the speedometer. The 5spd Stealth could top 70 in 2nd gear and 120 in third. That was quite useful

 

While we don't have the absurd 1st gear the STi owners have to live with, I'm still pretty sure the change in axle ratios wouldn't hurt first gear performance that much. The turbo will spool that much quicker (more load), yielding the same results.

That would be really interesting....too bad the front diff swap isn't so simple.

[Prod]

John M: I make no promises, but that's what my math tells me (and 110 @ 6500 in 3rd). Do your own math before you buy though, just to be sure. The idea is not entirely mine - years ago I heard from a guy with a BMW track car who put an AT diff in his MT car, though if I remember right that got him better acceleration and lower top speed. (Whole other set of ratios in that case.)

 

If the speedo gets its signal from the wheel then the diff swap would not affect it. That's an "if," not a promise, I don't have any idea where the sensor actually is.

 

Prod: The automatic and manual transmissions themselves have different ratios in 5th gear, in addition to having different ratios in the differentials. I'm not saying that a stock AT should do 90 @ 3k, I'm saying that an MT with AT diffs probably would.

 

Oh right, I never thought of the different combinations. But how will the AT diff hook up to the manual drivetrain? Subaru's AT AWD is computer controlled vs the viscous clutch type AWD in the MTs.

[NSFW]

I make no claims as to the feasibility of this swap... Nutbucket seems to think there's a problem with front diff compatibility, and I'll defer to anyone with an opinion. I haven't even looked at the underside of my own car yet, let alone compared it to someone else's AT.

[NutBucket]

No, not with compatibility; just the effort it would take to pop it open and find out it doesn't fit:p

[NSFW]

Good point.

[Vimy101]

^ This discussion is silly. Taking a rear diff out of an AT and putting it on a MT?

 

Just get yourself Spec.B OEM size wheels and do 75 mph @ 3k rpm and see how you like it. Its cheaper than screwing around with a Rube Goldberg contraption (how would the center diff react BTW?) and you will see better mileage not to mention better handling if you upgrade the suspension.

[bosco]

just to throw some fuel on the fire and if memory serves me correctly my '03 Z06 6spd turns 1800 rpm @ 70. bosco

 

 

 

 

3:42 rear end

[NSFW]

^ This discussion is silly. Taking a rear diff out of an AT and putting it on a MT? [...] (how would the center diff react BTW?)

 

I gather you missed the "s" in "differentials" and also those messages about the front diff.

[Vimy101]

^ Yes, I did. Still....

[edmundu]

Now that's an interesting proposition, especially if you can actually do it (bolt-on swap) and not screw up things like the speedometer. The 5spd Stealth could top 70 in 2nd gear and 120 in third. That was quite useful

 

While we don't have the absurd 1st gear the STi owners have to live with, I'm still pretty sure the change in axle ratios wouldn't hurt first gear performance that much. The turbo will spool that much quicker (more load), yielding the same results.

 

I don't know for sure about whether the Auto front diff will work with an MT, BUT what I do know is that you WILL severly weaken the drivetrain in doing so.

 

You will be adding tons more stress on the tranny gears, where it previously was being put on the much stronger differentials. I think part of the reason why WRX's are still plagued with tranny problems is due to this...They run a 3.7 final drive, in the '06-up and 3.9 in the 02-05's. So while they may have increased the tranny strength for '06, they then weakened it by dropping the diff ratio.

 

The way I see it, if you want better hwy cruising, then you either need the '07 SpecB drivetrain, or PPG gears, along with either the 3.9 or 3.7 diff's.

[John M]

I plan on living with it as-is for a good while. I wouldn't spend any money in that dept until all the other needs were addressed.

 

I just wish Subaru would quit thinking all 6 gears are for acceleration and produce a 6spd with a deep overdrive. Like I said - I don't care about what it does for mileage; I want a quieter cruise.

[edmundu]

^^^ ...Maybe not too deep, but certainly one gear higher than the current 5mt.

[NutBucket]

Bigger wheels + recalibrated speedo FTW. Now, if only we knew how to adjust calibration beyond the SSM.

[robust]

[PhilT]

 

So a Spec B can go 205 mph ???

 

I guess Vimy was right all along

[robust]

I took each gear to redline, not to say you'd get there. ya jackass j/k