Beefing up a Sporto to handle a 2.5SS?

[crater64]

I recently had a 2.5SS motor built with the intention of running it with a stock 905/20 Sportomatic gearbox. Upon assembly and testing, the builder found that at peak torque between 5500-5700 rpm, the box couldn't handle the torque. He tried replacing the stock 905 pressure plate with a Kennedy performance plate but experienced the same issues. Upon disassembly he saw no harm done to the plate, so he's speculating that the torque converter might be the weak link.

We're now trying to figure out next steps. One option might be to use a 925/xx box in place of the 905/20; another might be to use just the torque converter from a 925. A manual swap is out of the question because reasons, so please don't suggest it.

I'm aware there have been Sporto equipped cars out there that handled 2.7RS and 3.0SC levels of power, so I have to think that a properly prepared Sporto could handle the ~200ish hp from this short stroke motor. What I don't know is how to ensure the box is strong enough to handle that power. I know it's a long shot, but if anyone has any experience in this area, I'm all ears. Thanks.

[stownsen914]

There was a fellow named Ron Case who raced 911s with Sportos in IMSA. It must be possible. Let's see who chimes in.

[30westrob]

First I preface my comments that I have never worked on a Porsche Sporto, but as a general rule the more torque, the more torque converter required.

The pet shows in 72 Porsche specked a new converter for the 2.4S engine 925 116 005 00. As your 2.5ss approaches or exceeds the output of the 2.4S, one speculates that you need more converter.

Looking forward to other inputs and your solutions. Rob

[47silver]

925 116 005 10
Same number for 2.7 liter from pet

[crater64]

We may have a solution, albeit not the most elegant one. We managed to find a low-mileage 3-speed 925/12 from a middie that we'll convert to a 4-speed while keeping the rest of the guts. Here's a gear chart for the proposed custom gearing.



The T 4th may get swapped for a U, not 100% sold on that yet.

Kind of a shame to put the rebuilt, numbers-matching 905/20 on a pallet but it's there if ever needed.

[stownsen914]

One thing to note is T is a hard to find racing-only ratio, at least from the factory (I.e. $$$). If you are buying aftermarket gearsets, it doesn’t make any difference of course. U is more common as it was in production gearboxes.

[crater64]

I have access to both. Trying to decide which will be better for stretches of highway,

[30westrob]

Curious: is it difficult to change a 925 to a 4 spd, beside the gear ratio changes. Rob

[crater64]

Not terribly; it's been done a number of times. This thread 4-speed 925 sportomatic contains some details including factory diagrams showing that spacers in the 3-speed replace the gears of the 4-speed.