2.7 Head Porting

[6771911esses]

Can anyone recommend head port sizes for a modified 2.7 CIS. I am planning on going with JE 9.5:1, stock CIS injection, 964 spec cams. I am cleaning the chamber and ports but would like to know if, and how much larger I can make the intake/ exhaust ports. I would like to hear from anyone with living proof.

Thanks.

Mike

[dd74]

Seems to me that you can't port the heads unless you get larger CIS runners. At least that's what I found with my 3.0.

[6771911esses]

I haven't looked into this yet, but I was think of match porting my intakes with the CIS runners, and gaskets. Diddo for the exhaust ports. I will "clean" the ports anyway and polish the exhaust ports smooth. Just thought I could go to 36 mm intakes like the 2.7 RS, but I don't know if this will be too "rich" for the CIS. I know the CIS has limitations.

Mike

[dd74]

I'm not sure there were differing sizes of CIS for a 2.7. I know there were 4R and 2R sizes for the five-year run of 3.0s.

If you're going to 36mm and 2.7 CIS won't fit the intakes, what will you do? Hone out the CIS?

I might consider an alternative fuel management - Webers or cooler still (but expensive) throttle bodies.

[Bobboloo]

There were two basic 2.7CIS motors 74' thru 77'. The 2.7 (normal) which had 32mm in. and ex. ports, and the 2.7S which has 35mm in. and ex. ports.

What's your intended use for the motor? Is it for a street or race car?

If your motor is a 2.7 normal then porting to 35mm and locating 2.7S CIS setup my be an option for a street motor.

In 78' Porsche went to 39mm in. ports on the 3.0 CIS motor but then they turned around and made the ports 34mm in. 80' thru 83'. I don't know if this was because they felt that it made a better street motor or if it had something to do with air pollution but I know they don't just decide to make less powerful motors on newer models. My guess is that they felt it made a better street motor.

A healthy 2.7S is a nice street motor. I've got one and I imagine it would be even nicer with a higher CR.

[jluetjen]

Looking across the whole range of 911 motors, it seems like the CIS motors don't generally like big ports. My theory is that because of the slight reduction in pressure downstream of the airflow meter, bigger ports don't get you anything. In fact smaller ports (compared to a carbed or MFI engine) will actually help to keep the intake gas speeds up which will help to keep the fuel suspended and improve the turbulance in the cylinder.

But as was mentioned by Bobby, Porsche went from some pretty big ports on the early SC's to smaller ports without a big (or even measurable) drop in performance. They even were able to increase the HP to over 200 in the European SC's using the smaller ports. It is most likely safe to assume that you could go from small ports with CIS to large ports with CIS (all else being the same) and not have a measurable gain.

[kstylianos]

Competition Engineering opened up my 82 SC intake ports and runners from 34mm to 36mm.....not by my choice, but their recommendation. Exhaust ports stayed stock.

Other mods inculded SSI's, SC330 cams and 9.7:1 compression.

[6771911esses]

To all:
I am working with a 77 911S core, that has 35mm in/exh ports. Anderson's book shows that all 2.7 engines have 35mm in and exh ports. According to his book there are no different sizes except for the intake on the RS 2.7. This is a whole different animal. The CIS 2.7 engines in his book all have the same hp/ torque output. So really, this S is no different than a non S. I think the S badge may have been more of an appearance/ marketing ploy rather than a true performance distinction.

I am trying to do every tweak possible w/o running into issues with the CIS setup. I want the reliability and ease of CIS.

In essence, I should be able to clean up the intake ports a bit and smoothen out/ polish the exhaust ports w/o adversely affecting induction. Polishing the valves and exhaust ports can only improve exhaust flow which can only aid, though the effects may be minismal.

More thoughts??

Mike

[jluetjen]

Mike;
There were at least two versions of CIS 2.7's. the "normal" with 32 mm intake ports and 150 HP and the S's with 35 mm intake ports and 175 HP (excluding the CA versions). It sounds like you have the better version.

Now compare that with the fact that Porsche was able to pull enough air through the late SC's 1 mm smaller intake ports to generate over 200 HP. Sure the engine was bigger, but HP is a function of how much air you can burn and all of that air needed to go into the engine through six 34 mm intake ports. You have six 35 mm intake ports and thus should already be able to ingest more air then the 200 HP European SC.

Sure you can "clean up" the intake ports, but I doubt that it will net you anything for your efforts. The downside risk is that you might change them enough to hurt rather help the induction of air into the motor.

If you were to ask me, I'd suggest checking out or trying to pick up a used SC induction system. If the air flow meter and throttle body are larger then the 2.7's, I'd expect that you might be able to free up some airflow that way.

[emcon5]

For what it's worth, I spoke with Steve Weiner about the small port/large port question for an SC. His opinion is that unless you are using a "real cam" not a wimpy CIS compatable one, it won't make that big of a difference, and even then only up high.

Tom

[dd74]

Smaller intake runners and head ports in later SCs resulted in a more streetable engine which required less acceleration to reach the powerband.

As well, older SC motors fuel mileage were in the high teens for miles per gallon, while later SC motors averaged in the low 20s.

[6771911esses]

Yet I see some screaming 2.7 with CIS. We must be missing something *&&*^!@$#

[dd74]

You know the 2.7 in the 911S is only 5hp shy of any SC motor, right?

And you know some 911S with the 2.7 have been clocked from 0 to 60 in 5.8 seconds.

I think a healthy 2.7, especially an S version is quite nice. And probably quite underrated.

[jluetjen]

Ditto dd74.

[dd74]

John - I'm curious as to how you concluded 4R intake runners with properly-matched head intakes in an SC engine (these are the larger style runners) may not be as adequate as the smaller 2R intake runners? Is it that because of the 3.0's size that the large intakes and heads might over compensate the engine's ability to utilize the extra gas flow?

I'm thinking of swapping out my 2R intakes for 4Rs, boring the cylinders heads to port match the 4R intakes, and then installing 20/21 cams. The compression will remain 9.3.1, which is stock for what I have - an '83 3.0 engine.

All advice is welcomed.

Thanks.

[jluetjen]

John - I'm curious as to how you concluded 4R intake runners with properly-matched head intakes in an SC engine (these are the larger style runners) may not be as adequate as the smaller 2R intake runners? Is it that because of the 3.0's size that the large intakes and heads might over compensate the engine's ability to utilize the extra gas flow?

DD74;
I'm not really up on the different flavors (4R versusu 2R) of intake runners beyond their gross dimensions. I'm looking at it from a comparative view point vis-a-vis other engine configurations. Conceptually I've also drawn a line between Pre-CIS engines (my interest) and CIS engines because the CIS does seem to fundimentally change the charactoristics of the engine and it's ingestion of air.

Anyhow; my logic is this...

Torque ~ air consumed in 1 rev.
HP ~ air consumed in a certain amount of time ~ engine capacity * rev's.
Gas speed ~ cylinder capacity * time / intake size at smallest point

Bottom Line:
Late European SC has smaller intake ports (and associated runners) then a 2.7S, yet it makes more HP. As the FIA sports car series will attest, if you have a fixed choke size, a larger engine will not create any more HP. It will create more torque at lower rev's (which is why big engines are more competitive in the FIA series), but when you increase the rev's (remember that torque * rev's = HP), both engines will develop about the same peak HP. It's just that the larger engine will do it at lower rev's then the smaller engine.

So if the smaller runnered/ported European SC can create over 200 HP, the runners and intake ports are most likely not preventing the 2.7S from making more then 200 HP. If they're not preventing you from making 200 HP, then changing them will not help you get 200 HP. You need to look somewhere else for that HP.

[dd74]

I see your point, John.

Essentially, what I'm looking at is a way to get out of completing what might be needless work on my engine, which is the headwork and the swapping out the smaller intake runners for larger intake runners. But in the same light, I'd like to increase the revs on my motor (thus the 20/21 cams) while continuing with CIS (because of its reliability).

What I'm getting at is for the 3.0, it seems that the larger intakes and larger bore size in the heads may not make as much difference as cams and (of course) a free-flowing exhaust.

Is this a correct assumption.

As far as hp goes, 200 is fine with me. I'm currently at 193, which is good for my car's weight. I just want higher revs and power when I reach those revs.

The target for me is 6,700 RPMS.

Your thoughts, please?

Thanks.

[jluetjen]

DD;
My intuitive guess -- take a good look at the CIS. What's the throttle body diameter? How about the air sensor plate? What's the spring pressure? I'd be real interested in see what would happen if you were to hook a vacuum guage up downstream of the air sensor plate. Take it out to the highway and run it up to redline. Are you still pulling a vacuum at WOT? How much?

Also, what sort of CR are you running? Airflow restricted engines respond to higher CR's. Nascar restricter plate motors run some "insanely" high CR's (14:1+) at first blush. But if you consider that at wide open throttle they're pulling a vacuum, the extra CR ends up making up for the lower pressures in the cylinders. One thing that I know for sure is that if you do go down this path, you should seriously consider a mapped ignition system or at least a reprofiled distributor. And have the ignition/dissy profile tuned on a dyno.

Failure to do so WILL result in a holed piston or worse.

That's my $0.02.

[afterburn 549]

everything in my cis eng. shines from the boat tails to the intake ports....the computer thing you hang on the window says 0-60 4.8 sec could this be right????

[Wayne 962]

Hmm, I do agree with Steve on this one (kept quiet until now). Unless you're ditching the 2.7 CIS system, then you're not likely to see many (if any) gains...

-Wayne