School me on gear ratios

[super9064]

I have a 71 911 with the original 901 trans with a 3.2 motor, I am just wondering what the difference in character between the two are. How would it be different with a 915 or a G50.

[RoninLB]

until someone w/more smarts come along

915 & G-50 has different gearing

unless i was only on interstates i'd go w/915

a G-50 is a more complicated [ie:expensive] install also



ps: it should be a good strong 915

[Steve@Rennsport]

I'd be thinking about installing a 915: even if a 901 transmission gets all the needed modifications, that 7:31 will not go the distance with a 3.2 driving it.

There are many choices in 915's that vary by gearing, strength & shift quality. I'd be looking for a '77 & later one.

G-50 conversions are VERY expensive to do and hardly worth the expense (unless you are racing and its legal for the class). A properly set up 915 is a superb transmission to operate.

[Flieger]

G-50 will require extensive modifications and is heavy and unecesssary for your car. I think it has longer gears, so less acceleration.

915 might also require some hammer blows to get the thing to fit in your car.

The best gear ratios would be the earlier A-F-M-S-X from a 902/01 transaxle. They are shorter.

Your 911 transaxle was a two year only part and so anything else requires modification. Using an earlier 901 transaxle means a new clutch.

[RWebb]

your 1st gear is way too low - I felt like I could pull stumps with a similar setup.

the sad thing about a 915 is that you will lose not only the nice light shift feel, but also lose the wonderful road racing shift pattern

maybe Steve will say it ain't so & he was just kidding...

[RoninLB]

i paid big bucks on a 915 pro rebuild meant for a 3.4/250hp

it's the best connection between driver and car possible imho

w = wevo

[trekkor]

The best gear ratios would be the earlier A-F-M-S-X

Hey that's what I run!!

Well used to. I have a 'C' first gear now.
Good to 45mph!


KT

[Flieger]

Me too.

I would like to try a car with AEJQX, which is supposed to be a real hillclimb box with the X for a highway cruising gear.

[twin plugged targa]

My '71 twin plugged targa had a 2.7 engine putting out more HP than a stock 3.2 by a long shot.
The 901 should be fine without modification for road use.
Just baby it in 1st as I am informed it is unsupported and can disintegrate if pushed hard.
Having said that my 901 tranny gave me the fastest accelerating car I have owned and my favourite 'driving car' - blast, wish I had not sold her!!!!

[PCA7GGR]

Here's what a 3.2 (or larger displacement engine) can do to a 901's 7:31 pinion gear. It's not the horsepower, it's the torque that does it.

[YTNUKLR]

Well, between say a 911/01 '70-71 box and a '72-74 915/02 (or 06) you will not notice much difference in gearing, just the difference in 901 vs. 915 shifting, pattern, etc.

For a 3.2, I would recommend alu. case, wevo one-piece bearing plate, 8:31 R/P, and then 3rd-5th gears to suit ... depends on how you want to drive, but you probably don't want stock 915 ratios from an 8:31 box (probably want shorter ratios).

[super9064]

So what I'm hearing is that its impractical to put in the G50, and that the 915 will have the same driving characteristics as the 901, but without the shift pattern that I prefer.

The 901 is not as durable as the 915, even with the up grades I have done, The issue for me is that I already had the trans rebuilt before the transplant.

I do have a project car that I could use the 901 in, if I decided to get a 915.

So, the recommendation is a 915 rebuilt with A-F-M-S-X gears. What other things are involved,besides the shift lever, new fly wheel? clutch? axles? do I need to do the same upgrades as the 901?

[Matt Monson]

Super,
How are you going to use this car? Is it a street rod? A track toy? A combination of the two? Give us a bit more details about your application and I'll add my two cents specifically tailored to you...

[super9064]

Originally Posted by Matt Monson

Super,
How are you going to use this car? Is it a street rod? A track toy? A combination of the two? Give us a bit more details about your application and I'll add my two cents specifically tailored to you...

Its strictly a street rod, I use it for fun runs, rallys, its just to nice to track. I use it in a lot of heavy traffic, stop and go daily driving, so it needs to be very drivable.

I do drive rather "Sportingly" when ever conditions allow

[Matt Monson]

I was hoping you were gonna answer like that. You say that it's fresh and in good condition. Leave it as it is and have some fun with the car. If you were racing it, that 3.2 would be too much for it. For a weekend toy to drive through the country you're not gonna blow up that gearbox if you drive it like an adult, as I like to say. Don't drag launch and don't nail the downshifts too hard and you'll do just fine.

For the engine you are running, 1st, 2nd and 3rd on your current gearbox are probably pretty well geared. If you did want to crack it open and toss in an S 4th and Z 5th, it would be a bit more fun in the 90-125mph range. But the stock ZA 5th is a great highway cruising gear if you spend any time on the interstate. If it were my personal car, I would probably call it good enough and just drive it as it stands until it's time for another refresh on the box.

[super9064]

That's good to hear, thanks.

I've heard so much about how weak the 901 is, but figured driven sensibly it should last quite a while. I've also seen 914s with V8 and 3.6 conversions making huge HP, using a 901 ( the only option), and figured it couldn't be that much of a hand grenade.

Its not that I don't belive the 915 is stronger, I'm sure it is. I just don't want to have to worry about it when I'm gettin on it a bit.

[Matt Monson]

Well, just think about it this way... Porsche used the 911 mag case box like you have in the first year of the 911st. That was between a 2.2l and 2.5l twin plug race engine that was making 250+hp. Yes, it wasn't the torquiest of engines, but it was still a pretty potent engine being thrown at that gearbox. That was the outer limit of what the factory considered ok for that gearbox in a racing application, and shortly thereafter they introduced the 915.

You're at the limit of the gearbox really, but there's bunches of guys out there putting 350 Chevy V8's against that gearbox in the 914 chassis. Some of the blow up, but the ones that are driven sanely hold together for a reasonable amount of time. You've got the torque to rip 1st gear right off the shaft on a hard launch, but if you keep your spirited driving to 2nd through 4th, you'll do just fine.

[Bill Verburg]

I'm not at all sure what the ratios on a '71 901 might have been, for some reason my records skip that year, but here are some of the suggestions that have been thrown out
901AEJQX, this would make a good race only box


901AFMSX, I don't like the short drop into 5 here


901AFMSZ, I don't like the long drop into 5 here but some cruisers might


901AFNVZD, I like this best all around



and here are most 915s

[Matt Monson]

Bill,
His box is currently A, Ga, O, V, Za if you want to chart it.

[Bill Verburg]

Thanks Matt, is that a T? E? S? US? RoW?

anyway here it is, 901 AGaOVZa, as usual for stock Porsche transmissions, pretty nice all around box