Port size?

[Patrickprice]

Thanks in advance for your opinions.

I am a professional Mechanic and serial restorer of European cars. I am currently building a 1974 911 to be used by me on some long retirement drives.

Here is my question:
I am building the original base 2.7 with new 8.5:1 "S" P and C's, Web 20/21 cams, recurved distributor and a lot of durability mods (New TRW valves and guides, case savers, bypass mod, 4 rib pump etc). I am keeping the CIS. The car most likely will never see a track so I am looking for a good all around performer. currently the heads are 32/32 on the ports. since I own a milling machine I can set them up and punch them out fairly easily. The question is should I and if so how much? Is there a sweet spot between maintaining port velocity and allowing the engine to breath at the higher end?

[PeteKz]

Patrick, although you have a lot of mechanical experience, how much of that is in early air cooled 911's? The engines in particular are not like other cars.

How many miles were on the engine before you removed it? Where are you in the disassembly process, and what parts have you already bought? How have you made your decisions so far? Who is doing your case/crank/rods machine work? Be sure they install "case saver" inserts for the head studs too.

Two books to get, if you don't already have them: "Porsche 911 Performance Handbook, 3rd Edition" by Bruce Anderson; and "How to rebuild and Modify Porsche 911 Engines" by Wayne Dempsey.

For a street car, you are smart to focus on durability and reliability. The CIS is very durable and reliable, and can be tuned to work better than stock, and better than individual carbs (that's a personal opinion from someone who has stuck to CIS and become familiar with its advantages and drawbacks--the "carburetors uber allies" fans won't agree). CIS will also deliver better startup, drivability, and fuel efficiency--no argument there.

However, you can do better than 8.5:1 CR. 10:1 is about the limit with 92 octane gas. you should aim for somewhere in the 9.5 to 10 range. Other pro builders here can offer specific parts suggestions for the 2.7, as I have spent my engine rebuilding efforts on 3.0/3.2 engines. When you get to the reassembly point, keep posting your progress. Pay very close attention to getting clearances in a particular range, both for performance and durability. I shoot for .030"/0.75mm as the piston to head clearance.

Heads: Yes, there are sweet spots for port and valve sizes and cam combinations. The good news is that for a street car, you don't need to do head mods to get a good torquey engine. Those are only useful to racers and those who push 6000+RPMs. You can see from the heads in your hands that the ports are nearly straight to the valves, so there's not a lot of magic lurking in changing the port shapes. If you do decide to do port work, only use the few reputable shops who have done this over many years and can tell you exactly what combinations of cams, ports, valves, and RPM work to give best mid-range torque. Remember it's a street car.

Valves: There's a recent thread here about the poor quality of recent TRW and other brands of valves, including obviously different quality under the same TRW name. There could be fakes in the supply chain. If your original valves are in good condition, reuse them. Otherwise, be careful what you buy and from whom you buy it. If one seller has a price substantially below other sellers for the same parts, they are probably cheap knockoffs.

https://forums.pelicanparts.com/911-engine-rebuilding-forum/1158172-valves-should-i-stay-away.html

Rockers: Don't forget these while you have the engine apart. They almost certainly need rebuilding too.

See this thread on current best engine sealants:

https://forums.pelicanparts.com/911-engine-rebuilding-forum/284737-complete-engine-sealant-thread.html

Otherwise, save your money for other mods, especially for suspension refresh and improvement. After 50 years, suspension bushings, shocks, brakes, and other parts need renewing too.

[PeteKz]

And post a picture of your project too!

[targa72e]

My testing has found that heads with 36mm intake and 35mm exhaust, compared to stock smaller ports, made more power everywhere you are likely to care about. You can see some of my dyno testing in the thread below.

https://forums.pelicanparts.com/porsche-911-technical-forum/1151593-dyno-summary-testing-early-2-4l-2-5l.html

john

[PeteKz]

John, thanks for posting that again.

[eastbay]

As mentioned, if replacing pistons go higher compression, 9.5 at a minimum.

Standard 2.7 "S" build is 35mm ports intake and exhaust from the factory.

Also remember if you port your heads you need to change the CIS runners from 0Rs to the larger 2Rs. May as well port match them to the heads, makes a difference.

edit: Instead of recurving the old distributor, put in a 123 with bluetooth and then your curve can be whatever you want it to be.

[Patrickprice]

Thanks all for the good information. Based on the Dyno info I will most likely go out to 36 / 35. Luckily, in my bin of CIS stuff I do have some 2R runners so I can hopefully uncork the intakes enough to make it fun.