HP gains 350 Iron Heads: Ported vs Not

Discussion in 'Small Block Tech' started by MrSony, Aug 13, 2020.

  1. MrSony

    MrSony Well-Known Member

    Anyone have any real data on how much power is gained by porting iron heads? I have a spare set laying around and I'm curious. My current setup us right around 300hp (TA intake, 310 cam, headers, 9.5:1).

    Heads on engine are bone stock '71s with a valve job. Potential heads are off my exploded SP 350. Stock, home valve job.
    Any input welcome.
     
  2. Fox's Den

    Fox's Den 355Xrs

    about 70 hp
     
  3. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    How do you know they are ported? Got any flow data on them? Hard to say how much power they are worth. It depends on the rest of your combination.
     
    Dano likes this.
  4. gscalifornia

    gscalifornia Small blocks rule!!

    I had a local shop port my 350 heads and gained about 50hp. Hoping for a little more with the current rebuild!
     
  5. MrSony

    MrSony Well-Known Member

    Potential heads i would port. Both are stock port wise.
     
  6. Stevem

    Stevem Well-Known Member

    Just also note that once a head is ported to a certain level and of the Intake and Exh ratio shifts enough you will then need a different Cam with different overlap placement specs to reap the full benefits of the flow numbers that may have been gained!
    Basically a good rule of thumb of a motor like yours is that for every 4 Intake cfm @ 28" gained over stock the motor will make 1 additional hp.
    This assumes also that the Exh to Intake ratio stays in about the 72% range.
     
    MrSony likes this.
  7. alec296

    alec296 i need another buick

    Actual valve job can change flow vs stock. Polished valves, backside cut, flat valve design vs concave .
    A lot can change flow. my opinion, big valves blended right , pro valve job will give best gains, for stockish engine.
     
    Dano likes this.
  8. Mart

    Mart Gold level member

    So, if you gained 50 intake cfm over stock, ÷ x 4, you would only gain 12.5 h.p.? Doesn't seem to add up. Even adding 50 cfm for both int & exh together ÷ x 4 gives you only 25 hp.?

    I like Grumpys estimate better.:D
     
    Last edited: Aug 15, 2020
  9. Fox's Den

    Fox's Den 355Xrs

    somebody had to give a number.
    I ran 14.90 before engine rebuild and went 13.31 after rebuild. that extra hp came from somewhere.
     
    MrSony likes this.
  10. Dano

    Dano Platinum Level Contributor

    Looking at this KB 350 build from the 80's, big valve ported heads picked up .25 sec.

    I did several of the same upgrades in the 80's & 90's w/very similar results. When I did a complete rebuild in the 90's, the big changes were the heads (big valve/ported), cam (KB Mark 2 to C118) and stock '70 compression to true 10.5 SCR and it picked up about .75-1 sec. which added up corresponds pretty well to their results.

    Some of the results here are very interesting esp. rear gears (I went from 3.23 to 3.42 to 3.73), transmission (my car was a factory 4-sp), and compression (per above) all of which corresponded pretty closely to my experience. I never went to 12.5:1 or 4.88's & of course every build is different but neither made much difference on this one. I suspect it ran out of rpm w/the 4.88's.
     

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  11. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    When I bolted on my TA Performance heads, I went from low 13's (my best was 13.22@ 101) to 12.40's @ 108. With the SP1 and 1000DP carburetor, it went 12.11 @ 112. That is over a second and 11 MPH
     
  12. Mart

    Mart Gold level member

    Was that pre- roller cam motor?
     
  13. LARRY70GS

    LARRY70GS a.k.a. "THE WIZARD" Staff Member

    Yes. I went from iron Stage1 heads (untouched) and iron intake. Engine had KB118 cam, MT headers with 2 1/2" exhaust. Just adding the heads and Performer intake put me squarely in the mid 12's. The SP1 got me a best of 12.20 @110.65 with the Q-jet, and 12.11 @ 111.98MPH with the 1000DP carb.
     
  14. Sebambam

    Sebambam Well-Known Member

    I have machine shop ported heads, i have no flow # since the port job was actual mistake by the shop, they supposed to just do Valve job & somehow they ported the heads completely.
    if you have a spare set of heads i would port them myself. lay down the valve boss mainly, clean the bowls and work on the exhaust runners.
    then get a radius Valve job, with bigger valves . Mill the heads to pump up the compression. Thats exactly what i have on my SBB 350 heads.
    and i have a 464 build @ the machine shop right now where i DIY the portjob and let them handle the rest.
    You have to expect costs of arround 1300$ for both heads.
    If you think about porting do it all the way incl. the radius valve job that will absolutely help the performance a lot. with the right cam combo 70-80hp and a good curve.
    HOWEVER, you could also safe up some money and get the TA 350 Alu heads. out of the box with the Stock valvetrain they will be also arround +70-80hp over stock heads. But you really gain power if you get the ALU heads ported with a roller setup i think 100hp + is right there.
    downside is that you will be arround 3500$ in the hole. which is a nice lil turbo setup ;)
    And nothing beats boost on a street car.

    PS: all the gains you get with ported or Alu heads NA you will also gain them to a boosted setup, on top of the boost.
     
  15. gsjohnny1

    gsjohnny1 Well-Known Member

    you can port the iron and alum heads, but what do you get? still an engine that struggles to turn above 6k and still struggles by adding a bigger cam.
    just like accident investigation, you have to start where the problem is. that is on top of the engine. a qjet and a d/p on the 350 is ok for granny.
    but if the youngster wants to run, a s/p intake and a d/p carb is needed.

    been there and not going back.
     
  16. Stevem

    Stevem Well-Known Member

    Yes, what's so hard to understand here?
    To use your example if the heads where ported and you picked up 50 Intake cfm @28" and also the needed greater Exh flow numbers , and then assuming that you ran enough Cam lift to use all of that new higher lift cfm ( and the Cam was changed to make top usage of the ported heads flow ratio) then yes the motor should make a addition 12.5 hp per cylinder, or 100 hp in total!
     

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