Optimum Exhaust Port Size Question

[fast0911]

I am in the process of rebuilding my engine and plan to port the heads to better match my new setup.

1982 911SC
3.2L Short Stoke
DC-40 Cam
46mm PMOs

It was recommended that I enlarge the intake ports from 34mm to 40mm (by Richard from PMO). This is straight forward.

However, the exhaust port I am not sure about. The factory heads for this year had a 35mm (1.377in) Exhaust port but I know the 82SC was restricted due to emissions.

I attached a photo which is interesting. As you can see from the photo there is a
"Step Up " in size OUT of the exhaust port. The ID of the port goes from 35mm to the a larger ID of 38mm.

Question: Should I enlarge the Exhaust Port to 38mm (1.5in) and completely remove that transition from 35mm-38mm ??

These new specs would basically mirror a 1984 Carrera .

My guess is that I need to "even out" the Exhaust port to a 38mm or 1.5in.

However, I wanted to pose the question to folks with a lot more knowledge in this subject before I do anything...... Any input will be appreciated!!

[Flat6pac]

The recess is the exhaust pipe recess, 80 to 83 uses the recess to keep the gaskets from blowing out.
If you want more flow,you might look at a set of Carrera heads..but you do have to build up the intake

Bruce

[joetiii]

I guess it depends somewhat on your actual application. If you are building a track motor where you want primarily top end power, then porting out the exhausts to 3.2 carrera head dimensions is good. You could match them up to the header size or use 993 flipped flange heat exchangers for ultimate scavenging.

If you're building a dual purpose car that will spend most of its time on the street, I wouldn't be too quick to hog out the intakes and exhausts. Small intake ports help keep the intake velocity up at lower RPMs and help mid range torque. There are alot of posts on this board that discuss this decision/dilemma. here's one to start...small port vs large port

I too am planning a 3.2SS rebuild on my late 3.0 and when I did some port velocity calculations, I came up with an optimal 37 mm intake size. This was based on using a slightly less aggressive cam than the Mod S that peaks out around 6200 RPM. I am keeping the std exhaust port sizes as well as SSI heat exchangers which are well tuned for better mid range power on a street car.

I understand the Carrera heads are great right out of the box, and you might find a good used set for less money than having to enlarge your cuerrent setup.

Sorry if this didn't directly answer your question.

[fast0911]

Bruce & Joe ,

Thanks for the input..... I have not done anything to the heads since I am still gathering information from those with more experience ...

This will be a dual purpose car. I do want to have some decent low end "if possible". I wonder if I can have a smaller intake port than the recommended (40mm for my set up) by tricking out the heads for optimum flow. This may give me the flow rate of a 40mm but with a 37mm or 38mm intake port with higher velocity at low RPMs... What do you think?? You ever hear of others doing this?

George

[joetiii]

George,

To answer you question directly. Yes.

Do some more searching on this site and you will find there are definitely two schools of thought regarding port size. Henry at Supertec and Steve Weiner both have shown some great head work that make good horsepower with smaller port sizes. The port shape helps make the power up top without giving up much from 5k+ RPMs. 46 PMOs will def be able to deliver the fuel at that RPM range. On the track that you run, do you find yourself pulling out of corners needing more rpms? If you do, plan on tricking out the heads with Performance springs and retainers to be safe at 7k. This gets into spendy territory and is a questionable investment if you are an occassional tracker. I think you can talk to either guy above and get more experienced advice on a dual purpose setup. BTW, is this a daily street driver , or a weekend canyon carver?