HP plateau; Pulling Vacuum
HP plateau; Pulling Vacuum I finally made it to the dyno this week after having the absolute worst experience with uhaul. Ended up putting down 435 hp and 421 tq. the power just levels off at 6k rpm and doesnt drop till 6800 when he lets off. changing fuel and timing wouldnt change it at all, increase or decrease. the map readings were low 100s (dont remember exactly) then at 6k start dropping to 90 kpa. the tuner said this was due to a restriction. we ran open maf, hoping it was a dirty air filter, and same thing. my induction and exhaust setup is stock maf, trickflow elbow (replaced torn elbow), summit 58mm tb, ai street/strip ported intake, ai 190cc heads, kooks 1 3/4" long tube headers, kooks 3" ory, borla 3" catback with midsize plate (~2" opening). any idea what could be causing this? or how to further isolate the problem?
from doing more research i found that usually this is caused by an intake restriction since exhaust wont cause it to pull vacuum but just be down on power (i think). So that leaves me with the stock maf, trickflow elbow, intake manifold, or heads. I will post the dyno sheet tomorrow if that will help people visualize whats going on. anybody have an idea?
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Joined: Jul 2005
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Heads and intake manifold won't change that since they restrict flow downstream of the sensor location. Did you bore the intake manifold to match the 58 mm TB? You could also try removing everything off the front of the TB and see what happens.
didnt think about running open tb. forgot about it being able to run on sd if the map isnt plugged in. i could be wrong but couldnt the map still sense the restriction in the intake since its measurement is based off the pressure inside the manifold? not sure about the heads though.
edit: forgot to mention yes the manifold is ported for the 58mm as well as the runners are massaged
edit: forgot to mention yes the manifold is ported for the 58mm as well as the runners are massaged
Last LT1 383 I tuned made almost 25 hp pulling the elbow and MAF off.
i dont have anything to log with at the moment so i was gonna use the track as my tool. if i speed up with open tb then i have found a restriction. my question though is if my tune will be affected by running in sd (just unplugging the maf and removing everything)? i dont want to stomp on it if the tune will be off severely. this is my chart for those that want to see it. http://s190.photobucket.com/albums/z...9302008076.jpg
In very rough numbers 100kpa is about 30in.hg. so 10kpa is nearly 3" of vacuum. Seeing as you said it went from "low 100s" to 90kpa it sounds as it it may be pulling nearly 4" of vacuum.
Old rule for street carbed stuff was 1.5" was OK but for race they aimed for .5", since we don't have the "overcarburating" problem to be concerned with we should aim for low as possible, .5" would be nice. Maybe a 4" MAF and monoblade are in your future??
Old rule for street carbed stuff was 1.5" was OK but for race they aimed for .5", since we don't have the "overcarburating" problem to be concerned with we should aim for low as possible, .5" would be nice. Maybe a 4" MAF and monoblade are in your future??
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I would bet it is your stock MAF.
But you need to be tuned for speed density. It will liekly run terribly with everything unplugged and a non optomized SD tune. Have Jeff make you up a tune for SD.
But you need to be tuned for speed density. It will liekly run terribly with everything unplugged and a non optomized SD tune. Have Jeff make you up a tune for SD.
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