The primary issue with 100mm "3.5" cylinders was the very thin cylinder spigots. These thin spigots were susceptible to side loading. 3.5 engines used a 3.2/3.3 74.4 stroke crank with very short rods which created excessive side loading.
The other issue with the 3.5, 100mm cylinders was that they were "bore in" requiring a machinist to bore the case to match the cylinder spigot. Two things were common. The case bore was too tight creating a stress at the join. Thermal expansion requires movement and if the fit is too tight, the cylinder shear at the deck.
I also saw a number of cases where the machined hole had a sharp edge (no champher) which caused the cylinders to crack when the cylinder transitioned through the heat cycle.
The cure was always pretty simple. Proper case preparation, longer rods or a shorter stroke crank with better rod length to stroke ratio.

This I have been thinking about too.
But why bother with the air cooled cylinders and a fan to cool them. Why not just fit the entire GT3 engine and get all the benefits with a great crank, rods, oil pump and oil tank.



Do you have an opinion on why blowers hasn't been used more?

I have a plan to use a centrifugal Rotrex supercharger on my 3.6. I know its not optimal from any perspective, but its small enough to fit where the AC compressor once was and wouldn't require much modifications.

Why bother is a question you could attached each and every air-cooled 911 Porsche choice. They cost too much, performance is 50 years old and parts are harder and harder to source.
Why bother? I love air-cooled 911 engines. They have been my life for 40+ years.


I don't know why super charging haven't made it on many bucket lists. There is performance there but Porsche never traveled down that Porsche and the community is pretty devoted to "what would Ferdinand Anton Ernst Porsche do".

I have a friend who is an incredible metal worker that was instrumental in a huge number of Slant Nose conversions. Although it was a factory customer option, the slant nose concept hasn't aged well. With that in mind he actually told me doing all those slant nose cars is his singular greatest regret.

I love the air cooled engines to, I have no interest in water cooled 911's. :)

Tell him he is wrong! :) The slant nose are cool.

Immaculately restored long hoods are the norm today. I actually like an 80's conversion as long as they are in good condition, more interesting. They are getting rarer and rarer due to the restoration hype. And cars that someone put $100.000-200.000 into rarely gets used as it should be... which is a pity.

He's pretty convinced his efforts were wasted....He is a true artist with metal so who am I to argue.

I could not agree more. That is why my GT3 has 83,000 miles on it. Remember I walk to work, so none of that is commuting.

My latest 80 something conversion:

Back dated 87 Carrera: 338 hp @ 6500, 301 ft/lbs of torque at 4600 on 91 octane.
3.9, Supertec ITB and Haltec , the "magic" is John Dougherty's 43X, 109 ls cams.
The ITB manifolds were cast in a friend's garage using an MFI throttle body as a pattern and Volvo Mahle pistons for the aluminum. They can work with EFI or MFI.



http://forums.pelicanparts.com/uploa...1750257180.jpg
http://forums.pelicanparts.com/uploa...1750257180.jpg
http://forums.pelicanparts.com/uploa...1750257180.JPG
http://forums.pelicanparts.com/uploa...1750257180.JPGhttp://forums.pelicanparts.com/uploa...1750259065.JPG

Now , that is what I was talking about, Very nice!!

From my time w/ a 3.8 in my '76 w/ both a Mg case 915/44 and an Al case 915/67, JMO but a 915 trans needs a lot of beefing for more than a 3.6, The issue is the torque multiplication in the lower gears can misalign the cwp gears and cause, at the very least, weeping.

Besides that who needs 400hp in a 2400# package, 320 or so w/ a pouncing leopard torque curve(as opposed to a Camal shaped one) is a ball.

I've got some old Alusil pistons if you want to turn them into a set of itbs!

Well done!
I have a friend that has a GT3 with more than 310.000km (~200.000 miles) on it, most of them by the previous owner that traveled a lot between track days, I think that car has between 500 and 1000 laps on the Nurburg Ring.
BTW if you have any ideas why it sometimes rattles significantly on startups and can't pass emissions because of hydrocarbons in the exhaust at idle. We're thinking its the variable cam timing gear that is worn out. Pretty much everything else is replaced.


Impressive low and mid (3000-5000) on that 3.9 engine! Is that the "cam magic"?

We chased that rattle in my GT3 for years and replaced everything with no improvement until we learned about the check valves in the cam housing/heads.
These little check valves are rarely discussed and are not available through Porsche.
We sourced the valves with research through an Asian car company. Kia/Hyundai ?

http://forums.pelicanparts.com/uploa...1750344995.JPG
http://forums.pelicanparts.com/uploa...1750344995.JPG
http://forums.pelicanparts.com/uploa...1750344995.JPG

The cam "magic" was across the board but it was most noticeable in higher rpm.
The 993SS cams fell off dramatically above 4600. These DRC cams (43X 109) brought the engine to life and pulled all the way to 7K.
We're being told these cams won't work with stock Motronic but we're hoping to find something that produces similar numbers with 964/993 Motronic management.

On another thread about a very cool old school build, I saw a request to use this racing upgrade on my new project.
It's a hybrid cam oiling system that incorporates center feed and spray bar technology.
The challenge would be fabricating a unique chain box cover.
The drawback of using the center feed technology was/is creating a seal between the cover and the end of the cam. Historically, this was accomplished with a spring loaded, friction contact, water pump seal. These seals generally had a very limited life span and when they fail, the engine loses oil pressure.

http://forums.pelicanparts.com/uploa...1750344031.JPG

We used a different technique to replicate the same outcome.
We gun-drilled the cam, drilled the rocker pad oiling holes and drilled & grooved the cam bearing surfaces to oil the center of the cams. With this technique there is no need to fabricate a special cam feed line or unique cover. It can be accomplished using any cam.

http://forums.pelicanparts.com/uploa...1750343356.jpg

http://forums.pelicanparts.com/uploa...1750343129.JPG

Thank you, I Googled that part number and got a hit on eBay for $10!!
I will pass that along to my friend.

I find it interesting that Porsche doesn't show in PET.

The cam cover used are from 906/rsr/935. I addition to a bigger banjo bolt they have the clearance to run the oiling system. Seals are the standard centerlube items.... available but kinda pricey for something that comes in a small package!

Sorry, the 906 is an animal all it's own. 906 used sand cast magnesium chain boxes and covers with seven studs.
The RSR and 935 used die cast aluminum housing with sand cast covers and 11 studs.

http://forums.pelicanparts.com/uploa...1750459602.jpg

http://forums.pelicanparts.com/uploa...1750460305.png

http://forums.pelicanparts.com/uploa...1750460305.jpg

http://forums.pelicanparts.com/uploa...1750460459.jpg

Studs makes sense. On the bride's orders to deplete stash I've sold off half a dozen sets of racing centerlube cam box covers over the last couple of years and all were mag and 70s vintage. I held on to the NOS aluminum set I'd been hoarding and used here because they were actually FLAT! None of the mag sets were. All told me the warped magnesium covers flatten out when bolted up but it just seemed like an oil leak waiting to happen.

I’ve always thought a turbocharged, narrow “G” body 911 without a rear wing or ducktail would be a fun project. The challenge would be the intercooler(s). I would envision a twin turbo setup (one per cylinder bank) with each compressor feeding individual intercoolers, located to each side of the engine fan housing. No A/C nor heating system necessary here so that would add some room in the engine bay. Ducting for cooling air into the intercoolers would appear to be the biggest undertaking. For the engine, I would suggest an air-cooled 3.3 liter, prepared by Henry with all the best internal components, and boosted to produce around 500 bhp at a reasonable rpm. Externally, the car must appear as a stock, unadorned (plain) 911. What a sleeper this would be!

25 years ago we built a 3.1 liter 70.4 X 97mm. It was built with 8:1 compression pistons and twin plugs. The idea was to build an engine that would run fairly well before the turbo engaged. Much like the early big port 911 SC. We chose not to use an inter-cooler to reduce turbo lag. 911 SC cams, 36 mm ports and a K27 finished off the project. The engine was put in a stock bodies 74 coupe with 7 X 8 inch wheels.
The engine product 310 hp and 265 ft/lbs of torque. The short stroke long rod engine really like to spin. We had a red line of 7400 RPM.

http://forums.pelicanparts.com/uploa...1750972491.jpg

The guy who ended up buying the car added flares, an inter-cooler and a tail.

for sure

Creativity lives in small Venn diagrams.