from a posting by Bart Lane to the newsgroup alt.auto.corvette with his permission
Note: This article is also available at http://www.defaria.com/Gears.html
Note: This article is also available at http://www.defaria.com/Gears.html
Six speed transmissions are faster for one reason: they rob less horsepower from your engine. Manual transmissions have parts that positively mesh and interlock such as dry clutches and gear teeth while automatics replace these parts with fluid driven hydraulic torque converters and wet clutch packs. You are just going to get more power to the rear wheels with a stick and there is nothing you can do to reverse this.
In a nutshell, the after market torque converter is more efficient and takes less power to operate which means more power to the rear wheels. Its primary strength however is that it doesn't begin to transmit power to the transmission until the engine is in its power band. An engine revs faster and more easily in neutral. Imagine revving a motor up to 2500 rpm or so in neutral and then dropping the shifter into drive.
Wouldn't this method leave quicker than just flooring it while in gear? Of course you wouldn't do this because it can damage the transmission and you have to wait for the gears to engage using this method anyway which takes time. The looser stall converter can simulate this while you are in drive already by not transmitting power, when the gas pedal is floored, until the engine comes up within its strong area.
Why donut the car companies do this to begin with? Well, there are issues with driveability and fuel mileage. I will cover all driveability issues later in this article and fuel mileage is iffy. Newer cars have computers that can physically lock up the torque converter. This is desirable when full engine power is being delivered to the transmission so the converter stops depending on fluid to turn itself but has real time positive mechanical lock up. This is much more efficient than fluid driving the car.
I put a 2900 rpm converter on my car which means that the converter will not lock up until the engine reaches 2900 rpm. So, my car is being driven by fluid until it hits that engine speed. But, do you drive on the highway at 2900 rpm? I hope not, you would be going well within ticket range. So, if my torque converter is not locking up at freeway speeds and my rear wheels are being driven by transmission fluid isn't that less efficient and wont my mileage be terrible?
This is the concern but we live in the end of the twentieth century and enjoy computers and their benefits in every aspect of our lives including automobiles. Your computer knows that you are traveling at freeway speeds regardless of RPM and tells your transmission to lock up that converter for positive power delivery from engine to transmission with no loss. Problem solved.
There are still some very real concerns about around town driveability with a higher speed stall, which I will discuss at the end of this piece.
Shift firmness is internal transmission line pressure and that's what these devices control too. They can firm up the shifts, but not speed them up. My question is why bother? The PCM gives you maximum shift firmness at wide open throttle (WOT) anyway which is really the only time you need the boost. These devices fool the transmission into thinking you are at WOT no matter what speed you are at and thus give full firmness at lower speeds. This means nothing and doesn't help performance. It merely makes your butt feel like your car is going quicker at lower speeds than it really is. Useless for performance, donut waste your money if that is what you are going for.
What you want to do is to speed up your shifts. When a transmission shifts gears it remains in two gears at once for a short time to give smoothness to the shifts. This has two downsides though, it increases wear and slows you down. The only way to speed up your shifts is to add a real shift kit. This involves removing the transmission pan and the valve body to install new springs, balls, gaskets, plates, etc. This is the only way to go if you want to beef your transmission up for better performance. The only company I would think of getting a shift kit from is TransGo. They are the best and I have never seen a reason to switch brands. I got the TransGo shift kit for my year (94-99) and it cost me $130. This is a great thing to do to any automatic regardless of other modifications. You will experience crisper shifts, lower transmission temperatures, and a faster car.
Automatic transmissions on the C5 are stifled with two gear choices. One is so lame that it doesn't belong on your Granny's Buick and the other is barely adequate. The two gear ratios in order of descending suckiness are 2.73 and 3.15 If you have the 3.15 this upgrade may not be worth the money since that ratio is almost getting within respectable gearing. But if you have the 2.73 in your car like I do please read the following section closely because this final drive ratio is not good for anything other than saving gas.
The six speed cars have a ratio of 3.42, which is OK but still not what I like. I would prefer a 3.73 in an automatic and the 4.11 in the six speed. Unfortunately, the best ratio available (Even though this is a higher number it is considered a lower gear.) is the lower 3.42. This will make a big, big difference over the 2.73 gear but may not be worth the money for you performance axles owners.
Unlike the C4 Corvette, the C5 uses identical rear differentials, or carriers, for both the manual and automatic transmissions. This means you can just bolt up the whole factory loaded six speed carrier to your automatic transmission and get instant performance. Gains to expect? On my 94 I went from stock converter and performance axle to 2400 stall speed converter and 3.73 gears and got .8 seconds faster in the quarter.
On this car I am doing taller 3.42 gears but a higher stall speed of 2900 rpm so I hope to get similar gains. This would make it easily a 12 second car and who knows with good tires. This kind of speed does not come cheap.
Buying the carrier? I called every after market tuner there was and these things were just too expensive. Then I called the Chevy dealer direct and told them I had a business license and wished to buy a part at wholesale. I told them what I wanted and got 20% off list. My cost was $800!!! Nearly half of what the after market guys wanted. This is a tremendous value.
You will need something to recalibrate the speedometer though. I suggest picking up the Hypertech Power Programmer for $300 since it can let you reprogram for the lower gear ratio.
After thinking it over I realized that I would be better served finding an experienced guy to put the parts in for me. My search was on. I looked high and low within an 800 mile radius of my home to find a seasoned person. In the end the guy for me was less than an hour from my home. His name is Paul and he works for Roy Robinson Chevrolet. His love for cars, technical repair, and Corvettes really shines when you talk to him. He begged me to bring the parts in the Friday before I dropped the car off so he could look them over and read all the literature over the weekend, on his time!! That was great and really showed he was dedicated to doing a good job. He has C5 Corvette experience and is the main transmission guy for this dealership. I really liked him and trusted him right off the bat. This doesn't happen to me all that often so I took it as a signal to go with him.
Also, they gave me a firm, reasonable estimate of ~$900 total labor. No other place would even talk estimates. This is about the only time I have ever felt good about dropping my car off at the dealership for repairs.
When I picked the car up it was raining. Not just a regular Seattle rain either but a freak torrential downpour that made driving this new found power down right scary. I only got on it once on the way home. Two lane blacktop, straight and no cars in sight. I barely got into the pedal when the car lost it. I looked over and saw that the tail end was in the oncoming traffic lane and I was sliding sideways down the road. I was going 40 when I pressed it too. Fortunately it is hard to upset the demeanor of the C5 even with an act of stupidity like my own and a quick flick of the wheel and the release of the throttle brought me right back in line and I went home slowly the rest of the way.
To look at the bright side I was basically thrown into a den of tigers with this level of performance and such poor weather. I was able to observe driveability under the worst possible circumstances. I was mostly worried about how this thing would drive day to day but let me tell you, if anything it is better. There is really no decrease whatsoever. It feels different but not worse. I know now that this modification had all benefits and no negatives other than cost.
I was also worried a bit as to how hard it would shift. In a nutshell TransGo gives you four choices on the shift kit 0 to 4, 0 being basically stock (faster, but no firmer) and 4 being full race. I chose one because I drive on the streets most often by far. The 1 to 2 shift is very firm, you can feel a kick in the booty when it changes but the feel is very solid and purposeful. The other gear changes are less severe but still more firm than stock. All shifts are faster by far. I can set off the traction control going from second to third now.
When stopped you can take your foot off the brake and the car will not move forward if there is an incline at all. It will creep forward ever so slowly if the ground is level. You press the gas normally and the car drives off smoothly without jerkiness or high revving power antics. The gear shifts, other than being firmer, change quicker enough than you can tell, and you are on your way with little fanfare. The gears are quiet and 60 mph happens at 2000 rpm even. Mileage drops about 2-3 gallons because of the gears but maybe that's because I am flooring it more often to force a grin on myself and a look of terror on my passenger's mug. Power is addictive as the saying goes. If you drive an auto I sure wish I could take you for a ride in my car because you would love it! I do.
Once this is practiced you can rocket this car to 100 mph in no time at all. It pulls fast and strong the whole way and is easily the most powerful car I have ever driven or even ridden in. I haven't been to the track yet so I will save the numbers for the spring, I am quite sure they will be very impressive.
The search begins for adequate tires. I need some good drag tires for the track.
That's it for preparing the car for more power. We have a beefy brake system, good handling, and a great drive train. When you read the next piece the hood will be up and well see what kind of power you can make in your garage with a set of tools. Have to go for now, its Supra hunting time.