Cylinder head porting size
01-05-2004, 05:57 PM
#1
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Cylinder head porting size
Hi,
While I am doing some engine work, I am considering porting the head.
So would appreciate any input as to what size ports I should be enlarging it to?
The engine is pretty stock other than European fuel distributors/lines & I plan to continue using CIS. I'll be upgrading to the K27-HF and it has GHL headers with Borla muffler on it. And I'm planning on using a SC camshaft.
What I'm hoping to get is more usable range of power & torque (hence SC cam to help with off boost with a HF to compensate the top end) and not have to rely on rev-ing it to 7,000rpm all the time.
TIA...
While I am doing some engine work, I am considering porting the head.
So would appreciate any input as to what size ports I should be enlarging it to?
The engine is pretty stock other than European fuel distributors/lines & I plan to continue using CIS. I'll be upgrading to the K27-HF and it has GHL headers with Borla muffler on it. And I'm planning on using a SC camshaft.
What I'm hoping to get is more usable range of power & torque (hence SC cam to help with off boost with a HF to compensate the top end) and not have to rely on rev-ing it to 7,000rpm all the time.
TIA...
01-06-2004, 02:17 AM
#2
PT,
Can't really give any real numbers, as every application requires different flow potentials. If you already have an Exhaust system, then the Exhaust ports will have to match the flange size/pipe size, unless you intend to change these. As for the Intake, again it is up to the CIS manifold. Same as the Exhaust. What I can tell you from my own engine, the Head flow was critical to perfromance and Turbo matching. Given the same Intake port size, it takes more pressure to flow the same CFM as with a larger port with less pressure. Typically, 930 engines are constipated. The Intakes flow way more than the Exhaust can. Opening up the Exhaust ports helps a little, but the flow numbers show as the lift gets bigger the flow changes little. 930 Heads really like larger Exhaust Valves to help with the flow, and the Intakes take off with a larger one too. The Exhaust flow wants to be around 80-85% of the Intake. From memory, my heads were ported to give around 30% increase on the Exhaust over stock, and around 17% on the Intake over stock. This gave the flow balance required. I also had larger valves fitted to both sides. Again it comes down to the engine you are building. Sometimes the Larger Exhaust valve can effect the throttle response, but 930's and Turbo's in general love the change.
I wish I could remember the flow numbers, but I cannot, and I am sure I was never given them in any records I have. I do remember the porting was very important around the guide and behind it. The seat and its height is also very important. Low seats flow very poorly on these heads. Porsche Heads do not allow for low seats as do some other designs. Seat angles and margin widths also play an important role in the overall flow. Its a trade off between margin widths for longivity, and flow. Most Intakes have a 1.0mm seat, and the Exhausts are anywhere from 1.2mm up to 1.5mm. Above the 45, the seat widths, angles and below it become very important. Most modern Valve seats have a radius shape below the 45, and a very narrow flat top angle. It all is down to who does the repair or replacement. I know in my engine, they went for a narrower Exhaust seat, and ran a little more seat pressure to help with the heat transfer. Back cut angles on the Valves are also done. Again, widths and angles all play a part in flow potential. If you want the best, you need to go to someone who knows. You can make it worse in a heartbeat. I do not know everything here, just enough to understand. I do know that this is a really misunderstood art. Most think they know, but when you look at their complete work, the ports are done ok, but then the seat work is very poor and repaired in a production sort of way, where no consideration for flow is considered.
To be sure, call some Porsche race engine builders and see what a stock 930 head flows. It may flow enough for you needs. If not, they will be able to help as to what should be done. You can email me @ m42racer @aol.com and I will give you a name to call, and I will look for what I have in the way of flow numbers from my engine upgrade.
Can't really give any real numbers, as every application requires different flow potentials. If you already have an Exhaust system, then the Exhaust ports will have to match the flange size/pipe size, unless you intend to change these. As for the Intake, again it is up to the CIS manifold. Same as the Exhaust. What I can tell you from my own engine, the Head flow was critical to perfromance and Turbo matching. Given the same Intake port size, it takes more pressure to flow the same CFM as with a larger port with less pressure. Typically, 930 engines are constipated. The Intakes flow way more than the Exhaust can. Opening up the Exhaust ports helps a little, but the flow numbers show as the lift gets bigger the flow changes little. 930 Heads really like larger Exhaust Valves to help with the flow, and the Intakes take off with a larger one too. The Exhaust flow wants to be around 80-85% of the Intake. From memory, my heads were ported to give around 30% increase on the Exhaust over stock, and around 17% on the Intake over stock. This gave the flow balance required. I also had larger valves fitted to both sides. Again it comes down to the engine you are building. Sometimes the Larger Exhaust valve can effect the throttle response, but 930's and Turbo's in general love the change.
I wish I could remember the flow numbers, but I cannot, and I am sure I was never given them in any records I have. I do remember the porting was very important around the guide and behind it. The seat and its height is also very important. Low seats flow very poorly on these heads. Porsche Heads do not allow for low seats as do some other designs. Seat angles and margin widths also play an important role in the overall flow. Its a trade off between margin widths for longivity, and flow. Most Intakes have a 1.0mm seat, and the Exhausts are anywhere from 1.2mm up to 1.5mm. Above the 45, the seat widths, angles and below it become very important. Most modern Valve seats have a radius shape below the 45, and a very narrow flat top angle. It all is down to who does the repair or replacement. I know in my engine, they went for a narrower Exhaust seat, and ran a little more seat pressure to help with the heat transfer. Back cut angles on the Valves are also done. Again, widths and angles all play a part in flow potential. If you want the best, you need to go to someone who knows. You can make it worse in a heartbeat. I do not know everything here, just enough to understand. I do know that this is a really misunderstood art. Most think they know, but when you look at their complete work, the ports are done ok, but then the seat work is very poor and repaired in a production sort of way, where no consideration for flow is considered.
To be sure, call some Porsche race engine builders and see what a stock 930 head flows. It may flow enough for you needs. If not, they will be able to help as to what should be done. You can email me @ m42racer @aol.com and I will give you a name to call, and I will look for what I have in the way of flow numbers from my engine upgrade.
01-06-2004, 01:48 PM
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Thanks for the info m42racer.
One question though - based on memory from BA's book, I think factory 934/935s all use same exhaust port size as 930s but intake port is quite a bit larger.
That seems counter to your suggestion; though I have also heard from many that the bottleneck in the 930 is on the exhaust side.
One question though - based on memory from BA's book, I think factory 934/935s all use same exhaust port size as 930s but intake port is quite a bit larger.
That seems counter to your suggestion; though I have also heard from many that the bottleneck in the 930 is on the exhaust side.
01-06-2004, 02:16 PM
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PT, in theory your only good as the smallest hole and while the intake is 32mm stock the exhaust (inside pipe diameter is 35mm) that opening the intake more than 35mm might not gain you much. Of course people open them up to 41mm with unknown before and after results? At least none that I have ever seen. I think the big question here is what hp can the stock heads support without modification? If you don't exceed that hp then it would be a waste of time and money. I know I'm running alot of hp with stocks heads
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01-06-2004, 02:59 PM
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Brent, you pretty much said what I was thinking - its easy to say "while I'm there" but is there a benefit in doing anything...
I wasn't really into "let's change/upgrade everything" even though I do want to get close to the 400rwhp that many are getting.
I believe you are pushing ~450rwhp with a bigger turbo, cam, exhaust & CIS mods?
I wasn't really into "let's change/upgrade everything" even though I do want to get close to the 400rwhp that many are getting.
I believe you are pushing ~450rwhp with a bigger turbo, cam, exhaust & CIS mods?
01-06-2004, 03:38 PM
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It really depends on your goals to what you should or shouldn't do. Opening the heads wasn't apart of my goal but now wonder if I need it with the power I'm runnning? My 450rwhp is corrected numbers at 5000ft. above sea level. This would be less at sea level I suspect. Would still like to know in more detail what opening the intake will do? This will affect each car with a different setup in a different way I imagine. I think it's a very complex concept to figure out...lol One thing is for sure, very knowledgable people in the business say to open the intake to 36mm.
01-06-2004, 07:50 PM
#7
All Porsche engines are backwards. Its a widely known fact. Just get Flow numbers off a Flowed head and see the % difference between Intake and Exhaust. All other engines work with the Exhaust around 85% of the Intake flow. Porsche are no different. They just like to run large Intakes and keep the exhaust small for street use. As stated, if you don't need, don't do. All you will do is spend $ for nothing.
