Well I've been having suspicions of an aftermarket cam in my car so I decided I would try a vacuum test today. Vacuum showed rock steady 15 "hg at around 800-850 rmp idle. Hissin had told me to look for about 20 so would this be enough to show a cam? Not looking for anything huge but it does seem to have a lope. Thnx for any help.
Vacuum Test
- Thread starter 88-GT
- Start date
silverstripes
Member
The lower the reading the more the cam generally.
In some forms of racing such as dirt track they allow a certain amount of vacuum loss from a cam. For instance they will set the rpm at 1000 and the engine will have to pull at least 14'' Hg or higher to pass inspection. This will generally pic out the guys running too much cam than is allowed for the class.
At 1000 rpm if the 5.0 is fairly good shape it should pull 18-20 with the stock cam and the less the vacuum the more the cam.
*This is just an overall estimate for 5.0 engines so please no critisizm on above remarks, some really like to nitpick these replys.*
In some forms of racing such as dirt track they allow a certain amount of vacuum loss from a cam. For instance they will set the rpm at 1000 and the engine will have to pull at least 14'' Hg or higher to pass inspection. This will generally pic out the guys running too much cam than is allowed for the class.
At 1000 rpm if the 5.0 is fairly good shape it should pull 18-20 with the stock cam and the less the vacuum the more the cam.
*This is just an overall estimate for 5.0 engines so please no critisizm on above remarks, some really like to nitpick these replys.*
silverstripes
Member
Yea I was kinda more looking for whether this was a big enough margin from 18-20 to 15 to rule in that the cam is not stock. It will determine the mods that I will do so it is kinda important. Would rather find out this way then pull a valve cover and and check it off the lifter.
89MustangGX
I have nothing productive to add!
Would you build a combo based on a "mystery" cam anyway? Assume that it's stock and setup your combo the way you want it. 
FWIW, my 88 GT (SD) pulls 19.5" hg at 700 RPM.
You could just have a slight vac leak.
I doubt you would want to waver too far with regard to the vac characteristics of a cam or build-up, if you are still SD (we must have chatted awhile back, because this is not ringing a bell with me - but I am very tired right now [and get a few PM's from folks so my short term 'cache' can lapse]).
I would really look hard for a leak myself, before making further assumptions.
Good luck with it.
You could just have a slight vac leak.
I doubt you would want to waver too far with regard to the vac characteristics of a cam or build-up, if you are still SD (we must have chatted awhile back, because this is not ringing a bell with me - but I am very tired right now [and get a few PM's from folks so my short term 'cache' can lapse]).
I would really look hard for a leak myself, before making further assumptions.
Good luck with it.
