engine making vac under load and wot!
 

I have an issue with my car i just got running!

Brain teaser for you! Why am i making 20in hg vac at full throttle or even medium acceleration?

I have read that if your tb is to small it can make you have vac during a load, but not 20in, i am puzzeled but dosent seem to be affecting engine performance!

before you go out in left feild, i know this because i have a boost gauge hooked up in the car!

Im thinking that my intake dosent flow enough for the engine, but i`m at a must use right this min, i am waiting on buying the edl vic 5.0 intake, right now i have the tfs street intake!

i know choking it down horribly but its use it or let the car sit!

this is the engine specs:

347cu in
10.3:1 C/R
vic jr heads
comp cams nitrous cam
tfs street intake
30lbs injectors
75mm tb
1 3/4 in headers
2 1/2 h pipe

I have replaced all my vac lines, brake booster, master cylinder so it isnt a false reading!

Well it's hard for me to determine your definition of medium acceleration but an engine can still pull manifold vacuum while accelerating.

As for your too small of a throttle body question. At WOT you may be 1" or 2" of vacuum but as you said, not 20.

Make sure you are using a vacuum source right off the intake manifold. It must be before the throttle blade.

Ok what do you suggest as far as infront of the TB tapping the inlet tubing? all the vaccum ports are on the plenum or tb elbow!

Vaccum source right now is the source bar the brake booster etc runs off of, on the driverside!

i have noticed that my fp gets lower not higher under acceleration! by medium acc i mean halfway to the floor on takeoff!

the regulator vac is straight off the tb elbow!

any input to what the hell is going on is greatly appreciated! i guess in 2 wks we will know if the intake manifold is the culprit.
like i said the car runs good and pulls good up to 5300rpm flatlines wont turn any higher!

it isnt a spring issue there new hell everything is new exept the block and intake!

It can't be before the tb, but rather directly after the tb

there is no vac before the TB

sounds like it's not hooked to either a proper port or you have it inline with a joining vac source line that has a check valve in it, try a different location

Yeah, hook it directly to manifold vacuum source. If it's in the inlet tubing anywhere, or somewhere that air flows PAST the port, then the velocity of the airflow can create a high vacuum reading that's NOT the same as intake vacuum. Also check to make sure your throttle is opening all the way.

Ok thanks guys that is more of what i thought, the source right now is like i said the vac port on the driverside firewall that hooks to your brake booster etc, i`ll try a more direct hookup coming off the elbow and see if my reading changes!

I am thinking that the problem cant be a crack or leaky line, since that would cause a loss of vaccum, I`m thinking more along the lines of the intake is just too damn small, rationally if the TB sizing can cause a 1-2" hg reading then a intake that flows way too little could cause a greater reading!

the thing i am not understanding is the low reading at idle and the high reading under load where it is in normal operation exactly opposite!

I noticed when i have the car idling and i`m outside under the hood looking at my FPG when i rip the throttle the pressure goed down not up!

i have a farily new 255 intake pump and new fuel filter, flow is good, my regulator just appears to be working ass backwards dont know if this is a result of the vaccum issue?!

Thanks to all, will update later!

Sorry for the confusion. Considering which way the air travels I should have said after.

Manifold vacuum is vacuum between the intake port and the throttle body.

Well the grand update! no effect on vaccum no matter where the line is hooked!

Thinking logically the induction system is too small intake piping and the intake!

gonna upgrade to a c&l 76mm intake pipe and get the vic 5.0L intake will know then if thats it!

why not get a 90mm LMAF and 90mm TB with a 4" intake pipe...


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