This isn't exactly LT1 related, so I parked it here instead of in LT1 Tech. Fred, feel free to move it if you think there's a better spot for it.

I'm in the middle of a swap (which does involve an LT1) into an RX-7 which has a 4.10 ring and pinion to help out the (relatively) "torqueless" rotary engine it came standard with. 255 hp, 217 lb-ft. of torque.

The engine is an NA 396 that many of you are probably familiar with now, which made 640.8 on the base maps with ignition problems which are being straightened out, and 532.4 lb-ft. of torque. 389 lb-ft. at 2,000 rpm so far, and it'll only get "worse" after tuning is finished.

So the 4.10 not only won't cut it with a G92 6-speed (2.97:1 1st gear) as far as traction in lower gears are concerned, but it'll put an excessive amount of strain on the rear IRS and axles.

The only other readily available options are a 3.90:1 which came in the automatic cars, and a 4.33:1 (wrong direction) available through the aftermarket. Analysis has shown that I need a 2.xx:1 rear gear, probably ~2.80:1, according to Car Test 2000.

My problem is that I need to find a manufacturer who can produce a custom 8.0" (203mm) ring and pinion for me, and also (not necessarily from the same source) someone who can make a pinion shaft flange to accept a larger U-joint on the rear of the new driveshaft.

My main concerns are noise, which I've heard that Richmond gears sometimes have, and reliability. I've visited Mosler's site, but it appears that they have their gearsets built for them. Any other suggestions for a (relatively) quiet brand of gears that'll be tough enough to take a good bit of punishment?

Thanks in advance!

[This message has been edited by jimlab (edited June 22, 2002).]

 

Jim, I'll leave this here, at least for a little bit

Have you called places like Motive and Strange to see if there willing to take on this project?

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Ken Mook - 97' SS #2544
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Solve all the problems and get rid of the Stock rear in that car, it's gonna bust. Get rid of the IRS also. Put a ford 8.8 back there and all you problems will be solved. I can get you some pics of one we have done.

 

Thanks Ken. I was hoping someone who had used "standard" gears from an aftermarket company would speak up about durability and noise so I'd have a better idea of who to talk to first.

 

The IRS and differential will take a lot of power, since there are many 500+ hp single turbo RX-7s and quite a few with 20B 3-rotor engines making even more power. The low end torque will obviously be higher, but there are reasons why I'm not willing to sacrifice the IRS or the stock diff.

Believe it or not, it's not being built as a drag car. It's meant for track events, but will be used predominantly on the street, and will never have slicks on the back. When (if) I take it to the drag strip, it'll probably be on track tires, or drag radials at the most, but never slicks.

Weight is also very important. Keeping the curb weight of the car at or below the stock 2,800 lbs. is one of my primary goals. I have researched 8.8" and 9" Ford IRS conversions, and haven't found any that appear to offer the same level of performance, none that will be even close to the weight of the forged aluminum control arms and lightweight rear subframe, and none that wouldn't require some creative surgery to enable retaining the stock ABS system. It'd be far easier and cheaper to keep the stock setup, in my opinion.

By dropping to a 2.xx:1 differential, the strain on the rear components will be reduced somewhat, and with as much torque as my engine is producing at low rpm in such a light car, it's not going to require higher rpm to launch effectively. I expect that the tires will be the "weak link", not the differential, and the axles will be upgraded to 300M chromemoly.

So, given that... any ideas?

Thanks!

 

Have you tried other mazda models ?? Also you are right the rear will hold Mine does 200mm ring .Be wary of your CV's or 1/2 shafts out of the hole.IRS RULES enough said also the q45 and tt 300zx have a z31 over 9" ring gear.If I break the 200 its in.Also my friend in Fl runs 9's on the 200mm ring gear IRS.

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lil Japanese car BIG American motor
www.speakeasy.net/~lt1240z

[This message has been edited by LT1240Z (edited June 25, 2002).]

 

Unfortunately, the gear sets from other model years will not work in the '93-'95 differential, and even if they would, rotary cars all have gears in the 3.90:1 and up range to compensate for the rotary engine's lack of low end power.

Final drive on the '93-'95 manual in first gear is 14.28:1, and I need to end up with something near the low "8s", McLaren style.

Thanks!

 

the TT300zx and the Q45 both have a rear that may get you closer 3.54 in the infiti IRS 3.73 in the TT300zx.

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lil Japanese car BIG American motor
www.speakeasy.net/~lt1240z

 

Thanks, but even if they worked, they're still far too much gear for a 2,800 lb. car with 400 lb-ft. of torque by 2k rpm...

I'll just start calling gear set manufacturers and find out who can cut me a custom ring and pinion.

 

Hey jim good to hear the car is well underway..Funny, i never thought a few torque central memebers are here from the rx7 forum..

Have you sourced any info from overseas?Theres gotta be an alternative gearset overthere..If not then id suggest that you get the stock gears measured up so at least you will have solid info to fax or email to a manufacture.

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Have you considered only replacing the center section with a Ford 8.8", like out of a Supercoupe(or any of those year 'birds/Cougars). The Supercoupes have LSD's though. I am looking into this for my V8 Zcar. All you would need are custom halfshafts. Also, a guy has put the whole rear suspension out of a C4 corvette into a Zcar. I would say that is ATLEAST as good as your RX7 rear suspension. Also, the gear selection is pretty good. I'm sure a 2.xx ratio is available for the Vette rear. Also, I know guys who have thrown away 2.73 gear sets from brand new Mustangs that would go in the 8.8 IRS rear fine. I would check the local pick and pulls for a T-bird or Cougar. Get the center section and its axles. Then get an Auburn Pro LSD for it. It would probably have the 2.73 gears in it.

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Jamie Taylor
'94 Z28 M6
Mods: BBK 52mm TB, Granetelli MAF, MSD 8.5 wires, MSD coil, Bosch +4 plugs, gutted cat., In the final stages of HOT cam kit, ported heads, CSI H20 pump, Jet Hot Hooker LT's(thanks to the GP), custom 3" Y-pipe, and Custom CAI installations. It's nice being a welder, I can do all this custom tubing stuff easy!!!
1971 Datsun 240Z with V8 power. 500+rwhp

 

Try mark williams enterprises. They have not got anything in the catalog for you but might could tell you where to get it. They also have some pretty nice looking modular rear ends but I dont Know if you could make them work with irs?

 

consider looking at the carrier for the thunder bird IRS. i have heard of ppl using them in the mazda miatas when doing the 5.0 ford swap. you may want to research it.

good luck man

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It's a small world, isn't it?

All of the available aftermarket gear sets are "shorter" than stock, or in other words, 4.33, 4.50, 4.77... the wrong direction.

3.90:1 is the lowest (numerically) gear set available, and came stock in the automatic 3rd gen. RX-7s, however analysis hasn't shown that it would be of any advantage. If Car Test numbers can be believed (rolling start... I seriously doubt the standing start numbers are calculating "tire fry" correctly for my application) then a 3.73 would be "ideal" for highway sprints, since from 60-150, it's a two-gear shot and ends right past the peak power of the engine. Unfortunately, it looks like about $2,500 to have a ring and pinion custom made, and with only 0.6 sec. difference between the 3.73 and the 4.10s, it's not really worth it.

After talking to several people, some are recommending "starting in 2nd" and keeping the 4.10s. While this makes sense because final drive drops to ~8.5:1, some have also said that the clutch will have to be slipped excessively. In a ~2,750 car, I'm not so sure, but what is probably "real world" is starting out in 1st, short shifting (~2,000 rpm) to 2nd, and then flooring it.

I already have the 4.10s, and unfortunately, I lose 1st gear as an acceleration gear, pretty much, but gain 6th gear as a pulling gear. With a 2.xx diff, I'd lose 6th gear and gain first, so what are you going to do?

It was recommended to keep the 4.10 with the 0.62:1 6th gear to keep the engine right around 2,300 rpm (@ 70 mph) because (I was told) the engine would run better and more cleanly in its power band. I was comparing to my Z06, which "idles" at about 1,750 rpm in 6th at 70 mph (3.42 diff), but was told *not* to compare my car to anything existing, because the comparison isn't relevant. The engine (again, I was told) needs to be into its power band to run and burn cleanly, and the bigger cam and heads on my engine mean a bit higher rpm at cruising speed is more ideal, compared to an LS6 which redlines 1,000+ rpm earlier and makes power earlier with its smaller cam. Make sense?

And finally, it's been recommended that I try Racelogic's traction control system, since it will tie easily into the car's ABS sensors and should help control wheel spin in the lower gears.

Polecat Z: I did consider replacing the rear setup with an 8.8" or 9" IRS setup, but the research I did showed that it would be very expensive and although strength would certainly be improved, weight would increase significantly, and it's doubtful that handling would be improved. I also considered the C4 IRS, and while it is stronger, it is not superior to the RX-7s... if you want confirmation, look under the back of a C5... same setup as a 3rd gen. RX-7.

96ltz: I did contact Mark Williams and was told $25,000+ for tooling up (!!!), and told to just switch to a live rear axle. Luckily I know that someone else has had custom gears made and knew that the price, while still high, was much lower than $25k. Live axle isn't an option. I'm not willing to "back half" the car, or sacrifice its handling, unless it turns out to be absolutely necessary.

At this point, I've spent enough money on the car (the understatement of the year) and I'm having a very hard time justifying even more. If the T56 will work with the 4.10 well at highway speeds, that's fine. And I know that first gear is pretty much a lost cause, and that's fine, too. I'd rather spend $1,000+ on the Racelogic traction control system than $2,500 on a new ring and pinion which would gain me all 6 gears (3.23, 3.42, etc.) but be a compromise in performance from what I've seen. I'll try starting in 2nd just to see if it'll work.

The car had 217 lb-ft. of torque at peak stock, and a 14.+:1 final drive to compensate, so with an 8.5:1 final drive (2nd gear) and 300+ lb-ft. of torque at low rpm, it shouldn't be too hard to get rolling.

Thanks everyone!

 

I'm not suggesting replacing the whole rear assembly. Just the center section. Use custom halfshafts. The custome halfshafts don't have to be expensive if you are crafty. I have two choices on my Zcar. One is to use the 90+300ZXTT(same as Q45) rear(center section only). It is dang near bomb proof, since its 230mm ring gear is actually larger than a 9" Ford, plus it is Viscious, so no real wear item in the unit. Or, the Ford 8.8" IRS (again, center section only) These are dirt cheap. The gear sets are dime a dozen and the posi units fall out of trees. Both of these units are cast iron. So What? Even if it adds 30 pounds to the rear of your car, would you even notice it? My Zcar ways 2380lb with driver. Thats with a aluminum headed V8 and a Tremec 5spd. Granted our 200mm ring gear center section rears(cast iron, stock in 280Z's) can handle 500hp as long as no slicks(speaking drag racing here) are used. I've run mine on the street for awhile with the R200 rear and Yokohama A032R's 275/40/17's. Just food for thought. Some times people get caught up in trying to make stuff just too darn trick. When, driving the car is really what matters.

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Jamie Taylor
'94 Z28 M6
Mods: BBK 52mm TB, Granetelli MAF, MSD 8.5 wires, MSD coil, Bosch +4 plugs, gutted cat., In the final stages of HOT cam kit, ported heads, CSI H20 pump, Jet Hot Hooker LT's(thanks to the GP), custom 3" Y-pipe, and Custom CAI installations. It's nice being a welder, I can do all this custom tubing stuff easy!!!
1971 Datsun 240Z with V8 power. 500+rwhp