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Old 03-07-06, 04:18PM   #1
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Default Port Volumes vs. cfm

This has to be one of the best post I've seen by Jim Hand, he gives the results ( testing ) the difference between his iron head and the change
over to KRE's aluminum D-port head

I have never stated or claimed that higher flowiing heads with bigger ports would cause a performance "loss".

What I have said, am saying, and have proven conclusively is that larger ports with similar airflow capability will lose power as compared to similar flowing heads with smaller ports.

The fact that Cliff ran better with the KRE heads over his stocks heads is obviously a result of the added airflow and 50# lighter weight. And the modern chamber design also helps.

Let me make this as simple as possible without all the unrelated "possibilities"!

I ran iron heads that flowed 255 @ .6 lift, had 165 cc port volume, and had 10.08 static compression for about 6 years. I have excellent records on their performance with a range of cams. With no other changes of any kind including the cam, I switched to a set of KRE heads that I had specially prepared to duplicate my iron heads in regard to valve size and angles. After cleanup and opening the ports to match my previously prepared RPM intake, the KRE heads flowed 273 @ .6, and the exhaust flowed an honest 76.9 % of the intake as an average of each lift point. The final port volume averaged 186.5. The finished chambe size of the KRE headds made the engine 10.38 static CR And I might add that they lightened the car by about 50#.

And they "felt" great" The car launched well with no misses, stumble, bog and so on. And yes, it had good low end power!. But since my car does't spin, even on the street, I can't relate how far the tires spun!

But at the track, something went amiss. The car didn't launch as hard, even with that great feel on the street! The ET was almost .2 seconds worse. Yet the MPH increased by one to two MPH! Oh well, maybe it was the track, or the weather, or something else - at least that is what I wanted to think after about five runs.

Went back a few days later and exactly the same, if not worse, performance!
Went through all the normal timing and mixture checks - no difference.

Went back a third time - this time there was simply no doubt - my new higher flowing 50# lighter aluminum heads were just not making as much total power as the 255 iron heads. Yet, the car was running faster MPH which indicated it was recognizing the added airflow, and that indicated we had no unexpected engine problems of fuel delivery or ignition problems.

I had worked hard with the new heads, and added 30 degree intake seats, backcut both valves, and used the same seat cutters as we used on the iron heads - all to minimize the possibilities of some other change causing problems or significant differences. I wanted to run them - to save weight, to have new heads, to not have to worry about cracks or leaks in the iron due to age or porting. I had considered having the chambers and runners coated, but knew that could hide or distort any significant changes in performance.

Since the KRE heads had lost significant 60' time, had Bullet grind a custom cam of 233/235 with max possible lift on 108 LS in order to pick up the 60' time. It worked - did pick up almost .1 in the 60' but in the process lost 2 MPH, and the final ET was still .1 to .2 off from the iron heads.

Went back one more time for the fifth trip to the track with the new heads. It was a little warmer and the car would barely break into the 11's! That was enough for me!

I left them on while I did a little more work on the 6X heads that were on temporarily after one of the 64's developed a water leak after 6 years service. Since my engine recognized the added airflow of the KRE heads, I added flow to the 6X's. Since it lost so much low and mid range power due obviously to the 186.5 cc port volume, I was very careful with the re-porting of the 6X to keep the ports as small as possible. The result was average flow of 265 @ .55 with port volume of 170.

Off came the KRE and on went the 6X. Again, good street feel, strong low end, good throttle response, and all the other benefits associated with good fresh properly sealed heads with good airflow. Went to the track and what do you know? Even though the car was 50# heavier then with the KRE heads, and had about 8 cfm less flow, it ran quicker by almost .2 seconds, and was just as fast in MPH! And it picked up almost .1 second to 60'. (Last time out ran from 11.49 to 11.56 at 115+, 1.62 60', with 3.31 gear and hauling 4050# on 92 octane pump gas).

Skip, you can write as much as you want, you can quote others, you can estimate what Cliff's car "might" have done, you can tell me for the 10th time that chevy's use bigger ports, but I have the numbers! Bigger ports with equal airflow will give up power as compared to smaller ports with equal airflow! It is that simple. You have read it in most every good technical material, including McFarland, Vizard, Circle Track magazine, PHR Rod magazine, and countless other reliable sources. And Tom Hand wrote and posted a postulation about this subject with help from Harold Bettes of Super Flow in about 1999, and was 100 % correct. He compared the expected performance of stock E heads against my ported iron heads on my wagon, and the results he came up were exactly what happened with my current tests!

As I had since made the engine larger, I thought it could handle the relatively "small"port increase to 185 as compared to the 216 of the E heads, but unfortunately, I was wrong. As Tom wrote, and I have dozens of times, new heads of any kind will likely make any car run better, both on the street and at the track. But I am not talking just about running better, but rather what the real effects of port volume are on total power production. I am talking about what is the ideal combination of port size to RPM, the required flow, and all the other factors we both know that affect engine VE.

And I have also said dozens of times that any new head will likely add performance. The real subject of this topic is not that, but rather what effect port volume has on total power production throughout the entire RPM range. The fact that many cars aren't loaded at lower RPM due to converter selection, or manual clutch, does not enter into this discussion of actual engine power production. The fact that E heads allow us to spin our tires on the street is totally immaterial to this discussion. I once owned a 6 cylinder Studebaker with4.56 gears and OD that would spin the tires through the entire first gear, but still couldn't beat am El Paso city bus (loaded) across an intersection!

Also, the "amount" of lost power is not that important in this discussion - rather the fact that power is lost. Even with the loss, most cars that install KRE heads will run far better then they ever imagined. But again, I am talking about real technical effects of design attributes on performance. I recall several years ago a post that was copied off another unnamed board that stated port volume was not important. What wasn't discussed was that it came off a PRO Stock source - from the world of .9" valve lifts, and RPM that is never under about 7000 RPM and runs to almost 10,000! Sure, who cares about power at 2000 on an engine that won't even run under about 2500 RPM. But for those of us in the real world, 2000-3000 represents the typical cruising RPM and that is where we want and need optimum power and throttle response.

Finally, you can be sure that if I could have found a way to make my car run at least as well with the KRE heads without changing RPM, and associated converter and gear requirements required with added RPM, they would still be on. They are an excellent head, but did not match my car setup as well as my iron heads due to reasons noted above. And because most don't or can't prepare heads like mine (if true) does not change the fact that port volume does indeed affect total power production!.

Jim Hand

Last edited by Jim Hand : 03-03-2006 at 01:26 PM.
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