Porsche 917 flat 12 engine

[Foketry]

Really enjoying watching this build. Wonder why they dump the collector for the front three cylinders on the right side into one leg of the three rear cylinders and on the left side the front three collector connects into the secondary collector? Maybe there is something in the way?

I don't know the reason for this choice, I know that the exhaust manifolds must have the same length for each cylinder to have the same efficiency , example Ferrari and Honda 12V

 

[a41capt]

Really interesting exhaust system all the way around. Back in the 60s, while racing two-stroke motorcycles as a teenager, I experimented (trial and error of course) with exhaust gas flow and back pressure. Of course the piston-ported two stroke engines of the time required that timed back pressure to ensure a proper cylinder charge and still enhance the flow of exhaust, I wasted a lot of sheetmetal building, cutting apart, rewelding, and testing for the best configuration to fit my re-porting and carburetion. As a 16 year old kid, I was pretty proud of my accomplishments, but those pale in comparison to the advances in modern exhaust design.

I’m sure the Porsche engineers had a whole lot more knowledge and computer tools than I did, and their strange collector system must’ve been well engineered for performance, and yet compromised some efficiency around structural components as needed.

 

[ddmckee54]

The problem with a tuned exhaust system is just that, it's tuned. It will work well at its' dedsigned RPM range, outside that RPM range - not so much. I would imagine that there were some rather "heated" discussions between drivers, engineers, and mechanics, regarding what RPM range to tune the exhaust for.

Don

 

[Jan Dressler]

Really enjoying watching this build. Wonder why they dump the collector for the front three cylinders on the right side into one leg of the three rear cylinders and on the left side the front three collector connects into the secondary collector? Maybe there is something in the way?

Yes, the clutch / gearbox housing is wider on that side because of the starter motor which is located there.

 

[Foketry]

12 pistons, diameter 26 mm (1.02 in) , material: 7075 (Ergal)
Before making all 12 pistons I built 1, assembled on a cylinder, rotated the crankshaft to the TDC and checked the compression ratio via a graduated syringe and oil in the head.
The real value obtained is similar to the calculated value, I have slightly moved the wrist pin hole down to increase the compression a little
Now it is 7: 1 , I don't want to exceed this value because a very high torque would be needed to start the engine. My cordless drill does not have the necessary strength
Compression ratio of the real Porsche 917 engine is 10: 1

first and second turning step

Milled, drilled and threaded pistons

 

[CFLBob]

Do you tap under power on your mill? Using a tapping head or something like it, as opposed to manually with a tap wrench?

Just curious.

 

[Foketry]

Do you tap under power on your mill? Using a tapping head or something like it, as opposed to manually with a tap wrench?

Just curious.

being very small threaded holes I use 2 cordless drills, first one with roughing tap, the second with finishing tap , both with torque set to the minimum.

 

[petertha]

That's an interesting solution. So will your wrist pins have a slight flat or notch for the set screw to clamp against?

 

[Foketry]

That's an interesting solution. So will your wrist pins have a slight flat or notch for the set screw to clamp against?

my wrist pins are made from hardened commercial pins, I will make a slight ground flat

 

[Willyb]

Yes, the clutch / gearbox housing is wider on that side because of the starter motor which is located there.

Thanks Jan. I figured there must have been something to make the one side different for the other.
Cheers
Willy

 

[Foketry]

Finally ,after 2 broken tungsten saw blade , I finished milling the cooling fins of the heads

 

[Foketry]

12 finned heads

 

[CFLBob]

You know, with 12 heads, each with 12 slots, 9 of which appear to be around their entire perimeter, breaking two blades just doesn't seem that bad a price.

 

[Peter Twissell]

What was the cause of blade failure?

 

[Foketry]

What was the cause of blade failure?

both cases it was my mistake, once I moved the blade up still inside the cut, the second break I didn't fix properly the head to the jig , tungsten blade are very fast but very fragile, then I used HSS disc .

 

[michelko]

Those are beautyfull parts.

Michael

 

[Foketry]

Head assembly test , after threaded spark plug hole and counterbore 4 fixing holes

 

[Foketry]

The original Porsche engine

 

[Foketry]

The valves
I used a lead steel bar 36SMnPb14 dia 12 mm
It is a steel with medium carbon, moderate hardenability, high manganese, sulfur and lead content for good machinability.
Normally I use stainless steel, but in this case, given the large quantity of valves to be turned, I tried this material, already used on the Bugatti model engine
Several processing steps were required , cutting, centering, turning, grooving, sanding , parting.

first and second processing step

24 valves + 5

 

[michelko]

do you machine them conventional or with an cnc lathe?

Regards Michael