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Manual transmission impact from high HP and Torque
Hello - new to the forum and first post.
I previously owned a '87 RX-7 (manual tranny) and put on 160k miles before saying goodbye to it. Great car, fun to drive. I put on a borla exhaust around 90,000 miles - otherwise, completely stock.
At 140K miles, I trashed the gearbox during an aggressive shift from 3 to 2 (gotta love the rotary - stuck in 2nd gear and running 60 mph for 25 miles to the mechanic w/o sweating it). As we determined I was going to need to replace the tranny, I learned that the metalurgy in the 2nd gen RX-7 tranny was at the upper end of its capability with the stock HP - any mods would exceed the design limit of the soft metal and gear teeth would start to break off. I could pick up a brand new crate motor for $1000 from a Mazda dealer, yet it would be more than $3000 for a new tranny...if one could be located. As it turned out, I ended up getting a used one with 40k miles on it for $1700 (not including labor).
Question for the group - I have seen turbo kits under development, exhaust systems, and ECU fuel remaps delivering more power out of the drivetrain, however, is the tranny still a weak link? How much HP and Torque is the tranny rated for?
Thanks for the feedback and info in advance.
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Makes sense - not sure how much more design margin is remaining after the Stage I mod (which will have to be snuck in once the new car is safely parked in the garage - in spouse negotiations right now!)
I think a lot of the problems with the old NA tranny weren't from the design of the tranny, but the fact they have a LOT of miles on them. It's also common for the front cover to leak gear oil VERY slowly, and you don't realize you're driving with little or no gear oil!
Damn, $1700 for a replacement! I got one off Ebay a year or so ago for $30 .
Anyhow, I've known guys that installed old Cartech turbo kits and the like on their NA's back in the '80s and never had a lick of trouble. I think the issue here isn't power holding capability, it's longevity - the NA tranny doesn't have the longevity of, say, the Type R gearbox from the TurboII and FD. That sucker's strong.
Putting a good quality gear oil in the gearbox (Redline, Royal Purple) will also help reduce wear and insure the synchros get the right coefficient of friction to engage and disengage properly. Don't try to rush shifts, and don't speed shift or crap like that and you'll get many years of life out of the tranny.
RX-7 Bad Ass
'94 RX-7 R2 Brilliant Black - building the perfect FD
If you look in the racing forum, you'll see that Judge Ito has repeatedly blown up his transmission... and that's with a stock RX-8. The Judge has said he's going to be looking into strengthening the tranny, but I have not heard anything on his progress.
Others have said that the tranny in the RX-8 is the same unit as the one used in the Miata, and history with mod'ed Miatas has shown that this transmission is only good for around 300 WHP. If this is indeed the case, then us RX-8 owners who are modding our cars will soon have a problem, and the tranny will be a limiting factor to how much one can do.
Miata owners who have exceeded 300 HP have apparently had to go to an older, stronger, 5 speed unit... something I personally would hate to have to do. I think a better solution would be to rebuild the RX-8's tranny with stronger internals, getting away form the soft metal that it's apparnetly built with now. Perhaps some manufacturer will come up with a rebuild kit for this tranny, or perhaps even the manufacturer already has one. I have no idea of how to run this down, or even who makes the tranny for the RX-8... but I think this would be the next logical step.
The thing that many people don't realize is that it is abuse not power that will kill a transmission. You could bolt up a 500 hp Viper engine to the RX-8 tranny and not have it break as long as you don't abuse it. Obviously if you stomped on the gas it would probably fly apart but the point is made that if you are good to it you'll be fine. Judge Ito has always been hard on cars. If you go around drag racing your car and are really hard on it during the launches of course you'll run into trouble sometime.
Here's something to consider. A transmission endures more torque in first gear from the stock engine than many turbo kits produce in 2nd gear. Your gears are torque multipliers and first gear gets the most abuse and stress. A 3rd gear with a gear ratio of 1.4 will develop 40% more torque at the wheels than a 4th gear ratio of 1.0. Just an example don't chastise me! That extra torque is also getting felt on the gear sets. When alot of people have a problem with one gear, they usually lose 2 or 3 others as well. There are only a couple of shift rods in the tranny. One rod may hold the synochros for 1st, 3rd, and 5th gears where the other may hold 2nd, 4th, and 6th. Alot of strain on 1st gear isn't helping out 3rd or 5th either in terms of synchros. The key is to just not dump the clutch from 8000 rpm. This applies to the stock engine as well. If you abuse it, you'll eventually kill it regardless of engine power. If you add a mild forced induction kit or ecu retuning, you shouldn't have any more issues than you do now. Just get out of 1st gear OK and you should be fine. If you are already in gear and just want to step on the gas and accelerate harder, there shouldn't be any issues. If you notice who has the most trouble with transmissions it is drag racers. Those few seconds are harder on your car than almost any other form of racing. Anytime I have ever had issues with my transmission or clutch it has been drag race related. I have never had an issue with auto cross or road racing in terms of drivetrain.
I agree about the abuse factor - ask for more out of the drivetrain, and at a certain point something will fail.
In the aviation world, there is usually at least a 1.5x design margin built in. If an airframe is rated for 6.5 g in normal flight regime, around 10g it will have reached its design limit and a major structural part is going to give way. Maybe not the first time, but it won't take more than a few events to surface.
Similarly, the 1.8T powerplant in the VW Jetta, Audi A4 can safely handle around 265 Hp before the stock engine set-up will start having issues (not even addressing longevity) - blown seals, etc.
Looking at www.supercharger.com website, particularly in their primer section and most manufacturers designing kits are expecting that the supercharger will be the only mod accomplished. Throw on larger diameter wheels with larger contact patches, dual exhaust, and headers and the stock tranny is within 6 months of total failure.
When you upgrade your drivetrain, there is a reasonable expectation that you are going to want to take advantage of it - your power application becomes more aggressive and you want to move through the shifts quicker, as opposed to being more deliberate / methodical.
Alternatively, you utilize the same technique for power delivery as when the car was stock, however with more power available, there is greater stress on the drivetrain.
Flyin' Miata's FMII turbo kits can easily double the RWHP - they highly recommend upgrading the clutch and flywheel, but have not reported any issues with the tranny itself - then again, that could be because I was only looking for ~ 225 RWHP at most.
I like the idea of having the absolute best lubricant in the box, however, my A8L's tranny uses an exclusive for that tranny lubricant and I wonder how sensitive the RX8's might be to Royal Purple or Redline. (Incidently, my A8L's tranny and the S8 both max out at 350 Ft-lbs of torque - they are made by ZF, which are installed in a number of platforms, including Mercedes and Jag.
That would be interesting indeed if Mazda used the 6 speed out of the Miata for the 8.
SOrry for the rambling - I've been very impressed by this forum and the depth of knowledge - gives me confidence in enhancing the 8's performance!
There are several factors that determine the strain applied to the transmission. They include:
1. Vehicle weight - the Miata is lighter than the RX-8 so there is less torque reaction applied to the transmission.
2. Tire traction - sticky tires will apply more torque through the transmission - if the tires don't stick well they might just spin before you get the torque high enough to break something.
3. Wheel hop is death for the transmission and diff. - reduce it with smaller wheels and higher sidewall tires if you're into burnouts/drag racing.
4. Torque curve shape - if there is a sudden jump in torque and you use that area to drop the clutch you could exceed the ability of the hardware to sustain rotation . We are fortunate this is not a problem with the rotary renesis engine.
5. The torque seen in the drivetrain is not constant - a "bounce" can multiply it by at least 2X (wheel hop for example).
6. The driver can influence the transmission longevity more than you may think - smooth operation is the byword.
9,000 Mi and on my 2nd tranny. I was coming to a stop and shifted into 1st at approx 10 mph. I felt a pop from under the car. I took off slowly and drove home listening to the growling noise that had now developed get louder over the next 2 miles and as I was pulling into the garage I could hear the tranny chattering. Needless to say they replaced my entire tranny assy.
Now I love my car OK, but let's face it, the RX-8 is assembled out of low-buck parts that seem to be borrowed throughout the Ford Motor Co. parts bin. I don't abuse my car or drag race it, I have another car for that. But if this car is going to continuosly break down and have piddly problems MAZDA USA will lose potential customers.
I have had the rear diff replaced as well at 4500 miles for a constant whine noise at 46 mph. 2 RH window regulators, the silver paint on the shift **** wore off at 2500 miles during a 1200 mile trip (all hwy) to Atlanta, and I also had an oil leak from a seal at 3700 miles.
The quality or durability of the parts needs to be stepped up a couple of notches, I knew that it had to be too good to be true, a car of this supposed quality at such a low price.
Just my $.02
Keep It Clean '04 Ti Gray GT, Auto Exe, Mint Blue, Tein, Borla, BFG's, 19's, Your Mom, Tint, K&N V2, RMagic, Clear Markers
I know this thread is dead, but I have a question about the 6-speed transmission. How do you strengthen the transmission? I don't think anyone answered how much power, or more importantly IMO, torque that the transmission can handle. I am starting to wonder the same thing. How much torque can the 6-speed handle? I wonder if anyone has found out that information by now.
I have yet to experience tranny trouble. If someone could send photos of damaged trannys I can tell you where to modify the stock parts for greater strength. I have an idea for strengthening the case, itself, but I have yet to see the inside of this thing. If anyone is interested in making a "crashbox" all they need to do is grind off every other synchro tooth and they will engage more quickly. Not recommended for anything other than serious drag racing and the RX isn't built for that, anyway.