How's My Compression - w/Mazda Results
 

Motor has 1,500 miles on a rebuild and I just got it back from having a compression test from Mazda. It looks like they either plotted my results backwords on the graph or they plotted specs instead of results, they are at lunch so I posting for the forum's input.

Here is the rebuild:

Extremely low mileage stock port Irons.
Really good condition stock port rotor housings
New 2mm ALS Apex Seals
New short and long 2mm Apex seal springs
New Side Seals
New Side Seal springs
New corner seal springs
New corner seal rubber inserts
New Viton oil control rings
New inner and outer coolant seals
New dowel pin o-rings
NEw rear stat gear o-rings
Atkins thermal pellet
Mazdaspeed metal front cover gasket
New front bolt o-ring
NEw front and rear main seals.

Any chance these numbers get better with more miles? Here they are, how'd it do?

#1 Rotor @ 246 RPM - 6.6/6.1/5.8
#2 Rotor @ 237 RPM - 6.8/7.1/7.0

Is that roughly 85/90 Front 100 Rear?

Thank You

Was the engine warm when you performed the test? It's supposed to be. If they didn't do the test wrong, and these numbers are true: they look pretty bad.

It should improve with more miles. Who did your rebuild?

Test was done at Mazda by the head tech, said he drove it first to get heat in it, looks like he put 2-3 miles on it.

Here is more info: Performance - Car has DP, MP, Cat Back, Supra Pump, light weight pulleys

I drove it like ms. daisy, varying speeds and rpms along the 1180 miles I put on the total of 1500. Never saw boost until about 1350 miles. Car would go on 28 mile trips through and around my town about half in town half on highway, I tried to vary gears, rpms, and speeds as much as possible while on the highway. I am stretching here but maybe I was too nice, lol, looking for anything.

Car starts up first try everytime, idle runs to 2000rpm for about 5-10 seconds then tries to die, a touch of the gas peddle and it sits in about 1100-1400rpm, and pulls -16 vacuum. Once the motor starts to warm the idle sits at 1200 and doesn't move vacuum is -18. Usually the first couple minutes of being driven the idle will search around between 900-1500rpm only when stopped and if under load (lights on, A/C, etc) it sits at 1200 rpm. After it has a little drive time under it it idles at approx 1200rpm vacuum -18 just pegged there all day long. Engine seems to run well and strong, I was expecting the numbers to be closer to the rear housing. Its been a long time since Ive owned a 7 so I am looking for more confirmation on the quality of this rebuild. Thanks

This is one of my rebuilds. Im honestly a little disappointed and surprised by the low numbers. I would expect to see between 105-110psi after a break in. Im confident the numbers will get there with time but they should already be close at this time. I recommend a good long interstate trip keeping the rpms up around 3500 or more for a couple hundred miles. If it hot starts just fine I imagine it was good and heat soaked when the test was performed. Either way, no excuses, I would like to see higher numbers. The ALS seals usually give good compression results with little break in time. Just keep on driving it and I am sure they will improve. I always stand behind my work so if any issues arises you can rest assured I will do what I can to make it right.

Always great to see builders who stand by their products =). :icon_tup:

It could be full of gunk after 1100+ miles of babying. Clean her out and try again!

Give it time and those numbers will come up. Also, many Mazda tech don't know how to do a simple compression test. If it starts and idles fine I wouldn't be worried.

Just curious, does the car have any trouble starting when hot (e.g. after a fill-up at a gas station)?

6.6 (96psi) 6.1 (88psi) 5.8 (84psi) @246

6.8 (99psi) 7.1 (103psi) 7 (102psi) @237

Well if that compression test was done properly then it's time to get another rebuild. Makes me wonder if the compression test was done properly. I definitely would get another compression test done where the engine was built.

Mazda manual says that it's rebuild time if compression is below 100 psi @ 250rpm cranking speed. Your compression numbers adjusted for 250rpm cranking speed are:

6.7 (98psi) 6.2 (89psi) 5.9 (85psi) @250

6.9 (101psi) 7.2 (105psi) 7.11 (104psi) @250

For comparision, my compression numbers on a new engine from Malloy Mazda are:

@250 rpm
front(psi) 120,118,113
rear(psi) 127,124,117

The engine had 2k break in miles.

I'd get that compression test done again where you had your engine rebuilt. Almost sounds like the tech took out all the spark plugs insead of just the trailing ones:dunno:. Who knows, but obviously you engine shop is standing by there work which is awesome.

Also, during the break-in my compression numbers actually went down a few psi.

Just did an at home compression test. The only starting issues I've hard are sometimes when attempting to start the car just does nothing, acts like its dead as hell. Roll the key back to off, try again and cranks right over.
Not related but the car is only making 7lbs of boost running parallel so I def haven't over boosted it.


Just did an at home compression test almost how banzai says on their site. A trigger starter was used and test was completed from each trailing plug spot, 1 out at a time and with both out just to see if there was a difference, there wasnt. Throttle was pressed down, piston compression tester was used.

Front Rotor: with schraeder valve held down all bounces were even at approx 65 psi. With schraeder valve closed bounces were 100-95-95. Which its my understanding this is not an accurate reading per face, just overall compression
Rear Rotor: with schraeder valve held down all bounces were even at approx 65 psi. With schraeder valve closed
Bounces were 105-100-100.

Also did the test with out the accelerator pressed and the numbers were lower but I'm just posting as according to the how to I used. I have heard good things about the tech at Mazda and he's been there 24 yrs, owns an rx7 himself but these two tests are too different.

I've heard the senders on the Mazda compression testers go bad over time, it's quite possible it's one that isn't so hot.

Bigger question - how many inches of vacuum do you pull at idle? That's a pretty good indicator of general engine health and sealing.

If it starts easily, idles smooth, and runs smooth, quite possible your engine is just fine, could be a fault in the procedure or the testing instrument.

Dale

Dale: -16 at first start and -18 once warm. I've never seen it outside -16 through -19 depending on engine temp.

I wonder if he followed the compression testing procedure to the letter? You're supposed to remove the trailing plugs for example. Also, there should be correction for altitude. Your location is listed as Illinois so that's a minimum of 600 feet above sea level.

https://www.rx7club.com/attachment.p...1&d=1309396276

look closer at the altitude compensation chart:

https://www.rx7club.com/attachment.p...1&d=1309396624

I think there was an error in the FD FSM though. After checking the older service manuals, the units in { } should be meters, not feet. I'm not sure if altitude is high enough to make much of a difference in compression here. Your altitude might be dropping your readings by 5 or less psi.

Yes...the units, meters and feet, just need to be swapped-- 2X10^3 feet is approximately 609 meters (0.6X10^3).

18 inches at warm idle is about what I'd expect on a motor with used housings. Getting to 20" is REALLY tricky, I don't think I've ever built a motor that tight.

Signs where I'd be worried would be long, hard starts when hot, rough, shakey idle, not wanting to hold a proper idle, lower than 15" vacuum at warm idle.

Dale

Turn the idle down from 1200rpm to 720rpm where it should be and tell us what the vacuum reading is.

Sometimes the obvious isn't seen until it's pointed out, I will try this soon.
Feel free to read the rant at the end or just the purposeful part of this post.

DJSeven and I spoke last night and at this point I am going to adjust the idle to check the vac, put some more miles on the car how he suggests, and test again. The lead tech from Mazda stated today the at home compression test is 100% not valid. I saw a few videos of people doing compression tests and when their schraeder valve was held open they were getting pulses in the 85+ range and I was getting pulses at 65 psi, according to the How To these pulses do not indicate compression...any clarification???

Mazda really hosed up how they plotted compression on their graph, I was lead to believe the 2 plots were my test, once looking at it, the 2 plots have to be their spec numbers...that they labeled backwords. The head tech did state their compression tester was an older version. I am really hoping the car just need some more miles..

A long time friend of mine recommended I go to the local Mazda dealer because he was familiar with the head tech and the head tech is supposed to be very rotary knowledgeable. This friend witnessed the at home compression test last night and went to Mazda today to order some gaskets and to talk about my car. Right away he said the head tech told him "I knew that motor was bad the second I heard and drove the car, before I even tested it." He also said that the numbers were really low for the miles and I needed a rebuild. What pisses me off is that Mazda would not tell me this to my face, I asked 5 times the head service guy and the lead tech if the motor was ok. Each time they gave me a stone faced looked and didn't answer the question. I told this friend last night that the people at the dealership did not like my car. The tech told my friend they could not tell me any information in regards to the condition of the car because it had been modified. I could have left there thinking everything was super dandy duper and my shit blows up 6 months from now and I’m out of luck.

If you dont mind take a video of the engine hot starting or idling. I dont put much faith in the Mazda techs no matter how long they have been with the dealership. Especially on a car with the stock ecu and emissions deleted and non-sequential. You will hear no noticeable difference when an engine is running from one that makes 100psi or makes 120 psi. The fact that he said he knew the motor was bad is just ridiculous. If it was running on one rotor, spewing white smoke or oil smoke out the exhaust, or struggling to idle then maybe I could believe him. I saw this car hot start and run when it had about 250 miles on the engine and it sounded just fine. Unless there is a fluke failure on one of the coolant seals or there is some insane boost spike I have a feeling you will be enjoying this engine for years to come. If that is not the case, you can gaurantee I will make the situation right. :nod:

Ive had one other situation where the ALS seals gave some uneven and sligtly lower than I would like to see numbers after a break in on one housing. That same motor went on to set the all time 1/4 mile record on stock twins. It is still running strong last I heard after seeing 20+psi of boost for the last couple of years from the stock twins.

Turning the idle down and getting a good vacuum reading will show a lot. If for some reason something has affected this engine you can send it back and Ill rebuild it for free assuming the irons, housings and rotors werent damaged from an object passing through.

I'm going to say this again for posterity:

I think your engine is full of goop and gunk and junk after being babied for 1100 miles. Let her sing some, or steam clean it, and report back.

Also, the vacuum reading at 800rpms is pretty key, as other's have mentioned.

DJSeven has really been stand up to this point I fully appreciate the time he took to speak with me yesterday. I am going to do as mentioned above and work on getting that video, then still drive the thing this weekend and make another vid if there has been any changes...I really just want to see the compression even out or come up a bit...to be continued
And I am going back to Mazda tomorrow to get much more from them.

^ I remember cleaning the dipstick out of my coolant overflow tank the first 500-700 miles which was caked full of Vaseline and engine assembly lube... over time it diminished more and more.

Exactly. Not sure what type apex seals were used but they may need more time to seat. I have seen engines with less than 65 psi run for years and make good power.

Also, make sure whoever does the compression test opens up the butterflies all the way while the motor is turning over.

Cold start after 21hrs of sitting, yes I was stupid turning the video sideways
 

After a short drive and turning the idle down, approx 20min after the first video.
 

I used an original flip recorder so the quality is poor. At cold start the engine revs to 2000rpms and then tries to die, this is where I tapped the accelerator and the car took over. Once temps increase idle is about 1100-1200 rpms and vacuum is approx -18 if not a little lower.
After the first video I took the car for about a 2 mile drive to make sure temps were up, then the idle screw located at the bottom of the throttle body was adjusted. Idles approx 750 with vacuum around -17.5. I tried to provide details of the car running at different temps, from the inside, exhaust noise, and engine noise.

I'm thinking the car is going to struggle to stay running at initial cold start.

Any input on the starting idle dive??