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BeckyD @ James Martin Chevy


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Old 08-02-2017, 10:58 AM   #1
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GM CNC heads Flow Tested.

I got my hands on a set of GMPP CNC ported heads today and I am going to flow them against my heads later today when I get home.

Last edited by PRAY; 08-03-2017 at 04:47 AM.
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Old 08-02-2017, 11:48 AM   #2
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Wow, thank you for doing this!
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Old 08-02-2017, 02:35 PM   #3
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Okay, as usual, back to back unbiased results. I pulled the GM head off and put my head on. It is hot as hell in my garage and I had just about every outlet running so I couldn't get a whole lot of depression out of my bench today. This was also a stock valve to stock valve test as well. So this is simply a runner contest. My custom cut intake valves are worth about 10cfm on my bench. I simply don't have any cut valves on hand for this test.

This test was done on my SF-110 flow bench, not a SF-600. So the flow numbers are going to seem low. I will label each head and runner and the depression each was flowed at. The SF-110 favors velocity over force to get around the valves. So runner size / shape and short turn become critical. At 28" you can get away with ALOT of errors in your sizing and velocity. I usually gain about 30cfm on a SF-600 flow bench at 28". This is how the flow bench data turned out.

I did not CC the GM runner. If I get time I will see if I can get it poured.

Test was done on a 4" bore with a clay radius inlet and a 2"x4" exhaust extension.

...................PRAY Int......GM Int..../...................PRAY Exh......GM Exh
Dep....Lift..................................../.Dep....Lift...................................
12"....300"...........214..........204.../.15"....300"............180...........165
8"......400"...........273..........264.../.12"....400"............228...........225
7"......500"...........320..........306.../.10"....500"............251...........249
5"......600"...........350..........328.../.10"....600"............258...........258

I have to eat some crow on the GM exhaust port. It did suck at .300 and it was turbulent as crap at .400 so I had to get a best case bump for it's number. I couldn't get it to settle at all. But at .500 and .600 it was moving some air. I would like to see how it does at 28". I think it would be limited by size once the volume picks up. I am going to mess with the throat diameter of my exhaust seat/bowl to see if I can gain another 10-12cfm up top. I keep it really small and radiused to pump up my .100-.300 numbers. I have to find a happy medium.
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Old 08-02-2017, 03:30 PM   #4
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Once again, proves to myself I made the right decision. Thanks for the data.
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Old 08-02-2017, 03:36 PM   #5
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Once again, proves to myself I made the right decision. Thanks for the data.
Once your heads had the valves done they flowed a bit better. These heads will too when I am done with them. I worked over some ports on the heads you sent back to compare to the GM head.

Last edited by PRAY; 08-03-2017 at 04:51 AM.
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Old 08-02-2017, 03:44 PM   #6
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Once your heads had the valves done they flowed a bit better. These heads will too when I am done with them. I worked over some ports on the heads you sent back to compare to the GM head. I am pretty convinced that these are PRC heads. I am going to call my guy over there tomorrow and see whats up.
Brett, do you have the actual flow bench numbers from my heads? If so could you send them to my email?
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Old 08-02-2017, 03:48 PM   #7
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Brett, do you have the actual flow bench numbers from my heads? If so could you send them to my email?
I have them off of my bench. They are +/- 3cfm or so from my heads. I work my ass off to maintain consistency from set to set. If you are going to post numbers somewhere just use the 28" numbers I posted from my heads.
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Old 08-02-2017, 03:51 PM   #8
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PRAY delivers yet again on the Datagazm, thank you!

It's really great to see some back-to-back data. It for sure proves you not only know what you're talking about, but that you're humble enough to admit when you see room for improvement too. Two big thumbs up.

I really look forward to when you're back in the game, because now I more than ever want some PRAY-ported heads on my 1LE.
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Old 08-02-2017, 03:59 PM   #9
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PRAY delivers yet again on the Datagazm, thank you!

It's really great to see some back-to-back data. It for sure proves you not only know what you're talking about, but that you're humble enough to admit when you see room for improvement too. Two big thumbs up.

I really look forward to when you're back in the game, because now I more than ever want some PRAY-ported heads on my 1LE.
Thanks for the kind words Sir. Back to back is the only legit way to test in my mind. I could have showed what they did against my previous best runs on my heads in the winter. But that would be unfair and BS in the end. I think this test shows the consistence I have been able to achieve by hand as well. This is the 4th set of heads now and they are all within a couple CFM. I am constantly learning and testing to bring forth the best product possible. I made some changes on these ports that I think are going to show some nice gains once the valves are done. I am going to comb over these heads to see what I can learn or validate as things I don't want to experiment with. You can always learn something from others hard work.
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Old 08-02-2017, 09:13 PM   #10
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The gm heads are nothing to do with PRC. Just send a email or call and ask. No need to spread false rumors!
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Old 08-03-2017, 11:16 AM   #11
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Appreciate you putting in all the work homie. Knowledge is power....well, in our case horsepower...hmmm maybe that was the original saying (begins to ponder origins). Anyways, your posts don't go unnoticed bro so keep 'em coming.

I did inquire about a set of heads in the spring and I am still interested, I was just hoping to get my diff swap results next month, and a 10 second slip, before I crack the motor open. Consider me still in line.
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Old 08-03-2017, 02:08 PM   #12
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Appreciate you putting in all the work homie. Knowledge is power....well, in our case horsepower...hmmm maybe that was the original saying (begins to ponder origins). Anyways, your posts don't go unnoticed bro so keep 'em coming.

I did inquire about a set of heads in the spring and I am still interested, I was just hoping to get my diff swap results next month, and a 10 second slip, before I crack the motor open. Consider me still in line.
Thanks brother, get your 10 second slip and then hit me up about the heads. I would love to do some work for you.
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