36/39 head porting questions

[Wu.wei]

Aloha,

Recently acquired a 2.7l I believe its a '77. The guy I got it from didn't have very much info on it for me besides "it probably needs a top end". Based on that, I was assuming that it was just a common 2.7 CIS. When cleaning up the motor a bit I found that the intake port was hogged to 36mm. Seems to be nicely done. Valves also did not have very much carbon on them. So I spun it over to check out the exhaust side. 39mm exhaust ports !?!? Air injection holes are also welded up. Again seemed like a decent job. Discovered RS pistons using a borescope as well as Nikasil jugs (stuck a small magnet in there). According to the cam ends they are 911...143.00 and 144.00. I will eventually try to figure if they have been re ground??? The head casting numbers are the ...342 with a 76 date on them.

Im planning on using zenith 40 TIN carbs with a set of PMO tall manifolds (36mm). a buddy has a set of 1 5/8" headers for me.

my questions are:

1. were these heads done for a turbo application?

2. are these heads useable with my 40 TINs?

3. what is the theory behind a larger exhaust over intake port (assuming for turbo)?

4. how does that theory translate to NA setups?

Im trying to not pull the whole motor apart as I am already about to send another 2.7 case to California to get it line bored back to std and spigot bores decked. I was hoping to use this to get the car on the road for the interim!

[stownsen914]

It is not common practice to make the exhaust ports larger than the intakes on a 911 motor. It sounds like someone either made a mistake or was misguided possibly.

[Trackrash]

Maybe the PO used large race headers? Matching the exhaust manifold for a street motor to the ports could be problematic for your usage. Perhaps someone will have a solution.

[roblav]

Exhaust port is too big for a street 2.7, even with S cams. You might be better off selling those heads. Your exhaust gas velocity will be too slow. The engine will be a dog at low rpm.

[dannobee]

Clean up the carbon in the exhaust port and remeasure a little further in the port.

[Wu.wei]

Maybe the PO used large race headers? Matching the exhaust manifold for a street motor to the ports could be problematic for your usage. Perhaps someone will have a solution.

yeah im mostly just curious on why it is that turbo engines are the only ones that use larger exhaust ports over intake. hopefully someone will be able to school me a bit and that will tell me where I should go!

Exhaust port is too big for a street 2.7, even with S cams. You might be better off selling those heads. Your exhaust gas velocity will be too slow. The engine will be a dog at low rpm.

that was my initial prognosis. however I think these mag cases tend to warp once the top end comes apart (spigot deck) so new heads on this aren't really an option for me. worst case I sell this engine, but regardless I think the new buyer will have questions as I do.

Clean up the carbon in the exhaust port and remeasure a little further in the port.

basically same measurement after cleaned. I need to get the bore gauge out and will measure a little further in today.

thank you guys for the thoughts!!!

[lvporschepilot]

I run 39mm/38mm ports on my 2.8L with 10.5:1 compression, twin plug, EFI, and DC65 cams. Revs to 7800rpm. Absolutely streetable, a kitten on the road.

[stownsen914]

To add to lvporschepilot's point, it could be an option to port the intake to 40mm, but it may not run well on CIS. I'm sure lv's engine is carbed. I have a similar engine with 40/38 ports with MFI (bored stacks).

[Arlo911]

I'm also interested to hear some expert opinion on small intake/large exhaust ports as I have a rebuilt small port 34mm intake SC engine that has the exhaust ports opened up to 38mm.
Would this have any detrimental or positive effects?

[stownsen914]

Having exhaust ports larger than the intakes is odd to me. I can't think of any advantage, or why someone would intentionally do it. I'd love to hear an explanation.

[Wu.wei]

Originally Posted by stownsen914

Having exhaust ports larger than the intakes is odd to me. I can't think of any advantage, or why someone would intentionally do it. I'd love to hear an explanation.

According to waynes book they are found on certain turbos. Im assuming it has to do with the balance of pressure on the intake side vs the backpressure caused by the turbo. But it would be a cool lesson from the OGs on here as to the theory. Hopefully someone will chime in!

Ive tried to reaearch it a little and it surely isnt for NA motors. Im thinking about hogging out the intake side to match but i wouldnt want to make things worse. 40 ports might be too big for stock valves??? Also a commitment to hog out my tall PMO manifolds.

Mahalo guys!

[targa72e]

the 73 2.4T CIS heads have smaller intake (30) than exhaust (33). the previous 2.4MFI engine was 32/32 in/ex.

john

[Neil Harvey]

Originally Posted by Wu.wei

According to waynes book they are found on certain turbos. Im assuming it has to do with the balance of pressure on the intake side vs the backpressure caused by the turbo. But it would be a cool lesson from the OGs on here as to the theory. Hopefully someone will chime in!

Ive tried to reaearch it a little and it surely isnt for NA motors. Im thinking about hogging out the intake side to match but i wouldnt want to make things worse. 40 ports might be too big for stock valves??? Also a commitment to hog out my tall PMO manifolds.

Mahalo guys!

I try to offer any advice and help I can, whenever I have the time. Hope this helps some. It a very simplified explanation that hopefully will give you some hope that your heads may be Ok as they are.

As a rule, when establishing the flow through the exhaust port, you typically aim for approx. 75% of the intake flow. This has nothing to do with the size of the port either. It is measured past the valve and out through the port. So, the best way to understand what you have is to flow both sides of the cylinder head. This must be done with the full Intake attached so the measured air flow is the same as when in the car. Otherwise the numbers are false and will change the specification of Camshaft and port sizes required.

Remember, the gases exiting the exhaust port do so differently compared to how the intake gas flow enters.

Here is a simple explanation hoping not to techie, that you will understand. This is based on most “stock” Porsche engine configurations.

Once the Exhaust valve opens, while the piston is still heading towards BDC, most of the gases pass through the exhaust port. Once the piston turns around BDC, most of the exhaust gases are in the exhaust system and behind this is a vacuum, that pulls the intake charge into the cylinder. This also helps to scavenge the remaining exhaust gases out of the cylinder. This is totally controlled by the camshaft and its timing.

The amount of exhaust that leaves the cylinder depends on the airflow capability of the exhaust port and the amount of time from exhaust valve opening until just after BDC. The total amount of exhaust depends, of course, on the cylinder displacement and volumetric efficiency.

These Porsche engines typically have a 127.00mm CCL rod. This would be classified as a short length rod. This play a big part on what the Exhaust port should flow.
Porsche engines have higher flow demands across TDC overlap and have reversion or over-scavenging issues to account for. They prefer less valve lift across overlap and they also demand more flow closer to TDC overlap with more intake duration after BDC. On the exhaust side, Porsche engines require less low-lift port flow.

Just because the port exit appears large, this does not necessarily mean the port is too big. Valve sizes play a huge part here too. The smaller the valve diameter, typically the more lift you will need to achieve maximum airflow. The counter of this applies as well.

There are other parts of the port that play a bigger part in the flow characteristics than the port exit diameter. The bowl and the critical area. The bowl is the area just below the valve seat, which can and does affect the speed of the escaping gases. The critical area is the smallest cross-sectional area of the port, which is the point at which the highest velocity is achieved within the port. These two parts are far more important than the exit diameter. By the time the gases have reached this part of the port, its sizes have little effect.

Pipe size and length play more of a role in gas volume and how this mass exits the port in “time” and the creation of the vacuum behind it. The timing of this event needs to occur at a certain crank angle to be able to use this energy to draw into the cylinder the intake charge and also help scavenge the remaining exhaust gases out of the cylinder.

There is way more to this than what I have listed here. Cam spec’s, cam timing, (opening and closing events) have a huge part in how the cylinder heads flows both on the Intake side and on the exhaust side.

Don’t throw away your heads yet. Do some checking first and if unsure have someone who understands air flow through cylinder heads look at them and give you an idea of just what you have. You may not be in any trouble with what you have.

[Wu.wei]

RAD! thank you Neil! what an amazing explanation! its slightly above me in some parts, ie when speaking of the valve timing and lift vs flow/overlap etc. but I think if I re-read it a few more times I will start to really understand. Mahalo again for your time!!!

oh and one other question if your time permits,,, what is the short answer as to why some Porsche turbo motors have larger exhaust over intake ports (taking into account what you have already explained)? is it some sort of balance of flow that is optimized when the intake side is pressurized?

[Neil Harvey]

There are certain engine config's that require the exhaust to be considered over the Intake. Remember too, its easier to build an exhaust system into a car than it is an intake system. You typically have more room.

But, in a Turbo application, I think its also common for some to make the exhaust system bigger than needed out of some concern about exhaust pressure and restriction. Then the port is matched to suit.

Too small a primary system and you restrict the exhaust flow. Too big and the low lift scavenging effect is lost. It all has to be thought out.

In your case, I think the most obvious explanation is, someone bought and fitted a primary set that was larger in diameter than the exhaust port and matched the port to suit.

[Wu.wei]

Thank you again for the help neil. I have a much better understanding on what im looking at now!

[Walt Fricke]

The 3.0 Turbos were 32/36, the 3.3s 32/34. Nothing stock about these (and I guess they aren't turbo heads, either). Maybe someone used them on a hot rod turbo?

[stownsen914]

It may be worth asking whether design decisions that Porsche made in its early turbo engines are really optimal given what has been learned in the ensuing 40+ years.

[Walt Fricke]

My 07 Turbo has no apparent lag and is a wonderful car/engine. But it has about everything electronically controlled, water cooled, 4 valves, two turbos (variable angle at that), compression ratio way higher than the 930s because knock sensors and all the electronics can keep a lid on things.

I'm not sure just how much more is known about pulling a lot more reliable power out of the K26 era motors. I tried to bone up on turbos, dreaming of building a 2.1L air cooled turbo race motor, like Porsche did. A VATN turbo, for instance, should work wonders. Two small turbos might have been possible too. Pat Williams built one of those 2.1s, copying I think mostly what Porsche had done, and it was a class winner as long as the driver didn't miss shifts. My wife put her foot down - we can't afford to learn by breaking. She was right about the affording, and probably about what would happen along the learning curve for me.

[rgofast]

We are running 37 intake 39 exhaust on a short stroke 2.7 with modified S cams and 9.4 to one compression, and it runs great.