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Porsche Crest 911T 2.4 to 2.5 SS Conversion?

Hi, all:

I'm taking the reins on a project for my dad, who's the owner of a neglected '72 911T.

He's been open to the idea of swapping the engine for something a little more modern, but he does realize the value of having a matching numbers car.

That said, I'm curious what you folks have to say about doing a 2.5 short stroke conversion on our 2.4. Is it viable? Any things we should take note of? Any direction or info would be super helpful. Thank you!!
Old 04-15-2015, 10:08 AM
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Why not make it a 2.7? You can use every thing you've got now except the cylinders.

-Andy
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Old 04-15-2015, 12:06 PM
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A 2,5SS has the older 66mm crank.

Your 2,4 has the newer 70mm stroke crank. With a larger bore you could make a 2,5, but it would not be the short stroke variety.

Boring it out further would result in a 2,7 or 2,8 or maybe even a 2,9.
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Old 04-15-2015, 01:07 PM
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Make sure the case is a good candidate. From what I understand, only the mag 7R cases make good candidates as the spigots need to be enlarged for either a stroker 2.5 90x66 or a 2.7, 90x70.4. I'm doing a 2.5 86x70.4 and was warned not to go any bigger on my non-7R case.
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Old 04-15-2015, 01:11 PM
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A local mechanic who is a regular PCA autocrosser here in the redwoods has a 2,9 in his '71.

It has a 70mm crank with 93mm pistons and cylinders. 9,8 to 1 CR with dual ignition on a 7R case. A dream motor for sure. He tells me that motors built on these cases are basically bullet proof, if you don't rev the ***** out of them.

He and his brother own a reputable Porsche repair shop in Marin county, so I think he knows what he is talking about.

He also mentioned the problem when you split the case on these motors....

edit: if I had a 7R case I might be temped to build a motor something like it.

BTW, I made a 2,5SS out of my 2,2 back in 1978. It still runs fine today after 90K miles and a hundred autocrosses.
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Last edited by Trackrash; 04-15-2015 at 06:00 PM..
Old 04-15-2015, 05:55 PM
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Thanks for all the advice. Do you guys think that a 2.7 conversion would be better than the 2.5? All we're looking for is a significant gain in power, and hopefully not dropping 15k on engine work along the way.

Last edited by PorscheLiebe; 04-16-2015 at 09:36 AM..
Old 04-15-2015, 06:22 PM
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If I had it to do over, I would have sprung for the crank to make mine a 2,7. That said in your situation, it's just a matter of how big do you want to make your bore. My 2,5 has E cams, ported heads, and 90 Mahle pistons and cylinders.

I believe you can bore your own cylinders out to 87 mm making it a 2,5.

After that you will have to have your case and heads bored for larger, like 90mm, cylinders. This alone will not sustainably weaken the case.

There are two books you may want to buy that have all the details. Wayne's How to Rebuild and Modify Porsche 911 Engines and Bruce Anderson's Prorsche 911 Performance Handbook. These two books have a lifetime of information.

BTW, yes you can drop 15K on the motor, but you can do a nice motor for a lot less than that, if you do some planning and can turn a wrench.
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Old 04-15-2015, 07:30 PM
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Thank you, Gordon!
Old 04-15-2015, 07:55 PM
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If you have a 7r case in a 72 then you don't have the original motor.
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Old 04-15-2015, 09:50 PM
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You're right; I didn't check before, but it's a 5R case. Am I restricted with what we can do since it's not as strong as the 7R?
Old 04-16-2015, 09:37 AM
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I had my '71 case bored for 90 mm barrels back in 1983. It is a 3R case. 90K miles and numerous autocrosses and DEs later and it still seems fine.

However I have the 66mm crank and 8.5 to 1 CR.

The big decision you will have to make is about splitting the case. Will it warp? Will you need to have it line bored? Can you get affordable oversized bearings?
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Old 04-16-2015, 10:03 AM
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Some reading...

GT3 oil pump - Oil springs.
2.4 refurb question
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Old 04-16-2015, 10:24 AM
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New 2.45 Engine Build

Good thread with input from one of the top air cooled engine builders in the country. Use your crank and bore your cylinders.
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Old 04-16-2015, 10:38 AM
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My 72 4R case was used to make a 2.7. It worked fine for 10 years as a weekend driver and about 4 yeas of racing. It finally cracked but that was due to over ambitious wind age mods inside. I wouldn't be afraid to use your 5R case to make a 2.7.

-Andy
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Old 04-16-2015, 10:18 PM
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Thanks for all the suggestions. We're now thinking that it makes more sense to just do a 2.7 conversion. Are there any resources for that any of you would be willing to pass along? Thanks again for the help, and once we make some moves I'll post some pictures.

Christian
Old 04-19-2015, 12:35 PM
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Why not just go 2.5 long stroke. You won't have to modify the case. P&Cs and cams will make a world of difference compared to your T motor. If it's matching number I would hate to see you modify the case. Maybe save some cash and just buy a 3.0 or a 3.2 and throw it in. Put the matching numbers motor on the shelf for safe keeping. An added plus, if you throw a rod through the case, you don't depreciate the whole car.

Last edited by bgyglfr; 04-24-2015 at 05:07 PM..
Old 04-24-2015, 05:05 PM
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See post #7
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Old 04-24-2015, 06:31 PM
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