Can small port SC heads be modified to be big port?
 

Can a standard 911SC small port head be opened up to big port specs? Just wondering as the small port heads seem to be more available. Seems to me, the only difference it the intake side? or am I missing something?

thanks!!

Steve,

Yes they can. Simple way is to open top of port to match your intake runners. Better way is to take advantage of the material and have them properly flow matched and ported so they are better than the big port heads.

Big is not always better. Big means you need to turn the engine at higher rpms to develop more horsepower. Not very useable around town.

This is my opinion and what I did to my engine.

1981 3.0L rebuild with the small valves. Since I have been doing autoxing and track days for many years I decided to go the other route. What pulls the car out of the turns is torque not horsepower. That's a big deal at an autox and depending on the track, the same. What I did was have a cam cut that lowered my torque peak while still developing reasonable horsepower, and the smaller valves aid this setup.

The net result is 225 ft/lbs of torque between 4200-5200 rpm and 245 hp between 6200-6800 rpm. With my 915 transmission when I shift at 7000 it drops to 4000 rpm right into the torque peak.

Easy to open them up by port matching to what ever intake you plan. The valve size is the same. Only difference is the small port "necks" down more at the manifold. So all you need to do is open up the top of the port where it meets the manifold while smoothing it into the valve seat area.

But as mentioned, first decide on the size that will be optimal for your set up. There are calculations that can be done to determine that.

First determine the use, and personality you want for your motor. Street, AX, or Racing? Do you want a motor that will pull NOW when you step on the throttle OR one that is gutless below 5K rpm, but screams from there to 7,500 rpm?

Then tailor every component in your motor to meet those goals.

FWIW, I THOUGHT that the 39mm ports that my motor came with were really to large for my intended purpose. Since I auto cross I wanted a motor that would pull NOW when I hit the gas, especially when I was coming out of slow turns.

It turns out that they worked just fine for my setup, but my choice of cams and manifolds help.

What cam did you go with Kent? I’m trying to get a game plan together for my 82 3.0 rebuild and as alluring as a high HP motor that spins to 8,000rpm is, I think something like you have with an emphasis on torque and performance between 3-5000rpm will be much more enjoyable on tbt street in the real world.

Dougherty GT2-102

Maybe I should add, Twin plugs, 9.5:1 pistons, webers 40mm opened up to 42mm, SSI into M&K 2in, 2 out. Mike Bruns from Florida, when he worked for J&B racing, helped me build the engine. We modified it to turn 8500 rpm, I wanted something that was "bullet proof". One accidental over rev on the track to over 8000 rpm was a none event.

Me too. Best cam for anything but a full on race motor for 3,0 liters IMO.

is that 245whp or crank hp? Sounds like a good setup.

What all was done to make it 8500 safe?

I’m assuming this statement has the caveat “non-CIS.” If not, would y’all mind elaborating? The 964 and Webcam 20/21 have been long lauded for CIS applications, but with some asserting that bottom end suffers. Would appreciate any input on the aforementioned cam. Thanks.

Oh yeah that was at the crank. Mike and I ran the engine on the engine dyno at J&B racing in Tavares, Fl for 2 days. The rockers and springs were updated for that high rpm. Also Mike wanted the crank modified. Apparently the center bearing is a weak spot on the race engines so it was modified to make sure it doesn't lose oil. Not sure what was done but we did that also.

Transmission lever slipped in the floor mount on the track at Portland one day about 7-8 years ago. When shifting into 5th gear the lever turned and I actually shifted back into 3rd at 7000 rpm. I remember the needle going past 8000 as I de-clutched and took my foot off the gas. Coasted down the back straight into the pits. The engine was just idling, wasn't making any bad noises. I rev'd it up and it sounded ok. Went back out, one lap just up to 3000, then one up to 4000 then thought I'd better take it home.

Called Mike back in Florida. He said pull the oil drain and check the magnet for metal. NOPE, and it has run fine ever since. BULLET PROOF.

CIS, cough cough, not on my car. It is for sale though.

My motor is a mostly stock '78 SC. Mods are JE 9.5 pistons in stock Mahle 95mm cylinders. CR actually measures out at 9.8-1. Cams are John D's GT-2 102. With those my motor is very lively and responsive. I can put my foot down at 3000 rpm and it pulls. Pulls to 7K with my limiter at 7,200. Eibach racing valve springs. Weber 40s and backdated exhaust with a modified Dansk sport muffler. Re-curved dizzy with no vac.

You can go to Dougherty's web site and see all the specs. Basically the GT-2 102 is a custom grind. It could be best described as a mod-solex on steroids. Less duration than an S cam, but with more lift. The standard GT-2 cam has a lobe separation of 112 deg, so my cam has more overlap due to the tighter LS of 102 degrees, which is suited to carbs.

Yea, no way with CIS or stock pistons.

Back to Steve's original question.

Since this is a club racer you might have restrictions on the build. Check rules first.

As mentioned, the port size can be calculated based on airflow velocity. large ports can cause Too low velocity and you give away cylinder filling at low rpms. This means less toque coming out of turns. Too small ports choke off airflow at high rpm and limit HP (high rpm torque fade). There is an optimal for your engine, compression, cam, power band, etc.

Jac, there are options for a CIS car...

Gordon and Jamie, thanks. Went back and read a thread where Jeff Higgins talked about what I assume was the genesis of the Dougherty GT2-102.