flyingpenguin

A few months ago I got lucky and picked up two non running Boxster engines for little money. One M96.21 (2.7 L) with a failed IMS and piston / valve collision. The other 2.5 had a dropped sleeve that had resulted in a shattered piston on #1.

In order to be able to "mix-and-match" remaining good parts I am trying to figure out (beyond part numbers from PET) what the real differences are in terms of basic internals between these two engines.

From comparing / measuring, I believe the following parts are identical:
- Case halves
- Heads with valves and valve covers
- Connecting rods and bearings, wrist pins etc.
- IMS to exhaust cam chains
- Chain guides, tensioners etc.

Slightly different (but interchangable) are:
- Crank carrier; the main bearings are mirror images 2.5 vs. 2.7, but offsets and angles are the same. Also the 2.7 has a different design for the chain that runs from crank to IMS shaft; gear vs. double row of teeth.
- IMS shaft, for the same reason as the cranks are different.

So AFAIK, the only real difference is in the pistons.

Kind of important to have this reconfirmed before mixing-and-matching parts...

Input from the forums (this one and also cross-posted on renntech.org) would be very welcome.

Joost

Macster

My WAG is that you're going to have to figure this out on your own.

I believe the problem is compounded in that during a engine build run, which can last/cover several model years (as clearly is the case with the 2.5l and the 2.7l engines) that changes are made on an ongoing basis that affect interchangeability. While the engine model number doesn't change, changes are noted at the various points of the engine serial number stream.

What someone else might have found works may not work with the examples of engines you have.

For instance that crank carrrier...you might think it is interchangeable but if its fit/clamping force is not right, while it may feel, look like it fits it may not hold up but move about. (Some factory assembled engines suffered from this so even the factory didn't get this right 100% of the time.)

You might up against selective fitting of these various assemblies that is easy for the engine builder on the engine factory assembly line with a number of parts bins to select from but with which you are stuck with what you have in front of you.

Sincerely,

Macster.

flyingpenguin

Macster,

Good point on the crank carrier. I assume that this moving about contributed / directly caused some of the problems with crank vs. case centering and RMS leaks.

I need to give some thought on how to bench test this before putting everything together.

Also need to do some testing with 2.5 vs. 2.7 pistons and compare deck heights. Just want to make sure the 6 mm extra stroke on the 2.7 does not cause any issues with the pistons and valves interfering.

Macster

Be sure the engine is assembled correctly, the cams are properly timed.

Then get out the clay (silly putty?) and put some on top of the piston and hand crank the engine over - very very carefully and very very slowly (stop if there's any resistance) -- and then measure the thickness of the clay to confirm the piston/valve clearance is sufficient.

What this should be I do not have any idea for the Porsche engines.

Oh, do this for each piston to make sure you do not have an odd ball piston or an odd ball (long?) rod.

Sincerely,

Macster.

flyingpenguin

I like your approach. Old school and simple. Will do!

Byprodriver

The main thing that changed in 2000 was the base engine received the "S" crankshaft adding 6mm of stroke & 200cc to make a 2.7, this required pistons with 3mm higher pin height to maintain deck height & compression.
In 2001 Porsche changed the IMS chain drive gear on the IMS & crank to drive a tooth type chain which is quieter & causes less wear on the tensioning paddle wear pad. Seems you have a 2001 or 2002 2.7 engine & I can't see any reason to use any parts from the 2.5 to rebuild the 2.7

flyingpenguin

The 2.7 I have, suffered from IMS bearing failure and in the process the chain and chain drive gear on both IMS shaft and crank got damaged, while the crank and IMS shaft from the 2.5 are in good shape.

The only difference between the 2.5 and 2.7 crank that I have been able to discern is that the paddle shaped counterweights on the 2.7 crank are larger. Diameter of journals, angles and offsets are all the same.

The final answer on all important deck height will be in a test install though:

2.7 case halves + 2.7 crank carrier + 2.5 crank and one 2.5 piston + connecting rod and one 2.7 piston + connecting rod installed. Then do a couple of full rotations and measure deck height for each piston.

Thanks for the insights received so far.

flyingpenguin

Quick update:
My assessment on the cranks having the same basic dimensions must have been incorrect. On a test install, using the 2.5 crank, with one 2.5 piston and one 2.7 piston (connecting rods have 100% confirmed identical dimensions between the two), the deck height of the 2.7 is lower (deeper) than the 2.5 by about 1.5 - 2mm.

This would result in a loss in compression ratio from 11:1 to about 9.5:1, so I decided to reuse the 5 remaining good 2.5 pistons and found another one on fleabay for about $100 shipped.

Ready to mate the 2 case halves back together and start fondling with the wristpin clips. Yay!

Byprodriver

I love the Stomski Racing pin clip injecter!

flyingpenguin

The tool set on Ebay has a very good version of the clip-launchenator:
http://item.mobileweb.ebay.com/viewi...id=42442583183

Flat6 Innovations

I have ruck loads of OEM parts that could be used for this project.. we generally throw away 100 pounds of parts from our engines to replace with upgraded parts, and I'd rather someone put it to use.

Many components will swap between engines. I have an interchange table in my book.

Macster

What book are you talking about? I visited your site and found no books listed. Is this book for sale?

Sincerely,

Macster

Flat6 Innovations

Its being published over the winter.. I have just finished compiling all the info and we are just tweaking the layout now. A true assembly and reference guide-

flyingpenguin

I am sure your book would have pointed out that the case halves from a 2.5 and a 2.7 are very similar, but not exactly the same.
I found out this weekend that the "peek hole" to install the wrist pins on the 2.7 is slightly closer to the crank than on the 2.5.
Makes it a lot harder.. Not impossible though.

Flat6 Innovations

Correct and the same for the later/ larger engines as well.

I also go over this in our engine build school and even show how to assemble the engine using some improvised methods if this situation strikes and the assembler isn't prepared for it.