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996.1 cab C2 manual

We have goals, but there is still a lot to be decided, and a lot to be discovered - what we can rev too without going boom is going to be key, for example.

We are also in the process of designing some parts, they need to be machined and tested.

=It's a fairly long way away from going in the car, but the engine cases are with Hartech at the moment waiting to be/being converted to 3.7 litre closed deck specification.

All very vague I know, but there are too many variables right now to make projections without risking looking rather silly.

I'll update as we hit particular milestones.

Overall the project might be described as 'what would the M96 look like if Porsche never had the Mezger?"
 
Do you have a recording of the 'noise' you hear in your radio? Mine is just signal, no noise that I can hear (and its a standard oem unit)
 
Engine:

Everyone knows (or thinks they know!) about the problems inherent to the M96 series of engines, the IMSB is the most commonly talked about (endlessly on the 996 Facebook group it sometimes seems) but there's also bore-score and crank-breakage, plus accelerated big end bearing wear. Terrifying eh?

Well - lets add one more to that. Enter The Tappet Carrier.

These do as the name suggests, they contain the tappets, sit in the head - what's to know?

The problem is that the design isn't great and the material used is crap.

What does that mean? It can mean this:

Fail1_zps2530d281.jpg


Fail2_zps3d85df2c.jpg


Porsche actually tried to rectify this when they introduced the X51 Powerkit - which some of you I am sure recall had an additional scavenge pump on one of the heads. This was there in order to remove oil fast enough that under heavy braking the tappet carrier didn't suffer hydraulic fracture.

Now with our engine we want to eradicate the weaknesses that Porsche left, so this had to change.

Therefore we scanned in a tappet carrier:

65abe1d71d7d43ec6f142c6a2ab96deefb6d4477.png


And went through many iterations of CAD model:

9838c9f3b1447f2c8f8b0d5c15e271e08e9d6998.png


To get to the point where we have something that we can now tweak, and then have machined from billet:

43167994824_6c7765c0d5_b.jpg


We'll likely have two versions of this - one for standard hydraulic lifters, the other for the solid/mechanical lifters that I'm going to run.
 
Nothing really constructive to add other than to say the level of detail and commitment you have with your car is incredible. Thank you for sharing it all on here.

:thumbs: :worship:
 
Thanks! I enjoy it, and am lucky to be able to do it.

Bottom end:

Two famous problems, IMSB and crank flex.

IMSB is pretty simple - I'll go to the final, big bearing design that needs the case splitting to fit, with Hartech's updated shaft etc.

Crank flex: this is a bigger issue, and one caused due to the lack of support at the flywheel end of the crank. Porsche tried to resolve this by moving from a solid crank that transmitted force through all the bearings, to a partially hollow design that loaded the bearings closest to the flywheel. Neither of these methods was, shall we say, optimal. Bearings wear at an accelerated rate, and in certain circumstances the crank itself can break. The other issue here is that the M96.01 is an evolution of the 2.5 litre Boxster engine and the bearings were specified to deal with that capacity.

What are we doing? We're fitting an additional main bearing at the very end, just behind the flywheel. This adds much needed support to the crank and should resolve the problems observed with the unsupported design.

When it comes to the crank itself we have a range of options - and this is quite interesting.

The cranks from the various M96.xx engines can be exchanged between themselves, so we have:

2.5 = 72mm stroke
2.7, 3.2, 3.4 = 78mm stroke
3.6 and 3.8 = 82.8mm stroke

(2.3,2.7 and 3.2 are all Boxster cranks).

Now when we remove the Porsche cylinder liners and fit the Hartech 100mm liners we get the following:

2.5 = 72mm*100mm bore, 3.393 litre
2.7, 3.2, 3.4 = 78mm*100mm bore, 3.7 litre
3.6 and 3.8 = 82.8mm*100mm bore, 3.9 litre

I'm not sure, right now, whether I'll use the 72mm/2.5 crank or the 78mm/3.4 crank.

I'd like to use lots of revs - and the shorter the stroke the lower the peak piston speed and the less risk you run. We'll be running lightweight rods and pistons, with high rpm use in mind, so maybe the 78mm stroke would be ok.

But! I'm not sure here, so I'll update when a decision is made.

These liners will be machined out:

27668146567_4e4813b4d4_b.jpg


And these will be fitted, closing the deck at the same time:


40730456940_f4f955a579_b.jpg
 
Gears:

Couple of things of note here.

Thing 1, the ratios in the box are long, this is compounded if you raise the rev ceiling significantly, as we plan to do. We need to shorten 3/4/5/6 up.

Thing 2, the rev-drop between gears needs to be looked at with reference to the powerband. What we have to avoid is, when changing gear, dropping the engine off the cam and having to wade through a couple of thousand RPM before it climbs back on again.

One route to shortening the ratios is to change the final drive - but there are two issues with this.

Issue 1, it makes 1st and 2nd fairly useless
Issue 2, all ring and pinion sets available for our gearboxes warn that their estimated lifespan is 40-60 hours. This is ok for racing, and of course that life would extend for a road car, but this doesn't apply to the OEM ring and pinion.

From discussions with a chap who builds 996 gearboxes the recommendation is to use a company called Albin: http://www.albinsgear.com.au/ from the land of Skippy the bush kangaroo.

First, however, the engine needs to be run on the engine dyno to establish the rev range and the power band within that rev range, then work back from there to specify what ratios are needed for 3rd, 4th, 5th and 6th.

96603_500x333.jpg
 
Some great info. I had a VR6 Corrado that had a swapped out final drive and what a difference it made!

There was something I saw on super finishing the gears to help but then not sure that is going to help here. What sort of mileage are you expecting to do with the car?
 
Probably no more than I do now - around 5,000 miles per year.

I did 3,800 between the last two MOT's, bringing the car up to the ~67k it's on today.

Part of my decision to go with solid lifters was based on the low mileage - I'd expect to have to check the lash etc every couple of years.
 
On the 964 you are meant to check the lash at around 12,000 miles IIRC, which is around every three years for me - probably a longer period than I'd leave it, but we'll see.
 
Amazing project.

May I please have a go when it's finished? :D You can have a go in mine.
 
What's involved in the additional bearing at flywheel end - sounds like a big job. Grind crank and housing, put in additional oilway into the crank?

Will Hartech start doing this mod too?
 
Fascinating stuff!
Further than I'll ever take mine but interesting nonetheless.
 

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