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Old 05-09-2009, 02:09 PM   #1 (permalink)
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Possible causes for high rpm boost drop?

My car is done and it is making some great horsepower. I am supposed to be picking it up tomorrow hopefully. I upgraded the turbo to a GT2871 but the turbo pulls around 20psi in the mid range and drops off to 16-17psi max in the upper RPM range. I am really surprised that it does not pull more boost in the upper range especially since I have a bigger turbo on it (It did have a GT28RS).

Matt believes it is due to pressure drop in the intercooler but the intercooler seems to be not to bad in size. It appears to be bigger than what forcedfed uses but maybe it is not as efficient.

Could it be a problem with the blow off valve, wastegate, or intake restriction? Ideas? The car has a cone intake filter on it so I would think that was ok, I know they changed the wastegate (still internal), but the blow off valve was (Blowing off of the pipe) at one time. It does however pull high boost in mid range.

I guess it is making about 355hp to the rear wheels, which is great but I thought it would be about another 25hp or so, and probably would if it pulled the 20psi at higher rpm. Comments or suggestions would be appreciated. If I just need a better intercooler, maybe next year I can invest but I don't want to spend the money if that is not the issue.
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Old 05-10-2009, 07:09 PM   #2 (permalink)
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I picked the car up today and it is scary fast! Matt did a great job on the car and it runs excellent. I honestly don't know that I will ever want more horsepower but after doing some research it does seem that the intercooler is probably the culprit for the boost drop. Matt tried it with and without the cone filter, and that did not make a difference and the blow off valve seems to be holding up so the intercooler would make most sense. Other thoughts?
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Old 05-10-2009, 07:13 PM   #3 (permalink)
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You try a boost leak test?
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Old 05-11-2009, 06:51 PM   #4 (permalink)
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Not that I am aware of. How do you perform one?
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Old 05-12-2009, 01:20 AM   #5 (permalink)
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I thought boost drop was normal because the second cam has a greater VE, so its flowing the same boost with less resistance, hence the drop. Changing the the VVTI settings and retarding the exhaust cam apparently helps.
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Old 05-12-2009, 02:15 AM   #6 (permalink)
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If you have an internal waste gate on that turbo it's very possible that the exhaust pressure is forcing it open and over coming the actuator. If this is the case you would need to upgrade to an external larger wastegate.

If you have an external waste gate you may need to put a stiffer spring in it for the same reason as above.
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Old 05-12-2009, 03:55 AM   #7 (permalink)
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355rwhp and 280 lb-ft...
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Old 05-12-2009, 05:37 AM   #9 (permalink)
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Quote:
Originally Posted by Bavarian Motorist View Post
355rwhp and 280 lb-ft...


From my experience in my car, that seems like the absolute limit Id want this chassis at(350whp/280tq). I mean if I can get power oversteer with 180ftlbs, I Imagine 280 would take alot more pedal precision, or else
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Old 05-12-2009, 08:35 AM   #10 (permalink)
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Depending on which GT2871 you went with, you may be at the turbo's limit.
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Old 05-12-2009, 06:12 PM   #11 (permalink)
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Which ECU are you using?
If you have a Datalooger, why not install a Backpressure sensor to have 100% sure, if is pressure drop or a small turbo?
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Old 05-12-2009, 06:45 PM   #12 (permalink)
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Do a boost leak test first before ripping stuff apart in your engine area. A tiny pinhole in your vaccum lines, piping can cause a significant loss of boost. Your shop should be able to do this for you. Or, if you doing it yourself, here are some instructions:

IC Pipe Tester

When the system is pressurized with air, listen for leaks around the clamps, vaccum lines. Don't pressurize the system more than what you typically run your boost at...
If you can't hear any leaks, spray some soapy water on your piping/clamps to see if you have any bubbles. Note, the instructions above said you can use a bike pump, but i tried that myself and couldn't pressurize the system enough. Best to use a compressor with an air tank, but don't have the compressor running while you're listening for leaks.

Or , you can google "how to do boost leak test" and you'll find a gazillion articles on it.

Here's a homeade boost leak tester i made for myself with $10 in parts

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Old 05-12-2009, 06:49 PM   #13 (permalink)
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Here's a dynoplot of a new Evo X with a boost leak versus leak fixed. The aftermarket intercooler piping clamps were loose causing a leak



Thread discussing the leak:
SoCalEvo.net : Got boost leak?


Quote:
Originally Posted by GST Motorsports link=topic=77447.msg7651978#msg7651978 date=1240783657
I'm sure most people know that boost leaks can kill power and cause drivability and tuning issues.

However it is nice to be reminded visually sometimes at just how much a boost leak can suck power out of your car.

This is a Evo 10 MR with the following mods:

MilSpec 100cell cat
Cosworth Drop-in air filter
Nisei hot side intercooler pipe upgrade
Factory ECU (no tuning yet)

Dotted lines are baseline with a boost leak, solid lines are after fixing boost leak. Amazing what a good boost leak can do eh? :uglystupid2:


So far, every single Evo 10 that has come through the shop with upgraded intercooler pipes have had boost leaks. Remember to properly install and tighten the clamps!
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Last edited by Mr. Know; 05-12-2009 at 06:56 PM.
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Old 05-17-2009, 07:09 PM   #14 (permalink)
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Thanks for the info on the boost leak test. It will probably be a little bit before I get to doing anything about it but that would make sense. As far as the wastegate goes, I thought that was only important to control your boost and keep it at a specific level. My understanding was that if the internal wastegate was being forced open it would cause boost creep to a higher boost, not limit the boost. Is that right or am I incorrect?

The car is very fast and I drive it at a lower boost level (usually at 14-15psi) which gives it well over 300rwhp but at the touch of a button, I have all the power available. I wouldn't mind figuring out if there was more available from this turbo as it would be nice to have that power in case I run across a new 600hp Viper or 630hp ZR1 Corvette on the highway. I know I could keep with them now but an extra 30hp would place them in my rear view mirror for sure.

By the way, as far as controlling the power, it is a lot to get used to but with the boost controller turned off, it runs 11-12psi and is probably 290-300whp with a lot of torque. I pulled off the fastest time of the day at the first autocross competition for the year. I was running street tires and it has stock suspension. This was also the first time I have ever raced the car and I have only been driving it now for a couple days.
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