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Thread: Rebuilt SN89 low boost

  1. #1
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    Rebuilt SN89 low boost

    Last year I rebuilt and installed an SN89 on a carbureted 306. The motor is fairly built with windsor jr heads, rpm air gap intake, long tube headers, etc. The cam is a comp cams 270H magnum. It is pullied to 40k rpm at 6k engine rpm. I am using the new high output impeller and rebuilt to specs. It has about 40 inch pounds of resistance. I'm only seeing 3 or maybe 4 psi max in the intake. I've tested for leaks and it's only leaking through the throttle shafts a little - could I be losing much boost through there or do you think my cam is bleeding off boost from too much overlap? I'm also at 5200 feet elevation. Could that be killing boost as well?

    I was thinking this would make 6-8 psi on my small 306. Any help or ideas would be great. Thanks.

  2. #2
    SCH Owner Michael's Avatar
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    You're right in saying that a properly rebuilt SN should see something in the neighbourhood of 6-8psi of boost, especially with the HO impeller. And from everything you've stated above, your rebuild was done correctly, so I doubt there's any issues with the rebuild itself. Even taking into account the small amount of bleed-off through the throttle shafts, that shouldn't result in a 3-4psi drop (maybe 1 or 1.5psi, depending on how bad the leak really is).

    Where are you measuring the boost pressure? At the blower outlet?

    If anything is causing your low-boost condition, it's more than likely a combination of the cam you're running and the amount of airflow your heads/intake are capable of flowing. I'm not overly familiar with the Windsor Jr. heads you're running, but if they're flowing good numbers (more than 200cfm), coupled with the cam and high-flow intake then the combination could be a contributing factor to the low boost you're seeing. The SN is good for about 750cfm at just under 40,000rpm. If you're engine is capable of flowing more than that, then the boost you're see will be lower than the expected 6-8psi.

    I'm also assuming you're not running a lot of piping or an intercooler between the blower to the carb either. If you are, that will play a role as well. The greater the volume the blower has to fill between it's own outlet and the engine's intake, the lower the boost pressure you'll see.

  3. #3
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    Do you have a port on your compressor housing? If so measure boost there, at the charge pipe, and at the manifold. If you are loosing boost pressure it will be lower after the loss than before. If you have low pressure at the compressor housing you either have the wrong drive ratio, a leak from the compressor housing at the seam, or too much clearance between compressor housing and wheel therefore it is not compressing just blowing. At least that's how every supercharger or turbo I have ever dealt with works. I am just learning about the SN series but would imagine aside from the ball drive the theory of operation has to be the same as any other.

  4. #4
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    Thanks for the input guys. I broke a rocker stud in the fall, and just getting sound to changing it now. I am measuring boost from the intake itself, below the carb. I have another boost gauge, I'll rig up something to see boost at the carb hat to see what the difference is.

    As far as the piping, it is maybe 1 ft total with a bov in the middle.

  5. #5
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    What should the impeller clearance be? I should double check that anyway.

  6. #6
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    Impeller to scroll cover clearance should be .016 - .020 inches. That is what Paradise Wheels recommends.

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    Quote Originally Posted by JPJP View Post
    Impeller to scroll cover clearance should be .016 - .020 inches. That is what Paradise Wheels recommends.
    Where did you get that info from? I have not been able to find anything on this site except for some torque numbers.

  8. #8
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    I was also told the same thing from Paradise Wheels. No less than .010 on the back of the impeller and .016 - .020 on the curved side.

    The real question is how the heck do i measure the clearance on the curved side between the impeller and scroll cover?

  9. #9
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    I checked the impeller clearance and removed .005 in the shims to put me at .015 on the scroll cover and .010 on the rear. I machined my throttle shafts to accept o-rings and fixed all the other little boost leaks. I even tightened the belt. It's still only hitting around 3 psi. I'm going a little rich under boost, but was still hoping for 2-4 more psi. Must be my camshaft that is bleeding off the extra boost unless anyone can think of anything else?

    I also swapped out the synthetic b&m I had in there for some original b&m and saw no change.

  10. #10
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    Ok, definitely getting belt slip. I thought my belt was plenty tight, but it was getting worn pretty bad after only maybe 1000 miles. I might be to the point where going to a larger SC pulley or decreasing the ball tension would actually give me MORE boost. Currently running a 5" crank pulley and 3.25" SC pulley.

  11. #11
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    A 3.25 pulley is pretty small. That may be causing it? I was told besides over spinning it a 3.75 SC pulley was as small as you can go due to belt slippage.
    I just want to reiterate, I was told this. No idea if its BS or not.
    Are you running a 6 groove belt? I'm running a 3.75 but its on the stock 6.25 crank pulley. I get no noticeable belt slip & Im over spinning it a bit too.
    SN92 w/upgraded impeller on a 93 cobra. Dyno recorded 6.9# when i got it tuned. Their dated as hell but they work.

  12. #12
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    Yes, I would like to run bigger pulleys, however since I am using custom brackets, I am somewhat limited in pulley selection. My crankshaft pulley is only 5" in diameter, so the 3.25" SC pulley gives me about a 40,000 impeller RPM at redline - which should be optimized. The belt is a 6 rib belt. 6.9 lbs is very impressive - I'm getting about 3 - 3.5 with the HO impeller. I'm going to loosen the SC case bolts a hair to decrease the pressure on the ball drive and see if that decreases belt slip any. If not, I might have to eventually build a new (larger) crankshaft pulley and invest in a larger SC pulley ($$).

    The car runs great and drives really well and has a ton of power - I just know there should be about twice the boost, so it's somewhat frustrating!

  13. #13
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    I am running a 3.75 S/C pulley on my '86 Mustang. I have the 6.00 crankshaft pulley and am getting no slip. I too am running the HO impeller. I highly recommend a Gatorback belt. They slip less and the serrations allow a tighter bend radius than other belts.

    A way to check for belt slip is to look for evidence of the slippage (belt dust) on your cooling fan blades and other accessories.

  14. #14
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    Quote Originally Posted by JPJP View Post
    I am running a 3.75 S/C pulley on my '86 Mustang. I have the 6.00 crankshaft pulley and am getting no slip. I too am running the HO impeller. I highly recommend a Gatorback belt. They slip less and the serrations allow a tighter bend radius than other belts.

    A way to check for belt slip is to look for evidence of the slippage (belt dust) on your cooling fan blades and other accessories.
    I am getting a little dust after I take it out and do a couple pulls. I'll look into getting a gatorback belt. Thanks for the recommendation.

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