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post #1 of 22 Old 11-25-2006, 01:12 AM Thread Starter
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Gears and Turbo question

I am looking into buying the Hellion turbo get for my 2001 Cobra, but I am currently running 4.10 grears. What gear is best for turbo charged 4V's, and why do people say a lower gear like 3.27's or 3.55's are better for spinning the turbo, please explain

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post #2 of 22 Old 11-25-2006, 10:37 AM
 
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because the turbo reaches max boost below 3500 rpm. so once you reach full boost it will go through the gears so fast you wont even have time to shift. so going to 3:27's or 3:55's will give you a lil more room for your car to pull. the less you shift the gears the more time your car is pulling and there for the faster it will be.thats why power glide transmissions work well with turbos only 2 gears and have a lot more top end to just pull and pull. i hope this helps. its kinda hard to explain but with a 4:10 gear it will be like every gear is 1st. gear, so quick, and worthless.

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post #3 of 22 Old 11-25-2006, 10:04 PM
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the numerically lower gear number(3.27, 3.55) verses higher puts more load on the motor. with more load on the motor the turbo(s) will spool faster. Less lag you have the faster your car will get up and move.

I vote 3.55's if the car will see street. that way it wont be a complete pig while out of boost

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Last edited by fullthrottledan; 11-25-2006 at 10:06 PM.
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post #4 of 22 Old 11-25-2006, 10:12 PM
 
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Quote:
Originally Posted by fullthrottledan View Post
the numerically lower gear number(3.27, 3.55) verses higher puts more load on the motor. with more load on the motor the turbo(s) will spool faster. Less lag you have the faster your car will get up and move.

I vote 3.55's if the car will see street. that way it wont be a complete pig while out of boost


very well said.

The load is what is needed, but what size tire you going to run?
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post #5 of 22 Old 11-25-2006, 10:30 PM
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good call I forgot about tire size.

smaller tire diameter will act as a steeper gear. If you are running smaller tires Id keep towards 3.27s. If you are running a tall tire that will act like you have a numerically lower gear so a 3.55 or even a 3.73 would be a good choice then. I assume the cobra has a 3650 in it right?(sorry, silly ls1 turbo guy stepping in, LOL)

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post #6 of 22 Old 11-25-2006, 11:13 PM
 
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yeah I would go 3.55 for 28" tall tire
or 3.27 for 26" tall tire
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post #7 of 22 Old 11-27-2006, 08:52 AM
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Just because you load the engine more in 1st gear does not necessarily mean the car will go any faster. It still comes down to moving X weight over X distance with X power. The gear you pick just makes your car more or less efficient over a certain distance.

As a general rule of thumb, any high power car is easier to drive with a taller gear like a 3.27 or 3.55. It's less shifting. If you don't mind shifting a lot, a 4.10 or deeper gear will almost always give more performance. I guess that also assumes power shifting on a manual trans. If you're a granny shifter, then you'll want a taller gear.

We did several experiments between auto and manual transmissions, and different rear end gears, and it still took the engine the same amount of time to spool all the way up to full boost from idle at WOT when compared to dyno numbers. It was very interesting to say the least.

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post #8 of 22 Old 11-29-2006, 10:18 AM
 
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I am running a 3.31 set of gears and it is perfect with the 17" drag radials. I was running 3.73s before and it made a huge difference to go from the 3.73s to 3.31s. Much more constant pull, better top end. Overall I would say anything around 3.27-3.55 would be okay depending on tire size and what you want to do with the car.

There are also many guys with turbo mustangs running the stock 3.08s with good results. Having a (numerically, like 4.10s) doesn't help out at all with a turbocharged car that will be on the street. It will only hurt the performance.
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post #9 of 22 Old 11-29-2006, 07:15 PM
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I was wonderin the same thing a few weeks ago.Im doing a 355 with a 28'' tire.

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post #10 of 22 Old 11-30-2006, 09:04 AM
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Quote:
Originally Posted by ATOMonkey View Post
Just because you load the engine more in 1st gear does not necessarily mean the car will go any faster. It still comes down to moving X weight over X distance with X power. The gear you pick just makes your car more or less efficient over a certain distance.

As a general rule of thumb, any high power car is easier to drive with a taller gear like a 3.27 or 3.55. It's less shifting. If you don't mind shifting a lot, a 4.10 or deeper gear will almost always give more performance. I guess that also assumes power shifting on a manual trans. If you're a granny shifter, then you'll want a taller gear.

We did several experiments between auto and manual transmissions, and different rear end gears, and it still took the engine the same amount of time to spool all the way up to full boost from idle at WOT when compared to dyno numbers. It was very interesting to say the least.
Loading a turbo up quicker in first gear certainly does mean the car *should* go faster. The sooner you car reach full boost, the sooner you will be in you meat and potatoes hp/torque. Not only will the lower (numerical) gears load quicker in first gear, but they should do it in all gears. When I switched from 3.73 to 3.27's I noticed an astounding spool up difference in every gear.

As far as your experiments go…unless it was on a load based dyno the results wont be accurate. The final h/p and torque number will be close…but the actual curve will not be represented properly with a turbo setup.


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post #11 of 22 Old 11-30-2006, 10:51 AM
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Loading a turbo up quicker in first gear certainly does mean the car *should* go faster. The sooner you car reach full boost, the sooner you will be in you meat and potatoes hp/torque. Not only will the lower (numerical) gears load quicker in first gear, but they should do it in all gears. When I switched from 3.73 to 3.27's I noticed an astounding spool up difference in every gear.

As far as your experiments go…unless it was on a load based dyno the results wont be accurate. The final h/p and torque number will be close…but the actual curve will not be represented properly with a turbo setup.
I'm sure it did get to a higher boost level before you ran all the way through 1st gear. Did the quarter mile time improve though? Did the boost actually come up quicker, or did it just seem quicker since you didn't have to shift?

I used to do calibration for performance and emissions for Navistar. Based on the boost levels we saw on the engine dyno lug curve, transient boost response vs. time in the vehicle was not sensitive to rear gear ratio or transmission changes. We also used on-board data logging to measure vehicle performance.

New let's examine why. Gear ratio changes only multiply torque not HP. You can also say that gear ratio changes do not change the amount of power needed to move a vehicle. The weight of the vehicle is the actual "load" on the engine. Everything else is just variables.

Boost is a function of engine flow restriction based on compressor speed. A slow spinning engine will create more boost than a fast spinning engine for the same compressor speed and air flow.

The moral to the story is that you need to look at a lot more than just boost when determining vehicle performance.

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post #12 of 22 Old 11-30-2006, 12:40 PM
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I'm in the same boat..currently 3.73s with a tko600

Any recc?

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post #13 of 22 Old 11-30-2006, 01:23 PM
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I'm in the same boat..currently 3.73s with a tko600

Any recc?

Will you be racing this car primarily or street driving?

If you are going to be racing, all I need is the rear tire spec and the mph and rpm you want to cross the line.

If this is for street driving, then it's more about preference. Some people like a really deep gear and don't mind shifting 3 times to get through an intersection. It really bugs other people and they want a taller gear.

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post #14 of 22 Old 11-30-2006, 02:13 PM
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Both...
I have done all the calculators.
I want more load...i cant keep a load for nuthin with these gears

Just seems 3.73s is abit much when I can roast 4th gear 70mph

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post #15 of 22 Old 11-30-2006, 02:14 PM
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But if you wanna take a stab

28x10.5 ET Drag
6000rpm
130s

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post #16 of 22 Old 11-30-2006, 02:49 PM
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Well, if you've already looked at the calculators I don't know what else to tell you.

3.27s won't work well with your quarter mile goal.

3.08s will let you run the quarter through the first 3 gears, but it'll make 5th pretty much useless.

If you're hazing the tires at 70 mph you either need more down force, or more tire.

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post #17 of 22 Old 11-30-2006, 03:16 PM
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I agree with the traction issues

I never mentioned 3.08s or 3.27s...I asked for opins

3.55s or 3.27s were the two in mind, I wanna be in 4th gear and 132-135mph no higher than 6k

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post #18 of 22 Old 11-30-2006, 03:22 PM
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3.27
119mph 3rd
153mph 4th

3.55
110mph 3rd
141mph 4th

3.73
105mph 3rd
134mph 4th


This is a 6000rpm shift


Seems 3.55s are ideal with room to grow, and get a better load on the turbo

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Last edited by 4Jenna; 11-30-2006 at 03:27 PM.
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post #19 of 22 Old 11-30-2006, 03:22 PM
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Sorry that it came across like that. I'm not a very good communicator

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post #20 of 22 Old 11-30-2006, 03:31 PM
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Also 3.55s in 4th @ 5700rpm is 134mph
@5800rpm is 136mph


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post #21 of 22 Old 11-30-2006, 03:33 PM
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Originally Posted by ATOMonkey View Post
Sorry that it came across like that. I'm not a very good communicator
No prob man...just lookin for idears from some fellow turbo guys.

I know the turbos love load, so was tryin to keep that in mind..why 3.27 had crossed my mind but I do think I may suffer a lil on 60' and not turn enuff rpms in 4th to really beni my ET.

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post #22 of 22 Old 11-30-2006, 03:35 PM
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Also guys

Real nice calculator here, was even able to input my exact trans

http://www.5speedtransmissions.com/gear_calculator.html

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