I am looking to install a 9" rear in my 87 Iroc-Z, wondering which is better 3.73's or 4.11's?? I will use the car once or twice a year to go drag racing otherwise the car is just for weekend fun. It still has the 700R4 in it, so I still have overdrive. I do not need a car to go 150mph so am I better with the 4.11's so as to get more launch out of my car?? The car has a 355, pushing 412hp and a 3200 stall convertor. If someone using 3.73's or 4.11's could give me speed and rpm levels I would appreciate it. Also ayone who has suggestions.
Thanks

 

being its carbed i dont see a problem with 4.11s

 

Welcome to TGO !

Once or twice a year for drag .. Id go for 4.11's, if not 4.56's ..

 

4.11

 

What are your cam specs and what does your convertor flash too now?

If you were using a TH350 or 400 the 4.11's would be a natural choice for your intended usage. With the hp you claim, the gearing of the 700r4 using the 4.11's will put you at a higher trap rpm, probably at your max redline. Using the 3.73 witll still get you super 60's with the 700r4 first gear and should be about right at the other end of the track, not to mention alot nicer cruising. 4.11's combined with the 700r4 first gear ratio is great for a loggin truck but you wont spend much time in that gear in your car.

 

Cool I think it is going to be 4.11's then. That its what I was originally thinking but then a fellow I know said that would be too much and too stick with 3.73's, but 4.11's it is. I mean I only drive the car 1500 to 200 miles a year anyway. so lets go 4.11's!!!
Also I am looking to buy a Ford9" for this car, hopefully something that will drop right in, anyone know of any good places to buy one. I was looking at Currie, but not sure if there is anyone cheaper out there or closer to me. I am located in PA. I know there is a fellow named Art Houser that does rears but he is a little more expensive, however he is only 1 hour away from me. So if anyone knows of a good company on the internet or a shop in PA that will build me one please let me know.
Thanks

 

You'll cross the line in 3rd which is the same for the 350,400, and 700r4. All 1:1 so your trap rpm will be about the same for each tranny.

 

I have 3:73's and wouldn't in my opinion want to go any lower (numericaly higher) Going from 2:73 to 3:73 was a world of difference. First gear is almost too low with my 2700 stall and street tires. Maybe with a set of D.O.T. street slicks and the strength of the 9" you will get the car to hook up and keep the rear in one piece. I am thinking that the 7.5 I have is doomed. I too would eventually like to get a 9".

 

I've done the A-B testing yrs ago between the 700r4 and TH350 and when set-up they will trap and mph the same. That was with the 700r4 and 2800 stall 3.42 gear then switching to the TH350 with 3500 stall 3.42 gear, same et, same mph, same rpm.

IMO 4.11's with the 700r4's 1st gear ratio it's just darn darn low to be useful for anything other than pulling stumps., unless of course you install a manual valve body and leave in 2nd gear, then the 4.11's would be a good choice.

 

I agree, but why did you say they will produce different trap rpm?

 

the 3.73's will produce less trap rpm than the 4.11's no matter what the tranny is in front of it. But you have to compensate with the TH350 by raising the stall to compensate for lack of 1st gear ratio of the 700r4. Either way it's impossible to say until we know more facts on the combo. The old school method of throwing bigger numerical gears out back is looong forgotten, it's all about making the best use of what you have.

It's so hard to say where he'd be optimized at since there is no cam data/rear tire size to get an idea of rpm band. Generally with 412hp and we'll call that at the flywheel I'm "assuming" a midrange torque band and a cam with say a 110-114 LSA, typically these will run a tad bit better with a little lower gearing so it stays in the optimum torque curve longer to utilize the motor instead of trying to wind it out to quickly and be left with noting at the end.

Edit:
If I can say one thing with 100% certainty, if more strip than street, lose the 7.5 diff. You will/are going to be fighting a losing battle and you will tire of swapping entire rears, carriers to keep running. Unless your running tires that will not hook, then you have a little more time. 4x's I went through this and even did the upgrades eveybody talks about, the things are just too small for repeated abuse.

 

ok, but you said the 700r4 will cause a higher trap rpm than the 350 or 400. You even said it may cause him to "redline" where the 350 or 400 wouldn't. They all have the same 1:1 third gear so they will all be pretty much the same.

 

Okay, as I said before I "assumed" and I "assumed" too much. As it sits there is no way any of us can give him valid info without knowing the rest of his combo. Yes, the cam consideration, convertor stall speed and actual slippage, as well as tire size will all play into which gear he should go with. You are right with the 3rd gear being 1:1, but given just that fact alone as the only consideration when choosing the right, gear, stall, cam, tire size is bunk. If he has a.. say 110LSA 246/246 actual duration he will be all done pulling around 4000rpm depending on his heads with a 4.11 gear and 28" tire after the 1/8th he'll be done, all done. I use that cam as an example since a buddy tried the Comp grind with those specs in his 400sbc/TH350 3500 stall 4.11 gear 28" tall tire in a 3300lb Malibu. It pulled like a son of a gun off the line and banged through the gears quickly and after the 1/8th mi marker it quit pulling period. He drove, I drove it, his brother drove it and all of us had the same result. Switched over to the another Comp grind with a 106LSA, 250/260 duration and now the car pulled all the way through. Torque and hp bands were changed and his combo with the TH350 that has the 1:1 3rd gear ratio you keep refering to was all the same.

At least we now know one thing, you are 100% correct on 3rd gear ratios of the three tranny's we talked about(700r4 350/400). Get out and race every weekend with guys running much of the same stuff kids on here are always asking about and you will be able to have REAL world experience on how things really work instead of what some newstand magazine says should work or what buddies of buddies say "should" work. I've always given advise based off real world experiences, I've personally spent/wasted alot of money trying out different ideas and am at the point now I just might know wtf I'm talking about. I'm not going to waste my time tooting my own horn about everything I've had to get this info, but most that know me here trust what I have to say.

AGAIN I say, none of us here have the info needed to correctly pick gears for the guy. Everything based off "guestimates" and just like most car pasts your better off to err on the small side. Heck, why not tell him to install some 4.88's or 5.13's....but I guess we're just concerned with 1:1 ratios's
Feel like I'm talking to a kid with a Honda thinks he knows but has no idea of wtf is going on

 

I simply took that statement and corrected it, that's all. Wow man, you need to really chill. So you told the guy that if he had a 700r4 he would cross the traps at a higher rpm because of the gearing. Since each of the transmissions discussed have the same exact 1:1 ratio it simply can't be true; They would all cross at about the same rpm. Now first and second gear would be a different story, and I didn't necessarily say I disagreed withanything else you said. I simply corrected some false info. Take it as you wish, but you really need to calm down and realize that everyone makes mistakes now and then. Chill!

 

Nuff said, internet typing along with intended explanations sometimes get mixed up.

 

I have a built to the max 7.625 10 bolt. Don't waste you time. I know it is going to handgranade any day. Get a Currie Enterprises Ford 9 inch.

 

I don't have my spec sheets in front of me for all my cam info but I know I have .592 lift not sure about anything else..will try to get that info on tomorrrow. As far as the tire size they are the factory size I believe, 265' and 16 inch rims I have 2.02 fuelie heads that are worked and my intalke is from 3500 to 7000 I believe. Hope this clears up some stuff, I will get my build sheet out tomorrow and post definate information then. Also I am thinking of going to a 750 Holley carb..not sure if that is too much or not, or if that makes a difference as far as my question is concerned. Hope this helps and thanks for all the info so far.

 

www.moserengineering.com

 

I did the 9-bolt to 9" swap about 4 months ago. I have a Currie stage III Ford 9" with all the GM brackets already welded to the housing, and it fit perfect. The gear I am running is a 4.11, but I have a Tremec 3550 manul trans, so I can't relate to the stall/auto discussion. The rear end is heavier, but I like it a lot, I'd say go for the 4.11's.