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Old 01-06-2007, 06:42 AM
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True head port volumes

Finally got a chance to catch up on my reading the other day. Was looking at the August 2006 Chevy High Performance where they tested 8 heads on a 383. I was totally surprised when a lot of the 170 cc heads actually were 180 cc's in volume. 180 cc heads were coming in close to 200 cc's! Made me stop and think about what really is a good port size for a given combo since a lot of head's ports are actually bigger in volume than advertised. Anybody have any thoughts?

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Old 01-07-2007, 02:05 PM
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Quote:
Originally Posted by cool rockin daddy
Finally got a chance to catch up on my reading the other day. Was looking at the August 2006 Chevy High Performance where they tested 8 heads on a 383. I was totally surprised when a lot of the 170 cc heads actually were 180 cc's in volume. 180 cc heads were coming in close to 200 cc's! Made me stop and think about what really is a good port size for a given combo since a lot of head's ports are actually bigger in volume than advertised. Anybody have any thoughts?

Depends on what brand and series of engine, etc. and what purpose it is built for.

For SBC figure, cubes x 50 % = intake port volume for street performance cars.
cubes x 55 % for high rpm use.

Go AFR heads. They flow more per runner volume = velocity = torque.
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Old 01-07-2007, 05:45 PM
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Er, the point was that you think you are going with the proper port volume by ordering 170 cc heads and the volume is actually 185.
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Old 01-07-2007, 07:43 PM
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Quote:
Originally Posted by cool rockin daddy
Er, the point was that you think you are going with the proper port volume by ordering 170 cc heads and the volume is actually 185.

For SBC figure, cubes x 50 % = intake port volume for street performance cars.
cubes x 55 % for high rpm use.

Go AFR heads. They flow more per runner volume = velocity = torque.


What they are labeled is irrelavent, isn't it? The engine can't read.

Last edited by xntrik; 01-07-2007 at 07:48 PM.
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Old 01-08-2007, 02:10 PM
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I think the engine can read a bit better than you can by the responses you have made. Follow your little formula and let's say you determine that a 170 cc head is the answer for your needs. You order them and they turn out to be 185 cc. OOPS! Now you have too big of a intake port according to your formula, you aren't aware of that because you are trusting the cylinder head company to actually have molds the size they advertise! Sheesh!!
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Old 01-08-2007, 05:20 PM
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Don't pay much attention to a few percent variance in volume. It's not going to kill the engine's performance. The only variance I'd be keen to pay close attention to is the cross-sectional area of the port.

Think about it this for a second. You have two tubes, both 2" in diameter or 3.14sqin of cross-sectional area. One of them is 12" long and the other is 6" long. The short tube has a volume of 309cc, the long tube has a volume of twice that amount, 318cc, due to it being twice as long.

The diameter, or cross-sectional area, plus the port/chamber shape are the determining factors in how much air may pass through at a constant pressure for both tubes. Volume is nothing more than a result of those two. Without taking surface friction into account, two tubes will likely be close enough in actual flow that it wouldn't be measurable a flow bench.

Here is a picture of a couple of port molds. Say that the average cross-sectional area, or "diameter" of the molds is the same. The only difference is the length which makes the longer port larger in volume but no different in it's ability to allow air into, or out of, the cylinder but the longer port definately has a larger volume.




The difference in the specifications listed for the heads you're looking at could very well be a combination of port shape and casting variation. Equal parts of each would be my guess.

They may have intended for the port to have a longer, gentler radius where the port turns down to the back of the valve. Maybe the molds weren't perfect where floor of the port, referred to as the short side radius due to it's sharper bend than the roof of the port, was formed. The result was a larger cross sectional area than they intended. That combined with the longer port roof cause the variance in volume the magazine guys found when they measured the volume.

I hope this isn't a bit too much to swallow at once but it helps to understand these things when making a cylinder head selection.

As for magazine flow numbers and using them as a criteria for selecting a cylinder head......that's a subject that needs a good book to be written about it, and not by a magazine editor.


Larry
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Old 01-09-2007, 08:27 AM
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What you have explained makes good sense. I just wonder now if 190-200 cc heads aren't really the best choice for a street engine given most of the heads that were compared seemed to be close to that size. In other words, mild street engines shine with 170-180 cc heads and the throttle response and low end torque that results from that size head. But are they really getting the low end torque and throttle response from a larger 190-200 cc head but they don't know it as they are under the assumption that the port volume is smaller? That was the whole point to my original post. I know there is an engine builder on e-bay that insists the best port volume for a street engine is 200 cc's, no matter what. I wonder if he makes that statement knowing that most cylinder heads seem to have an actual port volume 10-15% larger than what they are marketed as.
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Old 01-09-2007, 11:08 AM
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As I was trying to point out, the engine only knows what it "sees" not what the label claims.

Sorry you're having a bad day.
Use your head. Don't throw a tantrum. Think.

Cold and Cool:
First, the length of ports in a small block Chevy head will all be "essentially" the same length. So length is not a factor in this discussion.

Since they are the same length, volume is a measurement of the factor of cross sectional area x length. Larger area x the same length = larger volume.

Cross sectional area of a specific port length determines its area (and fluid volume). That volume/area relationship determines the velocity of the moving air at any given rpm. IF the piston is sucking 200 cubic feet per minute the flow through a smaller area port will be faster than in a larger area port. Smaller area/volume is a faster velocity.

Excess head port and manifold volume reduces air velocity per rpm which manefests itself as soggy lower rpm reponse and poor fuel droplet suspension and mixture control.

Head ports are designed with a widening of the cross sectional area at the valve bowl area to reduce the air velocity behind the valve so that the air can turn around the short turn radius without flow restricting turbulence.

Obviously a 305, 350, 383, 406 will need different size heads. 406 heads on a 305 or vice versa will be a disaster. Heads that flow for 7000 rpm will be soggy at 3000.

By the way, 5 % of 355 cubes is 18 cfm. That is a fairly substantial difference based on the industry wide experience. That's a difference of using a 177 cc head to a 195 cc head. That DOES make a significant performance difference.

As I was trying to point out, the engine only knows what it "sees" not what the label claims.

General rule of thumb for small block Chevy is what has been stated.

edited: I would be suprised to learn that the major popular head manufacturers had a production variation of 10 % in the ports of their as-cast heads. I haven't seen that article yet, but I wonder if each manufacturer had CNCd their castings specially for this test.

Last edited by xntrik; 01-09-2007 at 12:02 PM.
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Old 01-09-2007, 03:05 PM
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I dont mean to stir any coals, but I think the fact that the motor doesnt know what is stamped on the heads is his point. If a 170cc port is what will perform best on the motor, you would want a 170cc port. If you buy a head that is stamped 170, but really is 180 due to inconsistancies is the casting process, the motor is going to react like it has a 180cc head, not a 170 like was optimal. I think his main question is whether or not a variation like this is really that important and if so, what can you do about it. Are there head companies that actually offer heads that have a port volume that they advertise?

Adam
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Old 01-09-2007, 09:17 PM
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Thank you, Adam! That is my point exactly. Dart SS heads are supposed to be 165 cc and they measured 178 cc. Edelbrock Performer RPM supposed to be 170 cc, measured at 185 cc. GM Vortec supposed to be 170 cc, measured at 185 cc. Competition Products 180 cc head measured at 195 cc.

Heads that measured at the volume they are sold as: Dart Iron Eagle Platinum 215, World Motown 220, Trick Flow 195, RHS Lightning 200.

I don't know about the rest of you guys but if I determine that a 180 head is the right size for my combo, I want a head that is actually 180 cc's in volume!

Evidently Xntrik would buy a pair of pants that are three inches too long but still wear them everyday because the label said they were the right size and his legs can't "read" the label. Give me a break.
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Old 01-09-2007, 09:40 PM
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Quote:
Originally Posted by cool rockin daddy

Evidently Xntrik would buy a pair of pants that are three inches too long but still wear them everyday because the label said they were the right size and his legs can't "read" the label. Give me a break.

Did you flunk English. I have been agreeing with you.


Grandpa always said, "Never argue with an idiot, someone listening might not be able to tell the difference".
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Old 01-10-2007, 10:24 AM
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Calm down people.


I believe marketing has a lot to do mislabeling or "creative judgment's of description" . If your comparing heads in the 180cc range for a street engine one might be looking for the highest flow No. for a given port volume. That 190cc head labeled a 180 will probably fool you. Of course, those who buy heads looking at just one or possibly two things are asking for their money to be taken without remorse. We all know when you have 15 heads that will work for a given combo, but only one guy buying, the competition get's tough. They will do anything. Including lie.

Cam company's often sell a cam slightly different then they advertise to keep guy's buying cams that are to big with poor performance for the situation and blaming the manufacturers for it. The duration and lift No. are often slightly lower and cam are built with huge amount's of advance to get the cam to work in the rpm ranges the customer is using actually using. Not what he thinks.

Let's just see the fact's that most backyard assemblers don't do a good job picking part's because they rely on their friends and the negatives gestures they retained over the years to chose something they need. How many times does a guy hear the "292" cam in a 350 with a 4 speed was the fastest thing....When in fact, that 292 duration cam is actually way to big for street engine with any trans. It worked in the 60's with 11:0 compression but not in a 350 built in the 80's with 7.5 CR. But the backyard shed assembler doesn't know this. A cam company can fudge that 292 description and make the assembler happier. Then he impress his friends with it.

Now most of the time the manufacturers go the other way, slightly smaller to compensate for overzelous shopping. But in the heads, you go the other way. Never mind that some heads with lower flowing No. make the most power.

The internet has made this worse with so much info being traded within the time of the keystrokes it takes to make it up. Sooo much of it is untrue, and not even spread by word of mouth anymore either. I couldn't tell you how many time Ive found a web site created from copy/pasting information from another site that was wrong. Then you and me and Joe go to look up something and when we see it 5 times in different places, we assume it must be true. You can't trust very much of what you read...Before the internet, you couldn't trust everything you read.

I saw some heads on Ebay the other day that the info was pasted from a site. That site was so wrong, I feel for the person bidding 15,000 dollars on the heads was getting it in the.....other end. Makes me nauseated at times.

Last edited by johnsongrass1; 01-10-2007 at 10:30 AM.
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