is a reading of -14 in of hg at 800 rpm considered poor, fully warmed up, mid pipe and farrel catback. can a free flowing exhaust effect this reading?

thanks,
chris

 

IIRC, if you have a stock motor (no porting) that's a bit low. 16-20 is normal, depending on engine condition. However, your vacuum gauge could be inaccurate...

The exhaust will not affect the vacuum reading.

 

I've got a stock reman with about 20k on it and vacuum is reading ~20 inHg at idle, just for a reference point.

Joe

 

i got about 19in at idle (750-800rpm) and about 17in with the fans on (1000-1100 rpm)

 

your electrical system can effect it because it changes the voltage the igniton is getting. real noticeable on a FC.

 

man i have seen brand new rebuilds reading at 14 at 800 rpms. so no thats not low. anyhow thats where mine is at, **** i hope that isnt low. :-)

 

Correct: 14 is not low for a new rebuild, since the seals haven't had time to seat-in yet.

 

well im looking at -14 in of Hg with a car that has 51,200 miles on it. Under ebay im covered up to 1,000 miles, however im flying to get the car next week, which will be at least a 800 mile drive. im thinking of asking him to take it to the dealership to get a compression test done just to make sure. should i trust the dealership that they're capable of doing this without breaking any parts to get to the plus along with the test being properly performed?

 

I am sitting at 14-13 in. hg. at idle (750-800rpm). I have a street port and 3" exhaust all the way through with a midpipe. The engine has 4k miles on it. I hope that is not too low for a street ported engine.

 

depends on what seals you have as well. 2 or 3mm

 

Thats where mine sits and I have a large street port. I think some of that has to do with my old *** used housing and irons. Pretty much right on the edge of what people say is good. Although on my piston compression tester I see 105-110psi on both rotors all three faces so I'm not worried. 10,000miles on the rebuild so far.


Joe

 

From http://www.fd3s.net/boost_test.html :

> My car idles at about 750 RPMs. Is the vacuum reading a sign of how
> aggressive the porting is?

Yes.

* Normal stock motor will show 18" or better.
* Mildly ported motors show 14"-16".
* Heavily ported motors can go down to 10"-12".

 

so i should be worried, im on a stock motor, unless theres a vacuum leak somewhere or the boost gauge is faulty

to clarify on the compresion test, i remove the front and rear trailing plugs right, the ones on top, and with a piston compression tester with the check valve removed or depressed, should i get about 35-35-35 or is it at least 85-85-85, ive been trying to figure this out exactly for a while

 

I told you Ill help you with all this stuff man

 

I pull the leading plug and leave the trailings in, although this maybe incorrect.

However you should see 85psi or better on all three rotor faces with the check valve out.

With the check valve in you'll see larger numbers. I'm not sure what these should be off the top of my head, I also never check this part. Seems not needed to me.

 

Vacuum reading are going to be different depending on your sea level. So I won't use your vacuum reading as a way to see if you have good compresion.

 

My 93 has a pretty serious street port. It reads about -10Hg at idle.

 

whats this i read about 3 pulses being 30-35 psi then, thats whats confusing me

 

Are you reading that from here (go to the tech section the click the link for compression test)?:
http://www.rotaryresurrection.com/

Probably testing pressure differential between the tree faces.

Joe

*edit* after reading that again, maybe not. I'll let someone else answer that one.

 

I ported my motor aggressively, and have quite a bit of overlap. I see 12-14 inches vacuum at 900 rpm, oem 2mm 2 piece seals, ~8k on the motor.

 

here is a thread of a bunch of guys on heres vacuum readings.

https://www.rx7club.com/showthread.p...ghlight=vacuum

 

I have a 1st GEN old school 4 port with very agressive porting, my vacuum is @ 14 -16 in hg.
On my 3rd Gen which has a stock motor I`m around 17 in hg.

If you do your compression test with a conventional tester, shreader valve out, you will need @ least 3 seperate bounces of the needle at 70 PSI in each rotor chamber to consider the motor good.

As long are they are even... just continue to boost the hell out of it.

Good Luck,

Andre

 

i have about 4-5 in hg @ 1800 rpms.....


but i also have a half bridge. on my old street port i had about 14-15 @ 900