I bought a wrecked RX-7 with a fresh (3000mi) rebuild. It has one piece ceramic apex seals and a street port. My question is this; what should the vacuum be at running temp. Cold it was about 13-14. Warm it dropped down to 10-11. Is this still in normal range for the mods? My old motor was loosing vacuum steadily for a month before it blew. Just before it went it was about 10-11. Both cars are using Auto Meter gauges. If there is an issue I want to fix it while it's out. Pic was taken after a one mile drive to test boost pattern 10-8-9 as set on purple box. It should be mentioned the temp gauge has been linerized so at 180 deg it's at the bottom.

Thanks,
Steve

 

that doesn't sound good at all, normally it should be at least 15+, mine is at 19 the higher the better. As for the boost pattern it should be 10-8-10, it sounds like you got a vacuum leak and small boost leak somewhere. Do the silicon hose job and get new couplers for ur intercooler pipes (check for cracks on the pipe as well) list ur mods for everyone else to give you a better opinion.

 

If you have any doubts you may also want to get a compression test.

 

that boost pattern is normal. its just tapering off at higher rpms, turbos might be aging a wee bit too. However that vacuum is low. My 86 k mile motor pulls 16

 

The vacuum is dependent on the size of the port done. I've seen large street ports that only pull slightly over 10inHg in vacuum.

 

Yep, porting will definately change vacuum levels. Also, what altitude do you live at? The vacuum is extremely dependant on altitude. When I lived in Colorado (7000ft), my car was pulling ~12in Hg. Now I live in Ohio (basically sea level), and my car pulls no less than 16in. Basically, what I'm trying to say, is that there are many things that effect vacuum. Test your compression, check vacuum lines and hoses, and keep an eye on it. Unless you start experiencing larger problems, i wouldn't worry about it too much.

 

Where can you check vacuum?

 

hey saxyman990, did the lower vacuum effect the operation of the turbo system as compared to sea level?

 

What is a more reliable source of vacuum reading? The Autometer mechanical gauge or the read out from the purple box off the ecu. He showed me the ecu was seeing 16-18 but the gauge in my face was saying something totally different. I didn't know if the calibration on the vacuum readings are adjustable so my gut said to go by the gauge.

 

That's about right for a large street port. Mine reads 11-13 in. or 340-370 mmHg on the PFC.

Jack

 

Ive got a large street port on my t78 and I run 11-14in/hg warmed up with a motor with 3k miles on it.. like they said above.. if its ported it messes with it.

 

I would have thought the one piece apex seal would have been a factor but no one has even mentioned it. All I have herd about the ceramics loosing compression and thus HP due to poor corner sealing. So, I assumed it would affect the vacuum as well. Boy was I off base.

 

Yeah, it sure did. Well technically, it wasn't the vacuum that effected the turbo system; it was the altitude itself (lower pressure, less oxygen). Turbos would spool rediculously SLOW, and the transition between primary to secondary was horrible. Boost would also fall off tremendously at high RPMs. I tried fixing the boost with some ball and spring boost controllers, which helped a lot. But coming back down to sea level put the whole system back to normal operation.

I actually did a bunch of research and analysis on this (yeah, what can I say, I'm a big geek). I'll see if I can dig up all my info on it, if you want.

 

Yeah I'd love to look through what you have on this saxy!!

 

woahhh sorry misread about the streetport

 

Rob, I'm not sure what sort of research and analysis YOU did, but I noticed a difference when I ran a power FC between boost gauge vacuum readings and commander vacuum readings and a little bit better spool and power in dallas rather than Colorado Springs. That spawned enough curiousity to run some #s. I came back to find that the difference in atmospheric pressure converted to the respectively used units (kg/cm^2, inHg, psi) was the same as the difference I was seeing in the gauges.....one was reading gauge pressure and the other absolute pressure which is the difference for when I was at different altitudes. I used to play with it running #s when I was halfway home in Amarillo, trips to Denver, etc wich pretty good real world results.

The turbo is simply having to overcome that difference in atmospheric pressure. I think it was something on the order of 3.5 psi at 7250 ft ASL? So, there's your slower spool and reaction times on the secondary.

As far as vacuum readings, I actually see less difference with a larger port between differing altitudes....but with so many factors contributing in different ways with how my car is setup, I just don't freak out unless it gets too close to 10 and doesn't respond to tuning inputs anymore.
I get about 14-15 or so @ 1100 rpm on a good sized street port.

Vacuum readings can vary a lot between different cars for different reasons. To say that your motor is stronger than another just from vacuum readings is a little fishy I think. I can make my motor to change vacuum about 3 inHg just by changing my AFR at idle...

 

I think that it is definitely worth investigating. I would venture to say that your idle vacuum will be a little lower with a 1 peice seal, but maybe not affect top end power as much...

Corner and side seal effectiveness definitely will affect idle quality and vacuum.

Hopefully you can find someone else using the 1 peice seals and find out what they're seeing. Comparing your readings to those of us using 2mm mazda 3 peice seals probably isn't very fair.

 

Stock motor, I'm at about 19-20.

 

Nic, your results are pretty much exactly the same as what I had. As for the two different readings, you are correct: Power FC (connected to the MAP sensor) reads absolute pressure, whereas the boost gauge reads gage pressure. This leads to a major difference in readings if you are pretty high up in altitude (as we both found out). During my investigation I looked at a bunch of other stuff too (vacuum readings, boost levels, boost response and timing, power output, etc etc etc). Man it was nice to have that Datalogit system.

I'll see if I can dig up all the notes I had on this. They're around my place somewhere, or I left them at my parents house in Detroit. Either way, I'll find em sooner or later. Maybe I'll post my results in a seperate thread...

Isn't it great to be a geek, errrr, I mean "engineer" ?

 

Rob, I think I made a thread concerning this subject a few years ago. If it's still in the database, maybe you could revive it and add your research and results. Seems like something for an FAQ/sticky section...

 

oooo... good idea man. I'll check that out.

 

heh - found them - written back in 2001. (lol)

https://www.rx7club.com/showthread.p...acuum+altitude

good check i forgot with the PFC. Try turning the power on without the engine running and see what your boost reading is. That # was negative for me and Dane (DocWankel). He says his results in this thread : https://www.rx7club.com/showthread.p...acuum+altitude

So, that's what I found back then....

 

Good deal. Well, I have a whole bunch of new stuff (that goes a little more indepth) to add to that thread. Now I just need to find it...