rear end ratio with turbo?

[D347643]

i am planning to do a turbo project in my stang using a powerstroke turbo (or cummins) anyway i was about to buy some new gears for the rear when a thought occured to me. if i run 3.73 out back wouldnt i run out of rpm's too quickly if i have a turbo? since it will spool about 3k, shouldnt i run a lower rato like 3.55 to take advantage of the turbo? anyone have any ideas about this?

thanks,

Drew

 

[HO93LX302]

What kind of motor you putting the turbo on?? stock 5.0, 306? or maybe something else.. If you are putting on stock engine i'd go with the 355's but if you have a different motor in it and your reving higher i'd put the 373's in since you have all that RPM to work with.

 

[NKau]

You don't want to go too high because you want to be able to get the turbo spooled up quickly; for race purposes at least. That's the only real disadvantage to turbos is the "lag" before boost becomes a factor. It's not as big of a deal when you are working with a 5.0L engine as opposed to a 2.0L DSM motor where it doesn't really make torque without the aid of the turbo. I would think a 3.73 would be about right, but I'm not really an expert on the subject.

 

[Cammed90Notch]

I've always heard from reliable sources (at turbomustangs.com) that turbo cars like less gear. I have 3.73s, and am planning a similar project. I'll probably keep my 3.73s w/ the turbo, but if I had build the turbo system first, I probably would have less gear

 

[stangbear427]

ITS Turbo was recommending a 3.27 gear to me- but we were discussing a 750HP twin kit on a 427ci stroker. 3.73's are pretty low though, regardless. I wouldn't go lower than 3.55's.

 

[D347643]

the engine is a 306 with twisted wedge heads, stock cam (gonna have 1.7rr on exhaust side) blah blah comp lifters, the whole deal. Im thinking of changing my intake, i have a street heat. Maybe a boxed upper?

Drew

 

[stangbear427]

Yeah, I'll say 3.55's. With a 306, you'll still want some good low end torque to suppliment the turbo spooling up, but not enough to redline you before it does. A boxed upper wouldn't be a bad idea, depending on what size turbo you run and how many psi of boost it moves. Unless it's more than 10psi or so, the Street Heat should be fine.

 

[Michael Yount]

Is it an automatic or a manual tranny?

With automatics, lag isn't too much of a problem as the motor/turbo tends to stay on the 'boil' during shifts. With manual transmissions, that's not always the case. Consequently, you'd probably get away with running more gear with the automatic assuming you've got the traction to manage it. With the manual, people frequently run less gear (taller, higher, lower numerically) to maximize time between shifts keeping engine on the boost, and let that flat wide turbo-torque curve do it's work.

In any event, 3.55 and 3.73's are awfully close - only 5% difference in speeds/torque multiplication. I think the real question is whether or not you want something like the 3.27's that Stangbear mentioned.

How's the car gonna be used? If you're drag racing it and it's a manual, you could always go a bit higher numerically (say 3.55's) and then use tire height to 'soften' the gear if you need to. Just ramblin'....

 

[D347643]

well im gonna be running some 305/50's which are (if im not mistaken) a 27 inch tire and (again if im not mistaken) it would kill a 3.73 ratio to about a 3.55. I posted a thread before about "killing the gear" before and Mr Michael Yount came through, like he always does, and gave me the equations to figure it out.
I seriously gotta give you props man, with the info you post, its like you take it out of a book.

Drew

Oh, My car is stick, 5 speed.

 

[stangbear427]

Out of a book? Dude, Yount is the book. Or the library, more like...

 

[Michael Yount]

...somebody turn the page....

 

[D347643]

seriously though, how do you know so much? do you get stuff out of a book or something?

 

[JadeFalcon]

3.55's work very well with turbos. You want to keep the motor in its power curve as long as you can so you can get that turbo spooled, its commonly agreed that 3.55's give you just what you need.