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post #1 of 34 Old 07-06-2010, 09:27 PM Thread Starter
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Rear end gears and turbos

I understand that having steep gears (4:10, 3:73) are not good in a turbo car because of spinning. But what gears are good for turbo spool under load? Is there a such thing as having the correct gear for the turbo to spool better? If the car hooks doesnt this put an instant load on the turbo, so if a 3.08 or 3.27 gear is utilized doesnt this help loading the turbo as well as bring a more usefull rpm range? Any light in this subject is appreciated. Thanks


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post #2 of 34 Old 07-06-2010, 10:29 PM
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I understand that having steep gears (4:10, 3:73) are not good in a turbo car because of spinning. But what gears are good for turbo spool under load? Is there a such thing as having the correct gear for the turbo to spool better? If the car hooks doesnt this put an instant load on the turbo, so if a 3.08 or 3.27 gear is utilized doesnt this help loading the turbo as well as bring a more usefull rpm range? Any light in this subject is appreciated. Thanks


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post #3 of 34 Old 07-06-2010, 10:49 PM Thread Starter
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Im starting to figure the stuff out lol thanks man-
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post #4 of 34 Old 07-06-2010, 10:57 PM
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Originally Posted by zombiestang View Post
I understand that having steep gears (4:10, 3:73) are not good in a turbo car because of spinning. But what gears are good for turbo spool under load? Is there a such thing as having the correct gear for the turbo to spool better? If the car hooks doesnt this put an instant load on the turbo, so if a 3.08 or 3.27 gear is utilized doesnt this help loading the turbo as well as bring a more usefull rpm range? Any light in this subject is appreciated. Thanks
You have a 2 fold issue with a high gear ratio in a turbo car. First, much like a nitrous car, the engine is now a torque monster. It no longer needs a mechanical advantage like a high gear ratio to get the car moving. By going to a lower gear ratio, you are now utilizing all that extra torque and loading the turbo.

Secondly if running in the 1/4 mile, you typically maximize gears based on cam and head combo. So if you are running N/A and you needed a 4:10 to cross the finish line at 6500, which is where your cam and heads made maximum power, it will be different with a turbo. Now with that same gear you may cross the stripe at 7800rpm, well out of the power range of the cam and heads and probably out of the range of the bottom end you built to spin to 6500. I have a friend that runs 4.88s in his turbo car but he spins it to 9500rpm through the traps because that's where his particular setup makes its peak power.

Remember, its all about matching your parts. If you build a motor to spin to 6500rpm, choose gears and a cam and heads that will peak at that rpm. Also note that a turbo can stretch the peak power point out a little. So for instance if your cam peaked at 6500 N/A, you may make power to 7000rpms with a turbo. Its all a matter of math and science. Now let an engineer come in here and explain it better than I did......LOL
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post #5 of 34 Old 07-07-2010, 01:31 AM
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i have a 3.27 in mine
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post #6 of 34 Old 07-07-2010, 04:29 PM Thread Starter
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Im thinking a 3.55, I also have it already. I bought a 3.73, 3.55, 4.10 before doing the turbo set-up and still have them all. and still have the 3.08 that cam in the car. It seems when I nail the car in 1st gear i have to shift, its going thru the gears really quickly.
post #7 of 34 Old 07-07-2010, 05:10 PM
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i would try the 3.27..i have a 3.27 and c4 with 28'' tires and i can drive this car anywhere i want, and loads the turbo great..
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post #8 of 34 Old 07-07-2010, 06:20 PM
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i would try the 3.27..i have a 3.27 and c4 with 28'' tires and i can drive this car anywhere i want, and loads the turbo great..
hey quick question hows the driving on the highway how the c-4 and the 3.27 that what i have on my 95GT with a 351w and 76mm turbonetics turbo
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post #9 of 34 Old 07-07-2010, 08:32 PM Thread Starter
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Would the 3.55 be okay with a 5spd then? I also use 28" tires, If the 3.27 is a must then i will go grab one from the pull a part for $45.00,
post #10 of 34 Old 07-07-2010, 09:42 PM
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asap funds available will swap to 3.27 from my 3.73.


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post #11 of 34 Old 07-08-2010, 02:46 PM
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hey quick question hows the driving on the highway how the c-4 and the 3.27 that what i have on my 95GT with a 351w and 76mm turbonetics turbo
this car could run 80mph all day long and run about 28-3000rpms...it can be driven anywhere..
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post #12 of 34 Old 07-08-2010, 08:37 PM
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Depends on the car and power level. I'd say treat the gearing like any other car. Its just you have more lower rev power with the turbo so you can still pull like a freight train at lower revs and use the higher gear and still run good.

What i have found is with too low of a gear with a turbo. When you mash the gas, your turbo is going to take an amount of time to spool. and with the lower gear, in that amount of time your engine is going to rise in RPM, So your turbo is going to have to push more air to make that desired boost pressure at that higher rpm.

In a sense you can "outrun" the turbo.

With the higher gear its going to bog the motor more and the RPM wont rise as much in that same amount of spool time, so you can reach the desire boost quicker. ALso the abundance of midrange torque gives you a broader powerband so the high gear can work good.

IMHO its not really that big of a differnce. The turbo seems to spool the same, but like i said the higher gear "bogs" it more and keeps RPMs from rising. It "seems" like it spool faster.

With the low gear, RPMS rose and boost hit and just blew the tires off.

Dont fall into the cut and paste 3.27s mindset. It seems like alot of folks automaticially hear "turbo car" and instantly tell you to go 3.27s

If your making lower power like 400 hp with lower boost, IMO a 3.27s can be too high. If your making more boost and more power in the 500-600 range a 3.27 should be the ticket.

If its a track car, dial in the gear for the trap speed/rpm/power etc etc.
If its a street car, IMO you can run the higher gear and work pretty good since your using alot more of your midrange and etc to play around with rather than just the high rpm like a strip car.

Also another factor is the trasmission your running.
A t5 or tko etc is much lower and tighter geared than say a c4 or AOD, so with the auto you can run a lower rear gear to get the same overall gearing as the manual car. unless you get a lower ratio gear set for your auto.

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post #13 of 34 Old 07-08-2010, 11:44 PM
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I run 2.08 rear gears with 33" tires on my car...........

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post #14 of 34 Old 07-09-2010, 11:06 AM
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I switched to 2.73 rearend gears in mine and it didn't seem to hurt a thing http://www.youtube.com/watch?v=AQtmrxaRVTg

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post #15 of 34 Old 07-09-2010, 03:20 PM Thread Starter
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I switched to 2.73 rearend gears in mine and it didn't seem to hurt a thing http://www.youtube.com/watch?v=AQtmrxaRVTg
Nice acceleration , that thing comes to life !!!!
post #16 of 34 Old 07-09-2010, 06:36 PM
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Im running 3.55 with 17x315 tires with my turbo setup . I havent had a chance to go to the track yet But i plan on going next friday . The closest track is like 15 miles away but it is a 1/8 mile track . At least i can see what my 60 is with the new gears and turbo .

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post #17 of 34 Old 07-10-2010, 02:46 PM Thread Starter
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keep us informed on this-
post #18 of 34 Old 07-10-2010, 05:20 PM
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When Mine was running 11.0 @ 134 I went from 3.73's to 3.27s and it felt like I gained 50rw.

I still run them

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post #19 of 34 Old 07-10-2010, 06:36 PM
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I had a major issue with my car but there are a lot of variables involved.

With a 3.55 and a 27 inch tire I couldn't go faster then 9.50 utilizing 14 lbs of boost which is about 650 rwhp. It was not comfortable nicking the rev limiter through the traps.

With a 3.27 and a 28 inch, I was able to go 9.70 with only 8 lbs of boost because the car was working "right".

For that valve train with 28 psi, I would have likely benefited from a 3.08 because it would only spin 6400-6500.

I since changed to a larger solid roller to let it spin 7500. I kept my 3.55's just in case...I will let everybody know how it works differently.

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post #20 of 34 Old 07-11-2010, 10:26 PM Thread Starter
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I dropped the 3.55s in today, wow what a difference!!! 1st-4th Im seeing 14-14lbs as soon as I nail it and still cruise in 4th all day lol, man it feels good with the 325-50-15 m/t. The turbos seem to come on easier, not really have to work or maybe they were overworking before??? not sure
post #21 of 34 Old 07-12-2010, 09:06 AM
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I'm looking for 600 rwhp, and I'm going 3.27 or 3.31......

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post #22 of 34 Old 07-12-2010, 07:56 PM
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I dropped the 3.55s in today, wow what a difference!!! 1st-4th Im seeing 14-14lbs as soon as I nail it and still cruise in 4th all day lol, man it feels good with the 325-50-15 m/t. The turbos seem to come on easier, not really have to work or maybe they were overworking before??? not sure
What did you have in there before?

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post #23 of 34 Old 07-12-2010, 08:13 PM Thread Starter
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4.10s first gear was way too fast, rpm wise. I didnt see alot of boost in 1st but now Im happy.
post #24 of 34 Old 07-13-2010, 12:04 AM
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I currently have 3.73s and plan to run about 450rwhp so well see how that works but I'm thinking ill switch to 3.55s to get the best overall.

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post #25 of 34 Old 07-13-2010, 12:12 AM
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Procharged F1R with 4:10's horrible will be swtiching asap oh yeah at 24lbs i cant hook up with any gear
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post #26 of 34 Old 07-13-2010, 02:07 AM
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Procharged F1R with 4:10's horrible will be swtiching asap oh yeah at 24lbs i cant hook up with any gear
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post #27 of 34 Old 07-13-2010, 01:01 PM
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You have a 2 fold issue with a high gear ratio in a turbo car. First, much like a nitrous car, the engine is now a torque monster. It no longer needs a mechanical advantage like a high gear ratio to get the car moving. By going to a lower gear ratio, you are now utilizing all that extra torque and loading the turbo.

Secondly if running in the 1/4 mile, you typically maximize gears based on cam and head combo. So if you are running N/A and you needed a 4:10 to cross the finish line at 6500, which is where your cam and heads made maximum power, it will be different with a turbo. Now with that same gear you may cross the stripe at 7800rpm, well out of the power range of the cam and heads and probably out of the range of the bottom end you built to spin to 6500. I have a friend that runs 4.88s in his turbo car but he spins it to 9500rpm through the traps because that's where his particular setup makes its peak power.

Remember, its all about matching your parts. If you build a motor to spin to 6500rpm, choose gears and a cam and heads that will peak at that rpm. Also note that a turbo can stretch the peak power point out a little. So for instance if your cam peaked at 6500 N/A, you may make power to 7000rpms with a turbo. Its all a matter of math and science. Now let an engineer come in here and explain it better than I did......LOL
This is a very good explanation. The next question is, how high is too high? Should we all trade our aods and C4s in on powerglides? Is crossing the traps at redline really the best way or does a turbo require even more load.
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post #28 of 34 Old 07-15-2010, 03:40 PM
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Ive had a chance to play some with my turbo setup . Keep in mind that this is a just about stock motor running 8 pounds of boost and 3.55 gears and like zombiestang said as soon as I hit the gas im also making boost . 8 pounds of boost by the time im at 3500rpm. The only problem is there is sooo much TQ that the tires want to break free as the RPMs climb in 2nd and 3rd. On the hwy the gear seems fine 2000rpm im running about 75mph. All and all i am glad i went with the 3.55 i think its a good allaround gear.

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post #29 of 34 Old 07-15-2010, 03:52 PM
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i would try the 3.27..i have a 3.27 and c4 with 28'' tires and i can drive this car anywhere i want, and loads the turbo great..
This

Same here.

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post #30 of 34 Old 07-17-2010, 07:29 PM
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i was getting ready to do a search and found this thread. answered almost everything i needed to know. one thing though what about convertors. getting ready to do a turbo and hav aod with a 3500 stall. what gearing should i look for? just recently did the tranny and convertor so not looking to lose it?????
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post #31 of 34 Old 07-18-2010, 01:57 PM Thread Starter
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3.55 or 3.73, also depends on your power level as stated above. I got a chance to hit the track friday night, it rained in staging lanes, waited 30mins to clear and clean the track. it started lightning, so they did hand drops wtf lol i really wanted a time, it felt good, 2nd &3rd was really nasty. it pulled like a frieght train, I left on a 4500 2 step with4-5lbs boost with worn 325 drag radials, thats a no no , spun horribly, but caught a normal 7.20 car with no problem and was granny shiftin lol. Putting the 28x10 on for this friday and trying again!
post #32 of 34 Old 07-18-2010, 03:15 PM
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3.55 or 3.73, also depends on your power level as stated above. I got a chance to hit the track friday night, it rained in staging lanes, waited 30mins to clear and clean the track. it started lightning, so they did hand drops wtf lol i really wanted a time, it felt good, 2nd &3rd was really nasty. it pulled like a frieght train, I left on a 4500 2 step with4-5lbs boost with worn 325 drag radials, thats a no no , spun horribly, but caught a normal 7.20 car with no problem and was granny shiftin lol. Putting the 28x10 on for this friday and trying again!
sounds good, keep us updated on how it runs..
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post #33 of 34 Old 07-18-2010, 05:22 PM
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i would try the 3.27..i have a 3.27 and c4 with 28'' tires and i can drive this car anywhere i want, and loads the turbo great..
Most likely 60' better with a 3.55 at your power level. I know mine did.

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post #34 of 34 Old 07-18-2010, 06:00 PM Thread Starter
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Ive got a run friday night with a 03 cobra, its a buddy. Hes putting 527 to the tires, granite his car has a solid axle conversion its a heavier car, he went 7.2-7.0s with dragradials last yr, he will be on et streets friday, I will be on full slicks, my shifting will hurt me if anything. I havent raced a 5spd in 4yrs lol. Trying to get a c4 back in it before friday lol, I will post the video on next sat
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