why is my motor pulling vacuum at high rpm?
09-19-2011, 09:56 PM
#1
why is my motor pulling vacuum at high rpm? if im correct, this should mean i have an intake restriction. i recently had the intake ported. now at lower rpms, it gets even closer to baro but it still drops the higher i go. the maf is maxing out at around 400gps. it has a 58mm tb, 1le elbow, stock descreened maf, and lingenfelter cai. the maf was screened but i took it out trying to diagnose this but it didnt seem to help. any ideas?
09-19-2011, 10:30 PM
#4
What RPMs are you spinning your 396 to, and what heads are you running?
The TB nor the descreened MAF will be a restriction on an NA build using the stock computer, but a stock style intake might be.
The TB nor the descreened MAF will be a restriction on an NA build using the stock computer, but a stock style intake might be.
09-19-2011, 10:36 PM
#5
How much vaccum are we talking? Generally speaking, you will always see vacuum in the plenum of an NA SBC engine, especially using a front facing TB like most LT1s do. Your goal should be to minimize it, not eliminate it.
Just because a race setup using a carb can flash to 0 vacuum at WOT doesn't mean an LT1 manifold w/ elbow can ever get there.
PS: A good speed density tune is perfectly daily drivable, the problem is finding someone to take the time to fully tune the SD tables. I doubt that will help though, since your descreened MAF is not a restriction to your NA 396.
Just because a race setup using a carb can flash to 0 vacuum at WOT doesn't mean an LT1 manifold w/ elbow can ever get there.
PS: A good speed density tune is perfectly daily drivable, the problem is finding someone to take the time to fully tune the SD tables. I doubt that will help though, since your descreened MAF is not a restriction to your NA 396.
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09-19-2011, 10:50 PM
#8
Do you know how many millions of vehicles on the road are running speed density with no issues?
BTW it goes into power enrichment at WOT, not speed density.
I'm talking about a closed loop speed density tune... still uses the 02s for feedback and still corrects fueling, just doesn't need a MAF to do so.
Why do you think every aftermarket EFI system out there is Speed Density?
09-19-2011, 10:58 PM
#9
if i got it, id just pay for a retune and have my tuner email me the new calibration for it. same guy put one on his h/c ls1. ill go sd if i have to but i get bad enough mileage as it is. what would be the differences of sd vs maf? drivability differences?
09-20-2011, 07:54 AM
#11
what does the MAP sensor read if you shut the car off and leave the key on.......should be 99-101..............
what is the map reading you are seeing that makes you say you are pulling vacuum? we need a bit more info....
also there are many things that can cause an intake vacuum........from camshaft selection to cylinder head and intake manifold port characteristics......you have a large cubic inch motor.......it is going to take ALOT of cylinder head and cam to satisfiy it completely to reach 0 vacuum at high rpm.....this is a fuction of Volumetric Efficency.....it took alot to get my motor to run 0 vacuum and it doesnt' run it all the time...........you will notice your vacuum is probably lower around your peak torque point also and not your hp peak correct??
what is the map reading you are seeing that makes you say you are pulling vacuum? we need a bit more info....
also there are many things that can cause an intake vacuum........from camshaft selection to cylinder head and intake manifold port characteristics......you have a large cubic inch motor.......it is going to take ALOT of cylinder head and cam to satisfiy it completely to reach 0 vacuum at high rpm.....this is a fuction of Volumetric Efficency.....it took alot to get my motor to run 0 vacuum and it doesnt' run it all the time...........you will notice your vacuum is probably lower around your peak torque point also and not your hp peak correct??
09-20-2011, 08:49 AM
#12
ill check the key off maf reading in a few min.
im using datamaster and the graph is showing a very obvious dip after 4k. its starts at 100 kpa and by the time i shift, its at about 95. iv got a ws6 hood and have been considering a ram air airbox to go with it. the cam is 238/242 .582/.582 112
im using datamaster and the graph is showing a very obvious dip after 4k. its starts at 100 kpa and by the time i shift, its at about 95. iv got a ws6 hood and have been considering a ram air airbox to go with it. the cam is 238/242 .582/.582 112
09-20-2011, 09:24 AM
#13
ill check the key off maf reading in a few min.
im using datamaster and the graph is showing a very obvious dip after 4k. its starts at 100 kpa and by the time i shift, its at about 95. iv got a ws6 hood and have been considering a ram air airbox to go with it. the cam is 238/242 .582/.582 112
im using datamaster and the graph is showing a very obvious dip after 4k. its starts at 100 kpa and by the time i shift, its at about 95. iv got a ws6 hood and have been considering a ram air airbox to go with it. the cam is 238/242 .582/.582 112
09-20-2011, 04:19 PM
#18
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Why is your motor pulling vacuum?
Because it sucks!
http://www.youtube.com/watch?v=oShTJ...e_gdata_player
Jk
Because it sucks!
http://www.youtube.com/watch?v=oShTJ...e_gdata_player
Jk
