Ideal intake port size for an SC?

[911st]

How do we come to what an ideal port size might be.

If one keeps the stock 35mm exhaust port, stock cams, and ran a full race 1.5" exhaust, what might the ideal intake port size be?

Assuming stock SC gears on a track car. HP goal is about 220whp.

I suspect the small port 34mm are just a little on the small side and the 39mm big ports are probably to big.

Thinking about a 35 or 36mm might be a better fit.

[Gunter]

Depends on the year:

'78-'79 has larger intake runners and ports,
'80-'83 has smaller ...................................

Carbs or CIS or..............?

SSI's ??

Muffler: 2in/2out ? Or?

Why stay with stock cams?

[911st]

Sorry,

Probably PMO Carbs, maybe EFI or MFI.

With stock gears -- SC's on a 3.0 I believe are a great cam.

Also it works well with the stock pistons and compression. Not much to be gained going past that for what I want other than letting them do there job by improving the motor's breathing potential on the intake and exhaust side of things.

[JP911]

Great questions, ST. I've read posts over on the Engine forum where a few of the experts were commenting on the benefits of a smaller port to manage optimal velocity. The 'herd' says go with the bigger ports on the early SC heads, but maybe somewhere in between (as you mention) would bring even greater gains? Likely dependent on injection vs. carbs, and even the height of the manifolds, location of butterflies, ecu tuning when using injection, 3.6 plenum, etc. Might be worth a call to one of the minds in the air-cooled 911 engine community to discuss.

Jerry Woods and Jeff Gamroth have experimented with all kinds of different setups on stock 3.0s for spec 911 cars. I'd start there.

[RWebb]

depends on where you want the peak hp to be; how broad you want the torque curve

choose the cam and let the ports follow

[jpnovak]

Assuming your SC cams exhibit a peak HP around 5500 rpm then the 34mm ports are pretty close to optimal. I have not seen any evidence that using carbs shifts the peak HP or TQ values by much.

John Luetjen did a empirical analysis of all Porsche racing engines and found that they produced best VE with an intake speed at the port of less than 100m/s. Using this value I calculate your peak HP intake gas speed is 100.76 m/s @5500 rpm. Right within the given parameters.

If, however, you shifted the peak HP up to 5800 then you would need nearly a 35mm port to be optimal giving the other remaining constants. Of course, you might consider staying at 34mm since this would really boost the mid-range torque by maintaining port velocity.

Personally, I am a believer in smaller ports for most engine applications. All-out race motors are not included in this. The key is flow velocity not just volumetric flow. The port size must be matched to port velocity that must be matched to your cam's lift, duration, valve sizes (curtain and flow) and engine displacement.

Make them work harmoniously and you benefit when you put your foot down at any speed.

[911st]

Jamie,

That is the kind of info I am looking for.

SC cams when not on CIS make peek power at about 6200rpm. It can be moved up and down a couple of hindered by playing with the cam timing if the intake or exhaust are flowing really well.

If you come back at 36mm for the intake do you know if the 35mm exhaust ports optimal?

What do you think?

[Joe Bob]

Assuming you are going with SSIs?

[Eagledriver]

I think you'll find that with 46mm carbs the peak power (with 39mm ports) is above 6500 RPM. As with everything on a race car, port size will be a compromise. Tracks with long straights will favor big ports. If I can stay in 3rd gear until 7200 RPM and you have to shift at 7000 because of your smaller ports, I'll pull you on the long straits. On a twistier track like Sears point your extra torque at 5500 RPM will help you pulling out of all the turns. I suspect that 36-39mm ports are probably about equal on most tracks. If I had my choice, I'd try 37mm and see how that works.

-Andy

[joetiii]

Originally Posted by 911st

Jamie,

That is the kind of info I am looking for.

SC cams when not on CIS make peek power at about 6200rpm. It can be moved up and down a couple of hindered by playing with the cam timing if the intake or exhaust are flowing really well.

If you come back at 36mm for the intake do you know if the 35mm exhaust ports optimal?

What do you think?

I've been following the discussion on this board regarding port sizes for a while now and I have yet to hear anyone mention opening up the exhaust ports. I believe the stock size is good until you need to empty a 100mm cylinder. Those that race a highly modified 3.0 or 3.2 may use the 993 headers through stock exhaust ports to get some added HP up top. I understand it costs some loss in lower rpm response.

BTW, I have a stock 3.0 SC engine that has PMO 40s and SSIs. I have not played with cam timing but my HP peaks at 5500. I would love to know the timing specs in order to pull harder up top. It would save me from opening up a perfectly solid motor to add more cam, compression and magical port work.

[haycait911]

stay small for driveability and low end torque. I'm going to 36mm on my 3.2, but stayed at 34mm for the 3.0. see below.

3.0 dyno day - a pleasant surprise

[Steve@Rennsport]

Originally Posted by 911st

How do we come to what an ideal port size might be.

If one keeps the stock 35mm exhaust port, stock cams, and ran a full race 1.5" exhaust, what might the ideal intake port size be?

Assuming stock SC gears on a track car. HP goal is about 220whp.

I suspect the small port 34mm are just a little on the small side and the 39mm big ports are probably to big.

Thinking about a 35 or 36mm might be a better fit.

Well sir,....there are a LOT of variables here.

First, there is no such thing as a "full-race 1.5" exhaust" for a 3.0: thats a nice size for a mild engine but far too small for a moderately aggressive 3.0, much less one in full race tune.

With your stated goal of 220RWHP, you should be using the early, large-port SC heads (or Carrera) ones as those will do the job very nicely.

[911st]

Steve,

Thx for the help. I meant with Flowmasters Phase 9's, or such.

The big port used to have a poor reputation for flow, now it seems to be the go to head on the SC. Wonder if a properly done port job on a small port head might have any advantage w SC cams.

Joe,

I could be wrong. Maybe try timing them to 77 euro or 3.2 Carrera spec's. Maybe the port size or tune has something to do with it. Go to 911chips.com and look at the dyno section. Carrera's run ths same cams.

My friend and my 3.2's with cat bypass & chip peaked at 6200rpm and peak TQ was flat from 5150 to 5550rpm. All stock otherwise.

Again, not an SC but same cam.

I know the 930's that run SC cams re-timed make peak HP around 5500rpm.

[911st]

Originally Posted by joetiii

...BTW, I have a stock 3.0 SC engine that has PMO 40s and SSIs. I have not played with cam timing but my HP peaks at 5500. I would love to know the timing specs in order to pull harder up top. It would save me from opening up a perfectly solid motor to add more cam, compression and magical port work.

Joe,

Check this out. SC cams peaking at about 6500 on CIS but with a very good full race exhaust (larger than 1.5").

It is also a probably a good example of the large port head.

[Steve@Rennsport]

Originally Posted by 911st

Steve,

Thx for the help. I meant with Flowmasters Phase 9's, or such.

The big port used to have a poor reputation for flow, now it seems to be the go to head on the SC. Wonder if a properly done port job on a small port head might have any advantage w SC cams.

The small-port heads are OK with SC cams on a 3.0 litre motor. They will limit power and effective RPM to about 6K in that configuration.

The large-port heads work very well with a little more cam, induction, and exhaust without a big penalty below 2K, but it really depends on what someone is trying to accomplish.

Since the large-port version of the SC head support power to about 325BHP, they do afford a lot more flexibility than the smaller ones.

[911st]

Big thanks Steve!!!

So much to learn.

[joetiii]

911ST,

I think that the race exhaust, 39mm intake port and the Euro higher compression allows this engine to make more power over 6k. Like Steve says, it shouldn't give up too much below 3k.

[911st]

I'll buy that.

I am thinking mostly cam timing, larger euro port's, and a great exhaust being key.

With the added compression mostly moving the power bands up about 3 % everywhere.

[Gunter]

By "great exhaust" or "fuul race", I assume you mean 2in/2out. ??

If so, be advised that virtually all 2in/2out have the most horrible drone between 2 and 3000 RPM. Maybe track with ear protectors but not for street-driving, IMHO.

When I say horrible drone, I mean excruciating and bone-shattering.

I have tried several 2out types like M&K and Dansk and for the sake of my hearing ended up with 2in/1out and sanity.

Lots of others had the same experience; do a search.

It's not the noise, it's the drone.
When cruising on the Hwy. in 5th, you inevitably get the "treatment".

[Flieger]

I think the trick is to not spend much time between 2000 and 3000 rpm. My car idles about 1500 and I think 3500-4000 is my typical cruise. I have never heard such a "drone" at any rev range. I have a "gutted" "homemade" sport muffler: the banana with the baffles cut out and two exits welded on in the middle.