Are 340 and 300 heads the Same?

Discussion in 'Small Block Tech' started by James66Skylark, Aug 29, 2017.

  1. James66Skylark

    James66Skylark Well-Known Member

    I want to put duel exhaust on my '66 Skylark. Summit has great kits but they begin after the long tube headers. So I could buy the kit and have an exhaust shop make two pipes from the manifolds backs.

    I did find a complete duel exhaust system from the manifolds back from a '66 Skylark but it's a 340 car. The owner tells me the 300 and 340 heads are the same and this exhaust system would bolt right on with no issues. is that the case?

    Thanks.
     
  2. alec296

    alec296 i need another buick

    300 has a different deck height then a 340. Doesn't mean the exhaust won't work. But the kit from summit and having the exhaust shop make 2 foot section to hook up isn't a bad plan.
     
  3. James66Skylark

    James66Skylark Well-Known Member

    Thanks. I'll most likely do that.
     
  4. philbquick

    philbquick Founders Club Member

    There was a recent post about this. Someone put headers on a 340 car and due to the wider (and taller) block the one tube touched the shifter linkage from the column to the trans. He sent them back and the vendor was able to fix them. Yes, 300 and 340 heads are exactly the same.
     
    James66Skylark likes this.
  5. 67skylark27

    67skylark27 Brett Jaloszynski

    I was able to hook up my dual exhaust to my 300 and 340 just fine with no mods.
    I don't plan on putting headers on it, ever. Nothing to gain for me.
     
    James66Skylark likes this.
  6. James66Skylark

    James66Skylark Well-Known Member

    alec296 likes this.
  7. the funny thing is the vendor used a 67 GS 340 with a 340 and column shift to make the first set of headers which were the pattern for their current product. the set I got from them had header ports that were smaller than the ports in my stock heads.
     
  8. Jim Blackwood

    Jim Blackwood Well-Known Member

    I have aluminum 300 heads on my 340. So far it's working out just fine.

    Jim
     
  9. philbquick

    philbquick Founders Club Member

    Your ports are mismatched by 1/8" on all sides going larger to smaller. Yes it will run but I'm sure there's all kinds of weird turbulence going on at the mismatch step.
     
  10. Jim Blackwood

    Jim Blackwood Well-Known Member

    Um..., no they aren't?

    IMG_0003 (2).JPG
     
    300sbb_overkill likes this.
  11. philbquick

    philbquick Founders Club Member

    Um..Yes they are. What's your point?
     
  12. It certainly looks like he has a custom intake manifold on there under that blower and if he does you are probably wrong
     
    300sbb_overkill likes this.
  13. alec296

    alec296 i need another buick

    Looks like fuel injection
     
  14. Jim Blackwood

    Jim Blackwood Well-Known Member

    Don't have the right jpeg on this laptop or I'd show you the port layout. Yes, the intake is custom. I realize this doesn't apply normally but I sort of object to absolutes on general principals. So yeah, the ports do match. Not to say you wouldn't usually be right, just not this time.

    The intake consists mostly of:
    -a 1/4" aluminum plate bent to match the valley with ports and bolt holes to match the head
    -2 triangular chunks of aluminum above that
    -a set of rectangular tubes from the ports to the opposite sides joining the triangles with the top at the end away from the ports opened up
    -filler pieces to close off the bottom
    -a water channel across the front
    -ports for injectors and sensors
    -a box for the intercooler core
    -mount for the blower
    -removable rear cover
    -sneeze valve
    -and various ports and fittings

    Took awhile to make, and if doing it again I'd leave off the intercooler as I'm not running enough boost to need it. The scoop would then be below my sight lines when driving. In fact, later on I may make another one for just that reason. Just have to give some thought to even air distribution, currently the IC core takes care of that. It also would mean changing the injector location
    just a bit.

    But it sounds like the port mismatch has not prevented owners from mismatching heads and intakes in the past, apparently with quite good results. I wouldn't let that put me off.

    Edit: Which is also saying that the TA Rover heads would be a good option for a 340 or 300 that was built to make proper use of them.

    Jim
     
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  15. philbquick

    philbquick Founders Club Member

    Jim, I'll give you an A+ for creativity and workmanship! If you bolt a cast iron 300/340 intake manifold to a 64 aluminum head and you put a bore scope down the intake runner you will see a 1/8" step all the way around the port flange, it will bolt together and run. This has nothing to do with your custom intake application. 340/300 cast iron head headers will also bolt to aluminum heads just fine. The forced induction you are using will help nebulize the effects of the smaller valves. If you still doubt me I will ship you one cast iron head and one aluminum head so you can see the difference for yourself.
     
  16. 300sbb_overkill

    300sbb_overkill WWG1WGA. MAGA

    "If we had to venture out onto a defined ledge, a mismatch of 1/8-inch or less is not going to cause a problem. A wider mismatch where the ledge protrudes in 1/4-inch or more, this might be significant. But if so, this may also have more to do with the mismatch of parts for the engine rather than the ledge itself. A very large intake port feeding a smaller cylinder head likely will suffer from low inlet velocity and that could be the real reason the power would be reduced. But often, these questions come up because of intake manifold choices that are based on other concerns besides torque and horsepower."

    http://www.onallcylinders.com/2014/...-rectangular-port-intake-big-oval-port-heads/

    Good article that will help correct the misconception that because there is an 1/8" mismatch between the intake and heads with the intake ports being the larger of the 2 that the engine won't run good, that just isn't so. The above quote from the article hints that its not the mismatch hurting performance when the intake ports start getting to be 1/4" of "defined ledge" larger is referring to the intake's ability to flow more air than the heads can consume.

    If the smaller head ports can flow more air than the larger ports of the intake the intake would still be the cork in the induction system. If you have the aluminum sbb 300 heads ported by the shop that Dan Jones uses, they can get 200 CFM + intake flow out of them which would be better than any of the factory cast iron sbb 300/340 heads ever thought about flowing so even a ported sbb 300 cast iron intake would be the cork in the induction with an 1/8" or bigger mismatch.

    Of coarse if the air is stuffed in like Jim B. is doing then a mismatch like that can cause some problems vs. an N/A application.
     
  17. Alssb

    Alssb Well-Known Member

  18. Jim Blackwood

    Jim Blackwood Well-Known Member

    Phil, I'm sure you are right that a mismatch does occur, and you've given the best description of it I've seen so far. Of course on the exhaust side an oversized header tube is sometimes referred to as a reversion dam so no problems there.

    I wouldn't hesitate to use an iron 4bbl intake on an aluminum head motor though. I don't think you'd be giving up anything except for the weight. Do some mild porting to the heads and it might be a pretty good match.

    Jim
     

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