Factory LSD installation into 915 - stub axle issue

[Matt Monson]

Sorry if I misunderstood you since both those threads have examples of your peers' failures with the products you are considering.

It is my policy to never attack the other brand (and you can see me "GTgears" in that RL thread only defend my brand when attacked. I was concerned you thought I was bagging on the others.

My opinion? Giken and Kaaz make decent recreational LSDs. Each person has to make their own cost benefit and risk analysis. I never want to come off like I am imposing my personal bias on your decision. I'm the Norelco guy. Paul Guard made GT what it is. I'm the guy who liked the product so much that he bought the company.

[jager911]

I'm just a dumb schmuck who's lucky enough to own a couple of these fine autos.
I'm overly analytical by profession and by nature. I tend to research everything to death, taking into account opinions/info from my peers and pros in the trade. I'm not running the Daytona 24 Hrs but at the same time, I do want whatever LSD I choose to be the best value for my weekend track use, without breaking the bank. To that end, I'm open and willing to listen to any and all opinions. This started out with me considering a factory ZF and not sure which product I'll end up with yet so please keep the info coming.

[Porsche84]

I'm late to this thread but I'll offer a Guard TBD coarse spline which came out of a 72 915. It was in the trans for a couple years and only a 3.0 DE car, never raced. I replaced it with a Guard LSD. There is a difference with TBD diff under braking but I could not see a difference under acceleration. The TBD is a good unit for street/DE, less heat generated, less tendency to "understeer". I am offering it for $900 shipped within US. I'll try to post photos later. Here is a link to the website Torque Biasing Differentials | Guard Transmission

[jager911]

Jerry,
That's a good price, I will consider your TBD but really prefer a LSD, due to stability under decel and braking.
Sending PM.

[Porsche84]

I thought I'd post a photo of a ZF LSD that I removed from a 3.0 SC race car yesterday. For those of you considering a "factory" 915 LSD, remember they are between 25 and 50 years old. Yes, the clutches can be replaced but other parts fatigue.



I've installed OS Giken LSDs and thought they were very noisy and will not use them again. I've installed a couple Wavetracs and they were ok but the ramps are very small and fragile and I had one break. I've installed and used Quaife TBD and they were great for street and semi aggressive DE. If you are a very aggressive track driver you'll want a clutch type LSD. Guard is the very best IMHO.

Here are some photos of the Guard TBD that I have for sale.

[jager911]

Jerry, I sent you email on the Guard TBD, I am considering it if it's a bolt-in to my '74 and then I don't need to worry about swapping stubs and axles to fit a fine spline LSD.

[Matt Monson]

Just be aware that different people mean different things when they say "bolt in". Is Jerry's TBD the correct spec for your car? Yes.

I question any differential manufacturer who calls their part "bolt in". Every differential installation should have backlash reset and bearing preload checked. My parts are CNC billet at the same every time. 40 years ago Porsche's were not AND they have 40 years of wear on them. Always measure your installation.

[jager911]

Matt, by bolt-in, I meant it's the correct coarse spline for my '74 915 application. I still plan to have the backlash measured and shimmed by Gary Fairbanks. I'm not 100% there but this is an attractive compromise than changing stub axles, machining to fit etc.

[Matt Monson]

I run a TBD in my 914. Different chassis dynamics than the 911, so it doesn't need as much lock on braking as a 911, when we are talking racecars.

But my car is 80+ back country roads and just blasting around town. For that a TBD cannot be beat. And it doesn't have enough power to break 100mph on the fastest straight at our local track, so it is kind of outside tailwag speeds. Usually need to be going faster than that with hard braking to get a 911 to feel loose.

But feel and be are two different things. Lots around here know Chad Plavan (SN Plavan). His ST relica runs a GT TBD. There's a video down in the racing forum of Pat Long wheeling that car. And lots of footage of him racing it.

It becomes a question of Nth degree. Where does that lie for you? Plavan won a lot. He was the fastest guy in his class frequently. Could he have been faster with an LSD? Yes. But he was fast enough without.

We've tested our lsd back to back with some of the others mentioned here. We know just how much time one can gain with our LSD. But just because Guy Cosmo, who is a very repeatable expert gained x.yy seconds at Daytona doesn't mean I could. Racecraft is part of all this. If I'm not a good enough driver or don't track often enough is the price difference worth it to me? Wasn't on my 914. You will need to decide on your own car.

[jager911]

Originally Posted by Matt Monson

Just be aware that different people mean different things when they say "bolt in". Is Jerry's TBD the correct spec for your car? Yes.

I notice Jerry's TBD is marked "GT 915". From the Guard website I see model 915 is for coarse spline '75-'77 and model 914 is for 915 early including 1974:
915 915 (mid 1975-1977 coarse spline)
914 914/901 late (mag case)/915 early (1972-1974)

So in reviewing the "915" marking, is it the correct one for my '74 in case I decide to go with the TBD?

[jager911]

Originally Posted by Matt Monson

I run a TBD in my 914. Different chassis dynamics than the 911, so it doesn't need as much lock on braking as a 911, when we are talking racecars.

But my car is 80+ back country roads and just blasting around town. For that a TBD cannot be beat. And it doesn't have enough power to break 100mph on the fastest straight at our local track, so it is kind of outside tailwag speeds. Usually need to be going faster than that with hard braking to get a 911 to feel loose.
.

I'm seeing the tailwag when terminal velocity is in the 140 range, then hard braking to 40-60mph for a turn. To counter the wagging, I start to lightly brake earlier than I should. With a LSD, I'm guessing I'd have the confidence to brake later and carry more speed through the turn, and also get back on the gas earlier in the 180 degree turns.

[Matt Monson]

Jäger,
My bad. I thought Jerry's was a 914 model. Difference between the two is ring gear offset. 915 is for an 8:31 r/p and course spline. 914 is for 7:31 r/p and course spline. If you still run a 7:31 you want a 914 model. Too much brain damage to set up the later diff to run right with the early r/p version.

If you frequently track your car and hit 140 get an LSD. Lots of time and extra stability/safety to be gained in a car/track that fast.

[jager911]

Originally Posted by Matt Monson

Jäger,
My bad. I thought Jerry's was a 914 model. Difference between the two is ring gear offset. 915 is for an 8:31 r/p and course spline. 914 is for 7:31 r/p and course spline. If you still run a 7:31 you want a 914 model. Too much brain damage to set up the later diff to run right with the early r/p version.

If you frequently track your car and hit 140 get an LSD. Lots of time and extra stability/safety to be gained in a car/track that fast.

That's what I thought, so that option is out as I still have the 7:31. Yes, my car runs the big tracks in the NE, it will see 140 at Pocono when we run the bowl and maybe 130-135 at Watkins Glen. It's a tuned 3.2L in a 2400lb car so it gets there easily.
Here's the patient.

[KTL]

I think what you'll also find when you first start using the new LSD, you'll "stop short" for corners. I found myself doing that when I went from my '87 911 to '79 911. Even though the '87 was street, '79 is race, they have the same weight, same power.

I was amused the first few sessions with the straight 80% LSD. I would keep braking WAY early for corners and either coast or have to get back on the gas to reach my turn in points.

[jager911]

Originally Posted by KTL

I think what you'll also find when you first start using the new LSD, you'll "stop short" for corners. I found myself doing that when I went from my '87 911 to '79 911. Even though the '87 was street, '79 is race, they have the same weight, same power.

I was amused the first few sessions with the straight 80% LSD. I would keep braking WAY early for corners and either coast or have to get back on the gas to reach my turn in points.

Kevin,
So if I understand you correctly, your brain was still programmed to brake early/overbrake until you built up confidence to trust the LSD and brake later?
Another issue is I have some problem 180 degree sweepers where I get on the gas at the apex, then the car slides a bit, I lift a bit, wait for grip, then get on the gas, it slides again, I back off iteratively until I get to track out and get straight. Does the LSD help where you just put the power down at apex and hook up and go? I realize there are other factors but generalizing to help me make the decision to just bite the bullet and spend the bucks for a proper LSD so I don't have to do this twice.

[KTL]

Yep that's a good way to describe what I was doing. I was so accustomed to my braking points with the non-LSD car that I was finding myself approaching the corners in the same fashion as w/out the LSD. Didn't take but a few sessions to realize the increased stability of the car .

I won't go so far as to say it was a huge amount of braking distance change. But it was enough to notice I was stopping short w/out a doubt. Another point worth considering is the added stability under braking allows you to trailbrake into a corner w/out having to catch the car with a large amount of countersteering

We have a long sweeper carousel at one of our local tracks, Blackhawk Farms, that sounds like what you're describing. W/out the LSD i'd find myself shuffling the wheel & balancing the throttle, catching the car as I rounded the turn. W/the LSD I am much more steady with my steering and basically throttle-steer the car. Reason for the change in behavior, I think, is because of the understeer the LSD induces on throttle. Corner exit with the LSD is also much improved since I can put the power down better

Non-LSD car BHF 7-20-07 - YouTube

LSD car PCA BHF 8-12-11 Group 1 Session 2.wmv - YouTube

You can see in the carousel my hands are a bit more steady in the second video and the rpms rise more consistently. Whereas in the first video, my hands are a bit more busy and i'm not able to apply the power very well.

Not a direct back to back comparison since the videos were taken at different times and different cars are used. But set up and driven by the same dufus- me