TheQuietThings

definetely one of the best threads on the SL forums by far.

mfbenson

Generally speaking, automatics have closer gear ratios which can be an advantage for a forced induction vehicle because there wont be as much RPM drop with each upshift. However, this advantage is typically more than offset by the losses in power caused by a stock torque converter not locking up as quickly as a manual tranny can be shifted.

So if you go the FI route with an automatic, get an aftermarket torque converter.

Even if you don't go the FI route, getting a performance torque converter can get you performance gains. Took about a quarter second off my quarter-mile time on an old mustang I used to have that way... it had the C6 three-speed automatic. In a drag against a 4 or 5 speed manual, it would just keep on pulling while my opponent would drop back with each shift... Of course, that was a true "shiftable automatic", not like the automatic that scions have where you can't force it to downshift.

TheQuietThings

many cars, especially the automatic tC, do not have closer gear ratios compared to manuals. Manual transmissions have shorter gear ratios for heightened performance. It is also why the tC gets better gas mileage in the auto, rather than the manual. The ratio is so close in the manual tC that its very hard to find a low RPM when you are going WOT.

Of course there are exceptions to the rule, but not the tC.

as for the torque converter, torque converters only give an advantage off the line, allowing your engine to rev higher , and if you have forced induction, allowing you to build boost before you even more, as to manual which you could do, but would severly hurt the clutch. most stock torque converters will stall at around 2100RPM, and most aftermarket converters will stall 800RPMs or higher than the stock stall. They are used to improve 0-60ft. times and 0-60mph times. But as far as acceleration after their stall, they don't do help increase time. Changing transmission parts to stronger, lighterweight parts would help reduce drivetrain lose. Also, a shift kit and valve job will help your transmission shift in its optimal RPM range.

mfbenson

Ok but who said he's getting a tC?

Its why I wrote "generally speaking..."

In lots of cars the automatic won't even have as many gears as the manual.

And I'm pretty sure that in 5th gear at least the transmission ratio is significantly different. At 75MPH with an automatic and stock wheels/tires I'm at 3000 RPM. What's your tach at when your at 75MPH in 5th?

Engine RPM*Gear Ratio*differential ratio=wheel RPM.
Wheel RPM*tire circumfrence = speed.

Guess we'd need to nail down what the differential ratio and tire circumfrences are to really settle this. Its probably in the tech area somewhere but I'm not very good at finding things on this site.

TheQuietThings

exactly, you said manuals have more gears than automatics, which is why the autos gears are longer, and take longer to get into their optimal RPM range. the manuals gears are shorter since there are more of them. and this is another reason why manuals have better quarter mile times stock over the automatic transmission.

mfbenson

Yeah but in the case of a 4 speed automatic against a 4 speed manual, in most cases, the automatic will have closer gear ratios, which is the point I was trying to generally make.

Specifically for the tc its a 4 speed automatic ECT against a 5 speed manual overdrive. But the ratio of the top gear on the manual I think is still lower than the ratio of the top gear on the automatic, its why the manual is rated for 29MPG highway and the automatic is rated for 30MPG highway. I guess the top gear on the automatic tC is overdriven too, but they don't call it "overdrive" because people have come to think of locking torque converters that can be turned off as overdrive due to sneaky marketing by GM. Even the xB has that "feature". Also the automatic weighs an extra 65 pounds.

I guess what it boils down to is that for a forced induction tC, manual is the way to go. In fact manual would be the way to go on the tC period, except for things like slightly better mileage or the need for a spouse who can't drive a manual to be able to drive the car.

TheQuietThings

if they are both 4 speeds, they will generally have the same gear ratios. you are right about the TOP gears on the cars, not the same gears, thats where the difference is, maybe we were just confusing ourselves in what we were trying to say.

monSSter-tC

I'm probably restating a lot of things that have already been said.

Automatic transmissions usually have a taller FDR to facilitate the disadvantage of having fewer gears and still be able to accomplish similar top speeds, acceleration and fuel economy.

In a 4-spd automatic 3rd gear is 1:1 and 4th gear is always overdriven, it's not always "listed" as an overdrive, but it is. On some of the newer cars like a Mercedes with a 6-speed auto, 5 and 6th gear are overdriven. On a manual transmission usually 4th gear is 1:1 and 5th gear (and 6th if applicable) are overdriven.

As for torque converters, this can definitely be the single most important mod for an automatic car. The stall speed and stall torque ratio you use must be tailored to your particular car for maximum results. But any increase in stall speed and STR will still boost the performance of the car. A torque converter will typically just make a car launch harder but it does improve it's overall straight line performance. As for locking being viewed as an overdrive... I've never heard GM try to say a locked converter is an extra gear, in fact with most factory torque converter's you wont even feel it locking up.

ehong81

so whenever TRD releases their supercharger, do you think they'll also have a torque converter available for an auto tranny? or some type of modification that will allow the transmission not to get messed up?

or are they only targeting the manual cars?

TheQuietThings

i highly doubt it, the amount of power the TRD SC without modification isnt going to put out enough power to damage the transmission.

SquallLHeart

thank god... a topic that is actually useful even though it's far from answering the obvious question in the first place...

i just started laughing at first when i saw the title of the topic (just thinking "duh you dumbass..."), but all-in-all, everyone here has more than informed the rest of the community of alot of information regarding auto and manual transmissions and the advantages/disadvantages of upgrading certain things for them both. again... thank you...

monSSter-tC

I highly doubt TRD will release a converter, reason being that next to a tranny brake a torque converter is the part that is going to hit your drivetrain and suspension hardest. High stall converters and tranny brakes in good combination is what makes drag cars lift their front wheels off the ground on launch. My Camaro almost gets the front wheels off the ground with just a big stall. Basically the point I'm trying to make is a torque converter is hard on the car and will likely cause other parts to fail, for instance my Camaro is a solid rear axle car, 13 passes at the track later after installing my first converter (a smaller one than is currently in the car) I destroyed the internals (largely the differential, not the ring or pinion)... $2,000 later (12 bolt rear) the car is running again... but you get the idea. Given the fact that TRD works very closely with Toyota, isn't going to release something that while improving acceleration greatly is also very likely to cause other parts to fail.

If you're gutsy with modifying your car and looking for a huge improvement in off the line performance Hughs could probably build you a converter. No matter what kind of car you have a converter is probably the single best improvement for straight line performance you can do to a car with an automatic (except a power adder). A decent sized converter in my Camaro took me from a 13.9 to a 13.0!

spoolinTC

dumbest thing ever said! autos have LONGER gears! thats why they noly have 4 instead of 5. oh my hell some of you people are making scions look so damn dumb

TheQuietThings

i know, i said the same thing, but i just gave up trying to explain that to him.