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Discussion Starter #1
Ok for all you gear masters I have a question for you guys and sure it will school not only myself but others on the site as well that are like myself just getting into the turbo stuff.

What is the deal with choosing the right gears for a turbo application I hear different things on this such as these two.

#1. Going with a lower gear (example: 4.10) will get you into your power band faster there for building boost sooner than (example: 3.55).

#2. Turbochargers rely on exhaust gasses to drive the unit. Useable Exhaust gas is created when the vehicle is under a load. This is the reason many people recommend 3.27's or 3.55's for a turbocharged street car. These less aggressive gear ratios place a higher load on the car for a longer amount of time per gear. Of course for a straight drag car you want to pick the set of gears that puts you in your powerband for every gear and so you are passing through the traps at about redline in your final non OD gear.

It makes sense that a lower gear gets you into your power band faster expecially for the 4.6 guys like myself cause our motors make there power at higher rpm's then say a 5.0? So does the same concept account for NA motor to a Turbo motor? Again it makes sense that it may be possible that a lower gear like a 4.10 will kill you in say 1st and 2nd gear cause it may be possible that the motor may run out of rpm's faster than the turbo can fully spool and make full boost vs. a gear like 3.55!

All of this is being based on a street driven car like mine for example I am looking for gears to replace my 2.73's but I don't wana screw up and get a gear that is to low to help things out let's use my setup for example.....

98 GT 4.6 2V 5spd T-45 stock gearing
Single 67mm ball bearing turbo intercooled 8psi
looking for around 400rwhp daily driver but want it to have the getty up for street or strip when I wana get into it.

If you need more info for this on my setup just let me know I will add it.
 

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Discussion Starter #2
If a moderator thinks this will see more results in the power adder section or any other section please feel free to move it. Also this wouldn't be a bad sticky for the power adder section either I hope this gets alot of good info for myself and other members of the forum that are wanting to learn more about gears or just learn the right gears for a turbo application.
 

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To start off gear ratios are by far one of the most popular topics around here. As we all know the GENERAL rule of thumb is that 3.73 goes best with 5 speed and 4.10 goes best with automatic. Now, YES THERE ARE EXCEPTIONS! when you start to make big power, a gear such as 4.10 and higher might get to be a bit much. However like everything else there are exceptions, if your building a drag strip car you might like them. If you have the patience or luxury of doing this, i would suggest doing all of your power adders first and then making a determination of what gear ratio you think would be best suited for your application by analyzing how fast you are able to get into your power band. Good luck! And to every one that reads this this is just a guide line based on what i have done and what i have personally seen done. Not just hearsay :bigthumbsup
 

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Discussion Starter #4
Does the same concept go with a turbo setup as it would for a NA setup?

For some reason my gut is leaning me towards the number 2 statment in my first post. I don't think it is gonna be the same guide line as a naturally aspirated motor but you are on key with determination of what gear ratio I think would be best suited for my application. In my mind though I think the 2.73 gears are way to high. The way the car is now I can cruise at 100mph at a little under 2500 rpm's in 5th gear. Never have I got it over 135mph in 4th never got in 5th trying to top it out it seems the gear is so high it just can't push no more. I was thinking along the line of 3.55 gears but the reason I made this thread is to get some info from people who know alot in this area and can get the true info out.
 

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well your talking to the right guy, im in the middle of my supercharger project ( vortech v2) and i already have 4.10 gears wich are perfect. The norm with higher gear ratios is to be cruising at 65-70 at 2500 RPM. its not bad i do it almost every day. i might switch to 3.73 when im done with the SC. you must keep in mind that with higher horsepower and too high of a gear ratio you could skip some if not all of your power band. one of my friends has a supercharged 5.0 5 speed and hes running 3.73 gears wich i believe are the ideal gear ratio. a 3.55 wouldnt be bad either,your hardly going to notice the difference between the two. its hardly half a driveshaft rotation difference
 

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Discussion Starter #6
I agree with you on the most part but if I am going to see full boost at high 3k rpm's or right around 4000 rpm's wouldn't it be pointless to have such a low gear like a 4.10 and from what I take if I have the right size turbo which I should be really close to it if not on it wouldn't a guy like me with a turbo see full boost before a guy like your self with a centri setup? From what I understand a lower gear like a 4.10 is good for centri and roots setups do to you pretty much see boost at high rpm's higher than a turbo. Like number 2 states a turbo needs load so a higher gear such as 3.27 or 3.55 would be more efficent right?

I am not saying my statments are correct I am just trying to get this down and right so I know what gears to go with and can help others later on with the same deal!
 

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Turbo car works better with load on the motor. Turbo's need to stretch their legs a bit. I think depending on your powerband you should be good with 3.55 for a mild turbo setup street driven stang. Also take into account that your car doesn't have the grunt down low and will spin the tach a little higher than your normal 5.0.

If you were looking to get more power and push the turbo some more and make your engine a more efficient air pump with PI heads and so forth then even a taller gear like 3.27 could be beneficial.
 

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OK here is the story. When choosing Final Drives their are a lot of variables. Number one what is the first gear ratio of your transmission? This is very important Two for any turbo car like my Turbo coupe i run 4.10 Gears heavy car low torque engine. In my Ls powered mustang I run 4.56 Final drive ratio. 6 speeder. The key to a gear setup is to not spin first gear to short. Another words if the combination of first gear ratio is beyond 16 Thats about all you can go. Reason is the gears are to widely spaced. Their is to much RPM drop between gears. Years ago Close ratio Muncies were popular with 4.11 final drives. Today with 5 and 6 speeders the ratios are much easier to handle like 4.56 4.88. Bottom line with a turbo charged engine you want to keep the RPM UP. The turbo needs to spool up and normally none really come on until app 2800 RPM. That means stiff gears in the rear. Example a stock Turbo coupe 5 speeder came with 3.55 and the auto came with 3.73. Reason being the auto does not have as much low gear as the 5 speeder. So when choosing gears always look at the first gear ratio compared to the final drive. Basically a 5 speed car of moderate weight should run a 3.73 and a heavy car 4.11. With an auto 4.11 or 4.56 depending on your top speed desired and the OD ratio. Picking gears is not just a number game its an entire numbers game from first gear to top gear. Hope this helps
 

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http://i600.photobucket.com/albums/tt88/tomrenzo/Kitties138.jpg

Here i am at the top of FIFTH GEAR. One more gear to go. Remember also that the max rpm your motor can produce is also a factor in the choice. I can run this LS turbo pontiac to 200 MPH with 4.11 final drive gears. Ask me how i KNOW:bigthumbsup:bigthumbsup:bigthumbsup
 

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Discussion Starter #10
I don't care about top speed nor do I care about running the rpm's up on the motor if I see full boost at 4 rpm's again just like it is now what is the point turning 6000 rpm's or more on a daily driver street car with stock internals. No need to have a lower gear with a turbo in my mind it's a medium gear I think but like number 2 in my first post states for a "straight drag car you want to pick the set of gears that puts you in your powerband for every gear and so you are passing through the traps at about redline in your final non OD gear". Now that probably would be 4.10 gears but I am street daily driver not making big power yet either using a 67mm turbo my tranny is stock geared T-45
1 - 3.37, 2 - 1.99, 3 - 1.33, 4 - 1.00, 5 - 0.67. Also keeping the rpm's up and getting them there is a different story. How heavy are you talking? Heavier than my 98? Yeah and I bet your LS turns more rpm's than my 4.6 mine redlines at 6k. This is not gonna be a drag car and that is what most of your statments have pointed to! This is gonna be a daily driver good street/highway gears I am not worried about a straight line race right now with it being a daily driver. Now when I move off base and get my own place so I can do that with it then yeah I will be doing it.
 

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You are not getting this are you???

5000 RPM
.67 High gear
4.11 final
26" tire
Max speed 141 MPH

5000 RPM
.67 high gear
3.73 Final
26" Tire
Max speed 154 MPH

The difference is 13 MPH

The 4.11 gears would work perfect for you especially with the setup you have. Just me could be wrong but the numbers are NOT.:bigthumbsup:bigthumbsup
So what gears are you thinking about???
 

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Discussion Starter #12
No I am getting it just fine I understand where you are coming from on the gear factor. If I was staying NA I would go with 3.73 gears hands down cause anything is gonna be better and me see a world of a difference than the stock 2.73 gears. What I am trying to get at is mainly in your first two gears is the big factor I am concerned about with load and not reving to fast to get the proper amount of boost. So your telling me if I run 4.10 gears with a turbo setup with all the info I gave I will see no problems at all with that and that 4.10's are gonna benefit me better than 3.55 gears that I was thinking of?
 

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Discussion Starter #13
Just to throw it out there this is no turbo coupe comparing 4cyl to V8 is like apples to oranges!
 

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It works the same pretty much for FI or NA. what you have too look at is the shift points and what rpm your at, at the begining of a new gear.. thats where a dyno map will help. you want to get the most of the hp and tq not over shoot it , or under shoot it.
 

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dont forget shifting eats time... that why drag racersat the top use autos with fewer gears... top fuel has a slipping clutch that is fully engaged after so many feet.. then it WOT
 

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It works the same pretty much for FI or NA. what you have too look at is the shift points and what rpm your at, at the begining of a new gear.. thats where a dyno map will help. you want to get the most of the hp and tq not over shoot it , or under shoot it.

YOU SEE THIS GUY GETS IT. Thank you Sir:bigthumbsup:bigthumbsup
 

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Discussion Starter #17
Exactly and that is not the same thing as your calculations you posted before looking at the dyno results is a hell of alot different than calculating what speed you will be at in your final gear at a certain rpm. Obviousally you don't get what I am saying do you??? You have not stated a thing to me that has made sense except the gear calculation and nothing about how gear ratios react different to turboed mustangs than a naturally aspirated mustang that is why I made this post. Just because you know alot about rear gears does not mean you know everything man and it is just like comparing a naturally aspirated motor to a turbo motor, or even a turbo to a vortech centri supercharger. When you state something that shows me proof that gears go the same for na then I will believe you.
 

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Discussion Starter #18
in a n/a setup, shorter gears can result in faster accel.

with a turbo setup, load = boost. short gears mean less load. and boost = power

with a larger turbo , if the gears are overly short, there isnt always enough load to generate full boost in lower gears.

and if we are talking larger turbos, then every psi below the target, is power given away, generally in the midrange.

now a loss of midrange means slower acceleration even if the peak is the same.

shorter gears arent always better. that is my story!
 

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Discussion Starter #19

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Fordman, remember it's a forum, really there isn't any one "correct" answer to any build/application, there are many varying factors in how people want cars to respond...take what you can use and disregard the rest...
Personally I don't like low rear gears, it might help acceleration, but I don't like screaming around at the red line.
To me yeah, the faster the engine spins the faster you can build boost, but the trade off is that you have very little time to build that boost before you have to shift, so finding the "sweet" spot is the key
Listen to Tom, he is a real world expert and is just trying to say that by varying the rear gears with the trans gears you can dial it in better...
Good luck, have fun
 
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