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Dammit Reader
8/12/18 12:23 p.m.


WillHoonForFood New Reader
8/12/18 6:42 p.m.

In reply to Dammit :

Nice! The reason I asked is that I work for a 3D scanner manufacturer, and find it awesome to see regular people using for the exact reasons I would. Good stuff!

Was this by any chance a CT scanner?

Durty New Reader
8/12/18 7:44 p.m.

You're not "screaming into the void." There are plenty of lurkers and its great to go back through a well documented build. I really like the built NA motor ethos and I haven't seen many water cooled 911 builds like this.



Dammit Reader
8/13/18 1:41 a.m.

There are a bunch of reasons I chose this project:

- This car was new out when I was in my early 20’s and I really wanted the GT3 when it was announced

- The M96 variant 996 is the red-headed step-child of the 911 family, “those” lights, “that” engine, the car that killed the air cooled

- But they are truly great drivers cars

- And because of point 3 they are cheap, an affordable canvas that you can paint your perfect 911 on (in a way that you used to be able to do with the 964, before they became £70,000 cars)

- This project also allows me to explore an alternate branch of history- what if Porsche had never been allowed to homolgate the Mezger and had to use the M96 in the Cup cars?

- finally, this will cost more than simply buying a good GT3, all in, but this way I have my perfect car and for me the journey is part of the destination

Dammit Reader
8/13/18 2:37 a.m.
WillHoonForFood said:

In reply to Dammit :

Nice! The reason I asked is that I work for a 3D scanner manufacturer, and find it awesome to see regular people using for the exact reasons I would. Good stuff!

Was this by any chance a CT scanner?

We used these guys: https://www.t3dmc.com/

Xceler8x UberDork
8/13/18 11:31 a.m.

Please keep posting. I'm really enjoying this build. I'm very interested to hear about the next track day after the engine mods are done. 

Dammit Reader
8/18/18 12:19 p.m.

Glad to hear that you are enjoying the story - will be a while before the engine goes into the car though!

To reduce delays/enable us to crack on whilst I still drive the car I've bought a new Boxster S short block - these are 3.2, but the stroke (and, indeed the casting) is the same as the 3.4 litre 996 engine. The bore is smaller, but we're going to machine out those cylinders anyway, so that doesn't matter. 

In it there is of course a new crank, rods and bearings (we'll be using different, 100mm pistons), which means that the 3.7 will be basically a new engine. It also has a Hivo IMS chain and the large IMS bearing (as fitted to the M97 engines).

Dammit Reader
8/27/18 3:38 p.m.

(A lot of the following post has been copied from one written by the engine designer (Mike), I'll try to make it more of a conversation so that the context is present).

We'd been having a fairly in depth discussion about stroke length - it had become a critical point that we could not get past, and it had to be decided. There are lots (and I mean lots) of good reasons to go for a longer stroke, larger swept capacity, and be able to both pull the proverbial stump from the field at very low revs. However, for me that's not a sports car engine. I want what Mike calls an old school sports car - ‘ to feel the engine come alive at 4000 rpm rather than pull like a turbo diesel from low revs’. This means that we will be throwing away one of the M96’s strong points, namely the grunt it produces from very low in the rev range and replacing that with an extended rev range, with a goal of an 8000-8200 rev limit. 

For those reasons, various components have been investigated for suitability. The tappet carrier was identified as being weak upon our first inspection, a point verified by the acknowledged M96 expert, Baz at Hartech. As some of the increased performance will be coming from more aggressive cam profiles, it was an easy decision to try to eradicate this weakness, but its still ongoing - the CAD plans are now at a good stage and we hope to have some carriers ready for testing shortly.

With increased rpm and more aggressive cams, stab torque loadings imparted to the timing chains will increase, so some clever profile design is going to be essential. In order to achieve enough area under the lift curve with relatively short duration, we have chosen to go with a mechanical tappet, which also eliminates any pump-up at high revs. This is in hand, but very much dependent on final engine spec. 

We have looked at capacities ranging from 3.4 to 4.0 litres. Baz has been a mine of information here, as he knows all of the strengths and weaknesses of the various engine sizes, from 2.5 to 3.8 litre, and it was he that pointed us toward the possibility of using a crank from the smaller engines to create a short stroke rev monster. He also pointed out the difficulties involved with trying to go to a full 4.0 litre, namely, that of the difficulty of assembling the engine due to the design of this particular unit. 

With the deck height (the distance from the crank centreline to the top of the cylinder liner face) being fixed, give or take a few thousandths of an inch, it follows that a longer stroke requires a shorter rod and vice-versa. A short rod has more angularity during one revolution, leading to a greater force being applied to the thrust face of the piston, which in turn loads the cylinder face more greatly. Hartech’s Nikasil liner will be much more resistant to seizure than the Lokasil standard block, but the problem can be exacerbated at high rpm. A short rod tends to lift low-end torque, as the greater rod angle gives a larger mechanical advantage, but this turns to a frictional disadvantage at high revs. Another side effect is that the piston is accelerated faster from top dead centre, but hangs around longer at bottom dead centre, which has implications on the final cam design. The short rod also leads to higher peak piston acceleration vs its longer counterpart, which puts a higher load on both the big end pin and piston assembly, but nothing too severe at the target revs. 

So various capacities have been considered, comprising the following bore and stroke configurations. 

1. 100mm bore, 72mm stroke, giving 3.4 litres, utilising a standard Porsche crank and rods. This gives the maximum overlap between the big end pin and main bearing journal, leading to what would be most suited to very high rpm. 
*RPM@ 20.148 m/s, 8395 
2. 100 bore, 76 stroke, giving 3.6 litres. This would require a bespoke crank and rod set up, but a very good engine size to achieve Dammit’s goals. In EN40B, this crank would be very strong, and along with having the rods made from 300M material, would be happy to rev hard. 
* RPM, 7976 
3. 100 bore, 77 stroke, giving 3.63 litres. As above but with a fraction less pin overlap. 
* RPM, 7842 
4. 100 bore, 78 stroke, giving 3,675 litres. This is the stock 3.4 stroke, so could utilise standard components. The feeling is that the stock parts would not be up pulling 8k rpm, at least, not for long. A billet crank and decent rods would put that right. This was our initial favoured configuration. 
* RPM, 7745 
5. 100 bore, 80mm stroke, giving 3.77 litres. Again a bespoke set up required. 
*RPM, 7576 
6. 100 bore, 82.8 stroke, giving 3.9 litres. This is the stock 3.6 stroke, so a standard crank and rods could be used, but are unlikely to take kindly to the increased rpm. A steel version would survive, but there will be considerably less pin overlap than with the 3.6/3.7 configuration. 
*Base RPM, 7300 
7. 100 bore, 84.5 stroke, giving 3.98 litres. Potentially tricky to assemble, Baz has said that the clearance between the rod bolts and the bottom of the cylinders would require attention, also the window for gudgeon pin fitting could become an issue. As a one off experiment, we feel that an engine of this capacity represents a risky proposition at this juncture. Besides, it would lift torque at the low end and most probably run out of breath at the top, the exact opposite of the end goal here. 
* RPM, 7153 

*In terms of hitting the required rpm, Mike looked at the stock 3.6 996.2 stroke and calculated the mean piston speed. This works out to 20.148 m/s at 7300 rpm. Translating this number to the various configurations, a 4 litre would have a redline at 7153 rpm, whereas at the opposite end of the scale, the short stroke 3.4 could be pulled to 8395 rpm. These numbers are shown above for reference vs. the various stroke lengths. So keeping the standard Porsche mean piston speeds as a reference, my target of 8k is met with a stroke of 76mm, at 7976 rpm. Did anyone say Mezger?! With the components being fitted to this engine, these numbers are very safe in terms of bottom end reliability. For reference, a well-developed race motor would happily stand 24.5 m/s . 

Well, there it is folks. It would appear to be a toss up between a 76 or 78mm stroke, the latter leading to 7745 rpm, which with the calibre of components being fitted to this engine, should be capable of withstanding 8000 with little problem. 

There is another variable that will require attention, namely the compression ratio, which due to the relatively restricted deck heights will be hard to increase meaningfully. To achieve the sort of power required, the optimum CR is going to be north of 11.7:1, possibly even as high as 12.2:1, which is pointing towards a possible piston crown remodelling. Some careful consideration is required here. 

Of course the various capacities require different airflow characteristics to optimise each set up, but we are a little hamstrung here with the stock valve sizes, or at most, stock +1mm. The engine is still a road engine, not a race version, so compromises have to be made. We are at the mercy of the original Porsche valve centreline pitching, trying to move this would be difficult to say the least, so they are staying put for this project. 

The new shape torque curve will require a complete rethink on gear ratios, enter Albins of Australia: http://www.albinsgear.com.au/

In terms of induction, I took the decision out of Mike's hands, opting to go for individual throttle bodies, which will help contain the effects of larger mechanical cams, as opposed to the hydraulically operated stock ones, along with adding a dose of theatre along the way. The reason they work this way is because the throttle butterfly is closer to the valves, meaning that any reverse flow does not have the ability to get back to the plenum where it can mix with clean intake air, which in turn corrupts the quality of the intake air to any linked cylinders. If a single butterfly was being utilised, variocam plus would be required to try to stop the back flow during the increased overlap period of the larger cams, something that is sadly lacking on the 996.1. 

The throttle bodies have altered the way the ports have been modified in the heads, as the standard runners have a slightly curved form to the roof and floor, a sort of continuation of the intake manifold that sits atop them normally. At the time of writing, more work is required on the inlet ports in order to hit my self -imposed target. They are on the way there but not good enough yet, though the exhaust ports are on target. The port runners will be straightened out to take advantage of the individual runner intakes, the flow bench will dictate the final design. 

The last consideration is that of the cam drive. The increased revs allied to a change in the cam profiles will show any weaknesses up very quickly. The single chain between the two cams may lead to problems, time will tell. We have to hope Porsche over engineered the variocam and chain drives.

We're striving to do a proper job here, ending with a fully resolved engine that is both tractable and easy to live with, yet will pull the needle around the dial until it's pointing to 8,000 rpm with authority, whilst emitting a sauronic flat-plane crank howl through six individual throats.

Still a huge amount of work ahead!

Here's a picture of the car in the British Summer, at the weekend, just to reinforce some cliches:

Dammit Reader
8/31/18 5:08 p.m.

Now ready for machining. 


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