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400 or 305 heads on a 350---Which is better?

Discussion in 'The Hokey Ass Message Board' started by Johnny Bouncewell, May 2, 2013.

  1. Johnny Bouncewell
    Joined: Apr 8, 2013
    Posts: 7

    Johnny Bouncewell
    Member
    from ABQ

    Hey guys, first post here. Google kept sending me here for answers to my questions so I figured its a good place.


    I'm reviving a 1971 Jaguar XJ6 which I'm installing a 350 into to replace the 305 that is in there now. I pulled off the heads for the 350 and the casting number is 3951598. Much to my surprise they are Chevy 400 sbc heads around 76cc with 1.60 exhaust valves and 1.94 intakes.

    The 305 heads I have are 14014416 with 58cc chambers, 1.84/1.50 valves. I know I'm bumping up compression. The Jag is heavy at 3800 lbs so torque off the line would be nice which the small valves would help with. Punching them out to 1.94 would help the top end but would probably not be in the budget.

    The 350 I'm planning installing one of these heads in has flat top pistons but is a fairly low compression motor. It is bored 60 over making it a 361 if that makes much of a difference. Due to the Jag's frame rails, I'm limited to a rams horn exhaust manifold but its connected to a fairly free exhaust. I'm not sure how the rams horns factor into the equation.

    I'm broke and these are the two options I have. I can't afford much machine work on the heads so I'd probably install them as is. Both heads were pulled off running engines so I'm hoping they'll be still okay when I install them on the 350.

    Thanks for the input guys!
     
  2. sdluck
    Joined: Sep 19, 2006
    Posts: 2,678

    sdluck
    Member

    What size ram horns are in it,I believe the choices are 2 " or 2 1/2",what cam is in it,deck height,head gasket thickness,transmission.Some more information would be nice.
     
  3. classic gary
    Joined: Sep 24, 2009
    Posts: 497

    classic gary
    Member

    use the 400 heads and a shim head gasket, you'll be fine.......
     
  4. oldolds
    Joined: Oct 18, 2010
    Posts: 3,130

    oldolds
    Member

    Are you sure you have a 350 not a 400 block? If I remember correctly 400 blocks need heads with 2 extra holes. Without them a 400 will overheat. The holes can be drilled into other heads and cause no problem on a 350 block.

    P.S. I hope you read this before the thread goes away. You are way off topic with a 71 Jag.
     

  5. if you can find a pair of heads from a tuned port 350 youd be surprised how much it would wake it up
     
  6. davidbistolas
    Joined: May 21, 2010
    Posts: 962

    davidbistolas
    Member

    57ford is right. The mid 80s L98 heads are essentially what's used on the zz4 crate motor. The problem with the 305 heads is how they breathe, and the small 58cc chambers. Your compression ratio won't just get a LITTLE bump.

    Personally, I'd use the 400 heads to get it running but have a head swap planned in the near future.
     
  7. GassersGarage
    Joined: Jul 1, 2007
    Posts: 4,728

    GassersGarage
    Member

    By the way, where is ABQ?
     
  8. Gman0046
    Joined: Jul 24, 2005
    Posts: 6,260

    Gman0046
    Member

    ABQ is in New Mexico.
     
  9. mustangsix
    Joined: Mar 7, 2005
    Posts: 1,297

    mustangsix
    Member

    My OT daily driver is a Jag XJs with a 350. 300hp cam, T350 trans, edelbrock 600cfm carb.

    I had 76cc 350 heads, but switched to 64cc Vortecs. The difference in power and fuel economy was very noticeable. With flatops my CR is right at 9.5:1 and the engine will run on regular 87 octane fuel. No pinging and I'm getting 17-18 mpg in normal driving. About 20 highway.
     
    Last edited: May 3, 2013
  10. 400 heads are not good for much....have a history of cracking.....
    #450 -"305 heads are great as they increase compression..... the 400 heads have large combustion chambers and promote the opposite....On a street driven car/auto trans- at lower than 6000RM's you will not see any real dissapointment in using the #450's
     
  11. Johnny Bouncewell
    Joined: Apr 8, 2013
    Posts: 7

    Johnny Bouncewell
    Member
    from ABQ

    I'll take a look at rams horns later today. Cam is pretty mild and won't likely be getting something new while I'm in there. I'm not sure on deck height but the head gasket I have is a .048". Trans is a 700r4 with 3.31 rear axle ratio. Unsure of stall.

    Yes, I'm positive its a 350, I ran the block numbers and I didn't see the steam holes on the block.

    And yes, I'm way off topic but this forum seems to have a helluva lot of knowledgeable dudes. I'm used to wrenching on Euro stuff so I'm a bit outta my element. I didn't expect to get a SBC schooling through a Jag. I never imagined the mind boggling number of variations and possible combinations out there. Its kinda fun but can be overwhelming.
     
  12. poprockcrusher
    Joined: May 17, 2009
    Posts: 123

    poprockcrusher
    Member

    Buy some Dart 2 SR heads ..pretty affordable ..as opposed to crack happy 305 heads
     
  13. Johnny Bouncewell
    Joined: Apr 8, 2013
    Posts: 7

    Johnny Bouncewell
    Member
    from ABQ



    I'd love to do that but it'll cost more than I paid for the car, which I'm trying to avoid.

    I have both 2 inch and 2.5 inch rams horns available to work with. The 2 inchers will be easier since that is what was installed on the car.
     
  14. chubbie
    Joined: Jan 14, 2009
    Posts: 2,311

    chubbie
    Member

    I've used the 305 heads, and like them. I matched the heads and polished the exaust side, and cleaned up the intake side. It runs great and gets 20 MPG.
     
  15. Johnny Bouncewell
    Joined: Apr 8, 2013
    Posts: 7

    Johnny Bouncewell
    Member
    from ABQ

    What do you mean by matching the heads? I'm used to dealing with only one head. I won't have time to do any serious cleaning/polishing. Do you think they'd be okay as stock. I'm not looking for a super hi performance option here, I just want to pick the best head I have for the engine I have.

    Also, 20 mpg would be great in this car!
     
  16. Tnomoldw
    Joined: Dec 5, 2012
    Posts: 1,563

    Tnomoldw
    Member

  17. 56sedandelivery
    Joined: Nov 21, 2006
    Posts: 6,678

    56sedandelivery
    ALLIANCE MEMBER

    With that "thick" of a head gasket, your quench is already messed up. You figure the pistons are probably a rebuilder or stock type, and therefore .020 down in the hole to start with, so you're looking at a quench of a least .068, when you want around .040. You should probably go with a thin, steel shim gasket to get the quench into specs/range. Otherwise you'll end up with an Arniold Schartzenegger engine, "The Detonator". I've found with this economy we've been experiencing the last few years, a lot of guys are being forced into selling off good parts; many of them very good parts, and very cheap. Watch Craigs List in you area, I'll bet you can find a decent, ready to go, pair of heads for a good price; not a used pair of heads that are far from ideal. JMHO. Butch/56sedandelivery.
     
  18. 40Standard
    Joined: Jul 30, 2005
    Posts: 5,828

    40Standard
    Member
    from Indy

    if you're broke, why not just sell it?
     
  19. Johnny Bouncewell
    Joined: Apr 8, 2013
    Posts: 7

    Johnny Bouncewell
    Member
    from ABQ


    I was thinking the thicker head gasket would help limit some of the compression increase I would have when installing the 305 heads. Am I way off base here?


    I'm not that broke but I've have a pretty tight budget to work with for the engine. There is a lot of other work that needs to be done!
     
  20. I think he means port matching......take your gaskets that go between head and intake=scribe the gasket opening on heads and then intake and die grind the offending areas till they match the gasket[and each other]...this promotes smooth airflow from intake into heads much better[for amount of work[ for mileage or performance.



     
  21. Johnny Bouncewell
    Joined: Apr 8, 2013
    Posts: 7

    Johnny Bouncewell
    Member
    from ABQ

    Okay, so I measured my deck and it was big, 0.035 ish. I think it would be a better idea to go with the 400 heads since I know they'll work fine. I'm already screwed on quench space and will be in the .06-.08 range depending on head gasket thickness.

    With a pair of heads on hand, I just want to make sure I had the best set for the block. Hopefully I'm not overreacting and walking away from free HP with the 305 heads. I just want to make the best of what I have on hand. If I had 350 heads, I'd just put those on and forget about it.


    With the 58cc heads I'd be around 11 to 1 static compression with the thick head gasket. With the .02 head gasket someone recommended, I'd be around 11.5 which is waaay too much for a DD. 11 a lot too for that matter.


    Thanks for the input, sorry my car isn't totally JJ worthy but I've been impressed with the discussions on this forum (and am also a GJ reader). Just think of it as a technical engine question.


    Cheers,
    Johnny
     
  22. falcongeorge
    Joined: Aug 26, 2010
    Posts: 18,341

    falcongeorge
    Member
    from BC

    One word. Vortecs.
     
  23. indyjps
    Joined: Feb 21, 2007
    Posts: 4,097

    indyjps
    Member

    neither choice is very good, if youre determined to use those 2 sets of heads, I'd say whichever has the best valve seats and guides.
    used vortecs are pretty cheap and will give much more power than either the 400 or 305 heads, you will need a new intake, or modify the bolt angles on an old one, this is limited to what intake as to how well it will work.
     
  24. chubbie
    Joined: Jan 14, 2009
    Posts: 2,311

    chubbie
    Member

    OK, fair question....gasket matched
     
  25. Ever tried to sell a car that doesn't run?


    The scrap guy will buy it but not too many other people will.
     

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