2.5 Short Stroke Project
 

Dear all,

I am a new member and a great fan of early 911, specially the special models ie. R, T/R, S/T, RS and RSR.

I have got a 1971 911 S which I am converting into a 2.5 Short Stroke, currently I am using the following parts:

Original engine case
89 mm Mahle P&C
2.7 RS pump
S cams or Sprint cams
R exhaust

I am also replacing the following parts with fiber.

Front & rear bumpers
Hood & engine lid
front wings & Rear fenders

and would really appreciate if you could help me with the following, taking into account that I would be using the car for twisty velocity rallies, hill climbs and occasionally circuit racing:

Which cams do you think are better S or RSR?

According to the figures of the 2.5 ST it had 41mm of I/E, is it too much?

Regarding the parts I am replacing in the front of the car with fiber. I was thinking of reinforcing the chassis by making a cross in the fuel compartment like the 935s. have any of you experience lack of rigidity or feel in the front of the car after replacing the front part of the car with fiber?

Many thanks in advance for any advice

PS. Wayne, great book, it has been my guiding light.

Sounds like a cool motor.

Im not an engine builder, but I think 41mm ports are going to be a little big.

I thought about building a 2.5 for a while and was planning on using S/RS ports, S/RS MFI, S+ cams, 2.2 rods & crank, single plug ignition, and custom J&Es at 9.8:1.

You would probably open the ports up a little (maybe 38mm) if you were going to run a really crazy cam, custom MFI, twin plugs, and higher compression, but you would obviously be adding a lot to the cost of the build.

you may want to try to post on the engine rebuild forum. There are guys there that can offer a lot better advice than me on the subject of engine rebuilds.

good luck and keep us posted as you progress.

Ugh, is it a nice car? Have you considered getiing a worn out T as a starting point? The S cars are really getting scarce.

Sounds like a really fun ride wen you are done!

Welcome El Mallorquin! http://www.pelicanparts.com/support/smileys/wat.gif

You're asking some good questions, things that a number of us have struggled with before. Rather then rehash a lot of stuff that has been done before, a quick search of this forum and the Engine Rebuilders Forum will bring up a lot of information that will provide hours of interesting reading for you.

I don't think you'll ever find a "doing this is best" answer, but you will find a lot of insight that will help you make the best decision for your application. As far as cams go, I suspect that the three choices (S or RSR or Sprint) range from agressive street (S) to all out race (RSR or Sprint). You might want to read through this thread for some ideas how selecting the best cam for your application. A number of great people helped me out in pulling it together.

PLEASE use a case that is not an 'S' VIN. You'll thank yourself later. Uncut cases on matching 'S' cars are already bringing hefty premiums and a 'T' or 7R case will serve you just as well. Spending a few hundred bucks on a less valuable case now will put thousands in your pocket later should you decide to sell the car.

(edit) I just read your post more thoroughly. Geez, I hate to sound like an 'S' snob, but you need to reconsider this project. Hacking up an 'S' like this is like burning dollar bills. Every cut and body panel replacement you make on that car will decrease its value by the amount it would take to put it back to stock. Would you do these kinds of mods to a Ferrari Dino? I'd bet you could sell your 'S' at a tidy enough profit to build the race car of your dreams out of a less desirable car.

Go to the Early 911S Registry BBS and website for more info:
http://www.early911sregistry.org/index.html
http://d240157.u39.zeonhost.com/forum/index.php?

The "cookbook" solution for that engine configuration is to use 2.7 S heads, which already have 36/37 (mine miked at 36.5mm) intakes. This is a better choice IMHO for an engine that is going to be driven on the street. For a pure track car, you might benefit from going bigger, but this is the ID used on the 2.7 RS engine, which was no slouch.

As for the front tub reinforcement, don't weld anything to a '71S chassis! The hood and bumper of a 911 just hang on the tub. The fenders do add some rigidity. It's likely you can find places to bolt an X-member in place to accomplish what you want, tying the front of the tub to the upper strut mounts with existing holes.

Again, if you keep the car from undergoing permanent changes, you will thank yourself later.

However, I must echo Erik's post: you'll be better off to do all this to a T.

Here are the spec's on my 2.5L short stroke.
Specs-
The internals: 1967 alum case bored out spigots, piston squirter's, oil bypass mod, bearing mod, Std/Std counterweighted 2.0L crank, 2.0L rods, ARP rod bolts, Carrera oil pump, Boat tailed, Mooned cylinders, 89 MM Mahle cylinders re plated, 89mm J&E pistons 10.5:1, GE80 profile cams, Stock 2.7 valves, Heads ported and polished, 37mm intake, 39mm exhaust, CIS ports welded up, EBS racing springs with ti retainers, 46mm PMO carbs with their new super tall manifolds, plus all new zinc dipped hardware and powder coated goodies

This motor is very rev happy and sounds great.

Chad, 46 PMOs seem like overkill to me, at least with anything short of 906 cams..

Just curious: are you running larger venturis than you could have put in a 40mm?

This is what was reccomended to my from the Aase guys. The 46mm PMO's work flawlessly. I never tried running it with 40's so I can't compare the two. I do not remember the venturi size. I have to check my book for that.

I will repeat what others have said above...find a T! Don't destroy another early S...

techweenie; Chad's GE80 cams are roughly comparable to 906 cams -- they're full race cams. Given that Chad's engine is a full-race engine, it seems that 46's should be fine since they're allowed by the rules.

I have to weigh in on the subject. I would like to have a 911 Hot Rod as well, and the thought of putting together a custom engine would be really a dream come true.

But I have to ask, why would you use an honest to goodness "S" as the starting point? Any of these cars, S and even E's in good condition, original and matching engines are getting harder to find. If your "S" has a matching numbers engine, you could easily sell that "S" to someone who wants an original car, buy a "T" in good shape -- change the body the way you want, get your engine rebuilt the way you want, and maybe even end up with money left over.

Good luck with your project, but I hate to see a real "S" used to make a hot rod. They are kinda becoming rare birds.

larry

It's your car and you should enjoy it however you see fit.

Just realize that all the money spent doing mods will be money down the drain that you'll never recoupe and after the mods are done you'll have a car that will be worth half as much as it was before you started.

Excellent condition all original Early S cars are getting $40k and up in some cases. Modified ones are very hard to sell and I doubt couid bring you $20k.

The 2.2 engine case is not the best starting point for the motor you wish to build. You should ideally get an early sandcast aluminum case like Chad, or atleast find a 7R case as your starting point.

Fiscally you'd be way ahead of the game finding a nice roller because you'll end up replacing virtually everything on your car anyway. (interior, motor, suspension, brakes, sheetmetal etc.) The only thing left about the car that made it an "S" will be the "S" on the title.

Good Luck and welcome to the board.

Unless you are leaving the sheet metal alone - and making only reversible changes, you will be throwing away $15-$25,000 by altering an increasingly rare, sought after car. I would start with a T and go wild, but altering an S flushing thousands down the toilet.

If you sold the S, and bought a T in similar condition, that would help make you money for a good amount of your mods and help preserve a valuable car - but then if you did chop it - you are only increasing the value of remaining Ss.

I have to agree with the majority of the opinions here, leave the S alone. I have a 1970 911 that has been converted into a RSR. It is a beautiful car but even with the 10's of thousands of $$ it would be worth as much a stock S. I also owned a 1971 T with an S motor in it. I considered swapping everything over so I had an RSR T and a correct S but I figured my wife would of had me commited to a mental institution.
Phil

I suspect some trolling here. That said? Do as you wish. It's your car. Chop away, if that's your thing. My thing is to keep insisting that my future widow send the car though a crusher. I doubt she is listening, considering it's current value...;)

Paul, You might consider putting it in your will that you be buried in you car. :)

Wow!

Many thanks for all your replies and advise. I agree with you all that it seems a shame to use an original S rather than a T...but I have always wanted an S and want it for racing (eventually in the future I will put it back to its original shape as it is not my intention to sell it for the reasons you have given me, as it´s a rare find)

I love historic racing and some of the rallies I will be competing in Europe require a FIA competition passport where all numbers have to match and because mine is a 2.2 I can converted it into homologation Group 4 and increase the engine capacity to 2.5.

These cars properly done with the correct parts (Homologated ones) and well documented actually can increase the value of the car to some extent as they are difficult to find as well.

I am keeping all the original parts for when I grow older and can put them back together...just in case.

Again thanks to all and will keep you posted on my progress...now I only need to find a Twin-plugg ignition kit!

Christian

Search user name "Burn Bros". he has the ticket...

Many thanks Kenik, I will ask Burn Bros about it.