Ok i have about 430 miles on my rebuild now with used housings and plates. My compression test was 110 both rotors, with a standard compression checker. My vac still seems to be only 11 to 12 at 1100rpms. It does have a medium to large street port. Just seems wierd to have a good compression and bad vac.

 

Hey mate, was your engine ported before you got it rebuilt?

The bigger the port job - the less vacuum you will pull.
Im guessing you wernt ported before or had a smaller port last time then you do now, so now now you think your vacuum reading is lower then it should be.

Depending on your port size your vacuum sounds reasonable to me.
Best of luck to you and enjoy your car

 

Yeah stock ports before but still i read other peoples street ports having better vacume

 

I'm no professional engine builder, but I noticed a huge difference in vacuum when I enlarged the exhaust ports on my motor from medium sized (matched to the sleeve) to beyond that. I had the same intake porting template on both motors. I replaced a rotor housing on the current motor and the side seals, but the car now pulls 12 inches of vac instead of 15-15.5 with the older crappier condition internals. That's usually because there is more overlap between the intake and exhaust stroke.

 

It is hard to judge the health of a street ported engine by looking at vacuum. You will not get anywhere near the same vac readings you would on a stock port. Vac is pretty useless at judging engine health unless youa re seeing a boucning vac needle and it is sitting around 8-10 psi, that pretty much always tells you that a seal is chipped/broken.

 

There are lots of variables to vacuum readings:

1. Brand of gauge, Autometer and other cheap gauges have a larger tolerance, when 0 is a block taking up 4 psi, you know the gauge is sloppy. Test the gauge with a Mitivac.

2. Gauge line routing, with most cheap gauges you need to route a plastic hose into the passenger compartment. This has a tendency to get kinked and crack. This is a vacuum leak and will lead to lower readings on the gauge. Test the line for leaks.

3. Vacuum source on block. People have a tendency to route the line to the available nipple on the passenger side of the UIM. This is connected to one intake runner. Relocating the source to a nipple closer to the TB will give a more realistic reading of both vacuum and boost.

4. AFR's, if the car is running overly rich at idle the vacuum will be low, same holds true if it is overly lean. Tune the car.

5. Other vacuum leaks. This is obvious, if there are leaks at the gaskets, nipples, hoses, etc then the reading will be low.

 

Elevation will also play a roll in your vacuum reading (I've read 1 inHg per 1000 feet, but I cannot confirm that). In Montana, the lowest point is around 1800 feet above sea level as compared to say Missouri which has a lowest point around 232 feet above sea level. So if you have a ported motor and live at a higher elevation, your vacuum reading will be lower than others.

Your vacuum reading seems about right considering you have a ported motor, slightly higher elevation, and you are idling at 1100 rpms.

 

Excellent info, I can chim in of my story, my car has stock ports and always pull 19-20 inch vacumn even after beating on it since 1995, over the weekend I went out for meet in connectiuct about 230 miles round trip. Anyway I notice my car had bouncing idle when I came to a dead stop, did not think anything of it, because it came down to normal. When coming back from the trip I beat on it then I notice my boost sound weird? Anyway I got home park the car, everything is fine until I start it yesterday I notice high idle and low vavummn 15 inch?. I have check all couplers and vavumn everything seem fine, then I deceide to use old fashion trick use carb spray on while the car is idling, I spray everything until when I get to next throttle body and UIM the car start to bucking about to shut off, found my leak, throttle body gasket is shot.
Khris

 

Your vacuum reading is not even close to being correct.

For example:

Our stock ported motors pull between 16"-18" vacuum @ 720-750 rpm. We have even had customers report upwards of 20"
Streetported motors 14"-16" vacuum @ 800-850 rpm

10"-12" vacuum at 1100 rpm is half-bridgeport territory

Rx7aholic- the new TB gaskets are made out of metal, for that very reason.

 

FWIW, a local friend had/has a Pettit street-ported motor idling around 1100 rpms with a vacuum around 12-13 inHg (using a Pettit ECU). No vacuum leaks, and has been running strong for several years. As Banzai has mentioned, tuning plays a lot into it. From my experience, the rechipped ECU users tend to have a slightly lower vacuum reading because of it.

 

I think this is proboble correct a combo of the elivation street port, ecu. I also am going to relocate the vacume from the throttle body instead it makes sense. My boost gauge is an obx which isnt the best so give or take a psi. Well im not worried, im going to give her a few more miles and let it rip. Think ill wait till 750 miles.

Thanks for all the imput guys feel pretty safe now