My dad has a '72 911 2.4 and its due for a rebuild,
that I'm going to do.
He wants a few modifications, so we thought about a 2.5.

Information about modifying a 911 are plenty out there but non so ever about the 2.5

Can somebody help me with specs and details like:?
- What bore
- What stroke
- What pistons/where to find?
- What Cams posible?
- Would 40 IDF webers still work on it with big venture or only 44 or 46 IDFs?

Thnak you

Here a pic of the two

You will find lot and lots of info on the bird site under engine sections. A few 911 engine builders that come to mind are: Steve Weiner ( Rensport in OR/ WA and one other in San Diego area)

bird site?

pelicanparts.com

There were two versions of the 2.5. The first was 66mm stroke by 89mm bore, and the second was 70.4mm stroke by 86.5 bore. These were called short stroke, long stroke for obvious reasons. The easiest in your case would be the long stroke version. The pistons and cylinders drop right in to your case and you have the crank. The short stroke version requires boring out the spigots in the case for what amounts to 2.7 pistons and cylinders. A call to JE might be in order as a possible source. There are 85mm pistons available for your engine given you have iron or biral cylinders. These can bored out to fit the pistons.

Don't know how hard this is, but the 911 I sold had a 2.4 engine case with 2.7 P&C's (and MFI modified to provide more gas). IMO if you're going to take the trouble to build a 2.4, 2.5 isn't that big of a displacement increase.

Mine's a 2.6.

914-6 crank.

I'll dig up the specs a little later.


KT

The short stroke 2.5 generally used the 2.7 pistons and cylinders, but that provides a very low compression ratio.

Bruce Anderson has quite a few suggestions in his book, Porsche 911 Performance Handbook, on how to modify various models of the engine reasonably.

I've had 2 Short-Stroke 2.5s - Torquey little Fuchers

2.6 short stroke:

2.4 case
92mm bore x66 mm stroke

custom pistons/ 9.5:1 CR
Solex cams
"S" heads

Makes good power.
I hope to dyno it in a few months.


KT

torquey? or peaky????

short stroke kinda takes away the torque....

Torquey - my 1st one Engine Dyno'd at 216HP & 188ft/lbs of Torque at 5600RPM

Nice garage !

SS is fast revving.


KT

Sheaster gave me a ride in his 2.5 twin plug GT replica. Seemed to have plenty of torque.

torquey to me has always meant having a very strong torque curve from low rpm and linear though out the power band.

a diesel is torquey a f1 engine makes power but not a broad torque band.

If I were building something like a 2.5, I wouldn't use anyone other than Walt Watson at Competition Engineering in Lake Isabella, CA.

The Cap'n

I've got a 2.6 short stroke engine in my 70 911E - It's a great engine combo

Mine:
Early aluminum case w/ bored spigots and other mods
66mm counter weighted crank
92mm Mahle RSR P&Cs
Twin plugged 911S MFI heads
Early S cams
Twin Plug distributor from Supertec
40mm Weber IDAs

Just over 200 hp and about 180 ft/lbs torque
I'm at a high elevation, so that probably lowers the numbers a bit

Oh forgot about Walt...He's very good too. It's been 5 years since I've last saw him. I hope he is doing well.

The first question I would ask is what build level is the 72 2.4? T, E or S? The second question is what will the car be used for? Street, AX, canyon carver, track? The third question is what is it about the current motor does he not like?

Answers to all these will help determine the best path forward.

The 2.5 is a great motor. However, given the 2.4 starting point the 2.5SS (66x89) requires acquisition of new crank/rods/PCs and significant case work to bore the spigots. Given the magnesium case usually requires case work it may be a good time to upgrade. However, if the crank/rods are in good shape, leave the 70.4mm crank setup from the 2.4. No sense in the expense.

If the PC set is in good shape then I would consider just a cam change. A new cam can and will shift the torque band up/down depending on specs. This changes the character and driving feel. A T-spec engine will have lots of low-rpm torque and run out of breath at higher rpms. The S cam (or hotter 40, 60, etc) will make great high rpm but not have as much torque down low. Depending on weight and gearing this can make the car feel slow off the line. The E cam (or Solex, etc) is a great compromise for a general use car.

If you want more torque this can be done through compression or displacement. You could keep the 84mm cylinder and just add some new pistons to increase compression. A 2.4T had 8:1 compression. Not very sporting.

If you are going to have case work, have them bore the case spigots for 90mm 2.7 PCs and build a 2.7. With the right compression and cam these are fantastic engines.

btw, what induction does the motor have? carbs or MFI? If MFI, you may have a limitation in swapping cams due to the fuel delivery requirements of the pump space cam.

Don't worry, it sounds complex but there are many tried and true engine combinations that maximize your current engine while providing the best increase in performance.

You will probably not find anyone on here that likes the 2.5 ss more than me.
I have owned one for around 30 years.
Mine started as a very early 2.0 with aluminum case with solex cams.
The case was cut to hold 2.7 p/c as were the heads.. 2.0 crank was used with 2.7 Euro S pistons and cylinders The 40 webers had the venturi's opened to 44's I think. Solex cams or S cams work well with this combo.


That being said.. If the 2.4 in the car is the original engine.. I would think twice before modifying it in a way that can't be reversed. There are many more engine options today than when these engines were conceived.
For the price of the mods and parts needed, you can probably find something in a 2.7 or 3.0 and keep the stock engine on a shelf for future return to the car.
A turn key replacement also requires less time with the car sitting due to engine being built.

it sounds like your going to build this engine yourself
and it also sounds like it would be your very first 911 engine build

if that is correct, you really ought to read up a bit. Get wayne's book and also anderson's book

its great that everyone has idea's for you... but this is going to be a 8k - 10k build so you might want to research it a bit. And it can jump up to a 20K build if you go over the top and pay a pro.

brant

I got mine for $3k with one hour of run time on the engine stand with break in oil.

I forgot to mention that tasty little tidbit...


KT

Here are my specs & Costs for the Twin-Plug 2.5

Click to view attachment

First of all thank you very much all of you!

I do have both books and worked on 911 engines befor just never a complete rebuild or even build. My experience so far are type 4 engines taught by Jerry.

The 911 is a German import. It is a 72 911 T and it seems to have matching numbers,
but with that being said the car is not an "original stock" car.

It was build and then build again by some small shop in Germany under contract of the previous owners less than perfect.

For example: Front and rear bumper are original Porsche 2.7 RS fiberglas parts, as well as the front bumper. The car had at least 3 different colors and the best part about the 72 the outside oil filler has been moved to the inside and welded closed.

Also the engine must have some upgrades, it used to run better then a stock T.
But I wont know what till I actually take it apart.

The reason for a rebuild are sudden timing chain rattling, worn carbs and bearing shavings I found in the last oil change .

My dad loves the old "R" and "ST" Factory Cars so thats why we are thinking about the 2.5.

Also the car is a daily driver, to work or cross country trip, sunshine or snow, as well as mountain races...

http://www.youtube.com/watch?v=24ftFOZOFyI...ng&index=18

My 2.6 powered 914 on the track.


KT