PSI Compression numbers
09-13-04, 05:05 PM
#1
She's on Sale!
Thread Starter
Join Date: Feb 2003
Location: San Francisco, Ca
Posts: 208
Likes: 0
Received 0 Likes
on
0 Posts
PSI Compression numbers
What are good compression numbers on a motor? Mine runs a 90/90/90 on both rotors. Are these the numbers of a really weak motor? Please tell me if I need a rebuild. Also I did a search for "compression numbers" but could not find anything that would answer my questions. What are the PSI numbers of a good engine?
09-13-04, 05:12 PM
#2
Eats, Sleeps, Dreams Rotary
Join Date: May 2003
Location: MIA
Posts: 3,639
Likes: 0
Received 0 Likes
on
0 Posts
Ive heard anything under 100 and it means the motor is on its way out. Was this cold or hot? How many miles, just sounds more like a tired motor maybe within the year think about a rebuild, not necesarily that you'll NEED one but it would be something to start saving cash for.
09-13-04, 05:24 PM
#3
She's on Sale!
Thread Starter
Join Date: Feb 2003
Location: San Francisco, Ca
Posts: 208
Likes: 0
Received 0 Likes
on
0 Posts
It was cold. Well then time to go out and do a few WOT runs. Damn lack of boost control will probably send the motor to it's grave. Then I can get a rebuild with the 5 speed rear plate so the tranny bolts will match up. (plans on doing the 5 speed swap himself while the motor is out then plans on sending out the empty shell to replace the wiring harness. I'm really poor with wiring...)
Anyone else have any comments before I go do something stupid like go purposely blow my motor? I need to know for sure that 90 is weak. (Mostly because someone told me 75 psi is ok.)
Anyone else have any comments before I go do something stupid like go purposely blow my motor? I need to know for sure that 90 is weak. (Mostly because someone told me 75 psi is ok.)
09-13-04, 05:31 PM
#4
Insane Burnout
Join Date: Oct 2003
Location: Pleasanton, CA
Posts: 1,568
Likes: 0
Received 0 Likes
on
0 Posts
09-13-04, 05:32 PM
#5
Eats, Sleeps, Dreams Rotary
Join Date: May 2003
Location: MIA
Posts: 3,639
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Elegant Black Monster
It was cold. Well then time to go out and do a few WOT runs. Damn lack of boost control will probably send the motor to it's grave. Then I can get a rebuild with the 5 speed rear plate so the tranny bolts will match up. (plans on doing the 5 speed swap himself while the motor is out then plans on sending out the empty shell to replace the wiring harness. I'm really poor with wiring...)
Anyone else have any comments before I go do something stupid like go purposely blow my motor? I need to know for sure that 90 is weak. (Mostly because someone told me 75 psi is ok.)
Anyone else have any comments before I go do something stupid like go purposely blow my motor? I need to know for sure that 90 is weak. (Mostly because someone told me 75 psi is ok.)
09-13-04, 05:42 PM
#7
She's on Sale!
Thread Starter
Join Date: Feb 2003
Location: San Francisco, Ca
Posts: 208
Likes: 0
Received 0 Likes
on
0 Posts
The first time we did the test the motor was warm. Removed the EGI relay. Well the Alldata program has something that will tell you the rest of the numbers, 90 PSI is 5.75 kgf/cm2 which is damned close to 5.5kgf/cm2 (which according to Rob Robinette's site is on the verge of death.) Besides, 18 PSI will be a fun run. Also it's a good excuse to do the tranny swap. Well thanks guys. Were you at the Falken Drift show Fatman? I was the only AT there.
Also would the weak motor cause the sparkplugs to get fouled within 2-3 days? I am running Turbo II plugs which I generally change every 2 oil changes. All trailing.
I'm tired of the car stalling on my with the A/C on too. Damned thing... (goes outside and kicks the tire of the car)
Also would the weak motor cause the sparkplugs to get fouled within 2-3 days? I am running Turbo II plugs which I generally change every 2 oil changes. All trailing.
I'm tired of the car stalling on my with the A/C on too. Damned thing... (goes outside and kicks the tire of the car)
Last edited by Elegant Black Monster; 09-13-04 at 05:44 PM.
Trending Topics
09-13-04, 05:47 PM
#8
Eats, Sleeps, Dreams Rotary
Join Date: May 2003
Location: MIA
Posts: 3,639
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Elegant Black Monster
The first time we did the test the motor was warm. Removed the EGI relay. Well the Alldata program has something that will tell you the rest of the numbers, 90 PSI is 5.75 kgf/cm2 which is damned close to 5.5kgf/cm2 (which according to Rob Robinette's site is on the verge of death.) Besides, 18 PSI will be a fun run. Also it's a good excuse to do the tranny swap. Well thanks guys. Were you at the Falken Drift show Fatman? I was the only AT there.
Also would the weak motor cause the sparkplugs to get fouled within 2-3 days? I am running Turbo II plugs which I generally change every 2 oil changes. All trailing.
I'm tired of the car stalling on my with the A/C on too. Damned thing... (goes outside and kicks the tire of the car)
Also would the weak motor cause the sparkplugs to get fouled within 2-3 days? I am running Turbo II plugs which I generally change every 2 oil changes. All trailing.
I'm tired of the car stalling on my with the A/C on too. Damned thing... (goes outside and kicks the tire of the car)
yea I was there, but my FD was down, so I was with my dad and bro in the truck. Im running 9s all around and they usually last me 2 oil changes but i clean them ever oil change before the change. Have you removed your airpump? Does the car die only with a/c and the cars hot or when its really cold (like start up) and still dies with a/c on?
09-13-04, 06:35 PM
#9
She's on Sale!
Thread Starter
Join Date: Feb 2003
Location: San Francisco, Ca
Posts: 208
Likes: 0
Received 0 Likes
on
0 Posts
It dies only with the A/C on when the car is hot, also mostly in reverse. Blast, seems I won't be getting help from the family in case of a rebuild. Which means that I better not blow the motor just yet. So how do you do the home depot boost control... wait I think I remember reading up on it on Rob's site... Yes the air pump is still physically there but not connected mostly because the airpump is broken. (It sounds like a buzzsaw cutting something.)
Damnit! Being a student sucks! I'm always broke and no one will hire me because of my lack of job experience... bastards. How is one supposed to get experience when no one will hire you without it.
Damnit! Being a student sucks! I'm always broke and no one will hire me because of my lack of job experience... bastards. How is one supposed to get experience when no one will hire you without it.
09-13-04, 06:38 PM
#10
Eats, Sleeps, Dreams Rotary
Join Date: May 2003
Location: MIA
Posts: 3,639
Likes: 0
Received 0 Likes
on
0 Posts
Originally Posted by Elegant Black Monster
It dies only with the A/C on when the car is hot, also mostly in reverse. Blast, seems I won't be getting help from the family in case of a rebuild. Which means that I better not blow the motor just yet. So how do you do the home depot boost control... wait I think I remember reading up on it on Rob's site... Yes the air pump is still physically there but not connected mostly because the airpump is broken. (It sounds like a buzzsaw cutting something.)
Damnit! Being a student sucks! I'm always broke and no one will hire me because of my lack of job experience... bastards. How is one supposed to get experience when no one will hire you without it.
Damnit! Being a student sucks! I'm always broke and no one will hire me because of my lack of job experience... bastards. How is one supposed to get experience when no one will hire you without it.
09-13-04, 06:44 PM
#11
She's on Sale!
Thread Starter
Join Date: Feb 2003
Location: San Francisco, Ca
Posts: 208
Likes: 0
Received 0 Likes
on
0 Posts
Man, that boost control setup seems pretty complicated, especially when you get to messing with the precontrol. I have some K&N dual cone filters attached to some stock looking pipes and a 3 inch exhaust with straightpipe and downpipe. (Blames the previous owner.)
What is this FPR?
What is this FPR?
Last edited by Elegant Black Monster; 09-13-04 at 06:47 PM.
09-13-04, 07:15 PM
#12
All out Track Freak!
iTrader: (263)
Join Date: Jul 2001
Location: Charlottesville VA 22901
Posts: 10,672
Received 412 Likes
on
250 Posts
A cold engine will test higher which is why once its hot if the compression is bad it won't start. the easiest compression test is driving it hard for 20 minutes or so shut it off for 2 minutes and see if she cranks right up.
90 psi is not too bad and remember the lower the compression the less the chance for detonation. If everything is seated and intact it could run another 50k miles. Think positively
90 psi is not too bad and remember the lower the compression the less the chance for detonation. If everything is seated and intact it could run another 50k miles. Think positively
09-13-04, 07:21 PM
#13
She's on Sale!
Thread Starter
Join Date: Feb 2003
Location: San Francisco, Ca
Posts: 208
Likes: 0
Received 0 Likes
on
0 Posts
Well it has a little trouble starting up if anything it takes a second or two longer for the motor to turn over and then the idle rises to about 1.3k RPM and then down to 500... then up to 750 and stays there at neutral. In drive the rpms drop to 500 or so and stalls sometimes with the a/c on. Also with the a/c on in neutral the revs will occasionally climb and then come back down to 500 then back up to around 800/900. Vacuum with A/C on when the car is cold and in gear is 5. When hot and the a/c on it's at 1. 
I seriously doubt 50k more miles. Maybe 10k if I'm lucky... which would be about... 97,000 miles.
I seriously doubt 50k more miles. Maybe 10k if I'm lucky... which would be about... 97,000 miles.
09-14-04, 05:56 PM
#18
She's on Sale!
Thread Starter
Join Date: Feb 2003
Location: San Francisco, Ca
Posts: 208
Likes: 0
Received 0 Likes
on
0 Posts
Wow, well my friend watched the video again and he says it was more around 85 - 90. Well we'll see how long this lasts. Fatman, are you offering to help me with the boost controller? Because I don't understand those instructions. Perhaps it will keep me from overboosting and it will just die a slow death. In the meantime I need to start finding people who have done the 5 speed swap for tips and perhaps a write-up. Like the clutch/brake pedals install.
09-14-04, 10:34 PM
#19
Rotary Enthusiast
Join Date: Feb 2001
Location: Huntington Beach, CA, USA
Posts: 1,025
Likes: 0
Received 4 Likes
on
4 Posts
Originally Posted by Elegant Black Monster
The first time we did the test the motor was warm. Removed the EGI relay. Well the Alldata program has something that will tell you the rest of the numbers, 90 PSI is 5.75 kgf/cm2 which is damned close to 5.5kgf/cm2 (which according to Rob Robinette's site is on the verge of death.) Besides, 18 PSI will be a fun run. Also it's a good excuse to do the tranny swap. Well thanks guys. Were you at the Falken Drift show Fatman? I was the only AT there.
Also would the weak motor cause the sparkplugs to get fouled within 2-3 days? I am running Turbo II plugs which I generally change every 2 oil changes. All trailing.
I'm tired of the car stalling on my with the A/C on too. Damned thing... (goes outside and kicks the tire of the car)
Also would the weak motor cause the sparkplugs to get fouled within 2-3 days? I am running Turbo II plugs which I generally change every 2 oil changes. All trailing.
I'm tired of the car stalling on my with the A/C on too. Damned thing... (goes outside and kicks the tire of the car)
I'd agree 5.5 is definitely near death. The shop manual has 6.0 as the minimum, so 90 PSI (6.33) I would consider all clear, as long as the chambers are even. However, the test should be done hot. I have examples of a cold test both hurting and helping the numbers, and its irrelavant anyway because the proper method is to test hot. Another thing, I want to point out is a common misconception about how compression relates to engine life: Engines that "blow" do not have to be low on compression. That is to say, engines do not start out at 8.5 and then go down as the car ages and then finally blow when they reach 5.5 or any other number. They can blow at 8.5, at 5.5, or anywhere between. A "blown" motor is a rapid failure while low compression is gradual failure, but the two don't have to be related. If your engine has compression in the 5s, near-death means simply that at a certain point it will cease to idle properly, performance will start to really suffer, it will be hard to start, and so on, and eventually you get fed up, declare the engine "blown", and replace it. This is the common failure mode for N/A engines, while FD engines tend to fail rapidly due to a broken apex seal or coolant o-ring. So, don't interpret 5.5 to mean you should boost it like crazy just to push it over the edge, its a totally different thing.
09-15-04, 01:01 PM
#21
Spec for a new or low mile engine is 125psi. Anything testing 110+ is pretty healthy. Anything under 100 is getting pretty weak. 90psi, even 85psi, is still going to run and drive, but will develop hot start, idle, gas contamination in oil, gas mileage, and fouled plug issues.
There is a common saying that a turbo rotary has a quick way of turning 90 psi into zero.
Note that if you plan to rebuild your engine, blowing it up would be a ******* STUPID idea. You'll tear up a rotor and rotorhousing in doing so, which will add $$$ to the bill.
There is a common saying that a turbo rotary has a quick way of turning 90 psi into zero.
Note that if you plan to rebuild your engine, blowing it up would be a ******* STUPID idea. You'll tear up a rotor and rotorhousing in doing so, which will add $$$ to the bill.
09-15-04, 01:29 PM
#23
Some rotaries like older nonturbos, just run and run until theyre too weak to reliably start and run. Most of the time, 80% or more, they break a seal which leads to immediate power loss and internal part damage. This would be more common on turbo or modified rotaries. You stand your best chance of riding it out a little while if you drive it easy, not much boost or high revs. IF it's gonna break, its gonna break, there isn't a way to know when or why. IF you want to save yourself a few bucks at rebuild time, and you know you're getting close (I'd worry at 85psi), then you should probably not drive the car much until you can get the work done.