Ideal intake port size for an SC?
 

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.

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?

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.

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.

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

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.

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?

Assuming you are going with SSIs?

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

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. :)

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.

http://forums.pelicanparts.com/porsche-911-technical-forum/502996-3-0-dyno-day-pleasant-surprise.html

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.

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.

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.

http://forums.pelicanparts.com/uploa...1291342686.jpg

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.

Big thanks Steve!!!

So much to learn.

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.

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.

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". :(

I think the trick is to not spend much time between 2000 and 3000 rpm. :D 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.

Exhaust port size on a 3.0 ??
 

I remember intake port being 39 mm on the '78-'79 SC's and 34 mm on the '80-'83 but what size are the exhaust ports?

And for that matter, what are the sizes on SSI pipes for SC's '78-'83 ??

All kinds of info about SSI's on Pelican and Stainless Systems Inc. but nothing about pipe sizes. ??

Thx. SmileWavy

The exhaust ports are 35mm.

SSI's are also 35mm I.D.

Thanks Tyson,

Even though the '78-'79 has larger intakes than the later SC's, the exhaust size is the same for all 3.0 liter regardless of intake size?

'78-'79 intake 39 mm
Exhaust 35 mm

'80-'83 intake 34 mm
Exhaus 35 mm

Somehow, that doesn't sound right. :confused:

I know many have a exceedingly simplistic mental model of an engine being the same as an air-pump; but clearly, much more is going on.

Intake size varies to optimize the mixing of fuel and air for the combustion process, thru the rpm range.

Bigger intake/valves of the earlier SC's had the same peek HP of the later (smaller intake) 3.0's, But the later had better midrange torque/HP. --that's optimization.

I suspect to shift to small ports on the SC was probably needed to achive emissions goals.

CIS is an interesting system. It basically operates at a 100% duty cycle. The only way to vary fuel delivery quantity is to vary pressure.

On top of that the intake valve is only open something like 220 deg's with the injectors spraying for 720 deg's and fuel probably just sits there sticking to the wall of the ports resulting in poor atomization.

Thus, Porsche probably went to a smaller intake port to better accelerate the incoming air to get a better mix.

This is the same time Porsche adopted different emission controls shifting to a Lambda type system and away from air injection.


Think about it. they went from a 34mm to a 40mm intake port with EFI. That is a about a 40% larger port for a 5% larger displacement motor.


At least, that is my guess.

http://forums.pelicanparts.com/uploa...1443725489.jpg
http://forums.pelicanparts.com/uploa...1443725509.jpg

Port size
 

When I built my 3.3SS engine, I decided to go big with the ports and intakes. I used a set of Carrera race prepped heads with 41mm intake ports and 39mm exhaust ports. Valve sizes stayed stock Carrera. Pistons are Mahle at 10.5:1. For an intake, I used an early CIS air box and intake runners that measure 39mm inside. I opened up the intake runners 3mm to match the heads. Cams are 964s and ignition is by Electromotive XDi twinplug. Turns out, this engine has very good low and mid range torque and will pull quite strong all the way up to ignition cutout at 7500 rpms. I think that I set the cams at 1.30 but would have to refer to my notes to be sure. I installed an Innovate air/fuel gauge to use while tuning the CIS. I have it set to run close to stoichometric (14.7) for normal driving but can richen it up with a small turn of the adjustment screw. To make certain that a valve would not run lean and burn, I added a full throttle switch that activates the cold start valve to put a bit more fuel into the engine when it is being pushed. The extra fuel at full throttle lowers the a/f ratio to around 13.5 or so. If I drop the a/f ratio to 13.5 then it goes to around 12.5. This engine idles well, is torquey and is very strong on the top end. Just what I was hoping for! Having the extra displacement certainly helps as does the improved ignition system. Fuel mileage seems to be about what it was as a 3.0 SC. Runs well on 93 octane pump gas!

No such thing as one ideal spec

smaller ports raise velocity and help lower rpm performance, bigger ports flow velocity is reduced until higher rpm where they can help

same thing w/ manifold lengths longer is better for lower rpm shorter for higher

this applies to intake and exhaust