3.6 Conversions, are any changes to the transmission recommended?

[the_win]

A lot of people seem to be doing 3.6 conversion. I was wondering, those using 915 transmissions, do the gear ratios also have to be changed for the increase in hp?

Or is the 915 the kind of transmission that the standard gear ratio, can work well with the increase of hp?

If the ratio is to be changed, what is the best setup?

[ted]

anyone street/track a 3.6 with a 915/7.31 and close 3,4 and 5th.
The PO could not recall the gear stack ratios?
My car is 2200lb with me in it and seems nicely geared for the stock 3.2 in it right now.
Will the 3.6 be too much for a 7.31?
I'm considering an 8.31 to see if the final drive ratios would then be better suited to the torque of the 3.6, suggestions?

[Jack Olsen]

Different 915's have different gearing. Up until 1974, they were magnesium cased with a 7:31 ring and pinion, which had significantly lower gearing than the 8:31 boxes, which were still magnesium, up through 1977 (I think). The 8:31 gearing isn't as much fun, but it's a significantly stronger transmission. The aluminum versions that came later are even more durable.

With all of these, you have to balance how much fun you want to have (lower gearing) with how often you want to replace your transmission.

Some people prefer the taller gearing. This also allows higher top speeds, if that's what you enjoy.

Another option is to replace individual gearsets. For track use, especially, shorter gaps between the gears allows you to keep your motor right at the peak of its torque curve. This type of tranny is often called 'active' or 'close-ratio.'

[pete911]

I track a 73 911 with a 3.6. I am using an 8:31 915 tranny with standard gearing. Works great for the tracks I run. (Watkins Glen, Limerock, Summit Point). I've been told the 7:31 trannies don't hold up as well to the torque of a 3.6. The pinion gear with only 7 teeth is not as strong and will eventually give up.

Pete

[Bill Verburg]

The 7:31 r/p will have a very short life. The 8:31 will be reliable, aluminum cases are stronger, 84-86 have the tallest gearing, coolers are very desireable for track use, steel bearing retainer plates and side plates are desireable for racing,

Consider tires and usage when thinking of gearing, tall rear tires like 275/40 @ 25.7" will eat into torque at lower speeds, though they are desireable for track use. Probably shorter 3,4,5 should be used for these tires at the track.

For street or dual purpose taller gearing can be great. I have the tall 275/40x17 and the tall 84-86 gearing and absolutely love it. Every now and then I will drive around in 4^th (>.965:1 OD) and then am thankful that I have an even taller 5^th. For my old 3.6 I felt a tad overgeared and was looking for tires a bit shorter. For the more powerfull 3.8 gearing is just about perfect!

Here is a snippet of a ss that shows the effect of overall tire height on gearing for a 915/67 trans

[wevoid]

The loads and force axis from the 915 7:31 CWP are significantly different to the 8:31 CWP. The 7:31 with a high torque motor will fail, normally due to the mesh pattern becoming poor as the side cover flexes and the pinion bulkhead distorts from the gear separation force.

In the past 12 months I know of seven 7:31 CWP failures in race cars and only two 8:31 in the same period. All these cars have less torque than a 3.6, but more tire than a street car.

If you have a good 7:31 for a 915, sell it to the guys running small motors that it is suitable for. Take the money buy yourself one of the more bountiful and less expensive 8:31's and a new 4th and 5th gear to match the motor, the car and the intended use.

In my opinion, it's a good idea to crack check any used CWP assemblies you buy, we reject about 1 in 10 for defects that might or might not have been catastrophic if built up in a transmission.

Regards

Hayden

[the_win]

Thank you everyone for the information.

Geff

[ChrisBennet]

Originally posted by wevoid
The loads and force axis from the 915 7:31 CWP are significantly different to the 8:31 CWP. The 7:31 with a high torque motor will fail, normally due to the mesh pattern becoming poor as the side cover flexes and the pinion bulkhead distorts from the gear separation force.

In the past 12 months I know of seven 7:31 CWP failures in race cars and only two 8:31 in the same period. All these cars have less torque than a 3.6, but more tire than a street car.

Hayden,
I'm used to being embarassed so I'll ask; what does CWP stand for? C... W.. Pinion ?
-Chris

[HarryD]

Chris,

I can't answer your question but I have a dumber one.

What the deal with the final ratio? Why is a 7.31 "more fun" than a 8:31?

[Jack Olsen]

Over the model's lifetime, gearing was raised to help the cars meet sound and mileage restrictions, and also to increase top speed (which -- I'm told -- Europeans focus on the way we love 0-60 times). Shorter gearing allows you to distribute your car's power over a narrower range. Close-ratio gearing allows you to keep you revs right in the sweet spot when you're upshifting.

I remember reading that changing to a 7:31 r&p from an 8:31 r&p was like giving a 17% boost in effective torque. Now, it's not literally increasing power, but it's allowing you to maintain higher revs at lower speeds.

As I said, some people prefer it the other way. The characteristics of Turbo motors are better suited to fewer, taller gears.

[Bill Verburg]

I'll take a wild guess and say cwp - crown wheel & pinion?

I took the liberty of adding Jack's gearing to my ss because at one time I was considering doing the same. I think that this is great gearing for tracking a light 3.6. I'm sure that Jack can also comment. Notice his 5^th is ~my 4^th, that's the difference between a track setup and a street setup.


As I said IMO the engine is too revvy in typical freeway use but just what you want at the track with this gearing.

Some good news is that Michelin now has a 265/35x17 and possibly a 275/35(anyone near Bruce Canepas shop?)

The 265/35 is only 24.3" tall and 10.7" wide when mounted on a 9.5" wheel. This would be a great tire fora stock gearbox w/ 3.6

I'd also like to thank Hayden for commenting here, it is great to get feedback from the pros!

[wevoid]

Gents,

Sorry for the mindless abbreviation, Yes, CWP is Crown Wheel & Pinion.

"I remember reading that changing from a 7:31 r&p to an 8:31 r&p was like giving a 17% boost in effective torque."

To clarify, I think that Jack means - changing to the 7:31 from an 8:31, in which case you will be turning the crankshaft faster for any same road speed.

It is also ironic that the factory stuck with 11:35 for first gear - for all years of the 915. For sure with the early small engine cars and 7:31 CWP it would have been useful for climbing hills from standstill. As the motors increased in capacity and torque (and torque at lower rpm), 7:31 CWP and 11:35 first gear is really too low for most times, so the introduction of the 8:31 helped in this respect.

Also, the general "tradition" of switching to a 7:31 probably seemed like a good idea at the time when the 8:31 was introduced and the stock motors had a weakling 170 hp. I am sure a mid 70's 2.7 CIS motor would have really benefitted by a conversion back to the recently superceded 7:31.

Now enthusiasts like Jack and others are using motors with stock output of +100 hp and +70 ftlbs of torque over the 2.7's and these motors are making more torque at much lower crankshaft speeds. At some point the 7:31 CWP became a liability, for both durability and useful 1st gear ratio.

For all it's shortcomings in basic architecture and design philosophy, the 915 has and will continue to serve a large chunk of the classic 911 enthusiasts.

The 915 is easy to rebuild, parts are relatively cheap and there is great support from forums like this.

Regards

Hayden

[Jack Olsen]

Yes. Sorry about the typo. I mean going from the new version to the older one for the 'airport gearing' type of effect. I'll correct it in the post.

And since Hayden is being generous with his time, let me also say that the re-geared 8:31 915 I use also has the Wevo internal gate-shift, which I've been happy with.

Bill's right about my gearing. Unless you think it would be fun to cruise on the freeway between 4K and 5K, it's not ideal for street driving -- especially with all the sound deadening out, and monoballs, etc.

[pbs911]

I would go with a gear oil cooler regardless of the gearing you select. Heat kills.

[beepbeep]

Well if you are really considered by ill-effects of 3.6 motor on your 915 transmission, i think swap to much beefier 930 tranny would be pretty easy hack if you don't mind 4 gears instead of 5. G50 is even better but requires cutting the compartment for additional space and coilovers, which raises the costs. 930 is virtually undestructable (being even stronger than G50 or any other Porsche transmission up to date except maybe 920) so that would be the "final solution" as long as you don't care too much about burnout starts from red light. 930's very tall 1:st gear makes it usefull in real life driving, not just for starting from a dead stop.

That being said, i understand there are many who are scared by idea of having only 4 gears :-)

[wevoid]

I dont disagree with pbs911, but I do want to post this info about coolers for 915's.

Any 911 regardless of engine size will use something like 65 hp to cruise at 80 mph. That means the transmission is transmitting the 65 hp through the gears, bearings, CWP etc.

Fitting a larger engine will not put any greater tax on the ability of the transmission to reject the heat created by cruising at 80 mph.

The primary source of heat creation in the 915 are, synchronizer action, LSD action, gear mesh, bearing friction and heat induction from the motor. At cruising speed you can eliminate the first two.

Unless you have the chance to fully deploy the potential of the any motor, the demand for a transmission cooling system will not exist.

Track work, with the opportunity to accelerate at full throttle, shift gears at an average of once every 8 - 10 seconds, activate your LSD every 15 seconds and act as a heat sink for the engine making full heat,....... this will tax the heat rejection of a 915, regardless of engine size.

On a 2.4 liter car it probably is not critical, on a 2.7 liter car it is desireable, on anything over that it becomes more critical and on a 3.6 operating towards it's full potential - it would become quite critical.

The complexity of a good cooler system and the fact that another system prone to failure is installed on the car should be factors taken into consideration.

We have a number of customers running 915 dog gear transmissions, with spools. This elimintes the heat from synchronizers and the LSD. The heat being managed is now limited to gear mesh, bearing friction and heat induction from the motor. These are fast cars Sub 1:35 at Mid Ohio, and would be sub 1:35 at Laguna (for the West Coast guys reference). Yet they can run without 915 transmission coolers. Some of the advantage comes from running full synthetic oil, that is less satisfactory in a synchro 915, but most of it comes from taking away the heat from the synchronizers and LSD. - same condition as most of us driving down the freeway.

Not meaning to be contradictory, but choose your 915 cooling requirements in proportion to the heat you are planning to generate - regardless of engine size.

regards

Hayden

[beepbeep]

Hayden: great post! Saved for further reference.

[Jack Olsen]

Track work, with the opportunity to accelerate at full throttle, shift gears at an average of once every 8 - 10 seconds, activate your LSD every 15 seconds and act as a heat sink for the engine making full heat,....... this will tax the heat rejection of a 915, regardless of engine size.

It was eye-opening when I put a temp gauge on my tranny. A lot of what I assumed went right out the window. Never, in street driving, was I able to get the tranny up above 200 degrees (which is where most gear oils begin to break down). 190 was about max.

On the track, it was scary. Without cooling, I remember seeing 260-degree tranny temps with 235 engine temps. Needless to say, I got a cooler.

What Hayden says is interesting, in that it's likely that it wasn't my 3.6 that was causing those temps, but the simple fact of driving my street transmission on a track. If most track-driven 911's had tranny temp gauges, we'd probably see a lot more coolers installed.

People make a lot of noise about the durability of the 915. But if the car you're talking about has been tracked, you've probably got your explanation right there.

For spirited (U.S.) street driving, however, they're just not needed, IMO.

[Geary]

Point of reference:

The 200-2001 GT3Rs with transmission oil/air oil coolers saw oil temperatures in the range of 275-300F in a 24-hour race.

Introduction of the oil/water oil cooler in 2002 dropped the trans oil temperature down to 240F. The life expectancy of the low ratio gears in these cars DOUBLED with this reduction in transmission oil temperature.

[adomakin]

when i had my mk1 escort i just used to cane the poor thing till something went bang, then replaced it. what was so strange (read pathetic!) was that it was sort of cool amongst my freinds and i to "blow my box" or "f--k my diff". ahh them good old teen days.......................