Has anyone taken the fifth gear out of an early NA and put it in a turbo gearbox? I know it's been done but search is too stupid to work unless my keywords are red and huge.

How difficult is this process? What all needs to be swapped out? I want the taller 5th in the turbo to make it a nice cruising/economy/top speed gear after my project is completed.

 

Go to porschehybrids.com. 'PICTURE GALLERY" page 3" Marc's 86 951 ls1" It has pictures and all. A word of advice. Clean the bolt of oil and grease for lower gear. And the pinion shaft! spec of 51ft/lbs will snap bolt." lubed" bolt torque is much lower. Porsche book says bolt is" single use."

 

the 951's 5th isnt tall enough for you?

 

Why would the NA trans have a higher 5th, but a lower R&P ratio than the turbo trans.? I thought the turbo was higher geared in all the gears...

EDIT---------

Learn something new everyday.... Why would Porsche do this? Seems counter productive.....


'82 - '87 NA Turbo

1st - 3.600 ---- 3.500
2nd - 2.125 ---- 2.059
3rd - 1.458 ---- 1.400
4th - 1.071 ---- 1.034
5th - 0.730 ---- 0.829
--------------------------------------
Final - 3.889 ---- 3.375

Thank you Spencer .... I shall plan on this swap as well

 

I'd like to be able to cruise at highway speeds with the RPM much lower so that the exhaust doesn't drone (as nice as it sounds, after about 100 miles at 80MPH/3000RPM it gets annoying). While I don't have the trans calculator here I figure that with the NA 5th + 951 Axle it should be somewhere about 2000RPM. I don't use 5th often anyways, unless I'm on the highway, and there's enough wind noise and road bump noise as it is, I don't need the exhaust to make it worse.

Besides with that gear top speed if I ever got enough HP should be phenomenal. Gas mileage will be AWESOME lol

 

Turbo has higher final drive which makes all gears higher. However, it has the short 5th gear. When the 5th gear is replaced with the n/a tall 5th, you get 4 taller gears and 1 super tall gear. It works well as an overdrive since no one really use their 5th for hard acceleration even for top speed runs as the engine runs out of puff and will never hit redline in 5th unless your car is highly modified. I like the idea of taller 5th in the turbo box. I already have a turbo box with the 5th removed. I am just too lazy to install the tall 5th because I have to go find the rear plug seal I bought 2 years ago (no idea where its at) and the rear housing gasket from the dealer.

 

Fantastic, I have a short 5th that I want to install in my S2 tranny!

 

Exactly. As much as I like the NA box's ratios for acceleration they certainly hamper it when things calm down and I just want to cruise. Even the little 4-banger could get some better mileage I suspect if it could turn slower at speed.

I expect that even with taller gears I should be making enough power to compensate and then some for accelerating ( )but then on the highway it can just coast and take it easy and be quiet

 

It's popular for the LS1 swap cars. They have lots of low-rpm torque, and can push just fine in that long fifth.

 

I would think this would just be too much of a jump down in the revs when shifting from 4th to 5th. I still want 5th to be an accelerating gear. Guess it would be nice on the Hwy.

 

That's all I use 5th gear for really, cruising...even with the high drive ratio on the NA it still takes a long time to get anywhere in 5th. If I want to pass someone on the highway I currently drop to 4th or 3rd depending on my speed and it works great. However, considering that with the turbo's gearing and my tire/wheel package that I can do over 100 in 3rd and about 140 in 4th, I'm not too concerned about that anyways. I'll let the engine more than make up for the drop in acceleration gearing.

 

With 400+ ft.lbs. of torque going, I don't think accelerating in 5th will be a problem..... and any RPM.

 

Legoland951, do you still own the shorter 5th gear set? Interested in a swap???

 

I agree with Patrick. The MPG between the stock 951 5th and the taller N/A 5th isn't much. The 4th to 5th split is wider and lacks the insane pull of the stock .899 gear.

The .778 5th out of the S2 would be a good compromise. Slightly taller than stock, but a closer split than taller N/A.

Marc's how-to is great, as usual

 

Well of course it's a personal choice and what you want from your car. I think it depends on driving styles, conditions, and track characteristics (if applicable). I have almost the opposite setup with the stock 951 gears but the S2 final drive. It is much shorter than normal and I like it a lot, but it's a bit of a pain on the Hwy. Not unbearable, but I'd love an extra cog to shift up to. I will keep this setup initially when my new motor goes in, but I see myself going back to stock final drive with 500+ hp/tq on tap.
I'd like to see what the proposed result of Spencer's swap out will be like all the same. Horses for courses.