Porting heads...change intake runners?

JohnJL · 08-02-2005, 03:13 AM

I was just about to call the machine shop to remind them to port out the intakes to 39MM when I decided to go check my intake runner outlets...sure enough they measure 33.5MM.

I've read that the optimal shape is to have a 1.5%/inch decrease in the intake runner diameter to encourage density & air speed.

Hitting a 'shelf' on the way into the intake seems counterproductive. So to those of you who have 'ported' your intakes, did you also perform corresponding surgery on your intake runners also?

thanks

jluetjen · jluetjen's Garage

Having the intake manifold significantly larger then the intake ports won't be good for your flow, accoustics nor suspension of the mixture. Too big isn't good either, especially I suspect if you are using CIS. What sort of engine configuration are you building? Specifically size, rev range and camshaft choice.

JohnJL · 08-02-2005, 12:46 PM

Hi John,

Far from CIS! here's the build shortlist:

3.0 L
JE 9.5CR
Megasquirt II V3 board
EDIS-6 ignition
DR 20 cams
Building for 3000-6500 peak torque range

What's the correct setup for widened/streamlined ports. These started out as the late US heads (34mm intake i believe). The stud spacing and mating surface will clearly take a wider profile and I've seen others doing it here. Did you match your intakes to the wider ports? Seems like if you put narrow intakes in front of wider ports you'll create a LOW pressure area right in front of your intake port!?!?

thanks

William Miller · 08-02-2005, 01:05 PM

I'm posting as a point of data, and, I have a related question on this matter which I have never been able to figure out.

I have an 83 ROW. I know the 930/10 engine was replaced with a 930/10. (This is based on the engine number and serial number.)
I really believe the PO was taken to the wood shed.
The CIS remained original with the larger intake runners.

I rebuilt the engine about a year ago.
What I found made no sence. The intake runners were larger than the ports in the heads. This created a step reducing the diameter. I looked up the specs. in Waynes rebuild book and it indicated that the US and ROW port sizes were the same.
That just seems wrong to me so I opened up the intake ports in the heads to match the runners.

I have never been able to confirm if waynes book had a typo or if it was designed that way for some reason.

I do know that the engine seems to breathe better at higher RPMS, but no scientific measurements taken.

Also during the rebuild I timed the cams ROW specifications which I understand increase higher rpm performance at the expense of lower end tourque.

jluetjen · jluetjen's Garage

I'm not familiar with DR20 cams, but I'm assuming that the peak torque engine speed will be about 4500 RPM. If this is the case, then 39 mm intake ports will most likely be OK. You may find that the engine is a little weak off of idle, or it may not be significant. But on the whole the intake gas speeds for your engine with 39 mm ports falls within the "normal" range of 911 examples.

JohnJL · 08-02-2005, 03:37 PM

Thanks John, my understanding from other posters here is that the stock 34MM ports will choke the engine at higher RPM, which is my reason for wanting to open them up some. My question therefore is what to do about the intake runners which are still 34MM.

randywebb · 08-02-2005, 03:52 PM

I would try to inrease the dia. of the distal ends of the runners to match. Beyond that, you need to build yourself a flow bench...

jluetjen · jluetjen's Garage

I think I agree with Randy. (distal???) Starting on the manifold face of the heads you'll want to open the ports up to 39 mm to match the manifolds. Then gradually blend them into the existing diameter over the next 1 - 1.5 inches. I wouldn't go messing around in the bowl without having some good flow bench data.

randywebb · 08-02-2005, 04:17 PM

ok, the part near the port...

artplumber · 08-02-2005, 05:12 PM

Quote:

Originally posted by jluetjen
(distal???)

Don't worry Randy, we know what you meant.

randywebb · 08-02-2005, 05:31 PM

good -- after looking at this thread w/o an overheated brain from sceraping my house in the sun I realize I really should have said proximal.....

if the thread goes on long enough I will change to sagittal...

JohnJL · 08-03-2005, 01:21 AM

sceraping?

ha, thanks guys. would be interested in others' views...i guess this is how people end up springing for TWM ITBs, right?

randywebb · 08-03-2005, 02:14 PM

that one wuz jest a typo - scraping

Luckily, I am now down to priming. Unfortunately it is 10 degrees F hotter today.

dd74 · 08-03-2005, 03:24 PM

To widen or not to widen. It's interesting: I believe it was a Car and Driver road test of the 1983 Porsche 911 SC that suggested the 3.0 performed better with the 34mm ports than with the 39mm. Where it performed better, seems to be in low-end and mid-range torque, thus making the motor more optimal for city driving.

As to what engine builders suggest to do with the 34mm ports, some suggest adapting the older CIS to the newer 3.0 bottom end, which has the higher (U.S.) 9.3:1 compression, but to suffice for any loss in low and mid-range torque, include with this modification a higher lift 964 or 20/21 cam. Also, some builders suggests SSIs or 3.0 headers for better exhaust flow to compensate for the increase exhaust speeds due to the the higher profile camshaft and the fact the later 3.0 with the larger intakes will be gulping more air. This is an interesting enough proposition - I'm just wondering if these modifcations are worth it.

My engine is a later SC motor with smaller intakes and '74 heat exchangers. As an earlier poster stated, the smaller intakes - and I assume, in my case, the smaller diameter heat exchangers, could lead to a choking of the engine at higher RPMs. Oddly enough, I have never found this to be the case when compared to SC engines with larger intakes that I have driven.

I'm not looking for an answer, really, of even to make a statement, but just passing along what I've heard in conjunction of modifying a CIS engine to its maximum state. I'd like to hear viewpoints on this, though.

artplumber · 08-03-2005, 05:40 PM

Quote:

Originally posted by randywebb
if the thread goes on long enough I will change to sagittal...

Don't forget coronal, cephalad, caudad, etc.

randywebb · 08-03-2005, 08:31 PM

and crawdad...

back to the ports...

- regardless of the proper size & the quantitative detriment for being x amount bigger or smaller than that size, there is also the issue of a sharp mismatch in the tube the ari flows thru. That was what jumped out at me from the facts above. You want to avoid having a fluid hit a wall abruptly at the edge of the flow, or to abruptly "fall over a dam."

William Miller · 08-04-2005, 04:40 AM

dd74
the 81-83 ROW engines 930/10 maintained the larger intake runners, 9.8:1 CR and slightly retarded valve timing specifications.
As I stated above I have been trying to find real evidence that the ports in the heads are in fact the larger diameter to match the intake runners.

When rebuilding mine I had to open up the ports on the heads to match the larger intake runners.

Anyone have the later ROW heads?
I would think the 78-79 USA may also have larger ports because the intakes runners were larger. If not then there is a step where the air leaves the runner and enters the port in the head.