there is a general consensus of fd owners that would agree that very poor vacuum can be a determinant of a bad engine/poor compression.



general knowledge that i have acquired from my studies and readings on the various forums was that good vacuum readings on a street ported engine would read about 14~16.....give or take half of a point.....



i am now seeing vacuum of 17-19 on the boost guage at ~1000rpm. it depends on the idle as to where it really lies, but generally, it is ok....i was wondering, that since it is easy to assume that an engine is on its last legs at very low vacuum, is a 17-19 reading a good thing, and lets me know that i still have a healthy unit? i also see 14-16 in vacuum when the idle sits near 875-925....i use my avc-r idle reading to let me know what the idle is exactly....someone please inform me of this..



i have a pettit rebuild, street port, and other goodies. it seems that as the weather has gotten much cooler, my fd has become a different animal. with the block off plates and such on, that the car is somewhat stubborn to start when it has been sitting over night... i guess that it is due to the removal of all that warm-up and emissions crap.....oh well. does anyone else experience the same thigs? if so, please include your thoughts and similarities...



louis

My car is a pain to start up in the morning if its cold. If it sits over the weekend and is cold then it usually takes me two tries. I have a street port and my vacume sits around 15-17 at about 1000rpms. With electrical load my car goes down to about 800rpms and reads about 13 on the vacume. I believe that typically a stock ported motor will see 20 on the vacume side.

the outside temps and humidity have a lot to do with the vacuum reading, but basically the reading is your engines ability to pull air in, the more compression the harder it can pull. also the more rpm the harder it can pull. basically its the poor mans way to tell the condition of the apex seal/ rotor housing seal. those 2 parts wear to each other.



mike

that sounds about right. i also experience what i like to call "poor idle recovery"....typically, is hapens under decelleration, or when i am approaching a traffic signal. the idle drops downwards of around 850 and then slowly and lumpily regains is smooth steady idle....anyone else?





lou

You must also remember that vacuum is dependant on backpressure. If you exh. is free flowing, like most ours are, then your vacuum may be lower. If it's not so free flowing then your vacuum may be higher.

maybe the clutch switch? its up under the dash at the pivot piont for the pedal



mike

maybe the clutch switch? its up under the dash at the pivot piont for the pedal



mike[/quote]

idle recovery is not related to the clutch switch is it? mine is bypassed

i'm not 100% sure what the clutch switch does, but when its bad the car will stall



mike

well, since my car is not reall stalling, it is simply having a more difficult time cranking after sitting for an extended period, no more than overnight though....i simply think that it has to do with the removal and blocking off of the acv, aws, and other ****....



louis

Did you remove the ISC?