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Boost Issues

Discussion in 'SVT Shelby GT500' started by XP900, May 26, 2021.

  1. XP900

    XP900 Active Member Established Member

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    Location:
    NY
    I noticed the other day that my factory boost gauge was reading low,,,like 10-11 lbs when normally it is pegged (over 15) during a hard accel. Dyno showed 19lbs max last year with no changes since.

    I checked my pump later and saw it moving fluid in the overflow but that doesn't mean I don't have an intermittent pump or relay issue.

    I haven't monitored my IAT2 on my Aeroforce but will start to see if my temps are inconsistent. I don't have any faith in my Aeroforce boost readings but my OBD2 readings are usually accurate so it should read valid IAT2 values.

    Question - I have the VMP triple HE with a Davies pump with the larger VMP supply tank - VMP Gen2R blower. What should my IAT2 range during normal driving in 75-85 degrees outside temps?
     
  2. 2011 gtcs

    2011 gtcs GT500 poster Established Member

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    Heat will obviously play a factor in boost levels, but you may also be having belt slip issue's. How old is the belt? Do you have the stock tensioner.
     
  3. XP900

    XP900 Active Member Established Member

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    Location:
    NY
    I replaced most of my pulleys (had a whine) and belt just before my VMP SC Gen2R install. I have about 5500 miles on the SC and belt...and I replace the tensioner too with a Ford one and have a VMP 90mm idler installed. I don't think my belt is slipping....no belt dust either on the front of the SC.
     
  4. Nutty 5.0

    Nutty 5.0 Well-Known Member Established Member

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    I see about 30F over ambient with that blower and same HE. So figure 115-120F give or take 5.
     
    HKusp likes this.
  5. XP900

    XP900 Active Member Established Member

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    Sep 27, 2020
    Location:
    NY
    Update.....
    Still low on boost...

    Ordered new belt - old one has 10K but looks clean with no dust anyway and no cracks....tight with tensioner (< 10K on both) and 90mm VMP idler on 2.5 pulley - Gen2R

    At idle I have 18-21 in Hg vac,,,appears normal with no leaking and steady (changes slightly with engine rpm as AC clutches)
    Boost bypass valve had 19 - 21 when I was measuring the line going to it. Tested bypass with Mighty VAC with throttle body removed to see movement.
    I saw it start to open at 5 inches and all the way open at full vac. With no vac it is tightly closed by spring. I had ordered a new boost bypass but wasted my money...spare unit for future. Checked SC oil too - clean and at correct level.

    Engine heat not a problem....even cold I have lower boost with low temp IATs.

    So my options are getting really limited. I think boost pressure readings are purely mechanical...meaning I should have close to the same boost readings regardless of any timing issues even if some existed. I don't think my CATS are clogged...only 5K on them and if anything I would think it would up my boost.

    So if the belt is not slipping, the only other think I can think of is the supercharger itself. I'll change my belt in the morning,,,,Gates racing belt to replace the Gates Micro-V belt on it now. I'll be shocked if it changes anything.

    Any other ideas??


    One more thing....when I turned my engine off (even after running only a few seconds) I hear a WISHHH sound....like air / vacuum from a canister...Is there a vacuum release mechanism that opens up when the engine power is shut?? it sounds like it is coming from the drivers side...possibly the air mechanism near the strut tower......is that for vacuum operated controls in the dash??
     
    Last edited: Jun 13, 2021
  6. 2011 gtcs

    2011 gtcs GT500 poster Established Member

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    Just keep in mind the stock boost gauge isn't very accurate as it doesn't go off of engine vaccum but what MAF sensor is reading. If you want to know your true boost reading you need to install a mechanical gauge.
     
  7. Willie

    Willie Well-Known Member Established Member

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    As you may have read elsewhere, our boost gauge is not a direct reading, rather "inferred". The following is my opinion so take it for what it's worth. I believe this boost gauge is actually reading load, with Zero being a load of 1.00 (or 100 percent volumetric efficiency), or the point where positive pressure kicks in.

    I have a GM 3-bar MAP sensor. It is used for three things: 1) Reference for my methanol system, 2) Reference for my JMS fuel pump booster, and 3) For datalogging through one analog channel. Comparing my mechanical boost gauge with my MAP sensor values, I know they are basically spot on.

    I have also datalogged load values. Based on my belief stated above and datalogged values, I created a Boost vs Load spreadsheet. Have you datalogged any full boost passes, before and / or after your gauge started reading low? If so, this would be evidence that you are indeed not making the boost you have. If not, I think the only way to determine this is as 2011 GTCS stated, install a mechanical gauge temporarily and if you can datalog, compare the two, maximum load to what your gauge reads.

    My spreadsheet shows that 10# boost equals ~ 1.70 Load, 18.2# equals 2.15 Load.
     
  8. XP900

    XP900 Active Member Established Member

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    Location:
    NY
    I have an Aeroforce 3.5 bar map sensor (even though I don't have a lot of faith in that product) and it seems to align close to what the inside gauge is showing. Max of about 12 lbs whereas before it would peg the inside gauge and dynno last year showed steady 18-19 lbs. I can feel it somewhat in the pickup...tires also no longer breaking traction in second and third gears.

    I have a cheap but accurate mechanical gauge that I tested with a Mighty Vac that I will place inside the car and test with the belt change. I'm hoping it is the belt but there is no dust, no squealing and jerking of the boost gauges or car body under hard accelerations.
     
  9. SCGallo2

    SCGallo2 Balanced performance Premium Member Established Member

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    Southern MD
    Maybe your throttle body is not opening fully? If you can datalog, capture pedal position (accelerator input), ETC throttle angle commanded (by PCM), and ETC throttle position (actual) PIDS. Disable traction control; at WOT with pedal position at 100%, ETC throttle angle commanded and position should be similar values and both slightly above 80 degrees.

    Throttle position.jpg
     
    Last edited: Jun 14, 2021
    Vinnie_B and ShelbyGT5HUN like this.
  10. 69b302

    69b302 Active Member Established Member

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    Check for leaks, post MAF. Unmetered air getting into the engine would lower the boost guage among other issues.
     
  11. XP900

    XP900 Active Member Established Member

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    Location:
    NY
    Thanks for suggestions....

    Went over everything and pulled off, cleaned and reseated everything....did not find any air leaks.....installed a mechanical gauge tapped into the front boost line. Reading 20-21 inches at idle and pretty steady just before testing. Handheld the gauge and full throttled in 2nd (not any easy task while watching the road and old boost gauge too!). I am getting 16-17 lbs of boost at 65 degrees outside air temp. Feels better and brakes loose in 2nd with the 4S tires but it still doesn't seem to be as potent as before. Did not data log yet.

    I removed the TPS and swapped it from my old stock intake to the VMP intake but haven't road tested yet. Cleaned everything again. More oil coating on the backside of the throttle body than I expected to see but I ran it for quite a while.

    Will test with old TPS installed and try to datalog parameters listed above ...rain on and off next two days so waiting again for dry roads.
     
  12. SCGallo2

    SCGallo2 Balanced performance Premium Member Established Member

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    Once you verify that your throttle body is staying fully open during a WOT pull and drawing in "all the air", monitor AFR and timing for expected WOT values programmed into your calibration, since air, fuel, and timing dictate your power level.

    Keep in mind that there may be nothing wrong with your car, other than feeling down on power (butt dyno). Density altitude, humidity, and temp can influence power production enough to effect wheel spin in 2nd and 3rd gear. Reading your previous thread about your dyno results, outside air temp was 45-50*F, and I am certain that your hood was open providing more unrestricted air flow. Since then, you installed a larger diameter exhaust system, reducing some restriction. Between differences in weather conditions, exhaust restriction, and hood open vs. hood closed, I can see you having a slightly lower boost reading. If your calibration and hardware are doing what they are programmed to do, then it is what it is...
     
    Last edited: Jun 22, 2021
  13. XP900

    XP900 Active Member Established Member

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    Location:
    NY
    THANKS for all the info.

    My car seemed to be having other issues too that I had just noticed last week....like loss of power / shuddering between 1k-2k. Almost like a miss or bog around 1.5K which also seemed to coincide with what I thought was a loss of boost. Since the temporary mechanical boost gauge showed pretty high boost at high rpm I'm thinking the lower rpm issue is what makes it feel like it is down on power. I yanked out my Aeroforce garbage and installed a Autometer mechanical boost for future testing.

    I spent two days and pulled off the SC , injectors etc to check everything. Checked all hoses for leaks, vacuum pressure, bypass valve operation. Cleaned injectors, rails, intake, TB, PVC etc. Ran tank out and installed fresh Shell 93 V-power gas with new fuel filter. Swapped out coils with a spare set. Installed another set of Iridiums gapped at a larger .032" setting this time. Swapped out MAF, PVC valve, purge valve, fuel pressure valve, fuel filter, TPsensor with a spare set I had.

    Put it all back together and tested. There are no codes coming up - ever. Anyway it is idling really nice but I still feel what seems like a bog / power loss between 1 and 2 K. Over 2K and it runs fine.

    Not sure if datalogging can help but I am going to give it a shot since i haven't done it in two years. Also looking to see if I have another VMP throttle body to swap. I have to see my tuner in two+ weeks since he left some OBD tests tuned off when it was tuned last year. His tune ran fine for 6K but I am not ruling that out either. I miss the old easy days of carb tuning!
     

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