Degreeing a Cam and its Effect on Manifold Vacuum

Discussion in ''Da Nailhead' started by JZRIV, Apr 18, 2016.

  1. JZRIV

    JZRIV Platinum Level Contributor

    If you don't say so yourself. :laugh: And especially so when looking at a before pic!
     
  2. LARRY70GS

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

    I'd say you have a problem then:grin: Hopefully you can get more vacuum if the cam is in way off. I can tell you my brakes are fine with 12.5" in gear.
     
  3. CameoInvicta

    CameoInvicta Well-Known Member

    In regards to the minimum vacuum requirement, my Nailhead has a cam QUITE a big larger than yours, and last I checked pulled 10-11 at idle, and 6-7 in gear. With a rebuilt 67 booster and dual master setup, Ive got no issues running power brakes. I am using the factory vacuum canister however. Idling around theres probably only 3-4 good stomps of the pedal available, but the second the RPMs come up the pedal becomes soft again.

    In regards to the small verses large booster, its not that the smaller booster needs more vacuum, rather that the diaphragm area is smaller and needs more vacuum to produce the same amount of assist as a larger booster.
     
  4. Lucy Fair

    Lucy Fair Nailheadlova

    Jason, here is my cam card. It's making 18'' in gear and idles at 550 rpm.
    CAM SPEC CARD.jpg
    Peace,
     
  5. BuickMike

    BuickMike Well-Known Member

    I'll throw my 2 cents in here. As others said, you are best off degreeing with the .050 numbers to eliminate slight variances in the cam lobes at low lift. Note that on the TA cam cards (at least the one that I got) that you need to pay close attention to whether any if the numbers have a - next to them. For example, the card may have ATDC printed for a open or close number with a -2 degrees hand written next to it. That -2 really means that it is 2 degrees BTDC. I initially glazed over on this when I was degreeing my cam and could not figure out why the profile was off. Once I realized what was up, the profile lined right up I found out that my cam was 3 degrees retarded. I ended up fixing that with my timing set and also advanced it a couple degrees to boot since I have a 364 with a twin turbine so a little more low end will help.
     
  6. JZRIV

    JZRIV Platinum Level Contributor

    My degree wheel arrived today so I checked the cam as installed. BuickMike you addressed a question before I asked. My cam card says Intake opens at -1 BTDC. Does that actually mean 1 degree ATDC?

    Since this was my first degree check, I went through the process from piston TDC, intake open @ 050, close @ .050 4 separate times to make sure no errors. Here are the numbers

    Intake open @ .050 4 deg BTDC. Closes 33 degrees which I guess would be ABDC After Bottom Dead Center
    Anyone feel free to analyze these numbers.

    4+33.5 + 180 = 217.5 (cam card 218 duration) 217.5/2 = 108.75 -4 = 104.75

    Tappet lift was .294
     

    Attached Files:

  7. telriv

    telriv Founders Club Member

    And, another way to double check is to use the .050" before & .050" after as I stated in one of my earlier posts.


    Tom T.
     
  8. pmuller9

    pmuller9 Well-Known Member

    Yes -1 BTDC does that actually mean 1 degree ATDC

    The numbers are correct. the cam is advanced just over 5 degrees.
    Please take the time to check the exhaust lobe. There have been some problems with the lobe separation angle being less than spec, 110* in this case.

    Nice job on getting the number to repeat 4 times!

    Paul
     
  9. JZRIV

    JZRIV Platinum Level Contributor

    Tom - the numbers above do reflect .050 before and .050 after unless I am not understanding you.

    I will do this tomorrow evening. I was going to do the exhaust but decided to take it one step at a time. Curious Is being advanced better or worse for idle vacuum?

    Thanks
     
  10. pmuller9

    pmuller9 Well-Known Member

    Advanced is better for vacuum and low end torque.
    The intake valve closes earlier decreasing the pistons time to push the intake charge back into the intake port during the compression stroke and manifold at low rpm.

    If the lobe separation angle is less than spec, it allows more reversion into the intake port at low rpm which will also reduce manifold vacuum.
     
  11. gsgtx

    gsgtx Silver Level contributor

    it will give you more cylinder pressure and more vacuum for a better idle.
     
  12. 8ad-f85

    8ad-f85 Well-Known Member

    Another characteristic of a mild duration cam Like yours (sitting advanced) with a slightly tighter LSA is that despite a slightly worsened idle, they wake up with a snap just off idle...if they don't already have good idle vacuum.
    That overlap is supposed to help cylinder fill and VE by pulling on the intake tract.
    Combos like that really respond to mild porting, and can be turds if there's no considerations for air flow or goofed up valve jobs, etc.
    Your cam is quite small for the cubic inches, and should be a beast.
     
  13. JZRIV

    JZRIV Platinum Level Contributor

    Rather than dealing with suspense all day at work, I did the exhaust this morning at 6:00AM "pre-coffee" but checked it 3 times. Based on result I will need to followup this evening to be sure. Unless I made an error, it seems something is wrong.

    Exhaust Opens @ .050 45*, Exhaust closes @.050 6*.

    Based on calculations using Jim's link:
    Exhaust duration 45+6+180=231 I don't understand why doesn't this doesn't match the cam card spec of 218? (the exhaust tappet lift was exactly .294 same as intake)

    Intake duration 217.5* noted above
    217.5+231=448.5 divided by 4 = 112.1
    Intake open @ .050 = 4 + Exhaust close @ .050 = 6 (4+6)=10 divided by 2 = 5
    112.1 - 5 = 107.1 LSA
    I find it hard to believe the LSA could be off that much so will double check tonight but if it is that would clearly explain my problem.
     
  14. telriv

    telriv Founders Club Member

    I don't know if you rotated the engine completely. You find the point of HIGHEST lift on the lobe & then do the .050" before & .050" after. Maybe you already did it this way.


    Tom T.

    P.S. T/A/Schiender may just owe you another cam. Especially if the exhaust doesn't match up properly because it was ground WRONG. This wouldn't be the 1st. time something like this has happened.
     
  15. gsgtx

    gsgtx Silver Level contributor

    wonder if they used the ta-25 exhaust lobe which is 228* ta-25= 218* intake and 228* exhaust
     
  16. JZRIV

    JZRIV Platinum Level Contributor

    Yes Tom this was confirmed - Thanks

    I need more explanation in one area. The cam is 5* advanced based on spec intake opening @.050 of -1 on cam card......but the degreeing spec says set it up 4* advanced. So is it advanced 1* or 5* from spec.

    Based on calculations to figure intake center line 217.5/2=108.75 -4 = 104.75 which is only 1* for practical purposes from spec of 106*. Right???
     
  17. JZRIV

    JZRIV Platinum Level Contributor

    Final word on the cam.
    Called Mike at TA Performance to review the numbers. He picked up immediately on the discrepancy I had with the exhaust duration. He plugged in -6* instead of 6* for the closing and all the numbers came out correct to spec so that was my error not realizing it should have been -6. Cam is just over 1* advanced from spec installed straight up.

    At least now I know the cam was ground to spec and no problem with how it was installed. I also now understand the difference between overlap at .050 and overlap from the point the valve starts to open.
    Running the numbers, he felt the grind should make enough vacuum for the booster but not much margin for other variables that can negatively affect vacuum. He also felt the booster might be requiring more vacuum than it should to operate. I learned shortcuts were taken on the heads but the valves were sealing OK so really shouldn't have affected vacuum. I'm having them redone no matter.

    To insure success when it goes back together, I'll probably install a different cam that's guaranteed to have full stock vacuum. This car has the problematic auto climate control with many feet of vacuum lines, rotary vacuum switches, controls, etc and if I'll need every inch of vacuum I can get for any chance of the system working.

    Thanks everyone for the assistance and training.
     
  18. pmuller9

    pmuller9 Well-Known Member

    Jason

    The job was nicely done!

    Paul
     
  19. 8ad-f85

    8ad-f85 Well-Known Member

    Not that you would take a risk from reading posts on forums, but while you have the heads off...
    A little bit of pocket porting work goes a long ways towards taming down an aggressive cam, even making it much more driveable.
    Making sure the VJ is out towards the edge of the valve rather than in the middle (or worse...inwards edge) and an appropriate throat percentage to the valve OD is a great starting point.
    Without being overly descriptive like I usually am, I find that it draws the carb harder, which is always a good thing.
    It will enjoy more cylinder pressure, better atomization, mileage, and a powerband that starts sooner than one built without any porting.
    As far as a port is concerned, the valve itself is always in the way, and the strategy behind aggressive cam specs is quickly wasted by impeding what it's really trying to do...jump-start the flow down the intake tract and build cylinder pressure.
    On an A to B comparison, decent head porting alone does as much for low speed response and driveability as any increases up top.
    I think that aspect is not largely promoted by the industry.
    It's an inherent effect to what the aggressive overlap is trying to do in the first place.

    I build many combos that are nearly 100 cubes smaller with much more aggressive cam specs than that and still see higher idle vacuum than described by the post.
    The key is always the heads and intake tract. The exhaust is important too.

    Sealing is one thing...
    Last week I took in some heads that were sealing only because of the carbon and oil deposits.
    Freshly cut and great looking seats were only touching the valve 60% of the way around.
    Intake valves won't often burn the seats like an exhaust valve would.
    I see a lot of VJ's where they seem to only pilot one valve and then cut the rest without setting the spindle to each guide, unfortunately.
    Telltale signs are carbon up inside the intake port a-ways, as well as a rapidly flickering vac gauge.
    That never gets looked at until there's difficulty tuning the carb, etc.
    This usually gets overlooked by the radical rump of a big cam.
    That's my 2 cents. :cool:
     
  20. BuickMike

    BuickMike Well-Known Member

    I initially bought the TA-20 for my motor, but after doing lots of research was concerned about manners given the fact that these motors all came with split duration cams to compensate for the funky heads. I decided to swap it for a TA-112 after talking to Tom and Russ. I'm a long way from getting the car ready to reinstall the motor, but hopefully I made a better choice in the end with the cam.
     

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