How long is the true trac going to hold up on your car before it needs a rebuild? All those american style non-salisbury diffs dont go very long between rebuilds in the turning world (we're talking OEM last of the stick axle mustang ones (spicer trac lok) lasting less than a season of driving around in parking lots).

 

Last oem truetrac or wavetrack that I've seen fail on a 15 mustang took about 150 hard track launches on a 4000lb 700whp car before giving up the ghost

 

Breaking or just not LSDing good enough anymore?

 

that latter part

basically became an open diff

 

This is the diff I plan to buy, it's an Eaton True-Trac: https://www.summitracing.com/parts/dtl-913a561

It's an Eaton parallel-axis planetary helix gear LSD. No clutch packs to wear out like an OEM Trac-Lok. Also apparently the OEM Trac-loc's can break if launched hard enough as they are cast and not that strong. Supposedly these never wear out, and failure is practically unheard of.

I'm hoping you guys are thinking of the Trac-loc wearing out, not a true-track right?

 

The diff I bought has 3.55 gears. Some rough math says I can do 140mph @ 8,000 with those gears. About 150mph would take 8,500-8,600 RPM, with > 150mph requiring a parachute. I never plan to go that fast since it's a street car, I'm not doing parachute/cage/etc.

So I figure there is no reason to go to anything numerically lower than a 3.55, like a 3.27 or such. Am I missing anything? A 3.55 is good for a 13.4% reduction in RPM so that will be nice. It was acceptable with the 4.10s so I think 3.55's will be great on the highway.

I think going with a 3.27, or 3.03, etc, will just make the car slower for no real gain. What do you guys think? Or I could go to a 3.73 instead. I think that would be faster, but less highway benefit. What gears should I run?

 

I think I recall the drag racing answer to be whatever puts you just above your peak horsepower coming through the quarter at your given miles per hour. Or maybe it was just short of your chosen rev limit at your mile per hour in the quarter. Something like that. But since you don't know what your power and therefore miles per hour are going to end up being you would definitely want some head room.

It sounds like the 3.55 gears already put you in a nice place for streetability. The 3.73 may be faster. Life is a series of trade-offs.

 

how many hundreds of posts/pages before another picture is posted?

 

I have discussed this many times. The quickest drag cars are set up to be pretty much maxed out at the finish line.

At 103 MPH in the 1/8th mile, you can expect mid to high 120's in the 1/4. Turbo cars tend to have faster MPH up top, but not by much. The fastest you can go in the 1/8th without doing a full cage is about 108 MPH. That will put you in the low 10's to high 9's in the 1/4. MPH would be in the mid 130's at most, in which you would be shut down by track officials promptly.

These figures are assuming you start hooking really hard. Most well sorted cars running 6's in the 1/8th are in the low 1.5 to high 1.4 60' times. If you choose the wrong gear, tire, and convertor combo, you will never get there.

If it were my car, there would be now way I would run anything taller then a 3.73. If I got to the point of running out of gear, I would run a taller tire.

Here is another thing you need to keep in mind. Cars that have the perfect combo to make the car quickest, will not be gobs and gobs faster MPH than the same car with the wrong gear and tire combo. The MPH you have now will be in the ballpark of the MPH you get when everything's right.

 

Yeah the one I was talking about was what Leafy mentioned, which is clutch-type

 

Thanks. I'll probably run the 3.55 for now since I have it, and go from there.

It is kind of sad, I haven't posted many pics in months. I have pics though. I'll try and upload a bunch in a day or so. It's been December since I posted build pics....

Thanks for the info, it's appreciated!


Cool. I went ahead and ordered the true-trac.

 

Yeah I forget which ones is the gear and clutch type. Is the gear type actually good for drag racing?

 

From what I read they are about bomb proof. The only thing stronger is a locker or spool, it's stronger than every other LSD for the 8.8.

 

I didnt mean for strength, I meant for performance.

 

Motive Gear FS8.8-31: FULL SP00L FORD 8.8 31 SPL | JEGS

 

It's basically a torsen, which I've been using for years. They work ok to me. I think a locker or spool would be better for sure, but I drive this car every day and ain't about to put either of those in my miata.

 

Once the new rear end is in the car and I've raced with the 3.55's, I plan to swap the front A/R back from a .82 to a .63 to see what affect that has. The converter is too tight and the turbo slow to spool because of it, so the smaller a/r should help it get a better launch from spooling the turbo quicker. I don't think the drop in power will be much, bet the car goes faster in the 1/8th just from having boost sooner.

 

better launch??? lol. seemed to have a great launch as is. It sounded like you had more boost than you needed to launch it as is?

 

Impossible.

 

no such thing as too much boost
needs moar boost

Pat what's the most amount of boost you ran on this contraption? it's gotta be well over 35 right?