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To those with boost gauges...What Vacuum number do you see at idle?

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I'm curious what your boost gauge reads for vacuum at idle. I get 18-20 and Shad at Comptech seemed to think that was a low number.
 

Yellow Rose

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Disagree.....those are very typical numbers - regardless if FI or NA because the engine is at idle, right?
 
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Correct. I get 18-20 when the engine is at idle.

I just read through the old "CTSC underwhelming" thread and found that the guy who started that thread also saw 18 on the vacuum gauge. Sounds like my numbers are normal. Shad suggested that anything below 20 might indicate the timing belt being off a tooth (not sure what that means), but I would think that would create other noticable problems.
 
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Most "stock" engines will pull 20+ at idle if properly tuned and in good condition. If you have a low idle speed or you're at altitude it'll be lower (18 to 20). If your idle is correct and you're not at a significant altitude check your timing or idle mixture. Anything that makes the IAC open more than it should to maintain the proper idle speed will lower the number (including a low battery or other demands on the alternator). If everything looks fine do a compression and cylinder leakdown check. My motor pulls almost 22 inches which I consider good. (I'm assuming your gauge is good).
 
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Mark911 said:
Most "stock" engines will pull 20+ at idle if properly tuned and in good condition. If you have a low idle speed or you're at altitude it'll be lower (18 to 20). If your idle is correct and you're not at a significant altitude check your timing or idle mixture. Anything that makes the IAC open more than it should to maintain the proper idle speed will lower the number (including a low battery or other demands on the alternator). If everything looks fine do a compression and cylinder leakdown check. My motor pulls almost 22 inches which I consider good. (I'm assuming your gauge is good).

What RPM does your car idle at?
 
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I just checked my vacuum compared to idle and I get around 17 at 800 RPM. If I'm driving and let off the gas, vacuum drops to 20 or 21. Does is sound like I have a problem? Is timing something I can check myself?
 
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Vaccum at idle has always been an indicator of the overall health of the engine as it'll expose anything that lowers the pumping strength of the engine (assuming everything else is within spec). Tight valves, poor piston ring seal, air leaks, even a clogged exhaust system are some examples. Mechanics have used it for years as a tool for tuning. I'm able to tune the timing and AF ratio for best vaccum at my idle of 1000 rpm (which implies the least throttle/IAT opening and best combustion efficiency). If you're winding it out and suddenly lift you should see way more than 22 inches, 25+. Yes, with a timing light you can check it easily, just consult the on-line manual. You might want to check your gauge as well. Also, if you just installed the Comptech S/Cer, check the fuel pressure at idle. Should be about 40 psi. If it's higher, you'll probably be running rich as the ecu isn't in closed loop during idle. That might explain a low vaccum at idle, but it doens't explain the chopped throttle results.
 
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We tested this afternoon with a real vacuum gauge an saw 19, 19.5 at idle at the same time my autometer gauge read 18. My numbers were the same as the other NSX that stopped over. We then tried to check the timing, but it appears impossible with all the new CTSC pulleys blocking the view. Anyone have any tricks for checking timing with a CTSC installed?

My idle fuel pressure is 45 psi. The highest pressure I see on the way to redline is 86 PSI. I'm high on the low end and low on the high end. I'd be concerned about higher RPM results if I lowered the pressure at idle.
 
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My idle fuel pressure is 45 psi. The highest pressure I see on the way to redline is 86 PSI. I'm high on the low end and low on the high end. I'd be concerned about higher RPM results if I lowered the pressure at idle.[/QUOTE]

The comptech regulator should only affect fuel pressue at boost since your fuel pressure will (should) only vary from 40 to about 46 psi from idle to 0" hg. Above that (boost) the pressure should increase at a much faster rate from 46 to 90 psi or so from start of boost to 6 psi. I'm not THAT knowledgeable about the comptech FMU, does it replace the stock fuel pressure regulator?
 
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gobble said:
We tested this afternoon with a real vacuum gauge an saw 19, 19.5 at idle at the same time my autometer gauge read 18. My numbers were the same as the other NSX that stopped over. We then tried to check the timing, but it appears impossible with all the new CTSC pulleys blocking the view. Anyone have any tricks for checking timing with a CTSC installed?

The car sounded good, the idle was smooth and the vacuum was the same as my NA car. Oh and thanks for letting me drive your car :biggrin:
 
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