re-gearing 915 for 3.6 engine

[Danskeren]

Hi guys,
I just got me a very nice 73 911 with a 993/3.6 transplant and a 915 8:31 transmission. RMP-max is appr 6700. Im gonna use the car for street only.

The car has a real massive torque and just keep pulling all the way to max rmp in 5th - appr 153mph in no time. I can take off in 2nd with allmost no problem, and 1st seems allmost unnecessary. 3rd and 4rd are fine I think.

So my question is if it will make sense to make 1st, 2nd and 5th gear longer, to get a more smooth launch, and to increase Vmax and comfort when crusing in 5th.

So my questions to you guys are:

1) Could this work. What are the disadvantages ?
2) What would be the best rations to fit my setup
3) What parts do I need to change. And where is the best palce to get these?
4) What would be the number of hours to redo a 915 transmission for a guy who knows how to ?
5) Can the 915 and clutch handle the increased load ?

I appreciate any input if you may have. Thanks

Peter

[Bill Verburg]

It all depends on your usage.

tires also affect gearing


For my use a 915/44 w/ 245/45x16 in front of a 3.6 was too short. This is for mostly highway driving.

A 915/67 w/ 265/40 or 275/40 x17 is much better

It is no where near as satisfying as a g50/30 6 speed, but better than a g50/20 6 speed.

So how do you use the car? what transmission? what rear tires?

[911pcars]

For a street car, it sounds like you could change all the gears to take advantage of the increased torque. Make sure 1st is low enough to avoid slipping the clutch, yet close enough to 2nd to keep the gear spread fairly close.

What with gas hovering at or over $4/gallon, now might be good time to examine 5th gear and see if any rpm-reduction ratios would be beneficial to your high speed cruise speeds.

Sherwood

[911st]

Changing 1st in a 915 is expensive, about $1k plus other gears and labor.

Your redline is higher than a 3.2 Carrera. Not shure about going faster that 150mph in a 35 year old car.

Put a 3.2 in it and all your troubles will go away.

[84toy]

It also depends on the weight of the car. My 914 (street car) weighs 2200 with a 3.6 and a 915. First gear was useless and 5th was too low for freeway cruising. I have 245/45/16's

I re-geared 1st and 2nd and used a 5th out of an early 7:31 915. 3rd and 4th are stock. For my purpose it works awesome. 53 mph in 1st and it will still go sideways when I stand on the throttle. 5th is only used for cruising now and under 3000 at freeway speeds. Since 1st is totally usable you get 4 good street gears and a gas and engine saving 5th

You just don't need to rev the hell out of a 3.6. If you do a lot of stop and go driving or live in a hilly area you might want to chose a lower 1st. I don't have either.

[Joeaksa]

Another thing to realize is that Denmark is very harsh on speeding. So a major question is, will you be taking the ferry to Putgarden and driving the car in Germany or staying Denmark and not driving real fast.

Do you want high speed or fast acceleration?

Joe A
(enjoy some Aquivit for us Danes in America!)

[LakeCleElum]

I am so jealous, I should have your problem?????

[Jack Olsen]

You should probably spend the money on a reinforced side plate and a few spare axles. The 915 isn't an ideal box for repeated hard launches.

[Matt Monson]

Hello Peter,

For your set up, I suggest something very similar to 84toy's suggestions. However, given the cost of 1st gear, I would probably leave it stock unless you plan to race this car. I would go as follows:

11:35 3.18
20:32 1.60
23:28 1.22
26:25 0.96
29:22 0.76

It's a little taller in 3rd and 4th and one tooth shorter in 5th. In my experience, even at the track, most people are truly at their personal limit (not to mention the car's) up around that 160mph range. Unless it's a racecar that's specifically prepped for it and it purpose built for racing, it's just not stable or stiff enough for those speeds. In fact, personally, I wouldn't push my own vintage Porsche to even 160mph without first installing a cage.

I can provide you with any of the gear ratios that you might need, be they something I suggest or another set that someone else here convinces you is better. I have a wide assortment of both factory good used, NOS, and new aftermarket gearsets. This includes weld on 1st gears or if you prefer new billet mainshafts with slide-on gears. The above ratios I suggested involve a couple of new gears and a couple of good used factory gears.

To answer your question about time, my shop generally charges 8 hours of labor on a 915. This assumes that it doesn't require repairs to the case, in particular making sure that the bearings mount solidly in the case which is becoming a widespread issue on these old 915s. Case repairs would add 2 hours to the job. Assume that a first timer would take 1.5-2 times as long. That also doesn't count the time you spend reading and asking questions partway through the rebuild. That's just the time you spend at your workbench.

At the bare minimum, with your power levels, I would install a steel bearing retaining plate as well as seriously consider an aluminum billet differential sidecover. At about $350 each, these items are "cheap" insurance when you start putting 3.6l power against the humble little 915 transmission.

With respect to clutch options, there are many. Stock is not very well suited to those power levels for long term use. There are upgrades available with greater clamping force that will run you several hundred dollars all the way up to a Tilton unit that runs in the thousands. There's lots of options for upgraded clutches and I would suggest you start right here in Pelican's new parts section looking through their offerings and reading their product descriptions. Then you will be in a position to ask educated questions about their products and people can guide you as you further explain your application and usage of your car to us.

Kind Regards,

Matt Monson

[Danskeren]

Hi Guys - thanks a lot for all your inpunt. Very helpfull and much appreciated.

My intentions is only to use the car on street (but maybee I get addicted to tracking one day - who knows). Crusing the highways in 5th will be a large part. Noise, comfort, fuel consumption is important so 5th has to be longer for sure. Vmax is not that relevant - especially not here in Denmark, I agree but just another side of the same story.

But it is also important for me that I have car is performing well and I have a good feeling. Its still all about joy you know.

I have 245/45-16 and dont have space for any larger wheels.

Paul - I would say that your set-up would be exactly what I had in mind.
What parts do I need to get for that ?

Matt, I think Paul setup is more in line with what I need than your suggestion. This seems very similar to my standard 7:31 2.4S trans.
Would you also be able to provide the parts for a setup like Pauls?

I have a local Porsche guy here who says he can do it. He is good but Im not that sure how much experience he has on the 915. I will let him know what you have here advised - and we will see.

BR
Peter

[Bill Verburg]

No offense to Paul but I don't like that gear stack at all
here it is


One of the nicest track set ups is this one from an SC/RS



For mostly street use w/ a torqey 3.6 I like a 915/67



Of course there are lots of other choices too.

[Matt Monson]

Originally Posted by Danskeren

Matt, I think Paul setup is more in line with what I need than your suggestion. This seems very similar to my standard 7:31 2.4S trans.
Would you also be able to provide the parts for a setup like Pauls?

Peter

Hello Peter,
I have those gears as well. Though like Bill, I don't think it's the best option for you. Take a look at those other charts that Bill has put together. See how with each shift up the RPM drop gets smaller? You want that. The faster you are going, the greater the air resistance and the more important it becomes to keep yourself at or above the maximum torque of your engine. Once you get up over 100mph you just won't accelerate as quickly if you let the gap between gears get too large. When I put together a race box for a customer, you often see 2-3 around 1400rpm, 3-4 around 1100-1200 rpm and 4-5 800-1000rpm. You don't need it that tight on a street car, but you definitely want to pay attention to the drops.

The exception to that rule is when you are going with a final gear whose sole purpose is to give you good highway cruising. Then it's often ok to have the 4-5drop get larger at the expense of acceleration. The set up I have suggested puts you just over 130mph at the top of 4th gear. You will get to 130mph pretty damned fast. But with the 5th gear I put in there, you will be able to run down the highway at 3000rpm at 75mph. It would be just as easy to shorten it up, but you complained about wanting a taller 5th, so that's what I offered you.

Feel free to drop me a PM when you think you have made up your mind. I will pretty much have any ratio that you may be considering and will give you a package price if you are buying 3 or 4 gears at once...

[Bill Verburg]

here's an example of what Matt is talking about, 915/61(SC) which the owner used for dual use. He put in a little shorter/closer 3 & 4 for better track performance and kept a stock 5 which now has an unusually large drop for street cruise.

[pete911]

Hi Pete

That's my old car right? I told you it was a rocket (no rust too !!)

Pete

[RWebb]

This is helpful for those of us who don't know how to read these charts - or think maybe we do, but aren't sure.

I see 2 principles to extract from the above -
[1] the aero drag one that implies you should keep the drop in gears less and less with each gear

[2] the 5th gear exception to the above for cruising

- is the huge drop from 1st to 2nd a problem? (or is it ok, since he has a torque monster 3.6?)

- what else should one watch out for when examining these diagrams

[Danskeren]

Thanks guys. Interesting - Im learning here. Will get back when I have studied this a little more.

Hi Pete - good to hear from you. Yes - This is your old car. Just handed it over to the guy who is gonna strip it apart and prepare it for the repaint. He was too very impressed by the good condition, the engine and all the upgrades.

Did you ever do anything on the transmission and Do you know what gears it has ?

BR
Peter

[Bill Verburg]

Originally Posted by RWebb

- is the huge drop from 1st to 2nd a problem? (or is it ok, since he has a torque monster 3.6?)

- what else should one watch out for when examining these diagrams

Again it depends, usage, preference and engine tune are all important factors.

In San Francisco or Pittsburgh you might like a really deep 1, I like a tall 1 to get going from pit out.
here are the trans i've used in my own cars behind 3.6 & 3.8 all in @2700 -2800# cars


for street the /67 & /20 are better because of the shorter first and taller tops, the /30 is the nuts on track and acceptable on the street, the things that make it better on track make it worse on the street(short & close w/ increasingly small splits)

the gears used on most tracks are 3, 4 & to a lesser extent 5, at a fast track like WGI 6 too

w/ a 3.6 the /44 is just too short for street use but is good for track days where you'll use 3, 4, 5 whereas the /44 was geat w/ the stock C3 engine

[Danskeren]

Anybody who has or know where to find the torque curve for stock 3.6/993 non-varioram engine ?

Br
Peter

[Matt Monson]

RWebb,
The 1-2 drop is the least important for street use, even though it's still somewhat relevant. However, there's a huge difference in trying to accelerate from 4000 rpm at 40mph in 2nd versus trying to do it at 60mph in 4th. Try it yourself in both gears and time how long it takes you to hit 6000rpm in each of the two gears. At those lower speeds, it's not so much about the air resistance as the mechanical advantage of the gearing. As Peter indicated, he has enough torque to pull away from a stopsign in 2nd gear.

Now if we were talking about a racecar it would become a far more important thing to address. Then you want to strike a balance between two things in how you gear the gearbox. At one end, you want to look at what is the highest top speed you can hit at the end of the longest straightaway given your HP and drag. The other end of the scale is your slowest corner and pulling away from it once you've cleared the apex.

The latter is when your 1-2 shift can become important. When we go with a super tall 1st gear, it's what we call an active first. This is because you actually use it on the track beyond the pits and getting into grid. If you are going to be downshifting into first while racing, you want to be entering that gear somewhere around your peak torque, just like you would on an upshift in your other gears. The stock 915 1st is pretty short at 3.18, even with an 8:31 r/p you are topping out around 40mph. So, what if I've got a 45 or 50mph corner?

Even with a stock 1.83 2nd gear, you are down under 4500rpm to slow down to that speed, and it's already too fast for 1st. This gives you two choices; shorter 2nd or taller first. If you are looking to hit 150 or 160mph in 5th, taller first becomes the preferable choice and you put something like a 17:35 2.06 1st in there. But if it's a short track and you top out under 125 or 130mph, you go the other way with it and you stick with a stock 1st and run that same 2.06 ratio in 2nd instead. But even that really depends on the motor. These ratios might work well for a 3.6l with a relatively low 6700rpm redline, but if you've got a little 2.0l with an 8250rpm redline and a more narrow powerband you are going to gear it totally differently...

[ToddM]

Originally Posted by LakeCleElum

I am so jealous, I should have your problem?????

Not much of a problem to have IMHO. The worst gearing ever in my opinion was on the stock 993; heaven forbid you get a 993 with the pressed on 1&2 like I did.

Also, I'd rather have live with the reinforcemnts the 915 than the regearing costs of a G50-6... for me it ran $9K

I agree with Bill on the SC/RS