Gearing vs turbo

[PorterzSupra]

I have 17 inch rims with 40 sidewall profile. I recently noticed somethin very interesting and thought id share and get your ideas/comments.

As many may know im running a T61 turbo and the stock gearing with my setup seems very low and my cars cant stay in gear for more then a second or so. I did a little experiment and changed the wheel size in the back to change the drive ratio. I went to a really high sidewall tire on a pair of old rims. With the new raito my car has a much stronger more defined powerband, the turbo comes in alot harder and pulls for longer. I think overall the car is much faster with the longer gears.

If you notice all the really fast cars like the Z06's have much much longer gears and I think with proper hp the longer gears provide much better powerbands and its especially important on cars with larger turbos (I know the vette is a NA but it was an example)

I feel with a larger turbo like that when it winds thru a gear in a second its not taking advantge of the turbos output and the turbo is not reaching full spool for long enough. For stock HP the gears are okay but when your making doubble the hp you need taller gears in order to maximize turbo output and engine powerband. What do you guys think? Think of it this way.. its like taking a 4 wheel drive truck and launching to 60 in 4 wheel drive low range. The truck is gonna have more torq to the wheels, its gonna jump to about 10 mph faster but in the normal high gearset you will get to 60 faster when the powerband comes in. Its compairing apples to oranges but its still the same basic idea. I think is especially important to see a relation between the turbo spool and geartime

 

[cadman]

My general idea is this. For a light car, you need HP, High RPM, and not as much Torque, using shorter gears to maxamise your weight bonus. For something a bit heavier, like the Supra, you need longer gears, More torque that hits lower in the RPM and just enough peak HP to get you through your gears to your next shift.

Example: 1989 Honda CRX, B16A swap. 190HP, 138torque. (Crank) 9500RPM. Car weighs in at 1900lbs with me in it. Not as much torque helps me with traction so I don't just roast off the tires. (Even though that happens anyway) I have the RPM range to make this car quick. It doesn't need tall gears, it needs short gears to work with the RPM range.

1988 Toyota Supra. (it needs...) 350hp, 400lb ft. torque. Weighs in at around 4000lbs with me and all my crap in there. Needs much taller gears to take advantage of the torque curve, because of more displacement and the turbo. Longer gearing gives you a way to use the torque, and turn it into HP easier.

Hp is just how fast you can get Torque to move an object. A 35HP tractor can have 200 torque, it's all in the gearing.

Just my ideas. It's all in weight, torque, power transfer and traction. It's truely amazing how something can sound so simple and yet be so complicated.

 

[Doward]

Yep, gearing plays a very integral role in overall driveability.

If you've got a large camshaft, high RPM, you want to get into that higher RPM as quickly as possible, and also stay there as long as possible.

The quicker you get to that RPM, though, the shorter your stay - unless you increase revs. It's all about weighing both, and decided which is better for you.

Personally, I want a set of 3.4x range gears in the back of my supra. I'm trying to find a 3.73 to trade my 3.91 for, as I simply feel I have way too much gear!

Some people will say the gearing is needed in these heavy cars. Personally, I'd take a look back at some of the 2nd gen Firebirds/Camaros, for example. 170hp, 3700lbs, and 2.73 gears on a TH350 with a 2.52 first gear.

What's the 1st gear in a r154? Something like 3.4? I don't think we really need as much gear as we have, with the torque capability of the 7M.

 

[Nick M]

R154 has a first gear ratio of 3.25:1 which is very typical in a street car.

edit: yeah, a taller gear will put a heavier load on the engine.

 

[89turbosupra]

Having a smaller gear will put more load on the car and turbos love load. The gears you have now were over revving your powerband. Putting a higher gear numerically will not always make you faster as most people believe. The best example I can think of is the crown vic cop cars. They have the option of 3.23 or 3.73. The 3.73 cars are slower in 0-60, 1/4, and 60-100. They were simply over revving the powerband.

 

[89supturbo]

supras dont way 2 tons w/ the driver, they only way about 3500 pounds from the factory

 

[Nick M]

3800 lbs with fuel and a driver is really close to 4000.

 

[IJ.]

weigh.....

 

[Havoc13]

out of curiosity, for 1/4 mile drag, ive heard using a automatic tranny because of the gearing range true or false

P.S. it might of been for street racing. (?)

 

[Dirgle]

weigh.....

Ah for the days when you had mod privileges and could take care of the yourself.

As for the post, this would be the reason they gave the turbo cars a 3.73/3.91 final drive and the NA 4.3. The turbo cars stay in gear longer, and because of the heavier load placed in the engine tend to build boost better than if they were equipped with the 4.3

 

[Nick M]

Torque and speed are a trade off in a gear box. The 2nd post is not necessarily correct. If you have a light weight, high torque, tall geared car, it will accelerate easily.

The tall gearing creates a lot of speed, and the high torque has no problem turning that load into motion. Especially in a light car. Just like the mentioned Z06, Viper, and to go back in time, an old 5.0 Mustang. Lower torque, or engines whose torque isn't low in output, but higher in the rev range does need that gear to get going.

 

[Havoc13]

Ah for the days when you had mod privileges and could take care of the yourself.

As for the post, this would be the reason they gave the turbo cars a 3.73/3.91 final drive and the NA 4.3. The turbo cars stay in gear longer, and because of the heavier load placed in the engine tend to build boost better than if they were equipped with the 4.3

So is my question is valid then... Right?

Sorry about that post, i knew the answer, just had a brain fart, and forgot to remove post.

does anyone know the math table for the gearing equation?

Thanks for your help in advance.

 

[SevenMKIII]

Some people will say the gearing is needed in these heavy cars. Personally, I'd take a look back at some of the 2nd gen Firebirds/Camaros, for example. 170hp, 3700lbs, and 2.73 gears on a TH350 with a 2.52 first gear.

But many on this thread are talking about the R154, not an auto. Take a look at the 2nd/3rd gen Z28's (my last car) with the T56-stock gearing was 3.42 and a common mod for higher power was putting 3.73 or 4.10 in back to get it moving quicker, even though top end would suffer. And we all know 350's make their torque a lot lower than we do.

-Chris

 

[whenmunkysfly]

I kinda noticed this concept. An easy way to see it in action is...

If you watch your boost in 1st gear it doesn't even hit your set boost pressure.(at least in my car) but in 2nd it will. In 1st you have less load on your engine then in 2nd.

 

[Junior]

well I can't speak for everyone, but I'll often launch from second if I'm gonna be running farther than just across the intersection, maybe abit slower off the line (not much difference if you don't mind abusing the clutch) but it's 1 less shift to make and most drags I'll not be needing 3rd anyways.

I guess mine being manual tranny, hardtop and 4.3 gearing probably makes that a better option for me than most people tho.


I guess in short what I'm trying to say is that I agree, more gear can be very beneficial. having tons of power at the line just makes you spin anyways.

 

[Nick M]

does anyone know the math table for the gearing equation?

Thanks for your help in advance.

Which table is that?

 

[cjsupra90]

Gearing and shift points go hand in hand. The whole point is to keep the engine in its highest average torque area and the car will accelerate the fastest that it possibly can. This is why there are programs out there (mainly used for drag vehicals) that calculate optimum shift points based on the torque curve of the motor.
Yes, as it has been stated before, turbos do better with more engine load so taller gears will tend to make a car faster cause it spools the turbo better, but shift points are very important especially in a turbo car due to turbo spool characteristics. An example of this is in a Miata that I built a few years ago for a friend of mine. When the car was N/A takinging it to the rev limiter (7500rpm) produced the fastest times and trap speeds. When I built the turbo setup on it, he would run it at the track the same as if it were N/A (taking it to the rev limiter in every gear and the car ran consistant 13sec flat so I got in it and ran it once to the rev limiter in each gear and still 13 flat after a few more runs and trying different shift points mainly in 1st and 2nd and ended up with short shifting 1st at 5500rpm and 2nd at 6500rpm (due to breaking traction above that point) and then the rev limiter in 3rd and ran a best time of 12.24 @ 110.9mph. Moral of the story, car gained 3/4's of a second by changing shift point.

Once its all said and done, yes especially in turbo equiped vehical's, taller gears can make the car faster but mainly only in lower gears (trany wise). The car will be slower though in 4th and 5th where the motor is already load more then enough to spool the turbo instantly even with a shorter final drives. Example, when my car was still turbo, with a 4.30 N/A diff, the car was almost a second faster from 70mph to 100mph in 5th gear then with a 3.73 turbo diff, but with the 3.73, the car was deffinitly noticably faster through 2nd and 3rd gear.

 

[Nick M]

Gearing and shift points....

Agreed.

 

[WeDgE]

If you watch your boost in 1st gear it doesn't even hit your set boost pressure.(at least in my car) but in 2nd it will. In 1st you have less load on your engine then in 2nd.

This is exactly why I want to swap out my 4.30 for 3.73...I don't have enough load on the engine in 1st and 2nd gear.

Excellent thread. :naughty:

 

[drjonez]

turbo engines like a lot of gear....as the mullets used to say.

i've seen 2-300 RPM difference in spool time between a 4.30 and a 3.73 on a dyno.....i.e. lower (numerical) ratio produces more load, spools faster.