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Reverse Boost Creep??


Guest Anonymous

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Guest Anonymous

I think I finally got my other problem solved, (wastegate lines to b/c were too long), my boost bleeds down at high rpms. It will build up in low rpms fine, but then will loose around 5 psi before redline. example: I can get up to 14 psi in second fine but it will slowly go down to 9 or 10. It does this in all gears. Any suggestions? Thanks guys

 

Scott www.flex.net/~mgwilson

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Sounds like you have a leak somewhere if you ask me. Intercooler piping, manifold, etc.

 

I've been following your progress, and I was thinking that maybe you went too small a diameter on your I/C piping. You were using 1.5" piping, correct?

 

Lets say you have about 260hp. Thats approximately 400cfm of air. In a 1.5" pipe:

 

Airspeed Velocity = airflow/area

 

= (400ft^3/min)/pi(1.5/2)^2in^2 * 144/60

 

Airspeed = 543 ft/sec

 

That's pretty darn high, though smaller than I expected. smile.gif I was thinking that perhaps your small diamter pipe was causing an increase in drag, and therefore flow loss. It is still possible that this is occuring though. On the other hand, you should have great throttle response! smile.gif

 

That said, I doubt this is your problem, as you are getting 14psi in 2nd gear, but not 3rd. I would check and double check for a leak. I'd also plan on going to 2" intercooler piping when you can. smile.gif

 

------------------

Richard Lewis - 1972 240z, Powered by a Nissan 2.8L Turbo Inline 6.

Drax240's Turbo Site

Beginners Turbo FAQ & Answers

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Guest Anonymous

thanks drax, I was thinking that my i/c pipin was a bit small, but i used the size that was on the outlets of my i/c. 1 5/8" rad hose fit over it perfectly, so thats what I used. Even if I upgrade to 2", wouldnt that be the limiting factor? Im still going to have a restriction of 1 5/8" at the i/c, right? I was also thinking about my exhaust, which is basically stock downpipe and 2.5" back. Btw, I am getting fairly constant boost in every gear now, it was my lines to my boost controller being too long. I shorted them and it helped out quite a bit. Thanks again

 

Scott

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Guest Anonymous

Drax, you said to check for leaks. Well Ive checked everything at idle with good ol' carb cleaner and nothing shows up, but how do I make sure nothings leaking under boost? I know 15 psi is a lot of pressure, any tips of the trade? :-)... thanks again

 

Scott

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Guest Anonymous

Drax, you amaze me... Im gunna be as smart as you one day, im getting there though. Be my mentor :-). Well, my boost problem is back, it is still increasing with each gear. What gives? I thought it stopped doing it but now i have it set , and its 9psi in 1st, 13 in second, and pretty high in third, I always let off the gas once it gets too 15 or so. My a/f gauge reads rich even at this boost level, does that mean I shouldnt be worrying? let it go up til i hear detonation or it runs lean? TIA

 

Scott

 

know anywhere i can get an electronic boost controller cheap? like really CHEAP :-). What are your opinions on one, a fuzzy logic one or non fuzzy? i have no clue which ones best. thanks

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Im pretty sure i know what your problem is... look at your actuator and wastegate flap sounds like its sometimes sticking open causing you to kose booste and sometimes sticking closed causing it to creep. if youve checked all your connections and hookups and piping for leaks im thinking thats the only thing it could be

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Don't run it until you hear pinging. Sort out the problem without hearing any pings at all if you can. There just isn't that much margin for error there. (ie: by the time you let off the gas after you hear pinging, it could be too late)

 

As for my dizzying intellect, I haven't even begun yet! For my next trick, I'll explain how to put a universal defibulator in your intake tract, thus producting electromagnetic boost throughout the chicken flange. confused.gif

 

I'll quit while I'm not too far behind.

 

------------------

Richard Lewis - 1972 240z, Powered by a Nissan 2.8L Turbo Inline 6.

Drax240's Turbo Site

Beginners Turbo FAQ & Answers

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Hmm, thats tough... Boost isn't produced unless you are under load, which means you can't sit around idling to 15psi. Maybe get a friend to stand on your hood with a bottle of propane, checking for leaks as you rip up and down your street? biggrin.gif Hehe. No, don't do that.

 

Just check and re-check your clamps on your piping. Inspect your vacuum hoses for cracks or splits. You are right, its pretty hard to check at 15Psi without getting really tricky. (rigging up a test with compressed air to leak test your intake system at 15psi, you'd need to somehow block the air to the cylinders however, and that would be a real PITA)

 

As for piping size, its not so much a limiting factor by moving to 2". What the primary concern is as far as I know is air velocity. If you push air through your I/C too fast, it won't be cooled enough. I've read that your target air velocity should be around 450feet/sec. If you go too low, you have a lot of extra volume in your system, which will lead to lag. If you go too high, you end up choking things off, running the I/C in-efficiently, etc. I would want to err on the side of having it too big, rather than too small. (and granted, running 3" or something huge is terrible, when I say "too big" I mean 2", 2,25", etc)

 

Again, I doubt this is the problem you are having, but when you do get things sorted out with your boost you may consider this. One more thing: Get a second opinion on piping size... smile.gif Hehe.

 

------------------

Richard Lewis - 1972 240z, Powered by a Nissan 2.8L Turbo Inline 6.

Drax240's Turbo Site

Beginners Turbo FAQ & Answers

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