Compression too high ?

[easternguy2005]

7mgte, 2K on new rings, reconditioned head, new lower bearings and just got a BHG. Head has OEM bolts and OEM gasket which blew so that it's gushing out the overflow coolant tank at boost. Little white on oil cap.

Compression test results:

#1 = 185 psi
#2 = 190
#3 = 190
#4 = 190
#5 = 190
#6 = 185

Are these numbers too high ?

 

[Rennat]

Are these the numbers WITH a bhg?

head and block may have been shaved a lot, and on a stock HG, your not compensating for that different heights, so it moves the combustion chambers close to the piston.

i remember my n/a only having like... 180psi?... maybe you have n/a pistons in your gte...

 

[tbcmorris]

Are these the numbers WITH a bhg?

head and block may have been shaved a lot, and on a stock HG, your not compensating for that different heights, so it moves the combustion chambers close to the piston.

i remember my n/a only having like... 180psi?... maybe you have n/a pistons in your gte...

What?! Are you serious?

 

[steveyblack]

open throttle?

 

[easternguy2005]

Car came with the original head in the trunk (and reman unit on the engine).

These are the numbers I'm getting with BHG, warm engine, throttle plate open, EFI fuse out, valve position sensor unplugged. About 3 compression strokes for the cylinder being tested till the needle rises no more. Each cylinder tested 2 times.

Car doesn't lose coolant until I start boosting. I expected to see a significant difference.

The high numbers make me think the head got shaved and the gasket is too slim. I don't see these high numbers in other posts. The valves and rings are new so perhaps that explains it....just don't know.

What is a good step at this point ?

 

[Poodles]

That IS high, but doesn't look like a BHG...

Leakdown test and a block test (tests for combustion gasses in the coolant) is in order...

 

[suprahero]

What does opening the throttle body do for you during the compression test? I"ve never did that while I was doing a compression test.

 

[grimreaper]

^ me either. guess it is carried over from the ge to gte instructions. tsrm makes no mention of it for the gte.

 

[IJ.]

It allows full cylinder filling in the minimum number of compression strokes of the engine.

Eventually you'll arrive at the same number throttle closed but usually by the time you get to #6 the battery is down and this will skew your results.

 

[easternguy2005]

Yeah wide open throttle.

BHG ? Hell yeah here's why - One plug is washed and the others slightly coated white.

Did a "pressurize the radiator" test 2 months ago and it lost 5 psi in 24 hrs. Let it continue sitting pressurized and it lost another 5 psi over the next 24 hrs. No water to be seen. Not leaking out the heater core. Generally this is acceptable for some mechanics ( asked 5 and they said - if it holds it 1/2 hour it's good.) but I know something is wrong.

I installed a larger turbo (no coolant lines on this turbo) and now I'm boosting at 8 psi and pushing water out the overflow faster than before.

The problem got worse rapidly with the extra 2 lbs of cooler boost.

Found a post where someone was pushing 185 psi after squirting oil in to test the rings. I soaked up the mess under valve cover #3. But you just can't get the little bit that's right around the plugs themselves. Considering its a good warm engine that might have coated things.

I'm gonna spin it one more time cold tomorrow and see what numbers I get.

Now who is renting out one of those block lapping discs ?

Last time it ran hot suddenly and I turned the heat on.....NO coolant in the heater core.

 

[easternguy2005]

Pulled the head and sure enough - #6 was deformed at the water jacket and #1 slightly deformed at water jacket. I say deformed meaning the exhaust gases pushed on the copper seal of the water jacket but the cylinder seal was not deformed.

Put a straight edge on the block with it wiped down and I'm showing .003 down at the water channels.

Going to have the head looked at.

 

[Zumtizzle]

hmm.

Maybe the tester is off?

 

[easternguy2005]

hmm.

Maybe the tester is off?

Could be the tester off is very true. I borrowed this one rather than buy one. Can't think of a easy way to baseline it.

 

[wiseco7mgt]

Full props to you for not pissing and moaning about a bhg and just getting in and getting the job done.How about a mhg this time.?:thumbup:

 

[grimreaper]

Not a supra but a bhg none the less. Buddies grand prix blew one on a v6. We pulled the heads and low and behold the gasket was PERFECT. Nothing what so ever showing a bhg. ONLY sign was a tiny tiny line of rust on the cylinder bore. We found the crack in hg with a 5X microscope. The guys at the shop thought we were nuts for pulling the heads for such a slight over heating....

 

[easternguy2005]

I set it to TDC before popping the valve position sensor out and taking the belt off so wasn't about to retest. Meanwhile am I the only one that had the dowels from hell on the block. Teeth are easier to pull. I ended up drilling a hole in the center and extracting them both.
When I put the level on the block it was lower right where the breaks are on the headgasket. Picked up a slab of smooth granite (1' x 1') and some valve lapping stuff and after filling the cylinders with grease started rubbing the block. The granite (home depot for $10) was flat within .004 against my flatness gauge. Rubbed that around carefully to get the worst buildup off. Switched to the glass since the granite was heavy so I didn't press down, but more importantly the size was giving the center a workout.

Next used a smaller piece of plate glass with the compound and continued rubbing with a tile on top of it. Circular motion and making sure to cover tip to tip. Also when I was going back to front I didn't go front to back as that would be like double hitting - the guy in the middle gets twice as much. - same for the supra....even on the top.

My friends have suggested rubbing a file back and forth on it sorta like whittling it but I don't want scratches. Of course I hear "the scratches are so the gasket has something to bite"......yeah well I want that head flat not scratchy.

Taking the precision staight edge to it tomorrow. Arp studs are on the way.

I'm going to try a Apex composite gasket one more time. That was the dealer gasket that blew. I feel the composite should be holding me together at 10 psi. With these high compression #'s I may find myself doing this one more time with a metal gasket.

Saving the metal gasket for the next bottom end rebuild. It's freshly built stock pistons. rods, bearings.


Here's some pics - Notice the #6 has the most noticeable damage. That's right next to the turbo. The block side sort of looks worse than the head side of the gasket. Also have a pic of the #1 but not much action there. The picture files are named what they are a pic of.

Tomorrow more of that wax on - wax off shit with my block and checking the head.

 

[Rennat]

Just a guess, but arent those kinds of "cave-in's" consistant with detonation? you can see a little black trail from the fire ring to the copper one right where it pushes it...

why not just get a MHG and measure the block, and head to see what size you need to get your compression back to OEM?

 

[easternguy2005]

OEM ? The TSR doesn't really nail a max pressure. It says 142 or more with 15 psi difference. I fall in that range. Anyone have some wet compression numbers ?

The simplest math (most likely wrong) tell me 190/14.7 = 12.9.

Preignition ? I did show a code for faulty knock sensor and drove it another 15 miles to the garage before changing the sensor and fixing the wiring. I would think the ECU went into retard mode when it threw the error.

Months back used starting fluid only to find out later the MAF needed to be replaced but not without a few good bangs while it tried to start.

Why not a MHG ? They are even less forgiving than composite types. Running under 10psi I keep hearing works fine for this application with some ARP studs. There's plenty of horrow stories of MHG leaking after install due to the tighter tolerances.

When I orginally set the last HG I didn't take much time in prepping the surfaces and the OEM bolts were torqued in sequence using the sweeping arm type torque wrench. The bolts "snap" tighter in much larger steps. So after putting 60# on a bolt for instance the next "snap" up would be 70#. Never mind chasing the threads etc or moly lube !

Anyone have any wet compression #'s ?

I am at sea level here BTW

 

[easternguy2005]

Had the head looked and no cracks or excessive warpage. Trying to avoid shaving this head because it's been to the machine shop before already. I'll most likely lap it very gently.

Finished cleaning the deck, The pistons are flush with the deck. Checks out to within .002 in all points.

Going to finish cleaning up and get prepared for assembly. Got my ARP studs coming from California for $110 including shipping.

There's not much discussed on actual compression numbers so I've done some research. Considering only known facts it's a given that the 7MGE and the 7MGTE are the same engine displacement (2954cc). The compression ratios are different.

Factory numbers are as follows:
7MGE = 9.2:1 with Cylinder Pressure of 156 PSI
7MGTE= 8.4:1 with Cylinder Pressure of 142 PSI

I then did this -

156 / 9.2 = 16.95
142 / 8.4 = 16.90

so PSI / X = 16.9

190 / X = 16.9
X = 11.24


That is my estimated compression ratio. Correct me if I'm wrong.

Also hearsay read You can take off 1mm (40 thou) before the compression ratio hits 9:1

Got some pictures using glass to lap block and where I decided I had done enough - that's the #6 SOB that was leaking looking better. My friend came by and said I would be lucky if I got any carbon off but he was pretty impressed.

 

[grimreaper]

very interesting. Is there a way to measure CR with a gauge besides doing math on the clearances and heights of the combustion chamber?