Gear choices for twin turbo setup.....

[US-1]

Ok race fans....let's all see how fast I can shuck the rods out of this thing!!!

On second thought....let's just see which gear seems to be better for this setup. Y'all know the car is stock with the twin 54mm turbo setup. There will be a PI 3500 rpm stall speed, multi disc converter going in next week. We are also going to put a Borla after-cat system on the car. Currently, there are 3.31 gears in the rear end. Car will also be wearing a set of M&H D/R's in either 275/50-17 or 325/45-17 size. These are 28" tall so here is the gear question.

Stock 3.31.....3.55......3.73......4.10

And...why do you think your particular choice is the correct one? I'm leaning towards 3.55's to keep the rods from taking a vacation flight to Asphaltville. Keep in mind that we have already run 123 mph.

 

[don_w]

Process of elimination...

I think 4.10s might be too steep, or they might be just right. You'd probably run out of gear somewhere between 125-130mph. If you expect to trap over 130, I wouldn't do 4.10s. But, if you don't think you'd ever trap more than 130, then those would put you in your peak powerband as you hit the stripe.

Forget 3.31s... those would kill the 60' time too much IMO.

So, that leaves 3.55 and 3.73. At say 6400rpm in 4th gear with 28" tires, the theoretical top speed would be 150mph with the 3.55s, and 143mph with 3.73s. It's unlikely you'd ever trap that high anyway. So, I'd choose 3.73s between those two, just to get the better 60' times out of it.

Bottom line, either 3.73 or 4.10s would be my choice, depending on what you think the upper mph will be.

 

[Black Tooth Grin]

Seeing that the rears are going to be 28" tall, and given that you are going to be a little slow on your 60ft times because of it being a turbo and will be wanting to make it up on the big end, I would almost say go with 4.10s. With the rear tire height you are talking about that would give you close to a 3.90-ish gear ratio. So it will be short enough to help with the 60' time and still tall enough that you (hopefully) won't be redline in 4th going through the traps. If not 4.10s, then maybe 3.90s (yea, I know, Motive gears are noisey) which with that tire height would put you around 3.75s or so. I don't know if I would go with anything taller than that though unless you could can get motor to wind up pretty quickly to take advantage of the taller gear ratio.

 

[don_w]

Seeing that the rears are going to be 28" tall, and given that you are going to be a little slow on your 60ft times because of it being a turbo and will be wanting to make it up on the big end, I would almost say go with 4.10s. With the rear tire height you are talking about that would give you close to a 3.90-ish gear ratio. So it will be short enough to help with the 60' time and still tall enough that you (hopefully) won't be redline in 4th going through the traps. If not 4.10s, then maybe 3.90s (yea, I know, Motive gears are noisey) which with that tire height would put you around 3.75s or so. I don't know if I would go with anything taller than that though unless you could can get motor to wind up pretty quickly to take advantage of the taller gear ratio.

I was going to mention the 3.90s, but they weren't listed as an option. If this is primarily a track car, then I'd consider that route (with a locker rear end), because I wouldn't care about the noise. If it also sees a lot of street time, then I'd stick with 3.73s or 4.10s. Street traction will be a challenge either way, IMO... unless the M&Hs are also used on the street.

 

[don_w]

Car will also be wearing a set of M&H D/R's in either 275/50-17 or 325/45-17 size.

Side note... if you have a 9" wide wheel, stick to the 325s...

 

[Black Tooth Grin]

Tell ya what.. if I were auto, I would probably be jerkin' off all over my 4.10s. For the track they are pretty fun because you are winding the hell out of it, but on the street.. not so fun. Even with my rear tires being 28.8" tall first gear is worthless. About like first in a semi. Gets it rolling then you are instantly changing. Hell, 99% of the time I take off in 2nd. Much wishing I had a spare set of 3.73s laying around to throw in to it.

 

[US-1]

Side note... if you have a 9" wide wheel, stick to the 325s...

Currently....stock 8" wheel. The M/T's I had on at Orlando looked like crap on the 8" wheel. I hated it. If we can get the 9" wheels here before Reynolds we will go with the 325. Although some part of me wants to try the 275 on the 8" wheel to see if I can make it hook. The whole "stock appearing" thing. Then again...it would probably look like crap on the 8" wheel too. Have you or anyone tried the 275 on an 8" wheel, Don?

The 3.31 gears may be the main problem with the car right now. It simply won't rpm in the first sixty feet. One hundred feet out it hauls ass but up to that point it's a dud. The 2.12 sixty foot times showed that much. Given that the tires are 28" tall you may be right on the 3.73 gears. I believe the 4.10's will just be too much.

 

[don_w]

Have you or anyone tried the 275 on an 8" wheel, Don?

I can't think of anyone off the top of my head. But the tread width is only 8.8", and M&H recommends an 8" rim width, so I'd think they'd fit fine. But will they hook? Dunno. The 325s have a tread width of 10.3"... thats almost 20% wider. And they've been proven to hook with 1.4x 60' times.

 

[retfr8flyr]

I would recommend the 3.73’s that’s what I am going to run with the 325’s. With a turbo you want less gear to load the engine and build boost but with these heavy ass cars we still need enough gear to get the thing moving. One option would be go with the 3.55’s and if it’s not enough gear you can switch to a shorter tire to compensate.



Earl

 

[Buckman]

I would run 3.73's for all the reasons stated above already given the tire size. I would even consider a 3.90 with that tire.

 

[don_w]

One option would be go with the 3.55’s and if it’s not enough gear you can switch to a shorter tire to compensate.

My concern with that approach is if you go to a shorter tire on a 17" wheel, you're reducing the sidewall to a point where it may have a serious negative impact on traction. If it was a 15" wheel/tire combo, I'd be less concerned about it.

 

[silverpoison]

3.73's , That's what I currently have on my car, and when the twins go in they will stay there. 4.10's might be ok if you where doing only 1/8, but you do your racing on 1/4mile tracks. Also the higher your gears are the higher you will take to spool the turbos and that equals less pull in each gear. I would wait to see what the converter does for you and then do the gear swap.

 

[Fastmover]

I say with a 28 inch tire, go with a nice tall gear! I got the 325 M/Hs and I run a 4.30 gear. Crossing the traps at 6k, Right where I wanna. You wanna get the tall gear to help bring the revs up quick to make power!

 

[fin1]

I have a 28" tire and run w/ the 4.10's & turbo. I've trapped around 137 mph at about 6400-6700 rpm (and as high as 140 mph). Having a locking converter helps too.

 

[don_w]

Brad... what tire are you running?

 

[fin1]

Brad... what tire are you running?

Don,
The M/T ET Street Radial (part #3757R) - 28x13.5 R15.

 

[don_w]

I wouldn't expect those to grow much on the top end, so you must really be wringing it out to trap 140 mph. That would be about 6900 rpm. Nice!!!

 

[fin1]

I wouldn't expect those to grow much on the top end, so you must really be wringing it out to trap 140 mph. That would be about 6900 rpm. Nice!!!

Agreed. I'm not legal to run under 9.99 and don't normally run it this fast. If I did, I would probably go with 3.90's. But where I'm at now (135-137 mph), the 4.10's have been a good balance.

 

[silverpoison]

Brad, for how long are you sitting at the line to get to your desire boost for launch?

 

[US-1]

Ok...I'm seriously leaning toward either the 3.73 or 3.90. The car will see some street time (eventually) so I don't want it roaring with a gear that is too short. Just got off the phone with the guys in California and they're swamped right now. They have nine Mopars to get done before the "Mopars at The Strip" show at the end of the month. The turbo stuff will be redone, converter will be installed, and rear suspension stuff will be installed. The roll bar......we don't know if Paul will have enough time to get it done before we need to head out for Reynolds. If not....there's going to be a full tank of fuel and two hundred pounds of plates in the spare tire well to slow Butch down to 11.50 for Reynolds and Commerce. After those two races we can do the roll bar and get the rear end work done before the Baytown FFW race in May. Sure would like to turn this thing loose in Commerce though. Always good air in Georgia this time of year.