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Technical SBC non-stock tune up - does this seem right?

Discussion in 'The Hokey Ass Message Board' started by David Gersic, Apr 22, 2019.

  1. A couple of years ago, I did a camshaft in a '37 (SBC). We took the front clip off so we could work around everything and installed a spare radiator just to break the cam in and make sure it was going to be OK! Then we reinstalled the front clip.
     
  2. David Gersic
    Joined: Feb 15, 2015
    Posts: 1,883

    David Gersic
    Member
    from DeKalb, IL

    Wouldn’t change the idle... maybe a little change to off idle tip in.


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  3. jaw22w
    Joined: Mar 2, 2013
    Posts: 774

    jaw22w
    Member
    from Indiana

    ??????????????????? How do you do that?
     
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  4. David Gersic
    Joined: Feb 15, 2015
    Posts: 1,883

    David Gersic
    Member
    from DeKalb, IL

    Remove oil pan. Lay under car. Wrench on crank bolt, turn crank slowly while watching #1 piston. When it reaches TDC, check balancer line vs. timing tab. Found the line on the balancer right at 0 on the tab.



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  5. saltflats
    Joined: Aug 14, 2007
    Posts: 9,355

    saltflats
    Member
    from Missouri

    The only way to check true TDC is with a piston stop.
     
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  6. I'd even go lower with the cam, like a 256H since the CR is only 8.5:1. That may work well with a stock converter, but agree on it needing more gear. With the OD trans a 3.55 rear would wake it up quite a bit.
     
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  7. saltflats
    Joined: Aug 14, 2007
    Posts: 9,355

    saltflats
    Member
    from Missouri

    Have you checked cranking compression?
     
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  8. David Gersic
    Joined: Feb 15, 2015
    Posts: 1,883

    David Gersic
    Member
    from DeKalb, IL

    Comp doesn’t show a 256H?

    http://www.compcams.com/Company/CC/cam-specs/cam-search-results.aspx?sc=15&sm=By Engine Family

    252H maybe?

    [​IMG]

    If I go for a cam swap, I’ll need to get the engine specs and I’ll check with Comp before ordering.



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  9. jaw22w
    Joined: Mar 2, 2013
    Posts: 774

    jaw22w
    Member
    from Indiana

    Not exactly the accepted method of checking TDC accurately. Due to TDC dwell you could still be off plus or minus 4 or 5 degrees. Maybe enough to explain needing 25* of lead to run. I am in the process of building a new 383 SBC. While checking TDC with a piston stop, I found the original pointer and balancer to be off by almost 5 degrees. If you can get a timing light to shine on the balancer, you have enough room to check TDC with a stop. This can be done in less than 5 minutes, and gives you a known starting point for setting timing. Piston stops are cheap or you can make one from a gutted spark plug.
     
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  10. He might be referencing XE256H. It is a dual pattern, hydraulic flat tappet with a more aggressive lobe ramp, that gives a lot more torque, vacuum and mileage for the duration listed.

    XE series is a very good choice. You would do well with an XE260H. It just barely has a rumpety-rump. I use them all the time. ;) No I don't work for Comp...

    http://www.compcams.com/v002/Pages/386/XE256H-10.aspx
     
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  11. jaw22w
    Joined: Mar 2, 2013
    Posts: 774

    jaw22w
    Member
    from Indiana

  12. tb33anda3rd
    Joined: Oct 8, 2010
    Posts: 14,931

    tb33anda3rd
    ALLIANCE MEMBER
    1. Connecticut HAMB'ers

    loosen the valves up, and then adjust them with about ten thousands lash. you might hear it but it should "calm" down that cam.
     
  13. David Gersic
    Joined: Feb 15, 2015
    Posts: 1,883

    David Gersic
    Member
    from DeKalb, IL

    Slight delay while I acquired some parts and made a piston stop.

    [​IMG]

    Not perfect, it’s hard to tell if I got the marker exactly lined up with the zero on the tab, but I’m pretty sure the tab shows TDC correctly.

    [​IMG]

    14 3/4 is the water pump side of the tab, after TDC.
    16 is the mark on the balancer.
    17 1/4 is the other side, before TDC.




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  14. David Gersic
    Joined: Feb 15, 2015
    Posts: 1,883

    David Gersic
    Member
    from DeKalb, IL

    I haven’t. I’ll do that tomorrow if you think it’ll add useful information.



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  15. David Gersic
    Joined: Feb 15, 2015
    Posts: 1,883

    David Gersic
    Member
    from DeKalb, IL

    1. 140
    3. 135
    5. 140
    7. 145
    2. 140
    4. 145
    6. 135
    8. 135



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  16. Fordors
    Joined: Sep 22, 2016
    Posts: 1,987

    Fordors
    Member

    That is still a very respectable cranking compression for what appears to be a cam that has a lot of duration. Looking at what the two cam grinders use to measure cam timing shows a different picture though. Crane uses .001 and Comp goes with .006 which tames down the value of the duration. Crane is checking duration using a lower value and that is measuring more of the clearance ramps at the start and end of lobe lift creating a bigger “paper” number. Is the larger number used to sell cams with the old “If some is good, more is better” reasoning? I don’t know but a review of the specs shows the Crane grind to have a 216* duration at .050 and overlap of 70*. But, and it is a big but, the overlap is at that .001 checking value. If we knew what the O/L was at the same .006 that Comp uses to establish their catalog duration figures the Crane likely won’t look as big as it does on the cam card.
    70* of overlap sounds like a lot but I don’t think the cranking compression backs that up. It would be interesting if we could check the cranking compression of the Comp XE256H in David’s engine. None of this helps with David’s problem but it does show his cam to be more moderate than it looks at first glance.
     
  17. saltflats
    Joined: Aug 14, 2007
    Posts: 9,355

    saltflats
    Member
    from Missouri

    Put some advance in this cam and see what cranking compression is.
     
  18. I would guess that XE256H would bring it up 20-25# and make 18-19" of vacuum.
     
    Last edited: Apr 25, 2019
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  19. Fordors
    Joined: Sep 22, 2016
    Posts: 1,987

    Fordors
    Member

    In fact Comp grinds theirs with 4* advance.
     
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  20. treb11
    Joined: Jan 21, 2006
    Posts: 3,192

    treb11
    ALLIANCE MEMBER

    This won't help in the short term, but you might consider a set of Rhodes lifters that bleed off on the lower end. ALLEGEDLY improving drivability, vacuum, etc. In fact, Crane has an equivalent type. Hi Intensity 99377-16. $108 for the set at Summit.

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  21. Sooo here’s how this stuff works.
    In a factory service manual the spec a degree of initial advance set at idle speed all within the tune up specs. When they wrote that they assumed the distributor to be stock, the weights to be stock and because of that being stock the total timing would be where it was supposed to be, makes sense right? Ok now understand it first but then throw all that out the window.
    An engine should be tuned to RUN not tuned to IDLE. So, let your total timing take the place of supreme hierarchy. 36-38 all in by 2500 rpm seems to be a great starting place. Bring RPM up to 2500 rpm and set the timing at 36. Now take the rpm up to 3500 -4000 and make sure you’re still at 36,, THAT PART IS really important. At this point if your engine won’t idle right you ALMOST absolutely 100% know you’ll need to recurve the distributor.

    Total timing first, (this has got to be right)
    Initial timing secondary ( it’s got to be good enough)

    All of this work will be done with a timing light and pointers and marks. Make sure you’ve got a good light!!! Make sure the marks and pointer are correct and accurate otherwise wise you’re pissing up a rope, the very rope you’ll need to climb.

     
  22. Chavezk21
    Joined: Jan 3, 2013
    Posts: 552

    Chavezk21
    Member

    ^^^ this. I had that same cam in a mildly built 350 in a late model nova. THE BEST thing I did with that cam was take it out and throw it away. The duration made it sound huge. The L79 cam I replaced it with though sounded milder but had way better street, driving manners than the crane did, without changing converter, gears etc.
     
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  23. Black_Sheep
    Joined: May 22, 2010
    Posts: 927

    Black_Sheep
    Member

    Generally speaking, a cam with 290 advertised duration needs more compression, more converter, more initial timing and more gear. I don't think the cam selection matches the rest of your existing combo. A cam swap would be the easiest and least expensive remedy.
     
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  24. David Gersic
    Joined: Feb 15, 2015
    Posts: 1,883

    David Gersic
    Member
    from DeKalb, IL

    Consensus seems to be that ultimately I’d be better off with a different cam. I agree, but that’s not happening right now. I’ll crutch it with timing and adjust as best I can for the summer.

    For now, the car is in the transmission shop getting the screwed up 700r4 fixed. I’ll update this after I get it back and can work on it again.



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  25. tb33anda3rd
    Joined: Oct 8, 2010
    Posts: 14,931

    tb33anda3rd
    ALLIANCE MEMBER
    1. Connecticut HAMB'ers

    add some lash and see what happens.
     
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  26. Yep, cheap and easy.

    Sure would like to see a degree wheel on there.
    At least a down and dirty measure off the balancer.
    That cranking compression is pretty sad
     
  27. David Gersic
    Joined: Feb 15, 2015
    Posts: 1,883

    David Gersic
    Member
    from DeKalb, IL

    Already know what happens. It idled well and ran pretty good, with lifters that didn’t pump up, bent pushrods, and rockers that were so loose that one slipped off its pushrod. Not really looking to go backwards here.



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  28. David Gersic
    Joined: Feb 15, 2015
    Posts: 1,883

    David Gersic
    Member
    from DeKalb, IL

    At that point, I’d only be a few more bolts away from replacing the cam with a better one.



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  29. treb11
    Joined: Jan 21, 2006
    Posts: 3,192

    treb11
    ALLIANCE MEMBER

    Lash is for solid lifters, this cam is hydraulic.

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  30. tb33anda3rd
    Joined: Oct 8, 2010
    Posts: 14,931

    tb33anda3rd
    ALLIANCE MEMBER
    1. Connecticut HAMB'ers

    it would still work
     

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